Univox compressor growl issue

[tubegeek]

A new/hot LED will get dimmer if you use a larger series resistor. Even 10X larger.

Thanks. Will try that.

Any ideas where the oscillation might be getting in - and how to kill it?

[rankot]

I've built this compressor using schematic for Melos Mini Comp. It works with generic Chinese vactrol sold as VTL5C10, but it makes too much noise on note decay, which tells me that there's too much gain. It is also too quiet compared to what I expect (meaning VOL pot is useless), so I wanted to tweak it a little.

I tried to simulate it in ltspice and it seems to have -6 to +6 db amplification when COMP pot is at 1M, and +1.2db to +46.2db when COMP pot is at minimum. Regarding that, I wanted to modify it so it can go from +10 to +20db at max COMP and 0 to 18 db at min COMP.

The math says it should be built like this, with fixed resistor instead of COMP pot, and putting COMP (B500k) pot in series with LDR and 100k resistor, which gives me the range I want. So I built it that way, but it is distorting the sound, although it seems to be compressing, too. What do gurus say? 

Code Select Expand

Version 4
SHEET 1 3340 1008
WIRE -800 -48 -896 -48
WIRE -896 -32 -896 -48
WIRE -800 -32 -800 -48
WIRE -256 16 -256 -16
WIRE 976 16 976 0
WIRE 976 16 880 16
WIRE -896 32 -896 -32
WIRE 976 32 976 16
WIRE -800 64 -800 48
WIRE -704 64 -704 48
WIRE -704 64 -800 64
WIRE -800 80 -800 64
WIRE -704 80 -704 64
WIRE 880 112 880 96
WIRE 976 112 976 96
WIRE 976 112 880 112
WIRE 976 128 976 112
WIRE -496 144 -512 144
WIRE -400 144 -416 144
WIRE -352 144 -400 144
WIRE -256 144 -256 96
WIRE -256 144 -288 144
WIRE -112 144 -256 144
WIRE 208 160 -48 160
WIRE -800 176 -800 160
WIRE -704 176 -704 144
WIRE -704 176 -800 176
WIRE -512 176 -512 144
WIRE -112 176 -176 176
WIRE -896 192 -896 112
WIRE -800 192 -800 176
WIRE -256 192 -256 144
WIRE 976 208 976 192
WIRE 1104 208 976 208
WIRE -400 224 -400 144
WIRE 976 224 976 208
WIRE -512 272 -512 256
WIRE -176 272 -176 176
WIRE -80 272 -176 272
WIRE 64 272 0 272
WIRE 208 272 208 160
WIRE 208 272 144 272
WIRE 544 272 208 272
WIRE 640 272 608 272
WIRE 752 272 720 272
WIRE 816 272 752 272
WIRE 912 272 816 272
WIRE 1104 288 1104 208
WIRE 752 304 752 272
WIRE 816 304 816 272
WIRE -400 320 -400 304
WIRE -256 320 -256 272
WIRE -256 320 -400 320
WIRE -400 336 -400 320
WIRE -176 400 -176 272
WIRE 752 400 752 368
WIRE 816 400 816 384
WIRE 816 400 752 400
WIRE 976 400 976 320
WIRE 976 400 816 400
WIRE 1104 400 1104 352
WIRE 1104 400 976 400
WIRE 752 416 752 400
WIRE 208 448 208 272
WIRE 240 448 208 448
WIRE 320 448 304 448
WIRE 432 448 400 448
WIRE 528 448 432 448
WIRE 432 464 432 448
WIRE -176 496 -176 464
WIRE 432 560 432 544
WIRE -176 608 -176 576
FLAG -512 272 0
FLAG -896 192 0
FLAG -896 -32 9V
FLAG -800 192 0
FLAG -704 48 Vr
FLAG -80 128 0
FLAG -80 192 9V
FLAG 528 448 OUT
IOPIN 528 448 Out
FLAG -400 336 0
FLAG -256 -16 9V
FLAG -176 608 0
FLAG 752 416 0
FLAG 976 0 9V
FLAG 432 560 0
SYMBOL voltage -512 160 M0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR Value2 AC ac 1.
SYMATTR InstName V1
SYMATTR Value SINE(0 .1 1000 0 10)
SYMBOL voltage -896 16 M0
WINDOW 123 24 124 Left 2
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 9
SYMBOL res -784 64 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL res -784 176 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R7
SYMATTR Value 10k
SYMBOL cap -688 144 R180
WINDOW 0 24 56 Left 2
WINDOW 3 22 19 Left 2
SYMATTR InstName C4
SYMATTR Value 10µ
SYMBOL Opamps\\LT1057 -80 224 M180
WINDOW 3 -47 130 Left 2
SYMATTR InstName U8
SYMBOL res 16 256 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Rled
SYMATTR Value {Rled*1.5}
SYMBOL res 624 288 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R8
SYMATTR Value 5k6
SYMBOL res -400 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R11
SYMATTR Value 100k
SYMBOL cap -288 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 10n
SYMBOL res -240 288 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R12
SYMATTR Value 1Meg
SYMBOL cap 544 288 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C7
SYMATTR Value 33n
SYMBOL res -384 320 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R15
SYMATTR Value 1Meg
SYMBOL res -240 112 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R16
SYMATTR Value 1Meg
SYMBOL cap -192 400 R0
SYMATTR InstName C1
SYMATTR Value 330n
SYMBOL res -160 592 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R1
SYMATTR Value 100k
SYMBOL diode 768 368 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL res 800 400 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R2
SYMATTR Value 1Meg
SYMBOL npn 912 224 R0
SYMATTR InstName Q1
SYMATTR Value BC548B
SYMBOL diode 960 128 R0
SYMATTR InstName D2
SYMATTR Value RLED
SYMBOL cap 960 32 R0
SYMATTR InstName C2
SYMATTR Value 10µ
SYMBOL res 896 112 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R3
SYMATTR Value 2k2
SYMBOL cap 240 464 R270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C5
SYMATTR Value 10µ
SYMBOL res 304 464 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R5
SYMATTR Value 1k2
SYMBOL res 416 448 R0
SYMATTR InstName Load
SYMATTR Value 2k
SYMBOL cap 1088 288 R0
SYMATTR InstName C6
SYMATTR Value 33µ
SYMBOL res 160 256 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName COMP
SYMATTR Value 100k
TEXT -960 312 Left 2 !;.tran .2
TEXT -960 344 Left 2 !.ac oct 16 10 50k
TEXT -960 376 Left 2 !;.step param Rx 10k 110k 30k
TEXT -960 408 Left 2 !;.param Rx = 2k
TEXT -960 440 Left 2 !;.step param Fx 50 1k 300
TEXT 40 128 Left 2 ;COMP = 100k + B500k
TEXT 0 -24 Left 2 !.step param Rled 10k 1000k 100k


