1176 inspired compressor on a budget

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Mattnezz

Member
Joined
Feb 26, 2012
Messages
10
Anyone ever tried a 1176 on a dual 15V supply?
It was just that I could use another comp, I like the 1176 and had 4 dual 15V supplies and no way to go for the -10/+30V. So there was a challenge to make a nice comp with a sound with the parts I had lying around.
Today I got this prototype running and it even sounds good to my ears.
The input is a THAT1240 line reciever.
After which I kept the discrete first gainstage and  rectifier.
I used a very simple DOA to drive an OEP 2:1 output transformer. Like the Jensen 918.

Of course no spare Sifam meter, and that's where I'm stuck now.
I used an electronic opamp based rectifier for a $7,- meter, which works fine for metering output level... Problem occurs in GR-mode... I think I have to look over my PCB layout and design around the TL071...Because the TL071 driving the rectifier seems to act like a comparator.
I can see the GR on the meter, but i swings full left or right and is untrimmable. Not that it's over sensitive, more digital, like yeah, gain reduction is taking place or no you're below threshold... ;)
I changed some R's because the circuit around the metering is fed +15V/-10V instead of +30V/-10V, but that was mostly guesswork, I changed the 4k3 to 2k1 aswell as lowering the 10k under the 'zero-adjust' trimpot... I think I might have to lower/half the 10k source resistor of the control-fet...

Well I have a good night sleep and will post schematic/pcb-layout if I get the GR-metering to work if anyone is interested.
 
Mattnezz said:
Well I have a good night sleep and will post schematic/pcb-layout if I get the GR-metering to work if anyone is interested.

Maybe post the schematic and someone can help you get the GR working  ;)
 
:) Its working! (the GR metering) I made a PCB layout mistake, and connected the 100k feedback resistor to the non-inv. input  :-[ So there was my comparator...
Schematic is still a pencil sketch on some sheets of paper. But another slight headroom problem arised around the input section/ first gain-stage... Which also seems a bit noisy. I think it's there, but I'll have another look.
Now I come think of it, maybe this is normal, maybe anyone can verify this?

When I feed a 0dBu (0.77Vrms)/1kHz signal to the input I see quite some hard clipping at the output when I crank the input pot to say 3 o'clock. This starts at the bottom of the sinewave, but eventually clips a lot.
I thought the gain was only 20dB so the stage has to be able to handle 10Vpp...
Too bad, because apart from that, below the clipping threshold I get good THD and noise figures.
I'll check voltages, but have to recalculate them, because I use +15V/-15V instead of +30V/0V, but that should be easy...
 
Mattnezz said:
When I feed a 0dBu (0.77Vrms)/1kHz signal to the input I see quite some hard clipping at the output when I crank the input pot to say 3 o'clock.

The output of the unit, or the output of the preamp stage?
 
The output of the unit, or the output of the preamp stage?
I looked at the output of the unit with output volume nearly closed, so I think it's in the pre-amp. Will look at what happens at pin 15( preamp output only) later.
Measured the voltages and they were spot on relative to -15V.
I thought the gain was only 20dB so the stage has to be able to handle 10Vpp..
Measured about 27dB gain in the first stage, which seems normal.
Noise seems to come from the powersupply. I'll clean that up with proper regulators.
I think using BC107's with lower Beta won't change things, would it? These are about Hfe 500...

edit: of course this is with GR set to off... Maybe the fet needs better bias? I think I might have set the 'Q bias trim' with GR on... Check into that too...
 
Mattnezz said:
Well I have a good night sleep and will post schematic/pcb-layout if I get the GR-metering to work if anyone is interested.

Hi I would be really interested to see the Schematic Thanks.
 
Check out Igors F76 compressor:

http://www.groupdiy.com/index.php?topic=40009.0

It's all broken up into different boards witch smaller schematics, but it's basically the same concept.
 

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