My studio Mic compressor build

[jfrabat]

I feel like Wedge Antilles in A new Hope...  "Almost there....  Almost there... "

Tell you the truth after the 3rd release (in time) I don't know what happens in Star Wars  .

Lucky for you this happened in the original film!     (Wedge is the pilot who first fired the torpedo's at the Death Star but missed).

Yes the aggressiveness is determined by R88.   You might even try 180k.     You want it so when the Depth (RV6) is full (14V) it's about as aggressive as you need.   Then you might need to back-off RV6 to soften it. The spec is -60dB  but that's fairly aggressive anyway.   -40dB max would mean setting the R90 voltage to about 150mV.   

If you have to back-off Depth (RV6) too far to make it usable then maybe increase R88 further.   As you found, for sound it's OK to back-off RV6 but I think it stuffs-up the noise-gate LEDs so you would want RV6 to bet setting between half and full, preferably between 3/4 and full.

I went back to 270K.  I like how it sounds there.  So DEPTH is set.  But I feel TRIGGER is to sensitive.  If I set it past 10 o'clock, it will not open (at least not with the guitar!).   

As for the Threshold adjustment.  Don't worry about where it is positioned ATM.  The position vs when the Gate opens depends on the signal level, and the parts that set the sensitivity are not around the Depth control.

What do you mean ATM?  And Threshold you mean TRIGGER, right?

2 things I have noticed, though:

1. When the gate circuit is active, overall volume is reduced
2. When the gate opens, there is a little bit of "crackling"

I think the best way to let you know what I mean is showing you:

But the good news is that it IS working (still needs some tuning, but it is working!)

[Rob Strand]

I went back to 270K.  I like how it sounds there.  So DEPTH is set.  But I feel TRIGGER is to sensitive.  If I set it past 10 o'clock, it will not open (at least not with the guitar!).   

Yes, the more I looked at the design the more I started to realized intended specs were very aggressive.    Even worse is the depth pot was  trimpot.   Fixed 60dB!!!   I also didn't realize you use a pot for Depth.    The 270k won't give you the original 60dB but it's going to be far more usable.

What do you mean ATM?  And Threshold you mean TRIGGER, right?

ATM = at the moment.

Yes I meant Trigger.

Did you use a log taper or linear taper pot for the TRIGGER (RV5)?   A linear pot could be the reason why you have to set the Trigger so low.

The trigger circuit is fairly complex.  It starts as the output pin 14 IC5 and ends say at pin 7 IC5.

The big problem at this point is the crackle.   More the point, where is it coming from?  There's so many possibilities!
With all the LEDs flashing it could come from those.  It could also come from opamps in the trigger circuit.  It could come from the whole design of the trigger circuit.

I'm not overly happy about noise-gate LED circuit you might try lifting the side of R78 that goes to the Depth pot wiper.
At least that's one possible cause out of the way.

Does setting the Attack and Release to slow times have any affect on the crackle?

If the crackle is coming from the trigger then it's going to be a big job because we need to re-design is correctly.
I could suggest some "hacks" but they might not be the best thing to do.   

[jfrabat]

I went back to 270K.  I like how it sounds there.  So DEPTH is set.  But I feel TRIGGER is to sensitive.  If I set it past 10 o'clock, it will not open (at least not with the guitar!).   

Yes, the more I looked at the design the more I started to realized intended specs were very aggressive.    Even worse is the depth pot was  trimpot.   Fixed 60dB!!!   I also didn't realize you use a pot for Depth.    The 270k won't give you the original 60dB but it's going to be far more usable.

What do you mean ATM?  And Threshold you mean TRIGGER, right?

ATM = at the moment.

Yes I meant Trigger.

Did you use a log taper or linear taper pot for the TRIGGER (RV5)?   A linear pot could be the reason why you have to set the Trigger so low.

The trigger circuit is fairly complex.  It starts as the output pin 14 IC5 and ends say at pin 7 IC5.

The big problem at this point is the crackle.   More the point, where is it coming from?  There's so many possibilities!
With all the LEDs flashing it could come from those.  It could also come from opamps in the trigger circuit.  It could come from the whole design of the trigger circuit.

I'm not overly happy about noise-gate LED circuit you might try lifting the side of R78 that goes to the Depth pot wiper.
At least that's one possible cause out of the way.

Does setting the Attack and Release to slow times have any affect on the crackle?

