Updated October 2011.
Download the new page to replace the page from 2006 because
won't work and there is some new information.
8585 Amplifier in 2004 with 8 x
Picture of 8585 on a bench.
Fig 1, Schematic of one channel of
2006, input and driver stages.
Fig 2, Schematic of one channel of
2006, output stage.
8585 General description,
Local output stage cathode FB and B+ regulation,
Global and Local NFB,
Max Vrms output for various RL,
THD, Frequency response,
Speaker load value and Amp Output Resistance,
Inbuilt line level preamp,
Fuse replacement, Fuses within amp,
Fig 3, Schematic of
Turn on delay,
Protection circuit function for owners,
LEDS at front panel of amplifier,
Graph for power output vs load for 4 x
Fig 5, Graph
harmonic content for middle setting of volume control.
Fig 6, Graph of harmonic content for
volume control setting.
Fig 7, Graph of 3 THD
8585 THD for comparisons.
Graph of harmonic content for 12AU7
Fig 9, Schematic of
8585 amp as
supplied in 2004.
Fig 10, Graph of
2004 version of 8585 with 4, 6 and 8 ohm loads.
Fig 11, Schematic of
circuit and explanation.
Fig 12, Schematic of
block diagram form.
Many notes on NFB and PP basic operation,
Fig 13, Graph for
analysis for 4 x KT90 used with 12.5% CFB.
How the loadlines are drawn, Step 1 to Step 16.
Fig 14, Picture of
chassis. Following notes.
8585 amp without reading all these notes is possible but
ABOUT THE BIAS ADJUSTMENT AND FAULT
INDICATION OR YOU WILL
HAVE UNWANTED INTERRUPTIONS
TO YOUR MUSIC.
THE BOTTOM COVER OR TOP TRANSFORMER
ANY PERSON TO POTENTIALLY DANGEROUS
ARE AN EXPERIENCED TECHNICIAN !
The basic design of the push-pull 8585 was conceived in 1995,
several years of trials with various PP topologies to get the
subjectively pleasing sound, along with the best measurements
possible with the least use of
corrective circuitry known as global NFB.
The fundamental output stage circuit design of the 8585 is
based on the
use of beam tetrodes or pentodes with renowned Acoustical
famously exploited Peter
Walker in the 1950s in
and continued in the much later Quad-II-40 amps with the same
Andy Grove, and in Quad 80 amps.
The performance of my 8585 output transformers surpasses
for Quad amplifiers.
The October 2006 version 8585 is an integrated amplifier, with
line level inputs, balance control, line level SET preamp amp,
Incremental improvements since 1995 evolved the present fine
The only change to the 8585
since 2006 has been in the PSU
where the shunt regulator
using 4 x
75V zener diodes in series has been
replaced by the
in the revised PSU schematic below.
The string of 4 seriesed zeners were found to change
to less than the 75V nominal value, and probably from repeated
There are presently 4 x KT90 output tubes per channel, but the
the use of all the main octal based output tubes such as 6V6,
KT88, 6550, KT90, and now KT120, in 2011.
However, only the 6L6GC, 5881, KT66, KT88, 6550, KT90 and
are able to be plugged in and biased to suit the existing
The use of EL34 and 6CA7 are usable, but require minor
the circuit to ensure the correct grid bias voltage these 2
For 6V6 the anode and screen supply must be re-wired to
anode supply of
no more than +350V and a screen supply of about +300V.
There is a tap on the 180Vrms A-B HT winding which gives a
stage cathode feedback and B+ Regulation.
The output stage for each channel has 4 output tubes, two for
push-pull circuit, and these are configured to allow 12.5% of
voltage to be fed back locally to the output tube cathodes
a tertiary winding on the OPT.
With a 5 ohm load, the 12.5% of local NFB equates to about 7
The reason for applying two lots of negative feedback, with
one lot in
of local cathode FB in the output stage and the other
local FB in the output stage has
a very favorable effect on the spectral content
of distortion harmonics which are reduced to lower levels than
connection but without paying a penalty of requiring too high
the output stage tube grids. This local NFB around as short a
results in minimal secondary IMD distortion products being
Therefore less global NFB is then required to make up a total
and it is thus easier to achieve unconditional stability
critical stabilizing techniques.
If the output tubes were connected in pure beam tetrode mode
spectra would remain more complex and the amplifier would be
to stabilise, even though 20dB of global NFB would reduce all
The local CFB prevents considerable production of unwanted
needing global NFB to complete the work. To accommodate the
global NFB, the input driver stage has been designed to give
usual amount of THD for the wanted drive voltages.
The anode supply is +500Vdc nominally using an 800 VA toroidal
transformer, silicon diodes, and large value capacitors, with
The output stage screen supply is +330Vdc, and actively shunt
The output tubes have fixed grid bias voltage applied, once
cathode current is adjusted, see below.
The output stage is driven by a "long tail pair" differential
strapped as triodes and supplied with DC via a centre tapped
from each end
of the choke to the EL84 anodes.
(A choke is a winding of wire around special grain oriented
steel core aka
GOSS and it has high impedance to signal currents, but has low
a high direct current anode supply easily ).
