Anderton Compressor distortion

[Gyoon]

I am getting some distortion when I turn up the compression knob on the CA compressor I have just finishedl.  The distortion becomes noticeable when the pot is about half rotation up.  I think this distortion has something to do with the compressing effect because I can hear it kick in when I really hammer the notes at a higher compression setting.  When the notes are picked softly there is no distortion sound.  Would I have to reduce the amount of gain coming into the unit?  For hte record, I am using a CLM6000 with a rc4558 and an lm201, just as specified in the book.  I have made no modifications except for the .033 cap and a 3.6k resistor in parallel with the 2.2 uF and 8.2k resistor going into the second gain stage.  I should be clear though:  the unit DOES indeed compress.  And it sure does add GAIN!!! But the compression just comes out distorted.

I noticed on the schem modified by Stellan Lundberg that he actually adds a 250k  input pot in series with a 100k resistor.  It is marked 'feel'.  Is this soemthing I should consider for my circuit?

Glenn

[Gyoon]

I have changed the input resistor to 1M and it distorts only on the highest sensitivity setting.  I guess that's a good thing.  But something tells me I am not getting to the root of my problem.  Because it only distorts when the LDR compresses, is it possible that the op amp is misbiased?  Is it possible that the signal dips so much that it is actually going into saturation?  (I think this would be the right word; it is the right word for a transistor) 

Glenn

[petemoore]

Because it only distorts when the LDR compresses, is it possible that the op amp is misbiased?
  Wierd behavior and misbias often share the same circuit, voltage help tell the tales, as does audio probing and other things.
  See the 'Debugging' thread.

[MartyMart]

Give us some opamp voltage readings, that will speak volumes !

MM.

[Gyoon]

OK, here's the scoop:

I'm looking at the op amp, which is an NE5532 dual jobbie.  The EPFM circuit, for a refresher, is a bipolar variety.  All measurements were taken in reference to ground. 
The unit had plugs going into both jacks and an instrument plugged into the other end of the input cord.  No signal was fed through.  This circuit was made stock according to the EPFM book:

Pin 8    8.1v
Pin 4   -8.1v.

This thing is getting power from the batteries.

First stage is set up as an inverting op amp, as is the second, btw.

pin 5 and pin 6: 0.0 v
output pin 7: 0.7 v

Second gain stage:

Pins 2 and 3: 0 v
Pin one (output) 0.7 millivolts

The one that troubles me is the first stage.  From what little I know about op amps set up in the inverting fashion is that the measurements are supposed to be zero.  At least this is what my understanding is.

I am still getting more distortion than I would like.  Much more.  When the sensitivity pot is turned up to 9 o'clock  I get it kicking in.  I am using both a dual humbucker guitar and an electric bass.  All instruments are passive.  I used a 1 meg ohm resistor in my first stage feeding back into the inverting input, just like the book has stated.

Any thoughts?  I hope I have done this right.

Glenn

[MartyMart]

I'm not sure what you mean by "I changed the input resistor" ?
Did you mean R9 on the schem from the book ?
I would leave that " as is " and adjust R8 ( 1M ) down a bit, it suggests 220k for
a line level input, perhaps your HB gtr/bass are just a bit "HOT" for this so perhaps
try a 470k here in the FB loop and see how that sounds .
You could also use a 1M pot wired as a variable resistor, for adjustment when using
a less "hot" output guitar or for a "hotter" input also.

MM

[Mark Hammer]

Normally, the Anderton/EPFM compressor will have less output as compression is turned up.  This is because the effect of the LDR is to provide a parallel resistance in the feedback loop of that first op-amp and reduce gain whenever there is a peak in the input signal.  The 2nd stage is there to compensate for that volume drop by adding some recovery gain so as to maintain the same or some other desired average level when in effect mode.

Consequently, the distortion that results at higher compression settings would not be from either a) overdriving the 2nd stage or b) overdriving something after the compressor unless 1) the CLM6000 and rectifier circuit were not functioning properly or 2) the gain were turned way way up.

