PedalPCB Median Compressor

M

Mikeydubsj

New member
Hey guys, I am building a few Median Compressors, and I just finished my first one and had a few questions as I can't get it to work.

I've been building pedals for a few years, and have learned a lot, but this is my first compressor. There's a few things in this build that I haven't encountered yet. So I apologize if this i'm just missing something stupid, but here is what I've got.

*Main Issue
TR1 10K Trimmer potentiometer, 3362P type. The PCB itself has a 5 hole spot for this pot, however the only 3362 types of pots i've seen only have 3 poles. Am I wrong in using this? Is there a different pot that I should be using? If anyone has questions that would be amazing.

IC replacements
I've seen other people using the TL072CP chips as opposed to the OP1678. (I had originally bought the OP1678 and the through hole adapter, but the ICs I bought weren't the right size. I will update that in the future. But will this still work with the TL072CP?

Q1 & Q2
When installing, I used a socket for the transistors for easy access to reversing them in case I got it wrong. Is this a bad practice? I just figured it would make things easier in case I made a mistake.

I've been reading up on how to bias the transistors, but i'm not sure i'm even to that point yet since I can't get any audio out of this circuit. Even if I roll the TR1 from one extreme to the other, I get no signal.

Other than that, I linked a picture of my build in here as well. Any help would be greatly appreciated!
 

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Trimmer is good - that layout accommodates two different pin configurations.

I don't know about the TL072s personally, but if the word on the street is those work, they probably work.

Transistor sockets are a bummer because eventually after some number of stomps, the trans like to work themselves loose and start causing problems. In this case it was maybe a good move, since it looks like yours are upside down.
 
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Mikeydubsj said:
The PCB itself has a 5 hole spot for this pot, however the only 3362 types of pots i've seen only have 3 poles. Am I wrong in using this?
Click to expand...
You’re fine using that. The 5 holes are there to accommodate different sized trimmers.

Mikeydubsj said:
But will this still work with the TL072CP?
Click to expand...
It looks like it would based on the Datasheet. It will be noisier, but might be fine.
Mikeydubsj said:
When installing, I used a socket for the transistors for easy access to reversing them in case I got it wrong. Is this a bad practice?
Click to expand...
I used to socket my transistors early on. Eventually I picked up a tester and these days I usually solder them directly in. Using sockets isn’t “wrong”, but it can lead to connectivity issues if the pedal gets jostled around.
 
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you could also hit the trannies with the hot glue goo when using sockets to get a little piece of mind.. it's reversible if ever needed.... but yeah... get a cheap tester and test the pinout... then full send with an iron is the way to go
 
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As i have been told when I post a build report with transistors in sockets, once you know everything is working, you can add a bit of solder to keep them in place.
On some fuzz pedals I have transistors in sockets so I can experiment with different types, hfe, etc. When I can finally make a decision I will add some solder lol!
 
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Robert said:
I verified the prototype with TL072s before my 1678's arrived.

The bottom left TL072 in your photo does not look genuine.
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How can you tell? These are actually ones I've had sitting around in other pedals. Once I found out the original ICs I bought weren't the right size, I went and scrapped these out of old builds.

I'm still not getting any audio out of my pedal. Last night I went and checked every solder joint in the schematic and everything should be working, but it's not. So I was guessing either Q1 and Q2 are in the wrong way, or they were bad. I had extras so I did try out other ones and in both variations (just in case). But it still didn't work.

So now i'm thinking it could be the TL072 that might not be working. Maybe all of them? or could it be just that one?
 
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Have you adjusted the bias trimmer to measure 1.875V on Q1? It may be that your JFETs are out of range. Try searching the forum for related threads. There’s a lot of info available on this build.

“The original has a test point on the Drain of Q1 that measured 1.875VDC. You could start there, or take a look at the biasing instructions for an Orange Squeezer compressor.”
 
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mybud said:
Have you adjusted the bias trimmer to measure 1.875V on Q1? It may be that your JFETs are out of range. Try searching the forum for related threads. There’s a lot of info available on this build.

“The original has a test point on the Drain of Q1 that measured 1.875VDC. You could start there, or take a look at the biasing instructions for an Orange Squeezer compressor.”
Click to expand...
I tried doing this, but the highest reading I could get was .49VDC.
 
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mybud said:
That might be why you’re not getting any sound. The JFETs need that minimum voltage to function, as I understand it. IOW, those parts may not be within spec for this circuit’s requirements.
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So how do I get JFETs that are within spec? Because these are the 2N5457 that listed on the parts list. Did I get a bad batch? I just bought a tester, so I can dig in a littler more that way ig.
 