The novel and little used choke loading method in all my best
dynamic signal current change in the EL84 to about 1/3 the
to use of a pure resistance
supplying Idc to the tube.
The EL84 anode voltage swing is increased with the choke and
the EL84 having to waste its efforts to make signal power
where it is
or needed in low ohm value
resistors bringing DC to the anode.
EL84 anodes are loaded mainly by the pair of 100k bias
each side of
the PP circuit. Thus each EL84 anode has a capictor coupled
load of 50k
impedance which is many times higher than 50k at most audio
In this case, the use of the choke was found to reduce THD to
1/3, or -10 dB,
compared to conventional circuits using cheaper dc carrying
The input stage of the power
is a 12AU7 with both halves paralleled.
The dc supply to this input stage is via a pair of 100k
which means that although the anode signal voltage is up to
7Vrms, there is
not a huge anode current change in the parallel triodes.
The 12AU7 is set up as a single ended triode and there is 12
NFB from the output transformer secondary applied to the
cathode via a
At clipping the amp requires about 2Vrms applied to the 12AU7
and there is 1.5Vrms of fed back voltage applied at the
cathode and the
difference between the input voltage and fed back voltage is
to make up to about 7Vrms to power the following Long Tail
driver stage ahead
of the output stage.
This basic method of globally applied NFB has been used for 60
and there is a total of 19dB of NFB applied consisting of 7dB
stage NFB which reduces the output impedance of the output
The 12dB of global NFB reduces the 2 ohms down to about 0.4
which results in a damping factor of greater than 10 for a 5
The distortions in pure beam tetrode connected tubes is
reduced by the
output stage NFB by about 12dB and then the global NFB further
by another 12 dB.
Without using any NFB the amplifier would be useless because
resistance would be many times the speaker impedances and the
both audible and objectionable.
The output power is 84 watts into 5 ohms at less than 0.3% thd
the onset of overload or clipping.
Power is class AB1 with an initial
20 watts of pure class A.
There can be 57 watts watts into 8 ohms at less than 0.2% thd,
with the first 40 watts being pure class A.
Refer to graph BELOW for power output vs load to examine the
power output levels possible.
The use of 4 ohm loads and
signal levels into loads with the 3 output terminals
strapped together are as follows:-
33 ohms, 18W, 24.4Vrms, 34.5 peak volts,
15 ohms, 32W, 22Vrms, 31.1Vpk,
8 ohms, 57W, 21.3Vrms, 30.2Vpk,
5 ohms, 84W, 20.5Vrms, 28.9Vpk,
2 ohms, 112W, 15Vrms, 21.21vpk,
Three pairs of Quad ESL will have an impedance varying from
11ohms at 60Hz to 0.6ohms at 18kHz.
This may seem like an impossible speaker impedance to drive
but 90% of
the power in music is within the 60Hz to 3kHz band where Z is
11 and 3 ohms, and only a tiny amount of power is needed to
frequencies above 3 kHz, and since the 8585 has a power
4 Quad-II amplifiers in parallel and
considering that stacking the Quads will
increase their power sensitivity approximately 3dB, then
the 8585 will not
have more difficulty driving 3 pairs of ESL57 than a
single pair of Quad-II amps
will have driving one pair of ESL57 which are regarded as a
of amp and speaker since both were designed in the same era by
Even with just a 0.6 ohm load there is 60 watts of power
8585 which means that the output current ability without
10Arms, or 14 amps peak.
At any load above 3 ohms,
1 watt of power, thd < 0.02%,
4 watts, thd < 0.03%,
16 watts, thd < 0.1%.
80 watts, 5 ohms, thd < 0.3%,
See the Graphs BELOW with following notes for Harmonic
levels of THD with 5 ohms.
The frequency response for 1 kHz and 5 ohms is from 14 Hz to
80 watts, limited by saturation of the OPT at LF, and
limiting at HF.
The response widens from about 5 Hz to 68 kHz at ordinary loud
With a test load comprising and RC series network of 0.5ohms
with 6 uF
and with 5 ohms shunting the RC network there is no peaking in
Pure capacitance loads of any value between 6uF and 0.1 uF may
across the output terminals with the HF response showing less
at 20kHz, and not more than 6dB of peaking between 20kHz and
so the amplifier
is stable with any value of C load.
Tests were done on C loads at low output voltage levels of
that the diminishing impedance of C loads at HF did not cause
circuit to activate because of excessive dc anode current draw
For example a 2uF capacitor has 2.48 ohms of purely reactive
and if the output voltage level was raised to equal that with
resistance at clipping at 1 kHz, the amp will shut down within
Amplifier Output Impedance.
Any type of load is permissible, including dynamic, ribbon or
and the amp will drive any load above 3 ohms for an average
of 5 watts, which allows for peaks in the music to be 50 watts
There are 3 pairs of output terminals arranged so there is 0.6
resistance from each output to the common internal connection
so that if 3 pairs of Quad ESL are connected with each pair
of output terminals then each pair of speakers is fed via its
The output source resistance in series with the ESL is thus
of 0.4 ohms plus the 0.6 ohms giving a total of 1 ohm which is
for Quad ESL57 in order to get a flat response to 20kHz.
The 8585 will thus mimic the action of the Quad II amp but
stability and lower distortions.
