Dyna comp clone (aion schematics) - Help with the trimmer

[d3im0s]

Hi, everybody,
  I'm building a dyna comp clone following the schematics from aion electronics (https://www.dropbox.com/s/d7m5f9rbe6d68f2/aion-aurora-ross-compressor-documentation.pdf). So far the pedal works but I'm having troubles understanding what I should do with the 200k trimmer in the circuit. As far as I understand this trimmer does not replace the 2k trimmer of the original circuit (that's being done by the RX1 and RX2 resistors) to balance the signal at the two inputs of the opamp.
Do you have any idea on what it does or what could be a good starting position?

Thanks
Valerio

[antonis]

About 200k trimmer:
<Input trim is an internal trimmer that attenuates the signal going into the CA3080, allowing you to use the compressor with high-output instruments such as keyboards or active pickups>
and
<This input trim control was adapted from the Keeley Compressor. The original value in the Keeley is 150k, but this is a pretty rare trimpot value. Since it's just a variable resistor, you can use the more common 200k, 250k or even just 100k and the result will be the same—you'll just have a different available range. If you'd like to omit this control entirely, leave off CX1 and jumper pins 2 and 3  of the trimmer>

About 2k trimmer:
<The original has a 2k trimmer here to compensate for imbalances in the early metal-can versions of the CA3080. The later DIP8 packages were much improved, and so this trimmer has been omitted from this project and replaced with two 1k resistors.>

[Buzz]

If the trimmer is already soldered on the board, just could use it to "tune-up" your pedal.

Experiment. Turn it and find out which way makes the pedal louder. Turn it up until you get distortion. Then back it off a bit. Tweak it from there until it sounds like you want it to for your purpose.

If you are using a guitar with passive pickups you should be able to turn it all the way up and be a happy chap.

That's my two cents. Lets call it a semi-educated guess. I'm just a mug lair ( new guy ) with pedals.

Be prepared for me to be corrected by someone with greater experience.

[d3im0s]

About 200k trimmer:
<Input trim is an internal trimmer that attenuates the signal going into the CA3080, allowing you to use the compressor with high-output instruments such as keyboards or active pickups>
and
<This input trim control was adapted from the Keeley Compressor. The original value in the Keeley is 150k, but this is a pretty rare trimpot value. Since it's just a variable resistor, you can use the more common 200k, 250k or even just 100k and the result will be the same—you'll just have a different available range. If you'd like to omit this control entirely, leave off CX1 and jumper pins 2 and 3  of the trimmer>

About 2k trimmer:
<The original has a 2k trimmer here to compensate for imbalances in the early metal-can versions of the CA3080. The later DIP8 packages were much improved, and so this trimmer has been omitted from this project and replaced with two 1k resistors.>

Thanks, I must have missed those lines going through all the instructions 

If the trimmer is already soldered on the board, just could use it to "tune-up" your pedal.

Experiment. Turn it and find out which way makes the pedal louder. Turn it up until you get distortion. Then back it off a bit. Tweak it from there until it sounds like you want it to for your purpose.

If you are using a guitar with passive pickups you should be able to turn it all the way up and be a happy chap.

That's my two cents. Lets call it a semi-educated guess. I'm just a mug lair ( new guy ) with pedals.

Be prepared for me to be corrected by someone with greater experience.

That should be a nice way to go, I'll try to see what happens with the trimmer at max value and back it off in case of distortion. I don't know why but I instinctively thought it to be the original 2k trim

Thanks to both!

[aion]

Buzz is right on. Definitely start with it all the way up, and then back off if you need to, which you probably won't - it was an easy mod to add so I included it in the layout, but it's not going to be tremendously useful to most people. It just makes the pedal a little more versatile for those edge cases!

[d3im0s]

Buzz is right on. Definitely start with it all the way up, and then back off if you need to, which you probably won't - it was an easy mod to add so I included it in the layout, but it's not going to be tremendously useful to most people. It just makes the pedal a little more versatile for those edge cases!

Ok thanks
I didn't look too carefully and at first I thought it served the purpose of the original trimmer. I will post a build report when I'm finished

[d3im0s]

Hi guys, I don't want to open a new thread, but I'm starting to do some tests. The first test I though of is a check of the output vs. input voltages. I used a sine wave at 1kHz as a signal. I'm not sure I understand how the sust pot acts on the signal. What I see is that with the sust pot at zero the behaviour of the pedal is linear and it's actually attenuating with a coefficient of 0.3 (output is 0.3 times the input). Raising the sust pot just increases the gain of the circuit until it hits a threshold of ~0.66 mV (peak-to-peak) where the circuit starts to act as a limiter. The gain in the linear stage is ~1 when the sust pot is circa in the middle position.
Is this the expected behaviour or is there something wrong?
I'm attaching a plot of the test
(sustain at zero - open circles
sustain at half position - red circles
sustain at full position - black circles)