[rankot]

I couldn't make this work with proposed mods, really no idea why. So I returned to original design. Had some problems with motorboating on note sustain/release, after replacing hand made vactrol with Chinese VTL ST, that was fixed. However I still have a problem with noise, there's too much of it on sustain/release with Compression pot at minimum. I will try to reduce 3M3 resistor to fix that.

Mark Davis in Facebook group told me that original vactrol was MCD527 optocoupler is a 1K (Light) \ 2M (Dark) device. The closest match is an XVive5C4 with a 1.2K (Light) \ 2M (Dark) rating. CabibTechGlobal has the XVive5C4.

[Mark Hammer]

The model is helpful, but not necessary.  I just threw together an LED and LDR I had.  Since the LDR simply adjusts the gain of the op-amp, one can always play with the ground-leg value to achieve whatever response you want from whatever LDR you end up using, or having to use.

[rankot]

So, to conclude, I have finished this pedal and this is what works for me:
- I put 43k resistor instead of 6k8 which goes in series with Compress pot. That reduce amplification of sustain/release part of envelope, so less noise (and less sustain, but I can live with that).
- I used Chinese VTL ST vactrol, see table above for characteristics. Nice and slow release, so no motorboating.
- I put 1M2 feedback resistor instead of 3M3. That also reduce amplification of sustain/release part of envelope, so less noise and distortion. Maybe I could've left this as in original, but now I'm too lazy to try again.
- I put 180k resistor in series with photoresistor, so compression range is a little bit smaller, but also maximum amplificiation is 2.5 instead of 1, so now Volume pot has it's purpose.
- I moved clipping diodes to the very end of a circuit, so now they are AFTER Volume pot, and they can't distort sound if it is louder, you can reduce Volume and will not clip the complete tone, but they still have their purpose of cutting transient.

I'm a little bit tired right now, so I must ask instead of experiment - is it better to use A1M Compression pot instead of B1M I have?

[Mark Hammer]

Good to hear.  Again, I draw your attention to this:  https://www.diystompboxes.com/smfforum/index.php?topic=119185.msg1172734#msg1172734

[rankot]

Yes, I've seen that, just moved diodes after the Volume pot. And I have also used BAT46 for envelope rectification.

But I'm having hard time to understand how to add another diode in series with 5k6, I tried to simulate it, but wherever I put it, it seems not OK. I go to sleep now

[rankot]

Mark, I was just wondering, is it OK to reduce that input series resistor to 10k and completely remove 1M resistor to ground after that one, like this:


To me, it seems as a good idea to do this, I also simulated the circuit in ltspice and it seems that functionality is not altered and I hope reducing this input resistor will reduce noise, too. I know it is some kind of attenuator wired this way, but doesn't look crucial to circuit to work well. Also, putting a 330p cap instead of that redundant 1M will create a nice low pass filter for radio frequencies and noise.

[Mark Hammer]

Your pedal, so try it out.  I'll just note that the noisiness of many simpler compressors is not internally-generated noise, but rather amplification of external noise when one stops playing and gain is increased again.