If the crackle is coming from the trigger then it's going to be a big job because we need to re-design is correctly.
I could suggest some "hacks" but they might not be the best thing to do.

I just tested the second board, and it does not have that crackling.  Odd, because the only difference between the two is that I had to use clear LEDs for the second (ran out of diffused LEDs and could not find any locally).

To be honest (as I mentioned earlier), the main intention of this build is the compressor; the noise gate at least now is functioning, so even though there is some crackle, it is not the end of the world.  I don't really have a need for 2 noise gates in the signal chain anyway!  Don't get me wrong, I still intend to try to fix it if possible, but if I cant, I still feel this build was a win. 

TRIGGER is an A10K pot, so no, not linear.  Could it be that I need a C10K?  I think I got some in my stockpile I could try...  Both circuits are the same here (highly sensitive).

[Rob Strand]

I just tested the second board, and it does not have that crackling.  Odd, because the only difference between the two is that I had to use clear LEDs for the second (ran out of diffused LEDs and could not find any locally).

Well that's very strange.  Possibly even pointing to a fault or build issue (part value).   The LEDs themselves are highly unlikely to be causing the problem.  It will be the circuits driving them and the current through the LEDs, which doesn't change with the LED.

*If* the noise-gate LED circuit is a problem, and if you tested board 2 with the Depth pot on full, it might have helped hide that issue.

To be honest (as I mentioned earlier), the main intention of this build is the compressor; the noise gate at least now is functioning, so even though there is some crackle, it is not the end of the world.  I don't really have a need for 2 noise gates in the signal chain anyway!  Don't get me wrong, I still intend to try to fix it if possible, but if I cant, I still feel this build was a win.

It's up to you at the end of the day.   The good thing is you have one that works and it might be possible to compare behaviour, voltages, maybe oscilloscope waveforms to see what is different.

TRIGGER is an A10K pot, so no, not linear.  Could it be that I need a C10K?  I think I got some in my stockpile I could try...  Both circuits are the same here (highly sensitive).

A log pot for the trigger seems to be the best choice.   Definitely not C10k.

So one possible change is to increase R71 from 10k to 33k.   That will make the trigger less jittery.   If that's still no good try increasing R64 to say 100k.


My feeling is playing with R64 should be unnecessary.   It could hide a problem.

[Rob Strand]

I just watched you video again listening to the crackle then I realized the way the trigger is wired is clockwise is *less* sensitive.  So my recommendation are around the wrong way.   So what you want is to decrease R64.   Maybe to 15k.

And don't worry about changing R71.

[jfrabat]

I just tested the second board, and it does not have that crackling.  Odd, because the only difference between the two is that I had to use clear LEDs for the second (ran out of diffused LEDs and could not find any locally).

Scratch that...  I just had the volume lower.  I tried the second board today, and it is doing the same thing!

I just watched you video again listening to the crackle then I realized the way the trigger is wired is clockwise is *less* sensitive.  So my recommendation are around the wrong way.   So what you want is to decrease R64.   Maybe to 15k.

And don't worry about changing R71.

Got it!  Thanks!

[jfrabat]

I just watched you video again listening to the crackle then I realized the way the trigger is wired is clockwise is *less* sensitive.  So my recommendation are around the wrong way.   So what you want is to decrease R64.   Maybe to 15k.

15K worked perfectly!  Now the TRIGGER is much less sensitive (it is VERY usable now!)

Scratch that...  I just had the volume lower.  I tried the second board today, and it is doing the same thing!

Actually, now that I managed to put both on at the same time, the first one us MUUUCH more noticeable.  The second one you need to crank it to hear it.

Anyway, this is where we are so far:



Amazing the difference it makes to finish the aluminum and put the decals on!  You can see that clearly in that pic (the two compressors are identical otherwise)!

I still have some issues with the pre-amp and noise (hum), and I got the crackling from the 1st compressor that I need to fix.  But other than that, we are moving forward with the analog studio build!  I still got that other preamp to finish (the fife fish studio kit), and I want to do a de-esser after that (and call it done!).

Rob, any advise as to where to start looking for a fix to the crakling sound?

[Rob Strand]

Amazing the difference it makes to finish the aluminum and put the decals on!  You can see that clearly in that pic (the two compressors are identical otherwise)!

Yes it is an amazing difference.   The "brand", device name, and the lines help make it look more professional as well.   I remember the same effect when I did aluminum panels - I only did lettering, no lines.

