Which compressor?

[Greenballs]

Thanks mate. Am I right in thinking that Johan replaced the Jfets with a second opamp? Also, there's two LEDs and two LDRs there, how are they arranged in practice? All lumped together or separated in pairs?

An additional opamp was added for the VU meter, so adding a dual opamp (or replacing all with a quad) leaves one spare opamp which looks to have been used to buffer the bias voltage. It appears the Jfet stage was simply removed and not replaced with anything.

There are some comments about the LEDs and LDRs in that old thread that I linked to. My interpretation is that they need to be lined up and placed as closely together as possible. But read for yourself and see what you think.

Having now read that thread I concur with your interpretation.

Just two more questions and I promise I'll leave you alone! Does it matter which side of the second opamp drives the VU meter? And regarding the resistor and the 'BIG' capacitor in series before the LEDs, will a 10k trim pot suffice as per the 'Jfet' schematic and would something like 4.7uf be in the right ball park do you think? Ok, that's three questions, but I'm done now, honest!

[moosapotamus]

Each section of a dual or quad opamp is the same. Doesn't matter which side you use for what. Whatever works best for your circuit layout.

If you are trying the VU schematic, it does not have the trimmer between the two LEDs. So, I would probably just start with the values as shown in the Jfet schematic, 1k and 0.2uF, and go up from there if necessary. Yes, you could replace the 1k with a trimmer to dial in the best value and then replace it with a fixed resistor if it turns out to be relatively close to a standard value.

[Phend]

Orange Squeezer note, Instructions mention that using 2N5457 is the best, I can vouch for that, if you don't "all bets are off".

[Greenballs]

Well, I've had both versions of the LA Light schematic on the breadboard and, despite quite a sustained effort switching out opamps, LDRs and LEDs and ensuring that the jfets were matched pairs, couldn't get a peep out of either of them. No sound, no LED lighting up, nada. It's such a simple circuit design that I'm not sure what else I can do? Any suggestions would be greatly appreciated.

On another note, I had much better success with Bajaman's optical comp, which sounds great, and I curently have the Weak Joe Compressor on the breadboard but so far haven't been able to get that working either.

I've got a + and - 15v bipolar voltage converter built and am aware of a mistake on the schematic with it showing +15v going to the collector of Q1 instead of the correct -15v. Am I right in thinking that the +15v supplies power to all the op-amps also, as oppose to 9v? Also, the opamps are variously connected to ground via pins 3 and 5, do the opamps require grounding via pin 4 also? This is the first time I've encountered such a means of grounding an opamp and it is unclear on the schematic whether or not this is the case. If there is anything else I need to look out for with this build I'd be most grateful for any pointers.

[duck_arse]

assume nothing on that schem,Greenballs, get a complete circuit diagram. generally, if an opamp input is grounded, the opamp will be bi-polar supplies. pin 4 is not ground, but V- or Vcc-, and it always needs a connection to your supply.

[Greenballs]

assume nothing on that schem,Greenballs, get a complete circuit diagram. generally, if an opamp input is grounded, the opamp will be bi-polar supplies. pin 4 is not ground, but V- or Vcc-, and it always needs a connection to your supply.

So, and forgive my ignorance here, does that mean that pin 4 needs connecting to the -15v rail?

[duck_arse]

unstated. you need to show a schem with the power supply included. if it sez +15V and -15V, then your yes-ish, Vcc- would be -15V.

[moosapotamus]

Well, I've had both versions of the LA Light schematic on the breadboard and...

Sorry to hear that. It does require a bipolar power supply. Note connections to -V. Can't think of much else other than double checking everything several times. Sounds like you did give it a pretty thorough going over, tho.

[Greenballs]

Well, I've had both versions of the LA Light schematic on the breadboard and...

Sorry to hear that. It does require a bipolar power supply. Note connections to -V. Can't think of much else other than double checking everything several times. Sounds like you did give it a pretty thorough going over, tho.

I got it working today, once I'd paid attention and realised it needed the bipolar power supply!

It's a nice sounding compression effect, although I did find that it made various static noises/squeals with the pots in certain positions, which I had some success in stopping by changing a couple of resistor values. Its also very low output when the compression is maxed out and could use some gain recovery, which I also attempted to implement but found that it led to more unwanted noises and less range in the compression effect.

Due to the super low parts count I am tempted to persevere with it, but I think I'm going to move onto Merlin's Engineer's Thumb, of which I've heard good things.

[moosapotamus]

 

[eh la bas ma]

Did you consider  Guyatone PS-021 ? This one is pure magic.

I built Aion's project, called Meridian, with bass specs, and I use it on everything, guitars, keyboards, drum machine, etc.

https://www.musikding.de/Meridian-kit
https://aionfx.com/app/files/docs/meridian_documentation.pdf

I built the Engineer thumb (5 knobs version) and I almost never use it. It creates some unpleasant distortion quite easily, depending on how you set the controls. To the point that many builders think their build has an issue :

https://www.diystompboxes.com/smfforum/index.php?topic=129237.msg1248160#msg1248160

 I think it would be much harder to use it on other instruments than guitar, but I never even tried that. Some of the controls need to be set under 12' or 3' or the circuit won't work correctly. Maybe the 2 knobs version is easier to use....

