DB54 - 2254 style compressor {update 22.10.19}

[MoJo]

Some pics from my units which I built in 2019. Everything works as it should except for the meter tracking (which was reported before from other users as well). I used BC184C, BC214C and BC441 for the transistors. It might help.

I used BC184C (with 2 perfectly matched ones where specified), BC214LC (could not find BC214KC anywhere), and I don't know where the BC441 went so I can't tell you what I used there. Thank you Undos, every perspective helps to rule out a potential variable.

 

[Undos]

Transistor orientation looks good now on your unit. also no other errors detected at first glance. as far as the bypass off/on position is concerned, according to the instructions under point 6, the up position (LED green) has the compression switched on. but on my device it is the other way round, the compression is switched on in the down position. i am pretty sure that the instructions are wrong. I would assume that compression is switched on when the toogle switch is pointing downwards.

 

[MoJo]

Transistor orientation looks good now on your unit. also no other errors detected at first glance. as far as the bypass off/on position is concerned, according to the instructions under point 6, the up position (LED green) has the compression switched on. but on my device it is the other way round, the compression is switched on in the down position. i am pretty sure that the instructions are wrong. I would assume that compression is switched on when the toogle switch is pointing downwards.

I may have put the LED in backwards Undos; believe me when I say one of my nicknames is "Wrong Way Joe"! Hahaha!

 

[JMan]

I discovered something odd on my board. The bi-color LED appears to be of the common-cathode variety, meaning the toggle switch should determine which anode sees current. However, on my PCB, there is no connection between the lower pin of the switch (presumably compression OFF) and the second anode of the LED. In the picture, you can see that a partial trace was clearly laid in the design software, but it doesn't end up anywhere (and no, there's no via, and I have confirmed this lack of connection with a meter). So even though it is supposed to support a bi-color LED, in practice I'll only be able to use a single color or I'll need to make that connection with a wire. I wonder if I got some early revision of the board and this error was corrected later, since it sounds like others are using the bi-color LED without issue.

 

[MoJo]

I discovered something odd on my board. The bi-color LED appears to be of the common-cathode variety, meaning the toggle switch should determine which anode sees current. However, on my PCB, there is no connection between the lower pin of the switch (presumably compression OFF) and the second anode of the LED. In the picture, you can see that a partial trace was clearly laid in the design software, but it doesn't end up anywhere (and no, there's no via, and I have confirmed this lack of connection with a meter). So even though it is supposed to support a bi-color LED, in practice I'll only be able to use a single color or I'll need to make that connection with a wire. I wonder if I got some early revision of the board and this error was corrected later, since it sounds like others are using the bi-color LED without issue.

I am able to verify that I get green (switch down) and red (switch up) colors, so I may have a newer PCB, JMan.

 

[JMan]

Who knows. I've been reading back through this entire thread and while there's some positive feedback, the project does seem to be plagued with gremlins for many people, exacerbated by a very glaring lack of any schematic (it is asked about again and again throughout the thread and the question is always completely ignored). So, I'm not surprised to find a minor error on my PCB. Somehow that feels like it fits. Part of me is thinking I might put this back on the shelf. I don't want to drop cash on the transformers just to end up with a unit that is full of bugs. We'll see.

By the way, Mojo, shortly before your first post in the thread, someone else had the same problem with the unit only passing signal in one mode and not the other. His question was never answered, but it might be worth trying to get his attention to see if he ever got it sorted out.

 

[MoJo]

Who knows. I've been reading back through this entire thread and while there's some positive feedback, the project does seem to be plagued with gremlins for many people, exacerbated by a very glaring lack of any schematic (it is asked about again and again throughout the thread and the question is always completely ignored). So, I'm not surprised to find a minor error on my PCB. Somehow that feels like it fits. Part of me is thinking I might put this back on the shelf. I don't want to drop cash on the transformers just to end up with a unit that is full of bugs. We'll see.

By the way, Mojo, shortly before your first post in the thread, someone else had the same problem with the unit only passing signal in one mode and not the other. His question was never answered, but it might be worth trying to get his attention to see if he ever got it sorted out.

I also saw the frequent requests for a schematic JMan.