For normal speakers where the lowest amplifier resistance may
all three active terminals to each channel may be strapped
with a wire
the 3 active binding posts, and thus Rout = 0.4 ohms + 0.2
ohms = 0.6
which gives a damping factor of 10 with a 6 ohm speaker.
With the volume control turned to the middle 12 o'clock
4.9Vrms is needed for clipping power of 80 watts into 5 ohms
an ear deafening level of approximately 104dB SPL using 3
ESL57 based on being able to obtain 86dB using 1 Watt at one
ESL speaker from one channel.
At 80 watts the amp makes 20Vrms into 5 ohms and it does not
voltage rating for the speakers. The level with 1Vrms of input
expected from a CD player will reduce the output voltage to
4Vrms and a
output of 3.2 watts
per channel which would give an SPL of 94dB with the 6
stacked Quad ESL. The gain of the internal preamp has been
2.6 times or 9dB ( see below ) between its input and output so
will not be a problem yet there will always be a high enough
regardless of the
speaker variety used.
The gain of the preamp was somewhat carefully chosen because
there is no
ability to delete the preamp from the signal path.
The 12AU7 integrated preamp ahead of each power amp uses both
the twin triode paralleled, and there is an active constant
carrying load component to reduce the THD to tiny amounts
of the input levels which
can be up to 20Vrms before the preamp clips.
Even when the output level of the preamp is at 10Vrms with its
THD < 0.3%, and at normal levels where a CD player produces
level signals, the THD < 0.06%, and nearly all second
thus does not destroy the musical fidelity.
See the graph of THD for 8585 input preamp which shows the 2H,
5H and 6H as they rise above the noise floor between 0.4Vrms
The 12AU7 preamp has a mild 12dB amount of shunt NFB between
output and grid input to ensure channel gain remains constant
and to reduce THD and noise and Rout.
The input selector switch is a 2 pole x 6 position silver
supplied by RS components. The balance control pot is a cermet
by Farnell, and the gain control pot is a dual 50k stereo Alps
"Black" carbon track
pot which has been used in numerous quality amps for the last
which is available at RS and Farnell Components.
The four front tubes are 12AU7.
The next row of four tubes from the front are EL84 or 6BQ5,
the same type of tube, but with different commonly used type
The rear eight tubes are the eight matched octal output tubes.
and Home Service
which should be
replaced by the owner is the 3 amp slow blow
mains fuse near the IEC input mains chassis plug at the rear
of the amp.
The amp must have the mains cable removed from the amp before
other fuses within the amp and placed close to the
circuit point, and none should be replaced by the non
without a technician examining why the the fuse blew.
See the list of fuses below the power supply schematic, Fig3.
Fig 3 Power supply for 8585.
Mains fuse = 3A slow
type 3AG, for Mains = 240Vrms, 50Hz or 60Hz.
Mains fuse = 6A slow blow, type 3AG, for Mains = 120Vrms, 50Hz
Changes to Mains voltage settings are done by removing the
Mains fuses are externally accessible and mounted in rear panel.
Anode dc supply fuses, two 1A slow blow, one 0.25A slow blow;
removing bottom cover.
Bias supply fuse, one 0.7A or 1A slow blow; access by removing
transformer cover on top of the amp.
Heater filament dc supply, two 3A slow blow; access by removing
transformer cover on top of the amp.
There is a short turn on delay of a few seconds, only to
input current at the moment of switch on, and a relay click
after turn on.
The amp has active protection to prevent one or more of the output
from conducting more than 3 times the 33 mA of idle current
If this ever occurs, a second relay in the power supply will turn off the
the output tubes,
turned on with heaters
glowing, but unable to make any sound, but harmless in this
Without any anode current the amp cannot overheat any part of
During this "fault" condition, the two red LEDs at the front
of the amp
to indicate the fault condition.
the amp after tripping the protection during an accident such
turning up the volume with shorted speaker leads is achieved
20 seconds after
turning off the amp and then turning it back on.
Fig 11 further down this page
protection schematic and notes
about its exact operation.
at the front also indicate the bias condition of the
When the bias of each output tube is correctly adjusted, the
remain extinguished. A small variation of bias balance between
halves of the PP
circuit of each channel will cause the LEDs to light up.
During normal operation, the red LEDs should remain
extraordinarily loud music, the leds may flash at times due to
bias current imbalance. Should noticeable distortion be heard,
LEDs flash at an unusually low gain setting, something is
speakers or leads, ie, they may be shorted together, or there
may be a
A schematic of the protection circuitry is further down the
page in Fig
The amp uses fixed bias. This is misleading, since if it was
it has to be adjusted?
Well, once adjusted to the correct level, it remains fixed at
for at least 3 months, and sometimes for many years without
And there are 8 output tubes which EACH require separate
All the 12AU7 and EL84 are automatically biased and need only
during a yearly routine check up.
should be checked every 3 months by a
However, owners find
seldom needs adjustment unless red LEDs
light up to indicate some bias "drift has occured.
You will need a simple voltmeter and flat bladed screw
with a thin
shaft at least 150mm long.
Bias adjustment can be done without moving the amp from the
stand, and speakers and preamp maybe left connected.