One possibility is that if the smoothing/averaging cap between the LED input and ground was not connected properly, then the highest compression setting would be introducing a sort of envelope ripple into the equation, not unlike what Analog Mike noted as a potential mod for the MXR/Ross compressor.  Personally, I would not expect such an outcome to be as pronounced in the CA comp as it is in the MXR/Ross because the sluggishness of the LDR tends to serve some of the same function as a smoothing cap.  Of course, at this point we don't know exactly how much distortion is being discussed, merely that it increases with compression amount.

[Gyoon]

Hey Mark!  I'm in Toronto too.  High Park area. 

Anyways . . . the only capacitor that I see that's even remotely close to the LED is the 2.2 uF bypass cap that sends signal to the sensitivity knob.  The knob then sends the signal to the LM201 (in my case).  I changed it from a tantalum to an electrolytic of the same value and no change.  I'm not sure this is the culprit anyways.  The unit is compressing, I can hear it.  If I disconnected the capacitor there wouldn't be any other way for signal to reach the LED.

When the compressing action takes place it is accompanied by a fuzziness, especially on the tail.  The attack seems to be free of the fuzz.  When the sensitivity knob is more than a third of the way up I can hear fuzz on the louder notes.  When it is more than half way up I can hear fuzz on every note, particularly on the lower ones.  Guitar is the same.  The higher strings seem to be free of it, though. 

The distortion is more like a fuzz, not too much treble.  I've retouched all the joints that would make up connections to this area and still no change.

You mentioned about having an LDR that wasn't working correctly.  I started with a CLM6000 and cut it in half and put a water clear LED right up against the LDR.  I hastily decided to check to see if the unit was working by destroying it.  Not too smart, huh?

Anyways, I am thinking of taking MartyMart's advice and changing the resistor in the feedback loop.  It's got tons of gain, right?  I can always add more volume when necessary. 

Glenn

[Johan]

sounds to me like maby you have a LDR that recovers very fast and what you hear is a result of the single ended rectification you get with only one LED combined with a LDR that is less forgiving than what we would usually expect.
since you opened the optocoupler and now have that option, try to put in an other LED antiparrallell ( distortion box style) to the one you got and point that one too at the LDR for a sort of fullwave recto. Ive seen thees type of problems to and it is usually more noticeable in lower frequency's...even if this is not the problem you have got, I'd still do this..

johan

[Gyoon]

Johan, that's one hell of a clever response!  I'm going to try that ASAP.  I have tried the audio probe method as well and seem to be getting an audio signal when hooked up to pin 4.  That can't be right . . . Pin 8 gives me nothing. 

You're answer is interesting because I thought that the light only matters when it comes to the resistance of the feedback loop.  No audio signal is actually being sent through the LED.  Maybe this would solve some things, though.  I'm just a newbie.

Glenn

[Gyoon]

Just a little update:  There is NOT an audio signal coming from pin 4.  If I got one before I'm not getting one now.  There IS an audio signal (my bass) coming from pin 6 of my lm201.  As that's the output pin of the 201 chip I guess I shouldn't be surprised. 

So just a little clarification:  My signal voltage is provided by pin 6 causing the grounded LED to begin flashing analogous to my signal.   This causes the LDR to reduce in resistance.  By adding a second LED antiparallel to the first one there would be no more flashing:  The LDR would then see a constant source of light.  This would somehow cause less distortion? 

Glenn

[Johan]

it looks like you have a constant light, but in fact, it is flashing at the frequency of the signal. and with anly one LED, depending on wich way you oriented the LED you get light on ONLY the positive OR the negative part of the signal.
so what might happen, and I've seen this on oscilloscope, is that the LDR start to recover inbetween the flashes. and the result is nice smoth attenuation on half the signal and clipping on the other half. you and I, being humans, can't detect a flashing AS flashing at freq's above around 25Hz,for us it is just a constant light.  but the LDR sure can...
j

[Gyoon]

Johan, that's a great tip.  I'm just a little unclear as to where the clipping is actually coming from.  With my audio probe test I have found that the clipping is occurring in pin 7, the output of the first gain stage.  When the compression is off I can get a clean boost like there is no tomorrow.  It's only when the attenuation kicks in is when there is an issue.  The more compression I add the worse it gets.  What exactly is clipping?

[Gyoon]

I'm sorry, when I asked, "What exactly is clipping?"  I'm not asking what clipping is.  I'm asking where is the clipping occurring?

Glenn