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Mikeydubsj said:
So how do I get JFETs that are within spec? Because these are the 2N5457 that listed on the parts list. Did I get a bad batch? I just bought a tester, so I can dig in a littler more that way ig.
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This is a tough one. The problem is that 'normal' through-hole JFETs may or may not be within spec, given that they are no longer current (AFAIK). Let's see what info your tester gives us before jumping to conclusions (note to self).
 
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Mikeydubsj said:
Because these are the 2N5457 that listed on the parts list. Did I get a bad batch?
Click to expand...

Where did you get the 2N5457s? Unfortunately these are pretty regularly (intentionally) mislabeled/counterfeited
on amazon/ebay/aliexpress/etc.

They're really only safe to get at the guitar pedal community type sites (small bear, stompboxparts, aionfx, ppcb, etc) or
in SMD form from Mouser/DigiKey (MMBF5457).
 
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z2amiller said:
Where did you get the 2N5457s? Unfortunately these are pretty regularly (intentionally) mislabeled/counterfeited
on amazon/ebay/aliexpress/etc.

They're really only safe to get at the guitar pedal community type sites (small bear, stompboxparts, aionfx, ppcb, etc) or
in SMD form from Mouser/DigiKey (MMBF5457).
Click to expand...
Yes, it does seem like the SMD ones are more reliable.
 
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z2amiller said:
Where did you get the 2N5457s? Unfortunately these are pretty regularly (intentionally) mislabeled/counterfeited
on amazon/ebay/aliexpress/etc.

They're really only safe to get at the guitar pedal community type sites (small bear, stompboxparts, aionfx, ppcb, etc) or
in SMD form from Mouser/DigiKey (MMBF5457).
Click to expand...
These are ones I got from tayda Electronics. I have bought some printed enclosures from them and figured i'd save on shipping by buying these with them too. Is tayda a company to stay away from when buying components?
 
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Mikeydubsj said:
These are ones I got from tayda Electronics. I have bought some printed enclosures from them and figured i'd save on shipping by buying these with them too. Is tayda a company to stay away from when buying components?
Click to expand...
No, not necessarily. But when it comes to parts like JFETs, take @z2amiller's advice and try to get these from specialist guitar pedal places, perhaps. I regularly order from Tayda but not parts such as JFETs as a rule.
 
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I suspect your TL072 with the notch in it (bottom left position) is a fake, it might work, but it's not a genuine TI part. Someone might correct me, but to my knowledge there are no genuine TL072's with a notch in the top.

I've never gotten a fake or defective 2N5457 from Tayda, that doesn't necessarily mean they are in spec, but unless they are wildly out of spec I would expect you to at least have some sound.


Regardless, lets start with the simple stuff first.

Do you have sound in bypass? If not, full stop. Let's address that now. If so, continue on.

Do you have sound with the Blend control all the way down? If not, you can ignore the JFETs for now.

If you don't have sound with the Blend control turned all the way down try putting one of your OTHER TL072's in the bottom lefthand spot.


We're not concerned with the top left IC right now, so you can just leave it empty since you won't have enough TL072s, but make sure the other three sockets have the (hopefully) good TL072's installed.
 
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Robert said:
I suspect your TL072 with the notch in it (bottom left position) is a fake, it might work, but it's not a genuine TI part. Someone might correct me, but to my knowledge there are no genuine TL072's with a notch in the top.

I've never gotten a fake or defective 2N5457 from Tayda, that doesn't necessarily mean they are in spec, but unless they are wildly out of spec I would expect you to at least have some sound.


Regardless, lets start with the simple stuff first.

Do you have sound in bypass? If not, full stop. Let's address that now. If so, continue on.

Do you have sound with the Blend control all the way down? If not, you can ignore the JFETs for now.

If you don't have sound with the Blend control turned all the way down try putting one of your OTHER TL072's in the bottom lefthand spot.


We're not concerned with the top left IC right now, so you can just leave it empty since you won't have enough TL072s, but make sure the other three sockets have the (hopefully) good TL072's installed.
Click to expand...
- Yes, i have sound in bypass (that was my first point of troubleshooting) , I also took a multimeter to every solder point on the pcb (a lengthly process 😂) but everything has connectivity.

- I have tested blend & level controls in all positions with no sound.

- I will test with the blend down and different IC.

I do have new TL072 ICs arriving today, so hopefully that may fix things.
 
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