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Old 4th March 2013, 04:27 AM   #1
Dura is offline Dura  Canada
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Default Complementary Zen 5 Questions

I'm considering building a Zen Variations - Part 5 The Complementary Zen as a first attempt to build an amplifier.

This amp has feedback though a 4.75k resister and I am wondering about how many dBs of feedback the amp has? I would guess in the 10-20dB range since that seems to be about the amount of feedback that Mr. Pass uses AFAIK. I am considered using a 2k resistor with a 10k pot to control the amount of feedback. I would like to have the feedback knob calibrated for the amount of feedback used but don't know how to calculate/estimate this.

An interesting thing about the circuit is that the volt divider bias for the MOSFETs are to the output rather than ground. I suspect this would give a more "dynamic" sound. I may try both and experiment

In the article it says a class A PP amp can deliver twice the current of the idle current before leaving class A: I plan to idle at 2.5A with 20V rails so does this means that the amp dissipates 100W/channel before leaving class A? From the article the amplifier seem to output about 10W/channel (.5%THD) in class A and up to about 30W/channel in class AB mode but with high distortion (10%THD).

Brock
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Old 4th March 2013, 05:20 AM   #2
The Dastardly Dad of Three
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Quote:
Originally Posted by Dura View Post
I'm considering building a Zen Variations - Part 5 The Complementary Zen as a first attempt to build an amplifier.

This amp has feedback though a 4.75k resister and I am wondering about how many dBs of feedback the amp has? I would guess in the 10-20dB range since that seems to be about the amount of feedback that Mr. Pass uses AFAIK. I am considered using a 2k resistor with a 10k pot to control the amount of feedback. I would like to have the feedback knob calibrated for the amount of feedback used but don't know how to calculate/estimate this.
>> The way that I would do it would be first the measure the gain (with a 8 ohm dummy load) of the amp without the feedback resistor R2 [this is the Open Loop Gain] and then again with R2 in place. The difference in gain is the amount of feedback applied. You can 'calibrate' the knob by rearranging & applying the classic feedback formula: Close Loop Gain = Open Loop Gain / (1+B*Open Loop Gain). B = R1/(R2+R1)

An interesting thing about the circuit is that the volt divider bias for the MOSFETs are to the output rather than ground. I suspect this would give a more "dynamic" sound. I may try both and experiment
>> I asked the question once but did not get a reply. I suspect that the taking it off the output gives us a more stable output offset versus referencing it off ground. Just note that the bias resistor network can start to provide a feedback path also if you lower the resistances.

In the article it says a class A PP amp can deliver twice the current of the idle current before leaving class A: I plan to idle at 2.5A with 20V rails so does this means that the amp dissipates 100W/channel before leaving class A? From the article the amplifier seem to output about 10W/channel (.5%THD) in class A and up to about 30W/channel in class AB mode but with high distortion (10%THD).
>>The graphs seem to suggest that the THD is lower than you indicate.

Good luck!



Brock
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