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Old 17th August 2012, 04:04 AM   #1
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Default Two stages EL34 power amplifier

I have build EL34 amp with differential phase splitter and tubes run in SRPP mode. It sounds very nice and has undistorted and very warm sound. It seems to be under power. I think that I have applied too much global feedback. I have no idea how to calculate it. Any help in solving this problem would be greatly appreciated. Also the topography of my design may be wrong. I have never seen anything like this.
Here goes the schematic.
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Old 17th August 2012, 04:23 AM   #2
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Quote:
Originally Posted by voycieck View Post
... It seems to be under power. I think that I have applied too much global feedback. I have no idea how to calculate it...
Do you mean that you need too high input level to get full output power ?

You can simply measure the amount of GNFB.
Disconnect the GNFB line from the OPT, measure the sensitivity and compare to the sensitivity when GNBF is connected.

There should be no reason to have more than 16...18 dB GNFB, especially when you have UL-connected output stage..
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Old 17th August 2012, 04:40 AM   #3
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Thanks for reply.
When connected to scope 1Khz 0.2V sin wave produces 2V output. I don't know how many dB of GNFB I applied. How do I calculate it?
As to power, at full volume I hear no distortion and it seems to be long way from overdrive. I wander if less GNFB would improve sound quality.
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Old 17th August 2012, 05:01 AM   #4
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Disconnect the GNFB line coming from the OPT to 4k7 / 360p and measure what input level is needed to produce the same 2V output voltage.
Then calculate the ratio of 0,2 V and the level just measured. And transform to dB's.
The amount of GNFB is the amount of the sensitivity is reduced when the feedback line is connected.
Less GNFB means higher distortion, lower S/N and worse frequency response - assuming the original GNFB was correctly accomplished.

Last edited by artosalo; 17th August 2012 at 05:16 AM.
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Old 17th August 2012, 07:30 AM   #5
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Quote:
Originally Posted by voycieck View Post
Thanks for reply.
When connected to scope 1Khz 0.2V sin wave produces 2V output. I don't know how many dB of GNFB I applied. How do I calculate it?
As to power, at full volume I hear no distortion and it seems to be long way from overdrive. I wander if less GNFB would improve sound quality.

your closed loop gain of 10 is indeed small...at least twice that will give a perception of more power...

you play around with the 4.7k/47ohm feedback network, getting the ratio higher decreases feedback and increases your closed loop gain...
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Old 17th August 2012, 04:43 PM   #6
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Originally Posted by Tony View Post
your closed loop gain of 10 is indeed small...at least twice that will give a perception of more power...

you play around with the 4.7k/47ohm feedback network, getting the ratio higher decreases feedback and increases your closed loop gain...
I agree. The gain is now small, but there is only phase splitter before output stage and it is therefore one reason for the situation.

Play around with 4k7 / 47 but do that after you know what the amount of GNFB of your amplifier is.
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Old 17th August 2012, 05:39 PM   #7
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That design resembles a design by one finnish tube veteran, Mauri Pännäri. His design wasn't exactly as yours, but similar in that it also had a single stage up front - a differential amplifier as a phase splitter, just as yours does. It didn't have active loads, though. The power stage was pure class A push-pull, though. Yours seems to be class AB, right?

The first thing I'd do, is measure the gain and balance of the phase splitter. Although, if it does sound undistorted, as you state, then probably the balance at least should be great. By the way, in theory you should be able to ditch the balance pot from the input stage's cathodes. Check how the balance shifts with a scope when you turn the pot from end to end. Of course, disconnect the upper 4,7 kOhm resistor and the parallel cap before doing any measurements I proposed.

It still might be, that you need a preamplifier with more than 2Vrms output to drive this amp to it's limits. Although the amp by mr. Pännäri had an ECC81 up front, it was designed to be used behind a power amplifier. Then again, a gain of 10 should give you an output of 50 watts in to 8 ohms with full CD output (2 Vrms).

After that, there are several resistor values in the schematic I would like to see a bit smaller, but those have nothing to do with the perceived loudness of the thing. One thing I'd like to state, that with 330k grid leaks on the power tubes G1, you risk the power tubes running amok when they age. In the aforementioned class A -amp I had 330K grid leaks on EL34's, and I never got more than 4 months out of a pair of output tubes. After that at least one would go south by turning it's anode nicely cherry red. Switched the grid leaks from 330K to 220K, no more problems. What I observed when I dug into the problem, was that the grid was starting to go emissive with age and heat (it was class A, thus HOT), and the current caused the voltage at G1 to rise, which again worsened the situation, and the result would be a nicely glowing catastrophic power tube failure. I remember my findings being in line with what was written in Robert Tomer's 1960 book "Getting the most out of Vacuum Tubes".
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Old 18th August 2012, 12:01 AM   #8
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if we assume that the LTP has voltage gain of about 50, the EL34 around 5, so that is x250 or around 48db....assuming 4.3k:8 opt, turns ratio is 23 or 27db so that overall gain is 48-27 or 21db....so that closed loop gain of 20db gives a feedback factor of only 1db.....you can try operating without the feedback loop connected and see what gives....

more knowledgeable members, please check my math, i could be wrong....
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Old 18th August 2012, 12:59 AM   #9
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Default Thank You All for replies

I took some advice and I'm listening to the music now.
It sounds very good, nice bass and high frequencies. On the bench I run it without feedback resistor. I thought amplification was 48. Next I added 250k pot and run it down till amplification starts to drop. It started dropping at 20k, then I soldered in 20k resistor. Now amplification is 13.5.
I'll do it over in a few days. My scope is a cheap piece of junk from Radio Shack and I think I'm getting wrong readings. As to loudness, vol control needs to be at about 11 o'clock for comfortable listening and at 1 o'clock I don't hear my telephone ringing. Sound is good and beats my David Hafler amplifiers so I'll keep it as is for now, till I get better scope.
Once again, thank You All for help and cheers.

Last edited by voycieck; 18th August 2012 at 01:15 AM.
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Old 18th August 2012, 01:24 AM   #10
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good job......
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