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Old 15th June 2011, 04:10 PM   #1
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Default Edison 12 amplifier

Hi,

I wonder if anyone can answer some questions regarding this amplifier? It is an EL84 based amplifier run in pentode mode with feedback. It uses three ECC88 valves to provide the gain and driving/phase splitting. I am in the process of refurbishing the amp and do have a schematic but it is a little confusing. The amp sounds good but has very high gain and I notice that the mains transformer becomes extremely hot after using it for a while.

1. Conventionally in a valve power amp the feedback resistor is in parallel with a compensating capacitor. In this amp, however, there is a 4.7K feedback resistor in series with another 4.7K feedback resistor and the 270pF (220 pF in the schematic) compensating capacitor is in parallel with only ONE of the feedback resistors. Why is this?

2. In the schematic there is a 2.2K resistor in series with a capacitor and both are in parallel with the anode resistor of the input valve. Presumably this is to provide additional compensation. However this resistor is missing on the actual circuit. Instead there is a 270 pF capacitor (not shown on the schematic), just after a 1K grid stopper, from the grid of the input valve to ground. Is this to provide low pass filtering and does it replace the action of the additional compensation?

3. I have decided to apply some high pass filtering at the input of the amp since I suspect the output transformers are saturating at low frequencies (flabby bass). I also want to reduce the gain of the amp by using a resistor ladder network at the input. Would all these resistor cap combinations cause horrible phase shifts?

I hope someone can help.
harlequin
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Old 15th June 2011, 06:33 PM   #2
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Imagine how useful a schematic would be to someone attempting to understand your problem....
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Old 15th June 2011, 09:50 PM   #3
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Default Edison 12 Schematic

Hi,

I agree; here is the schematic for the amplifier. The coupling capacitors in my particular amp are all 0.68 uF, another difference from the schematic.
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Old 16th June 2011, 10:40 AM   #4
jakruby is offline jakruby  South Africa
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it does appear to be a bit of an oddball design. i am not surprised there is too much gain...there are too many valves to drive a pair of EL84s! also there is that 1M5 resistor bypassed by 22nF feeding the second gain stage...that will give a treble boost. how old is this design and what was it used for? If I where you i would bypass the first ECC88. connect your input directly to the second and connect the feedback to that valve's cathode. you'll probably have to fiddle with the values feedback series resistor and it's paralleled capacitor. If the power transformer is getting hot its probably just a bit small - the Dynaco power tarnnies always seemed at the edge of meltdown as well.
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Old 16th June 2011, 04:14 PM   #5
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Nothing oddball with this one!

This is from famous Audion and Audio Innovations designer Erik Andersson. The reason for the high gain was that it should be used with a passive preamp. This was called Edison 1 and had a good RIAA stage together with the passive line.

Very good special design OPTs.

This is Eriks latest:
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Last edited by revintage; 16th June 2011 at 04:19 PM.
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Old 16th June 2011, 07:13 PM   #6
jakruby is offline jakruby  South Africa
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if he designed audionote and audion amps i am sure he knows what he is doing! and thus i doubt the OPTs would be inadequate. just slap a volume pot on the input then. this amp having so much gain is not suited to be used with a preamp (if that is what you are doing). if you want to roll off the bass the easiest way would be to make the coupling caps to the EL84s smaller - try 100nF and see how it sounds.
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Old 16th June 2011, 10:47 PM   #7
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Donīt change the amp. The only thing needed is to turn down the input level a notch. The flabby bass is probably due to overload.

No need to worry about the mains transformer. There has never been a burnt one.
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Old 16th June 2011, 11:25 PM   #8
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Quote:
Originally Posted by harlequin View Post
Hi,

I agree; here is the schematic for the amplifier. The coupling capacitors in my particular amp are all 0.68 uF, another difference from the schematic.
these coupling caps seem to be the reason for the bass issue. If you calculate the cutoff frequency, it is way too low. For example, the first to second stage is -3dB at 0.23 Hz! Replace them with the values on the schematic, even go lower. And yes, the "treble peaking" thing in that area is something out of a Marshall amplifier, odd..
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Old 16th June 2011, 11:49 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by harlequin View Post
Hi,

I wonder if anyone can answer some questions regarding this amplifier? It is an EL84 based amplifier run in pentode mode with feedback. It uses three ECC88 valves to provide the gain and driving/phase splitting. I am in the process of refurbishing the amp and do have a schematic but it is a little confusing. The amp sounds good but has very high gain and I notice that the mains transformer becomes extremely hot after using it for a while.

Pretty standard for a budget amplifier biased into class AB.
Gain is related to the nominal line level, passive its quite low.


1. Conventionally in a valve power amp the feedback resistor is in parallel with a compensating capacitor. In this amp, however, there is a 4.7K feedback resistor in series with another 4.7K feedback resistor and the 270pF (220 pF in the schematic) compensating capacitor is in parallel with only ONE of the feedback resistors. Why is this?

This is phase compensation of the overall closed loop, its not related to
the overall compensation of the amplifier which is applied to the open loop.


2. In the schematic there is a 2.2K resistor in series with a capacitor and both are in parallel with the anode resistor of the input valve. Presumably this is to provide additional compensation. However this resistor is missing on the actual circuit. Instead there is a 270 pF capacitor (not shown on the schematic), just after a 1K grid stopper, from the grid of the input valve to ground. Is this to provide low pass filtering and does it replace the action of the additional compensation?

3. I have decided to apply some high pass filtering at the input of the amp since I suspect the output transformers are saturating at low frequencies (flabby bass). I also want to reduce the gain of the amp by using a resistor ladder network at the input. Would all these resistor cap combinations cause horrible phase shifts?

FWIW you have to understand the overall compensation arrangements.
You can attenuate the input, or reduce the closed loop gain by playing
with the feedback resistors. The critical parameter is the gain margin.
(how much you can reduce feedback gain without stability issues)


I hope someone can help.
harlequin
rgds sreten.
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Last edited by sreten; 16th June 2011 at 11:53 PM.
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Old 16th June 2011, 11:55 PM   #10
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The "treble peaking" is just another high pass filter and part of the phase compensation in the fb loop. The 3 dB point is well below audible so I don't know about the "treble" thing.
The value of the coupling caps is also part of the phase compensation and increasing the value to ,68u has probably put your amp on the limit to low freq oscillation, hence the flabby bass.

/Olof
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