Zen Exhibits Low Gain - diyAudio
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Old 1st August 2001, 04:21 AM   #1
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I just completed one channel of the Pass Zen amplifier. I've made the appropriate adjustments, etc. Output waveforms look good, but the gain seems to be about one half of the value specified in the article (I believe it was about 15dB). 3VPP in results in about 10VPP out. Any thoughts? I've gone over the circuit to check for obvious stuff like wrong resistor values, etc. I'm using the original circuit configuration with a 20V zener and an IRFP240 on the signal side. Could it be a bad MOSFET? Anyone else encountered this?

Thanks for your help.

Mike
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Old 1st August 2001, 04:53 AM   #2
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Mike,
What are you reading across R1, voltage/current-wise?

Grey
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Old 2nd August 2001, 01:41 AM   #3
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Grey:

Thanks for your reply.

IQ is 2.00A. Voltage across R1 is .657V. Voltage across R1 and Q2 is 16.97V. Supply voltage is 34.1V. P1 is about 70% CW.

My initial thought was the dreaded "leaky" zener, but I would expect that to "chop" the waveform rather than reduce the gain.

I'm using an OsziFox "Probe Scope" for my AC measurements. It isn't exactly a precision instrument, but I wouldn't expect it to be too far out of whack. My trusty 'ol Tektronix scope died the day I finished up the first channel.

I appreciate your thoughts.

Mike
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Old 2nd August 2001, 03:14 AM   #4
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I've since checked the circuit with my ACVM to be sure the "Probe Scope" wasn't misleading me. 1V RMS in yeilds 3.3 V RMS out. That's 20*LOG10(3.3) = 10 dB, if I've got the math right.
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Old 3rd August 2001, 01:04 AM   #5
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Mike,
I got curious and looked at Nelson's writeup on the Zen. He states a gain of 8.5 dB. Offhand, I'd say you're doing fine. If anything, you're getting a free 1.5 dB gain in your amp.

Grey
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Old 3rd August 2001, 01:22 AM   #6
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Grey:

You're absolutely correct! The gain for the Zen was quoted at 15 dB in the pre-text of the "Bride of Zen" article, but 8.5 dB in the original Zen article. Please forgive my oversight and thank you for your help.

Eureka! It actually works!

I can't wait to get the second channel built and hook this thing up to some speakers...

[Edited by mrothacher on 08-02-2001 at 08:24 PM]
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