I spent so much time trying to get rid of the distortion... I'll always remember the quote from Chuck at PPCB forum :

"The design is clever, I wanted to like it, but that never happened."

https://forum.pedalpcb.com/threads/engineers-thumb-compressor-question.12149/

I bought an expensive Walrus deep six, it's good and surely a bit more effective than the meridian on the attack and compression side.
But the Colour and the Exciter controls on the Meridian are very useful to shape the signal. I really miss them on the deep six. Exciter controls odd and even harmonics. This is amazingly good to give more precision and clarity to the signal. Colour pans between bass boost and treble boost, really useful on drums and many other instruments.

[amptramp]

You question may come down to:

1. How much compression do you want?

If you are feeding a distortion, you may want to maintain a constant level so the sustain is as long as possible.  If you are going on radio, they already compress everything.  If you are live, how much dynamic range do you want to retain?

2. At what level do you want the compression to start?

If you are in a studio, you may want a line level compressor.  If you are live, you may want something different for amp levels.

3. Do you need variable controls for attack and decay time constants?

Some people want this to be fixed so they know exactly what they are working with and can't accidentally move an adjustment on stage.  Some people want it to be variable so they can accommodate any playing style, such as in studio equipment.

4. What do you want to use for a variable gain stage?  FET?  OTA?  tubes?

FET's are easy to find but difficult to match with any consistency and this gets to be more of a problem with push-pull circuits that would be used in a studio setting.

OTA's are easier to use but more diffiicult to find.
 
My Hammond Solovox uses 6SK7 remote-cutoff tubes for its volume control and I did get a call one time from a guy on a fan forum of a band he was in to see if there were any 6386 remote-cutoff twin triodes in the US for his Fairchild 670 compressor.  I gave him a few names of tube purveyors, but he couldn't find any.  The 6386's are approaching unobtanium but a pair of variable 6SK7 / 6BA6 stages cascaded would probably give more compression with the added advantage that everybody and his dog carries them.


So you have some criteria here to select a compressor circuit.  I hope this list helps clarify what you need to look for.

[EBK]

You question may come down to:
....

4. What do you want to use for a variable gain stage?  FET?  OTA?  tubes?

Don't forget about VCAs.  These are readily available and are great for compressors.  THAT Corp. makes some very nice Analog Engines, which include a RMS detector and VCA in one package, and they have a lot of informative app notes and example circuits on their site. 

[FSFX]

Don't forget about VCAs. 

I thought virtually all VCAs are based on an OTA type of circuit or similar configurations using Gilbert cells either as discrete implementations or using ICs.

[Greenballs]

Did you consider  Guyatone PS-021 ? This one is pure magic.

I built Aion's project, called Meridian, with bass specs, and I use it on everything, guitars, keyboards, drum machine, etc.

https://www.musikding.de/Meridian-kit
https://aionfx.com/app/files/docs/meridian_documentation.pdf

I built the Engineer thumb (5 knobs version) and I almost never use it. It creates some unpleasant distortion quite easily, depending on how you set the controls. To the point that many builders think their build has an issue :

https://www.diystompboxes.com/smfforum/index.php?topic=129237.msg1248160#msg1248160

 I think it would be much harder to use it on other instruments than guitar, but I never even tried that. Some of the controls need to be set under 12' or 3' or the circuit won't work correctly. Maybe the 2 knobs version is easier to use....

I spent so much time trying to get rid of the distortion... I'll always remember the quote from Chuck at PPCB forum :

"The design is clever, I wanted to like it, but that never happened."

https://forum.pedalpcb.com/threads/engineers-thumb-compressor-question.12149/

I bought an expensive Walrus deep six, it's good and surely a bit more effective than the meridian on the attack and compression side.
But the Colour and the Exciter controls on the Meridian are very useful to shape the signal. I really miss them on the deep six. Exciter controls odd and even harmonics. This is amazingly good to give more precision and clarity to the signal. Colour pans between bass boost and treble boost, really useful on drums and many other instruments.

Ah cool, thanks for the tip on the PS-021! The addition of the Exciter and Colour controls sounds like it might be more suited to my needs, I'll definitely be building that on the breadboard.

[EBK]

Don't forget about VCAs. 

I thought virtually all VCAs are based on an OTA type of circuit or similar configurations using Gilbert cells either as discrete implementations or using ICs.

VCAs are usually based on Blackmer cells, which can be thought of as Gilbert cells redesigned for audio.

[Greenballs]

You question may come down to:

1. How much compression do you want?

If you are feeding a distortion, you may want to maintain a constant level so the sustain is as long as possible.  If you are going on radio, they already compress everything.  If you are live, how much dynamic range do you want to retain?

I'm not sure how much compression exactly, just a wide range, say from 2:1 - 20:1.