Volume control should be
to to zero volume level.
There are 4 test points on each side of the front panel, with
corresponding to an adjust screw on the top of the chassis
each output tube, when viewed left to right.
Each test point is a recessed brass plated philips head screw.
It is safe for an unskilled
perform the bias adjustment.
The positive red lead probe of the volt meter is held
philips screw head and the negative black probe is plugged
the black lead probe, or simply held against
The long flat bladed screw driver is used to reach down
tube cover to turn the shafts of the 8 bias adjustments.
Do NOT attempt to turn the
philips head screws;
these are not
The voltmeter is set to a low range of direct voltage, say the
and is used to measure the first test point voltage and the
on top of the chassis
adjusted in either direction so the test voltage measures
Although slightly awkward, an untrained person may hold the
of the voltmeter with red lead to the test point, and the
adjust the appropriate screw.
Make sure that the red lead probe is not shorting the recessed
screw to the chassis.
When replacing a tube or all tubes, always check the bias
tubes after turn on and turn the bias voltage measure to lower
if any read
Allow the amp to warm its tubes for 5 minutes and then set the
all 8 test points slowly and accurately to 0.7V dc from points
1 to 8.
After 10minutes, repeat the bias setting from 1 to 8 since the
of one or more tubes will affect the setting of the remainder.
minutes, repeat the bias adjustment again.
Both the red LEDs in the front panel should both remain
the balance in both channels will be correct because there is
in each tube.
If one or both the LEDs remains alight after the bias
done it wrong, and turned the wrong screw whilst measuring the
test point, or
you have a problem in the amplifier.
When correct, *all* test points should each measure 0.7Vdc
test point and chassis. I have found the bias voltage may be
which means Ia for each tube is about only 28mA and yet the
The relative rotation position of each adjust screw will be
with new tubes, and as the tubes age, the rotation position
If it is impossible to obtain enough adjustment screw rotation
to get a
reading for any output tube, and Vdc is higher than 0.7Vdc, or
0.25Vdc, it is possible that this tube has a fault, and it may
The output tubes are those most likely to ever cause
The output tubes run at about 150C temperature at the
top of the
each output tube has its filament power liberated as heat,
power input of about 16Watts, plus screen grid input power of
making a total of 27Watts.
An orange glow should be seen at the centre of the small
electrodes within the larger dark grey colored outer metal box
seen easily just inside the glass
This larger electrode within each tube should never appear to
If it does glow, there is a fault in the tube operation, and a
the tube will feel that it is running hotter than the tubes
Such overheating should be reported to the amp maker, but the
circuit has been designed to turn off the amp before the tube
red hot due to
too much current flow.
Highest power consumption occurs when the tubes used
are 6550 or
which have 1.8A rated filaments, and when the amp is working
But the power consumption will vary very little during
average power below 5 watts.
Power consumption from the mains is as follows, with
Output filaments, 8 x 6.3Vac x 1.8A = 91W.
Input 12AU7 filaments, 4 x 12.6Vdc x 0.15A = 8W.
Driver EL84/6BQ5 filaments, 4 x 6.3Vac x 0.8A = 20W.
B+ supply to anode supply, 510Vdc x 0.4A = 204W.
Bias voltage supply, -132Vdc x 0.020A = 3W.
Sub Total power = 326W. Allow winding losses of 6% plus
extra input power for AB operation with music up to occasional
additional 74 watts.
Total maximum mains input power = 400Watts,
and idle power = approx 350Watts.
The amplifier need a sturdy support bench and
only when turned off and with great care.
The 8585 is 42Kg, and above the maximum weight allowed by
standards for unaided lifting in a workplace. But many heavier
are available from ARC and VAC, and I like to think mine give
music and less smoke per Kg than my competition :-).
Never drop the amp to a hard surface because the weight of the
may deform the steel sheet metal cases or aluminium/brass
Due to the weight of tube amps, lifting them around can
produce a back
ache if not done correctly.
sure he'll enjoy a listen
never be built, but 85 Watt monoblocs will be.
To give minimum weight, ease of handling, and very best
the two monoblocs have cabling to a third and separate
supply chassis, so that each chassis is less than 15Kg.
General notes about 8585 output power,
THD measurements, and active protection.
Also some basics about NFB, and PP amp operation.
The available undistorted
power varies with
load. For the 8585
the recommended loading for
should average 3 ohms or
The Fig 4
shows the power output at less than 1% THD for
between 0 ohms and 33 ohms, using 4 x KT90 per
Distortion products in the 8585.
There are 3 following graphs with comments
tested, the results shown are for the right channel only
curves for each channel were remarkably similar.
Fig 5 shows the
components of 2H to 7H where relevant, ie,
increase with output voltage levels.
There is considerable 2H produced in comparison to the 3H
which would cover the average listening levels of 99% of
The amount is all
less than 0.03%, and lower than an SE amp, and utterly
Notice that the dreaded higher number 4H, 5H, 6H, and 7H
do not appear significantly until the amp is being worked
or 13.5Vrms of output, where they try to rise above the
Fig 6 shows the
harmonic products produced when the input
preamp has its output minimized by turning up the volume
This gave less 3H and more 2H especially at high levels,
and Fig 5
& 6 show the
effects of 2H cancellations when the preamp is used to
before the volume
I first thought perhaps the preamp 2H would add to the
power amp 2H
produced in the V2 12AU7 but this appears not to be the
case, so the
2H is otherwise
being produced in the power amp to what i expected.
