Help with Ross Comp, please!

[Jason D]

I just built a Ross Comp, from the Tonepad layout. The signal is really really weak coming out of the effect.

I traced it back to the output of the CA3080. The sound going into it at pin 2 sounds healthy, and then it comes out of pin 6 sounding weak and distorted.

Would that mean that the CA3080 is shot?

I can get another one from the lab at school tomorrow morning and see if that is it.

Any ideas.

The only thing I did different on mine was raise the value of the 1uF cap in the voltage divider to 47uF.

[Jason D]

how do you tell if the OTA is dead?

[Mark Hammer]

If there was nothing coming out, then that might mean it is dead.  But bear in mind that it is acting as a current-controlled amplifier here, and for the moment you have no indications of whether the current being fed to pin 5 is sufficient to produce a reasonable degree of amplification; i.e., if you are effectively *telling* it to keep things quiet, and it does, then it ain't broken.

If any of Q2-Q5 are misoriented, that might have some impact on current feeding the Iabc pin.  Likewise if your 500k compression amount pot is buggered up somehow, that could also have an impact on how loud things are.  Finally, just check to make sure the 27k fixed resistor is the stated value and not 270k or something like that.  I'm not questioning your competence, but from personal experience I would venture it may not be the first time in DIY history that someone looked at the first two colour bands and neglected the third in a rush to finish a cherished project, or looked at the component "upside down" and mentally flipped the order of two colour bands (I once installed 51k resistors as 15k and took a while to notice).

[Jason D]

Update...

I took the Ross with me too the school's lab (I go to a techinal college). I switched out the OTA with 2 others. Still no change. So I checked all the points that should be sitting at gnd. Nothing wrong with that. Checked all the 9v and Vb, nothing wrong with that.

So, I got one of my instructors to look at it. He had me test a few things leading up to pin 2 of the OTA. All look good, from what he could see.
We found another bin full of CA3080E's (RCA brand, looked old to me). Put one of those in and then we were getting a decent amount of gain out.

I thought we had it fixed. Though he said the signal looked unstable. I thought maybe that is what it should have looked like. Guess I was just excited that something seemed to be working.

Took it home, plugged in. Getting a really distorted nasty sound. I can get a decent boost. But it sounds nasty. Like a cold solder joint somewhere, or a something is up with one of the trans. Turning the 2k trimpot didn't help.

Checked all the components. They are all right. So, I guess its back to probing.

I am going to start with the output side of the OTA this time.

[Jason D]

Thanks for your help Mark.

I found the problem. There was a soldering bridge between the B and the E of Q3 (tonepad's layout). Causing the diode to short to ground, and the emitter to be shorted to ground. So, its working now.

Now, should the Ross distort or not handle the attack on chords very well?

[Mark Hammer]

CA3080's are not all that robust with signals greater than 100mv or so.  (One of the many reasons why many prefer the CA3280 and LM13700.)  Indeed, it was the inherent distortion characteristics of the 3080 that led Craig Anderton  to use it for a voltage-controlled distortion module in the AMS-100 series in DEVICE  (http://ampage.org/hammer/files/Device1-8.PDF).  The usual strategy is to tame the input signal to the 3080 and then boost afterwards.  In the case of the Ross/MXR, that second function is fulfilled by Q2.

Still, it is not unreasonable for an input signal, especially from a chord, using hot pickups, to push the 3080 a little harder than it can bear, despite the input attenuation and pre-cut/post-boost scheme.  That's part of the coloration that people like form the Dynacomp and part of why people describe other NON-OTA compressors as sounding "transparent".

So, all in all, the patient sounds fully recovered.

[Jason D]

Can the CA3280 and LM13700 sub for the CA3080?

If so, I will order some along with the CA3080's. I don't know how good the quality is on the RCA brand CA3080E I got from the lab are.

[Jason D]

nevermind....looks like both of those are dual OTA