I started looking for data by troubleshooting using the BA185 schematic. My limited schematic reading skills indicate that Pins U and V are feeding the 31267 style transformer, and Pins R and T are the output of BA185, feeding the 10468 style transformer. Here are my values (I am feeding a 1kHz sine wave into the XLR Input jack:

Compress switch in down position:

Pin U = 2.04VAC
Pin V = 2.02VAC
Pin R = 0.003VAC
Pin T = 0.003VAC

Compress switch in up position:

Pin U = 0.003VAC
Pin V = 0.003VAC
Pin R = 0.003VAC
Pin T = 0.003VAC

Does this look reasonable? If it does, then I will move on to the next board.

 

[MoJo]

I also saw the frequent requests for a schematic JMan.

I started looking for data by troubleshooting using the BA185 schematic. My limited schematic reading skills indicate that Pins U and V are feeding the 31267 style transformer, and Pins R and T are the output of BA185, feeding the 10468 style transformer. Here are my values (I am feeding a 1kHz sine wave into the XLR Input jack:

Compress switch in down position:

Pin U = 2.04VAC
Pin V = 2.02VAC
Pin R = 0.003VAC
Pin T = 0.003VAC

Compress switch in up position:

Pin U = 0.003VAC
Pin V = 0.003VAC
Pin R = 0.003VAC
Pin T = 0.003VAC

Does this look reasonable? If it does, then I will move on to the next board.

BTW JMan, I saw that person's posts and am trying to follow the suggested troubleshooting process, but cannot determine whether a 1kHz sine wave equals 1VAC. So I am just recording voltage values going n and out of the boards to see if you or anyone else notices a potential problem. The numbers I am getting don't really say much to me, being ignorant of the design in general.

 

[JMan]

1kHz is just the frequency of the tone - it's a common frequency for a test tone. The level you feed to the unit, rather than the frequency, will affect how you interpret subsequent VAC readings in the circuit. A good reference point is 0.775VAC = 0dBu, and 1.228VAC = +4dBu = 0VU (in many typical studio applications, although 0VU is not a true standard value but a reference level). This will not always have a consistent relationship to dBfs levels displayed within a DAW, but I seem to recall that the last time I was calibrating something a tone generator plugin set at roughly -18dBfs gave me 0dBu on my interface's analog output - I could be misremembering though. You can measure AC voltage between pins 2 & 3 on the XLR input to see what you're actually feeding the unit and adjust it accordingly. Maybe try starting with a 0dBu/0.775VAC signal.

I would say that 0.003VAC is effectively 0VAC, for all intents and purposes. So with the unit engaged, I would think having that on pins R and T might indicate a problem. That would mean that, assuming your reading of the schematic is right, no signal is reaching the 10468 transformer. (Sorry I am not double checking the pin out as I write this, I am on my phone and it's a little clunky to try to look through schematics on here).

 

[MoJo]

1kHz is just the frequency of the tone - it's a common frequency for a test tone. The level you feed to the unit, rather than the frequency, will affect how you interpret subsequent VAC readings in the circuit. A good reference point is 0.775VAC = 0dBu, and 1.228VAC = +4dBu = 0VU (in many typical studio applications, although 0VU is not a true standard value but a reference level). This will not always have a consistent relationship to dBfs levels displayed within a DAW, but I seem to recall that the last time I was calibrating something a tone generator plugin set at roughly -18dBfs gave me 0dBu on my interface's analog output - I could be misremembering though. You can measure AC voltage between pins 2 & 3 on the XLR input to see what you're actually feeding the unit and adjust it accordingly. Maybe try starting with a 0dBu/0.775VAC signal.

I would say that 0.003VAC is effectively 0VAC, for all intents and purposes. So with the unit engaged, I would think having that on pins R and T might indicate a problem. That would mean that, assuming your reading of the schematic is right, no signal is reaching the 10468 transformer. (Sorry I am not double checking the pin out as I write this, I am on my phone and it's a little clunky to try to look through schematics on here).

Thanks so much JMan. I will start checking the individual components of BA185 to see if I can determine where I am losing voltage. Does that sound like a reasonable troubleshooting plan?

 

[JMan]

Yes it does!