The point is that the 2H is a small amount of 2H for any
In PP amps the 3H is usually the most dominant but in this
design of PP
amp the 3H is very low compared to many other designs
using such low
Being able to obtain less than 0.014% of 3H at 4Watts is a
any tube power amp, and regardless of the volume control
In both Fig 5 & 6 the higher order harmonics are
very loud levels.
Fig 7 has 3 curves drawn above for THD for the
8585 with KT90
Tubes are biased lightly with Ia at only 33mA per tube.
Curve A & B were plotted in 2006 after changing from
schematic for the
Curve C is derived from the 2004 measurements for 4,6,8
ohms in Fig 10
and for a 5 ohm load and with volume control at maximum to
the least THD possible.
Curve A is for the 2006 above amp schematic and with the
the middle position. This means that any incoming signal
reduced in level by a factor of 0.156 times by the volume
The curve A was plotted by varying the input signal at 1
kHz until the
clipped at just over 20Vrms into 5 ohms.
Curve B was plotted with the same increasing input signal
with the volume
control set to the maximum level so to reduce the amount
the V1 12AU7
preamp and negate its probable effect on the thd
Notice that there is a difference between thd levels
of curve A
and B and it is
due largely to cancellation effects of the second harmonic
preamp and power amp.
Curve C is taken from measurements made on the older
presented in 2004.
There is not a huge difference between any of the curves,
there is a consistent
reduction of THD of at least 6dB between the 2004 and 2006
ie, between the curve C and A respectively.
This is mainly due to a reduction of 2H in the V1 12AU7
preamp by using
load for the anodes and due to 12dB of shunt NFB to reduce
The older 2004 preamp relied only upon about 4dB of
current NFB from
unbypassed 1.5k cathode R of the V1 12AU7.
At low levels the reduction in distortion due to a large
NFB does not
result in an exactly proportionate THD reduction when the
the power amp.
Fig 8 shows the THD result for the V1 12AU7 triode
into the 8585.
I measured the harmonics up to 10Vrms output only because
high output resistance circuits above 600 ohms my test
gear has to use
high input impedance
low distortion buffer between the device under test
and the analyzer whose maximum input
voltage is limited to 10Vrms.
There is no point in measuring output voltages above
because it is
extremely unlikely the input preamp would ever have to
voltage level. It is of course capable of about 60Vrms of
As can be seen the THD at 2.6 Vrms of anode output which
would be a
high level from a CD player input signal, THD = 0.04%, and
2H with ALL other harmonics below the noise
There are those who despise and discourage the use of NFB
output resistance and distortion on the grounds that a
small amount of
such as I have
used around this preamp will significantly raise the
order harmonic products above the
2H to become a serious sonic pest.
There is NO evidence that they are correct. I am convinced
that 3H or
other harmonics all at below the 0.001% level do not have any effect
on the sound quality at all.
The older 2004 schematic
channel of the 8585
here as Fig 9 for
Part numbers used in Fig 9
have no similarity to numbers used in Fig
and Fig 2 above.
9 above is
2004 schematic of the
8585 for the record.
Fig 10 is the THD
2004 with the 2004 schematic of Fig 9.
Notice that even without the improvements to the input preamp
the amp the THD is below 0.1% for any load above 4 ohms and
below 30 watts.
The further analysis of the THD spectra in 2006 in Figs 6
& 7 shows
there are very small and insignificant amounts of high
content above 3H.
taken with Ia = 40mA at idle per each output tube.
Should the bias Iadc be increased to 60mA, the THD into 4 ohms
worst would be reduced from 0.04% at 4 watts to 0.03%.
The best fidelity is gained by
using loads above 4
It is thought that this reduction of THD by biasing more heavily
would not be audible, and not worth the cost of wearing out the
an extra 76 watts of heat within the amplifier.
The 2006 version of the amp measures better than the 2004 curves
yet the idle bias current levels are slightly lower than the
11 shows the schematic on a
circuit board under the chassis about
150mm x 100mm with the above circuit to be able to make this amp
Vdc signals from each
cathode of L and R channels are fed to
a pair of differential amps.
The K1, K2, and K3, K4 signals are from of each side of
Considering K1, K2, Left channel, the two cathode signals are
through R1, D1, and R2, D2 to
then be applied through 4k7 R3 and to
the base of Q1.
A 100uF cap filters out nearly all the ac signal from the
and only the dc signal is applied to the Q1/Q2 differential
The highest dc signal from K1/K2 is what determines the dcV at
When all bias currents are correctly adjusted in each output
voltages at Q1/Q2 bases will be very close to equal but should
vary its current by
more than about +/- 10mA, there will be a dc change
to Q1/Q2 inputs, and this will be amplified
to produce a difference
in collector voltages, and thus turn on the red LED through D4
This will tell an owner immediately if there is a bias problem.
