EH - Low Frequency Compressor  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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mictester
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Post by mictester »

pedalgrinder wrote: I can't say iam a big fan of transconductance amplifier compressor they just don't do it for me.
if you look carefully at the schematic, you'll see that the OTA is used in the negative feedback loop of the op-amp - the gain of the OTA is increased to reduce the gain of the op-amp. The use of the 3080 in this way overcomes many of the limitations of the OTA (and keeps that nasty device out of the signal path). The range of gain control is limited (compared to an OTA in the signal path like the Dynacomp) but the quality is substantially higher!
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pedalgrinder
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Post by pedalgrinder »

what do you think putting it in the feedback path eliminates the hiss. I've tried both very little difference yes it does decrease the hiss and crap slightly in the feedback path. But basically it sounds like shit. The transistor would sound way better. In my opinion. I've tried both and i know which one i would pick. Putting in the feedback doesn't change much noise wise. if anything you lose more control of the unit.
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Post by borislavgajic »

:thumbsup Got one BlackFinger here...same PCB as 2 IC lowFreqCompressor.........
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borislavgajic
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Completed builds: ...script phase90,ETI phaser,Ross phaser,mutronC200,vox wah,WH10,FW-3,mutronIII,DrQuack,Penfoldautowah,Tubezipper,
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Post by borislavgajic »

there are different versions.....not 100% correct.........some resistor values........

Lucaslz
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Post by Lucaslz »

Hello, I’m new here, so greeting to all. I know this thread is old but I need some help fixing an early EHX Black Finger Distortion Free Sustainer (3 knobs in triangle version). There are no Ic’s inside, just transistors. My problem is that the initial signal cuts out with a loud pop then it comes back in with the sustain, every new note played cuts out then comes back in. I was hoping to find a schematic but nothing here seems to match up.
Cheers,
Lucas

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Post by Lucaslz »

Hello again, I had a look at the transistors and tried to read them. They are all round and black with 3 legs except for one larger round silver can with 8 legs. I will list all the lettering and numbers I could read.
List,
748CE 7209. 8 pin
FS37006 206, three of these
FS37000 143
FS36959 143, two of these
FS36999 143

I can’t find any information on them. I think the F stands for Fairchild.
Any ideas?

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Post by Scruffie »

The 748 is an Op Amp, sounds like it's this version https://www.freestompboxes.org/download/fil ... &mode=view

Your issue sounds like a bad electro, just replace them all, they're 40+ years old at this point.

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Post by Lucaslz »

Thanks for the response Scruffie. I think you are right about the op-amp. The schematic you provided has a lot of similarities and will be useful but it looks like I will have to trace my own schematic. I actually did change all the electrolytics, ( 10uf, 1uf, and three 5uf’s) but it looks like I missed a small one under the tone pot. To me it seems like a over clipped signal which is being shunted, so my suspicion is a bad transistor or out of spec resistor.
Correction; the 748 can has 7 pins and not 8!
Cheers
Lucas

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