Hairball Rev D Distortion Problem

Hello,

I'm having trouble with distortion of about 1.5% on my hairball rev d.  I recently finished the build and calibrated the unit, skipping the distortion calibration.  Everything went well and it sounded great.  The next day I noticed some distortion so I decided to do the distortion calibration.  It was measuring about 1/5% THD and turning trimmer had little effect.  I accidently bumped the trim pot and the distortion when down to .05%.  I inspected and decided that the pot may be shorted, so I resoldered the pot and everything now appears to check out.

However, the distortion is still here.  I tried re-calibrating and I noticed that the distortion levels are very reasonable until the q-bias control is brought up the the "1db or reduction" level as described in the calibration procedures.

I've tried searching this forum but I can't seem to find anything related.  If anyone can point me in the right direction I'd really appreciate it.

cheers
Scott
 

... Use the support thread.

And when you do, ask yourself if the distortion is happening:

Before the pre-amp stage (after the input transformer)
After the pre-amp (pad 15)
After the line amp (brown/black leads in the output transformer)
After the transformer

Man I can't for the life of me find the Rev D support thread.  A little help please.

:-\

franalog said:

Man I can't for the life of me find the Rev D support thread.  A little help please.
:-\

Click to expand...

http://groupdiy.com/index.php?topic=20058.0

From the description of your problem : Distortion triggers when the FET starts conduction, but is apparently not present when the FET is cut, it would appear that there might be something wrong on the biasing of Q1.

FETs are commonly used as VCRs (Voltage Controlled regulators), but they have the issue of having significant distortion (easily above 5-7% as Vds approaches saturation) if they are not linearized. This is what appears to be the case.

Contrary to what you may believe R86 has a small impact on the distortion, it allows to minimize it true, but in a well built compressor you should not have above 0.5% distortion with a 1dB drop of a 1dBu signal even if R86 was shorted.

Check if when you "un-calibrate" the compressor (with GR off) your distortion goes away. If that is the case, check the linearization feedback network in the pre-amp.
C28, R10, R15, R85, C9, R6, C3, C6, R7 and C22