D3, D9, Q14, D20 from
each of the base inputs of all 4 bjts Q1 to
Q4 are taken to a common
rail which detects the highest Vdc being applied
to any base input. The voltage on this rail is applied to Q7
Should the output of Q7 emitter rise to cause more than 0.7Vdc
SCR gate, it will latch on and the protection relay will open to
HT on the power transformer.
the amp off then on
after 20 seconds will reset the amp,
Q5 and Q6 act in a 4 second
delay circuit to shunt R41 on the power supply
but if the problem causing the
excess cathode current re-appears, then the
HT will be shut down again. With the shut down, both LEDs are
through D8 and D19.
schematic which limits the current input to the main B+ caps at
seconds the surge in charge currents when the relay closes to
the B+ up to its maximum is
slightly less than the initial surge.
This allows a more sensitive
fuse to be used.
Negative feedback occurs in many systems employed by
their daily lives.
There is even NFB used in principle in the toilet. Every time
the water empties from the small tank ( aka cistern ) on the
supply plumbing then begins to flow into the tank to refill
There is a float in the tank which sends a message via a lever
controlling the input flow of water to the tank.
When the water level raises the float, the lever sends a
message to the
to turn off the water, and the level of the water is cut off.
NFB is controlling the water level. Even while the water
its level slowly drops, the level will be maintained by the
Thermostats for air-conditioning work in the same way; a
sensitive device reads the temperature, and if too cool it
hot it turns on a cooler.
The message from float lever or thermostat is called negative
and negative feedback is where that fed back message is
applied in a
which opposes the action of the input message.
the water supply pressure increases, the valve will be forced
slightly and the float will rise to apply more closing
pressure on the
So even if input pressure doubles, the water level in the
tank may only rise
Fig 12 shows the whole
channel of the 8585 in the form of a
Block Diagram Schematic which is much easier to understand
to read off what I have posted above in Fig 1 and Fig 2.
NFB in preamp.
The line level preamp within the 8585
is a single 12AU7
has a "shunt resistance" NFB network. Such networks have (R1)
signal coming from a low resistance source and the tube input
(R2) is from
the anode output and the grid. The 8585 block diagram omits
which have such low impedance in the audio band that they
don't need to
displayed; only the essential gain setting elements are shown.
is effectively approximately 40k0, when R2, R3, and VR1 are
together. (R2) is actually R8 and 270k.
Beta = 40k / ( 270k + 40k ) = 0.129.
Suppose the THD of the preamp with operating voltages as shown
measured to be 0.03%. With
output voltage at anode =
1.8Vrms, the distortion
voltage = 0.54mVrms, and it isn't much, and let us say its
This distortion voltage is divided by the (R1) and (R2)
divider so that
Beta x VDn appears at the grid, ie, 0.129 x 0.54 mV = +0.07mV.
This is amplified by 14 times by the 12AU7 to make a resulting
voltage of -0.98mV appear at the anode. This seems absurd,
said there is +0.54mV at the anode. But what is actually
without the NFB (R1)&(R2) network connected, there was
+1.52mV of Dn at the anode for the same anode output and grid
What is happening is that the -0.98mV is the Correction Signal
the distortion without NFB from 1.52mV to 0.54mV, ie, the NFB
the open loop THD from 0.84% to 0.03%, a very welcome
in THD. And the effective anode output resistance Ra' of the
considerably reduced, and may be calculated,
Ra' with NFB = Ra without NFB / ( 1 + [ µ x B ] ).
With paralleled 12AU7, Ra' = 6k0 / ( 1 + [ 17 x 0.129 ] ) =
and the bandwidth of the 12AU7 stage will be in excess of
Distortion varies approximately with output voltage level for
For Hi-Fi, the maximum voltage from a CD player might be
average levels = 0.14Vrms, and we might might expect THD <
and well below audibility. I have always found the use of a
preamp like that shown to be entirely wonderful sounding.
NFB in Power Amp.
power amp within the 8585
has 3 cascaded tube stages
and an output
transformer which create a NON inverting voltage amplifier
with gain =
40x times without any NFB connection.
There is a "series voltage NFB Network" which is set up with
which in this case are R15, 47 ohms plus R28, 680 ohms between
output speaker terminal and 0V. The junction of the 2 R are
inverting input terminal of the amp, V2 cathode.
Beta = 47 / ( 680 + 47 ) = 0.0646.
with operating voltages as shown is
measured to be 0.025%. With amp output voltage = 2.0Vrms, the
voltage = 0.50mVrms, and it isn't much, and let us say its
This distortion voltage is divided by the (R1) and (R2)
divider so that
Beta x VDn appears at the V2 cathode, ie, 0.0646 x 0.50mV =
This +0.0323mV is amplified by -40 times by the open loop gain
no NFB to make a resulting distortion voltage of -1.29mV
appear at the
This also seems absurd, because I just said there is +0.5mV at
But what is actually happening is that without the NFB
connected, there was already 1.29mV + 0.5mV at the output for
levels of output and grid input signals, ie, THD without FB =
What is happening is that the -1.29mV is the Correction Signal
the distortion without NFB from +1.79mV to +0.50mV, ie, the
the open loop THD from 0.895% to 0.025%, a very welcome
in THD. The THD is reduced by a factor = 0.025 % /
or by -11dB approximately. This reduction in THD is also given
as CLG /
or A' / A.