I still have some issues with the pre-amp and noise (hum), and I got the crackling from the 1st compressor that I need to fix.  But other than that, we are moving forward with the analog studio build!  I still got that other preamp to finish (the fife fish studio kit), and I want to do a de-esser after that (and call it done!).

Yes, things are moving on.   That PCB issue put a stop to things for a while.

Rob, any advise as to where to start looking for a fix to the crakling sound?

From what you are saying you have narrowed the problem down to
- the problem is only build #1
- the problem is the compressor? (Initially I was thinking the crackle was caused by the noise gate.)

As a side issue:  Because I thought the crackle was from the noise gate I had a closer look at the detector circuit IC5c through to Q9.    I found some possible issues with the design.    It kind of only just works, or possibly doesn't work 100% correct.   To get to the point, when you set the Depth pot (RV6) to minimum ie. no noise gate, what is the voltage across R90?    There is a potential problem that when RV6 is set to minimum to noise gate doesn't quite open fully.   Ideally we want about 14V to 14.5V across R90 when RV6 is set to minimum.

[jfrabat]

From what you are saying you have narrowed the problem down to
- the problem is only build #1
- the problem the compressor? (Initially I was thinking the crackle was caused by the noise gate.)

As a side issue:  Because I thought the crackle was from the noise gate I had a closer look at the detector circuit IC5c through to Q9.    I found some possible issues with the design.    It kind of only just works, or possibly doesn't work 100% correct.   To get to the point, when you set the Depth pot (RV6) to minimum ie. no noise gate, what is the voltage across R90?    There is a potential problem that when RV6 is set to minimum to noise gate doesn't quite open fully.   Ideally we want about 14V to 14.5V across R90 when RV6 is set to minimum.

The problem is only with the gate engaged.  No issues with the compressor only.

Let me get back to you tomorrow with the voltages (I have the whole thing drying from the stickers).  I had to remake the front one, because I had the gate bypass and the sensibility switches inverted (did not notice until the sticker was dry).  I also opened the holes for the XLR/quarter inch combo jack in the opposite sides (it only uses 2 bolts, and I have it backwards...  AGAIN!  Did EXACTLY the same thing in the original!  I feel stupid admitting this, but it happened...)

The other thing I have not decided yet is if I want to change the green LED.  The new ones I got today are significantly less bright than the ones I had, and it is most noticeable in the greens (especially since there is 2 of them next to each other!).  So I think I will take the bright one out and put a new one in to have both at the same brightness.

[Rob Strand]

The problem is only with the gate engaged.  No issues with the compressor only.

So do both builds or only one have the problem?

The other thing I have not decided yet is if I want to change the green LED.  The new ones I got today are significantly less bright than the ones I had, and it is most noticeable in the greens (especially since there is 2 of them next to each other!).  So I think I will take the bright one out and put a new one in to have both at the same brightness.

LED colors and brightness is always a pain for production.   Good idea not to mixed LED types in the same unit it always looks bad.

I've had issues on a few professional products where a model/brand of LED has gone obsolete and we had to find a replacement.  I had to tweak the LED current on the builds with the new LED so the brightness matched the old products.   Otherwise you put the old and new builds side by side and the customer thinks one of the units has a fault!

[Rob Strand]

It would be good to check if the crackle is present when the Depth control is set to full and to minimum.   If the crackle disappears or reduces when the Depth pot s at full and/or minimum it means the noise-gate LED circuit is stuffing-up the signal (that would point to a LED drive circuit design problem).

After checking that,  it would also be a good idea to lift the one end of R78 (100k), the end that goes Depth pot wiper.   See if it removes or affects the crackle.

[Rob Strand]

Before I forget, the noise-gate has a fixed hold time.  Not all analog noise gates have an explicit hold time. 
C18 (15n) sets the hold time for the noise-gate detector.   Currently it's around 3ms, which is fairly short.
You could easily doubly that to 33n (6.6mS) which would generally improve things and pretty much have no negative impact.

[jfrabat]

Thanks!  I will give it a shot.  Unfortunately, it will need to wait until next month, as life got in the way of my projects, but it will.get done none the less!

But to understand, what do you mean by hold time?  I know attack and release, but not familiar with hold....

[Rob Strand]

But to understand, what do you mean by hold time?  I know attack and release, but not familiar with hold....