2. At what level do you want the compression to start?

If you are in a studio, you may want a line level compressor.  If you are live, you may want something different for amp levels.

I'll be using the compressor to do home recordings with. I'll be driving the unit from my Focusrite audio interface, so line level.

3. Do you need variable controls for attack and decay time constants?

Some people want this to be fixed so they know exactly what they are working with and can't accidentally move an adjustment on stage.  Some people want it to be variable so they can accommodate any playing style, such as in studio equipment.

Variable would be the ideal but I could live with fixed if I had to. It depends on the parts count and the cost involved. If I can buy a decent quad comp for significantly less that the cost of building one then I probably, reluctantly, will.

4. What do you want to use for a variable gain stage?  FET?  OTA?  tubes?

FET's are easy to find but difficult to match with any consistency and this gets to be more of a problem with push-pull circuits that would be used in a studio setting.

OTA's are easier to use but more diffiicult to find.
 
My Hammond Solovox uses 6SK7 remote-cutoff tubes for its volume control and I did get a call one time from a guy on a fan forum of a band he was in to see if there were any 6386 remote-cutoff twin triodes in the US for his Fairchild 670 compressor.  I gave him a few names of tube purveyors, but he couldn't find any.  The 6386's are approaching unobtanium but a pair of variable 6SK7 / 6BA6 stages cascaded would probably give more compression with the added advantage that everybody and his dog carries them.

At the moment I'm open to all options, but as I said, cost is the main driver. After cost its about sonics and how suitable the circuit is to the purpose. I still have research to do on compressing drum overheads, for example (I'll be using the Glyn Johns drum mic technique). Once I know what the desirable compression characteristics are for this application I'll be able to narrow my search criteria. I realise that I may be on a fools errand and that I may be trying to force a square peg into a round hole but they're both risks I'm willing to take!

So you have some criteria here to select a compressor circuit.  I hope this list helps clarify what you need to look for.

Thanks for your comments, they've certainly given me food for thought.

[Greenballs]

Can the Bajaman LA2A style optical compressor be modded to allow control of the attack and release of the effect either by means of a switch, between preset fast/slow modes, or a potentiometer? Or, if anyone has the time, can someone give me a brief analysis of the attached schematic please so I can try and figure it out myself? Thanks.

[Fancy Lime]

Can the Bajaman LA2A style optical compressor be modded to allow control of the attack and release of the effect either by means of a switch, between preset fast/slow modes, or a potentiometer? Or, if anyone has the time, can someone give me a brief analysis of the attached schematic please so I can try and figure it out myself? Thanks.

It can be done but frankly, the design has so too many issues to bother trying to improve it, imo. Too many superfluous, poorly chosen, and missing components. I would strongly advocate a complete redesign of the side chain. Controlling attack and release in a feed back opto limiter (which is what this is, despite the side chain forking off left of the LDR) in a predictable fashion is tricky in general because the LDR's rise and fall times limit the maximum speed, which makes the infamous dynamic overcompression problem of feedback limiters much worse and nearly impossible to solve on high settings. Also, the current through the LED messes with the release time (unless you add an extra buffer), and the resistance range of the LDR forces us to use high series resistance values, which is not ideal for noise reasons. For those reasons, I pretty much abandoned the idea of opto limiters. You can get the same results (if you want) and much more from JFET or OTA designs.

Andy

[Greenballs]

Can the Bajaman LA2A style optical compressor be modded to allow control of the attack and release of the effect either by means of a switch, between preset fast/slow modes, or a potentiometer? Or, if anyone has the time, can someone give me a brief analysis of the attached schematic please so I can try and figure it out myself? Thanks.

It can be done but frankly, the design has so too many issues to bother trying to improve it, imo. Too many superfluous, poorly chosen, and missing components. I would strongly advocate a complete redesign of the side chain. Controlling attack and release in a feed back opto limiter (which is what this is, despite the side chain forking off left of the LDR) in a predictable fashion is tricky in general because the LDR's rise and fall times limit the maximum speed, which makes the infamous dynamic overcompression problem of feedback limiters much worse and nearly impossible to solve on high settings. Also, the current through the LED messes with the release time (unless you add an extra buffer), and the resistance range of the LDR forces us to use high series resistance values, which is not ideal for noise reasons. For those reasons, I pretty much abandoned the idea of opto limiters. You can get the same results (if you want) and much more from JFET or OTA designs.

Andy

It sounds good though, feels good under the fingers and doesn't suck tone (at least, not until it's near maxed out, then it loses a little high end I'll admit).

So, its great to hear your opinions -

I get that the feedforward claim is possibly false, but I'm unsure why?
I understand that the LDR and the LED affect the attack and release times and that adding an extra buffer (somewhere? in front?) will solve the LED side of the problem but I'm guessing the LDR/high series resistance values issue is just one that we have to live with?
I'm unclear as to which parts of the circuit you consider superfluous, poorly chosen and missing? Or how you'd go about redesigning the side chain?

Genuinely interested in your response.