The output resistance of the amp without NFB is reduced by at
factor or more. I don't need to show how Rout is calculated
here but my
web pages do contain the formulas. With power amps, the NFB
bandwidth without NFB, and calculation of exactly how much is
scope of ths page on 8585 basic issues.
those still confused about relative phases of signals within
the + or - sign before Vrms
indicates the relative phase of the signal voltage,
and there is 180 degrees
difference between + and - phases.
The two phases are like two kids on a see saw,
and while one ascends,
the other descends, much to their merriment.
The phase inversion or non inversion of the phase of signals
has zero effect on the sound, despite what many people say.
of feedback used in amplifiers,
both positive and negative, current or voltage, shunt or
been written about it, and deeper explanation won't fit
into this very brief
action and Distortion
Push Pull amps have inherently low distortion even without any
NFB at all.
If one imagines two men each side of a log trying to cut it
with a long
their motion is smooth and regular with one man pulling, and
the saw, and the individual motional irregularities of one man
cancelled by the
other man's similar irregularities.
One man using the saw will tend to pull the saw with more
force than he
can push it,
so the cutting power delivered to the log is uneven for each
the cutting cycle.
This is like the second harmonic distortion in electronic sine
where the top
half of a sine wave has a different amplitude or height than
respect to the centre point about which the wave moves.
The irregularities of the two men's combined action when
to give equal
power reductions at the end of the saw's motion, so it is as
there is a flattening of each +ve and -ve peak of the sine
this is akin to
third harmonic distortion in an electronic sine wave. The
effect of having a
man on each side of the log gives a much less distorted motion
men working on the one end of the saw from one side of the
log. So the
effect of PP action with 2 tubes gives much less THD than you
two tubes in parallel.
The power in a push-pull circuit is liberated into the load in
with one tube increasing its current into its R load, and the
reducing its current
into its R load. The instantaneous current direction in 1/2 of
OPT is oppositely
phased at all times and one tube is said to be pulling while
although in fact currents do no such thing as we imagine
things in our
Power liberated in a load resistance = load voltage change x
ie, P = V x I.
Or Po = current change in load squared x load resistance, ie P
squared x R.
Either way of calculating PO is correct.
The two tubes swap "push" and "pull" roles as the signal
and negatively each side of zero volts.
This principle has been used since about 1910, and it confuses
people although two kids on a see-saw are using the principles
without knowing what it is, at least until they grow up and
The complementary contribution to the output power is enabled
output transformer designed to take the oppositely phased
wheras the SE OPT is designed to accept only one signal from
with only one phase of signal.
The driver stage of the 8585 has very low distortion, and the
stages also are set up for best linearity, and contribute a
order THD, ie, 2H, 4H, 8H, thus allowing the low measureable
PP amps, without
using a high amount of NFB.
Loadline analysis for FOUR x
PP class AB1 with 12.5% CFB.
not a determined attempt to confuse everyone, but you *will
to think about what it all means!
I suggest those who don't understand load line analysis should
in the Radiotron Designer's Handbook, 4th Ed, 1955, or my other
on load matching to PP tubes.
the 8585 amp is described
and has an OPT with load match ratio,
aka impedance ratio, aka ZR = 4,172 ohms : 5 ohms, to suit 4 x
arranged to power 3 parallel stacked pairs of Quad ESL57.
8585 load as specified may be plotted as follows :-
Draw up the combined Ra curve for TWO KT90 in parallel.
This is shown as a curve from 0.0 thru K, C G and swinging over
below Ia = 1,000mA. This curve is generated by doubling the Ia
for a given Ea change along the Ra curve for Eg = 0V shown in
for ONE KT90. The KT90EH I have used is very similar to KT88EH
6550EH and old original 6550 and KT88.
2. Draw the Pda
for 84Watts, which is twice the maximum
allowed Pda for KT88 and 6550. The 8585 is able to use 6550,
or KT90, so the load lines above are valid for all 3 tube types,
the more recent EH KT120.
Plot Point Q at Ea = +475V and Ia = 60mA. This is the Quiescent
point for 2 x KT90 on each side of the PP circuit.
Calculate the B RLa which is the Class B load when the operation
has moved above the Class A1 to AB1 threshold.
B RLa = 4,172 / 4 = 1,043 ohms.
Plot Point A vertically below Point Q on Ea axis.
6. Calculated Ea /
RL B = 475V
/ 1,043ohms = 455mA.
7. Plot Point B on
Ia axis at
Ia = 455mA.
8. Draw line from B
to A. This
is the loadline for 1,043 ohms.
9. Plot Point C
where Line B
to A intersects the Ra curve for Eg = 0V.
Plot Point D on Line B to A where Ia = twice idle current =
Read off the Ea vertically below Point C = +110V. This is the
point which sets the limit for Ea load swing with this RL, and
is E min
for the load voltage swing.
Calculate Peak Load voltage swing for RLa-a = 4,172 ohms.
Peak load voltage for RLa-a = 2 x ( Ea - Ea min ) = 2 x ( 475 -
110 ) =
Calculate Va-a in Vrms = 730 x 0.707 = 516.11Vrms.