Hold time is similar to release time but it has one important difference:  when the signal falls below the threshold the gain does not change at all until the Hold time expires then after that the release time kicks in and the gains starts to decrease.   

The main advantage is the hold time completely removes distortion on low frequency signals.  For low frequency signals, the trigger pulses off and on between cycles.  If you only have release the gain starts reducing *during the cycle* and goes back up again on the next cycle.   The gain modulation causes distortion.   With hold the gain is kept constant over the cycle and there is no distortion.

For a circuit like the ETI the detector is full-wave so the detector will get a kick after half a cycle.  100Hz is 10ms  so half that is 5ms, so a hold of more than 5ms will not cause distortion on a 100Hz signal.

[jfrabat]

OK, here is where we are at; the original compressor has some issue that causes distortion, so I took it out of the signal chain.  It works (compressor and noise gate), but makes your voice sound like a robot.  The second board works beautifully for the compressor, but the noise gate for some reason has stopped working.  I figured I must have done something when I put it inside the enclosure.  So back to troubleshooting!

EDIT: Apparently it is a design issue with the circuit itself; it seems the board does not recognize when you invert the sensitivity and the gate bypass switches and corrects accordingly as it should certainly be designed to do...   

Anyway, the new cap is in.  I also made my first recording today (my daughter singing an early 2000s rock tune - Fefe Dobson's "Take me away"; here is how that one is turning out - I still need to fix the guitars; I threw them quickly to record the voice and test the system!), and the whole "studio" gear seems to be working wonderfully!

Regarding the original board, I am tempted to just redo the board (with the changes I know are needed), and use all the off-board stuff I already got going (will need to use the same ICs, and I am short of some of them) instead of troubleshooting the thing.  What do you guys think?  It would be nice to have 2 compressors, and I do have the components to fill the board.  So go with a new one (kind of) or troubleshoot?

After that, I need to troubleshoot my tube preamp, as it is making noise through the XLR.  And after that, I need to finish the other preamp, before I even think about the de-esser!  But so far, we got 2 components working extremely well, and working well through 1/4 jacks.

[11-90-an]

...before I even think about the de-esser!

Just a thought, I drew a simple block diagram... given you have 2 EQ's




When you say "redo the board", do you mean just pulling everything out and populating again? I think the troubleshooting will just be easier now that you have a working one ro compare it with...

[jfrabat]

...before I even think about the de-esser!

Just a thought, I drew a simple block diagram... given you have 2 EQ's




When you say "redo the board", do you mean just pulling everything out and populating again? I think the troubleshooting will just be easier now that you have a working one ro compare it with...

I only have 1 EQ (I have 2 compressors).  I made 3 EQs, but one has that issue with the boost in the high frequencies, the second I made for my daughter (no way she is giving me that one back!  LOL!), and the 3rd one is the one for the "studio" (which is also the only one with the frontal panel; the other 2 have top panels).

What I mean by "redo the board" is take a new board, but new resistors, caps, transistors and IC sockets, and reuse the pots, IC's, LEDs, jacks and enclosure.  The old one I made some mistakes, and it is a bit battle scarred from all the fixing already (I even lost one side of the copper base for the gate LED from reheating it so many times to correct).

[11-90-an]

High frequency problem? Even turning the highest frequency knob down to 0 doesn't help? Besides, the de-essed signal will only need around 5-9 kHz range, so you can just turn all the other knobs to 0...

Yup, redoing the board seems good...

[jfrabat]

High frequency problem? Even turning the highest frequency knob down to 0 doesn't help? Besides, the de-essed signal will only need around 5-9 kHz range, so you can just turn all the other knobs to 0...

The first EQ I built worked, but if I boosted the 4th frequency (high frequency), and I had the frequency selector a bit past 2/3's of the pot range, and I boosted it, there was a high pitched wail.  Seems there was something causing oscillation in that board, which did not replicate in the 2nd or 3rd build of the EQ.  So I just stopped using it (or, rather, left it to use in with the guitar, as the panel is top mounted, but, to be honest, it is not even in my pedalboard!).

Yup, redoing the board seems good...

Just to be on the same page, the new board will be of the compressor, not the EQ (I am out of dual gang pots for that one).

But I was thinking the same thing.  The hard parts are already done (all the off board stuff and the enclosure), and I have everything I need to build it (including 3 boards, as OSH sent me 2 initially, then they sent me 3 more because they did not send 3 originally), AND we know the board with the mods works, as the 2nd one is running just fine.