AB1 power = Va-a squared / RLa-a
= 516 x 516 / 4,172 = 63.8 Watts.
14. Read off the Ea
below Point D = +350V.
This is the voltage point where the class A1 operation ceases,
class AB1 begins.
( Ea at Q ) - ( Ea at
Point D ) = 475 - 350 = 125V.
to Ea at Q, 475 + 125 =
Point E on Ea axis at 600V.
Draw straight line from D thru Q and it should proceed thru
Point E at
Line DQE is the load line for the class A portion of power
2 x KT90,
and is equal to a load of 2,086 ohms.
of KT90 on each side
of the PP circuit have the same Class A and AB loading.
The voltage between Point D and E is the peak to peak load
class A for
2 x KT90.
Calculate Class A portion of power for 4 x KT90.
peak voltage for 2 x
KT90 = 2 x 125V = 250V.
Va rms for 2 x KT90 = 250pk-pk / 2.82 = 88.65Vrms.
PO for class A RL a, 2 x K90 = Va squared / RLa = 88.65V x
= 3.77 Watts.
Class A portion of PO = 2 x 3.77Watts = 7.54Watts.
The speaker load to be used
the 8585 in this case has load ohm value
equal to 1/3 of the load value for 1 Quad ESL57.
The ESL57 has Z = 32 ohms at 50Hz, reducing to 8 ohms at 1kHz,
down to 1.8 ohms at 18kHz, so that the average Z of a single
for where most power is to be made is between 100Hz and 500Hz
about 15 ohms. The ESL57 was designed to be able to be driven
by the Quad-II amp produced before the ESL57 was marketed.
The Quad-II amp produced about 22Watts into 15 ohms.
Therefore an amp capable of 66Watts into 5 ohms should be
adequately powering 3 parallel ESL57. When the ESL are
one above the other, their sensitivity increases, and for the
less power is needed for the 3 stacked ESL. The 8585 described
used with the stacked ESL57 since 2005, and the KT90EH
test as new, despite
being used for long periods daily.
Now the original Quad-II amps relied on their pure class A
fidelity, because their power supplies are rather inadequate.
OPT is strapped for 15 ohms, the RLa-a is about 4k0, and the
class A power = 9 Watts approx, with an additional 13 Watts in
The KT66 struggle to achieve the 22 Watts and because my 8585
a much better power supply and input driver stage than Quad-II,
because stacked pairs of ESL are more sensitive, the amount of
power does not need to be greater than one Quad-II amp, and for
and crescendos there is 66 watts available from the 4 x KT90.
have higher Ia idle current at say 50mA in each KT90 which would
class A PO to 15Watts before the amp moved into class AB, but
bias it uses now it performs flawlessly.
At low bass frequencies, the stacked ESL Z rises to 10 ohms, and
no shortage of drive plus the amount of class A power increases.
there is very little energy in music, and the stacked ESL load
0.6 ohms. I have 0.6 ohms in series with each of the 3 stacked
so the minimum RL for the amp = 0.8 ohms.
Because the power is so little where Z is so low, the power
Fig 13 has load lines
= 2,086ohms : 2.5 ohms and also
8,344 ohms : 10 ohms. These load lines may be drawn using a
numbers 1 to 16 as for 4,172 ohms : 5 ohms.
Fig 14 is a photo taken
October 2011 of the 8585 under the chassis,
after a full service following 6 years of trouble free
the owner said he uses the amp daily.
All KT90EH tested similarly to brand new tubes, and showed no
of wear, ie, gassiness, cloudy gettering, or positive grid
All input and driver tubes measured perfectly, and gave no
The servicing was required because four zener diodes I had used
had degraded to having much less than their rated zener voltage
so the shunt regulator for the KT90 screen supply produced Eg2 =
instead of +330Vdc, and this turned off Ia in all KT90. I have
simple 4 x series zener diodes with the BJT based shunt
on the AL angle flange seen at the top of the picture.
Unfortunately, even though zener diodes may be rated for 5Watts,
can fail if subjected to only 1.5Watts of heat production, and
it is wise to never let their Pd rise above 1/10 of their
Other zener diodes in the 8585 have been retained since they
originally installed in 1996, and they show no sign of change.
The point to point circuitry is somewhat messy, but quite
it is easy to figure out where parts shown on the schematics are
Amplifiers I have made in recent years are a lot neater under
but will never be as neat as a well done printed circuit board.
Unfortunately, the use of printed circuit boards obstructs the
of air up and around the parts and up through the holes drilled
the top plate of the amp and past the glass of the output tubes.
If one looks into the finest samples of circuit production used
professional test equipment as manufactured by Tektronix, or
you will find 3 dimensional pint to point circuitry, and neatly
wire looms of multicolored wiring, with much thought and design
given to the location and placement of each item to maximize
density, yet disallow adverse stray signal coupling, and to
operating temperatures of everything. Fans were often used in
gear to reduce heat, but fan noise is always intolerable for
There is no need for a fan to keep things cool in the 8585,
transformers and tubes have been designed to run at low
Without the heat stress, there is only the music to behold.
For more ideas
matching and how tubes work
go to my list of educational and DIY pages.
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