Scott Hampton Class A Opto Comp with schemo and parts list!

[Ptownkid]

So I finally got around to building this, and I seem to be having some problems.

the led stays fully lit always...and it's waaaaaaaaaaay quiter than the level actually coming out of my computer.

I checked all the voltages and they read as they should.

Maybe I'm doing somehing wrong. I'm putting it between the right output on my sound card and my monitor, and it's like maybe 1/4 of the left channel (no compressor) at best.

I read on scotts page that decreasing r6 decreases overall gain and increasing increases....is that right. I switched r6 from 33k to 51k and i swear it got quiter.

Hellllllppppp meeeeee, hahaha, my brain hurts

cheers

 

[Ptownkid]

someone, anyone...

 

[Sleeper]

Hi PTown, I built a pair of these.
I put a hamptone JFET amp on the front end.
That sounds great for synths, instruments etc.

I think these need a pretty healthy drive to be happy. The nice thing, is that you'll already have most of the parts you need, transistors etc. because hamptone uses the same components here, AND the PSU voltage is the same. The JFET amps are also pretty well documented on this list so if you've got room, you should be set.

I'm not sure about the LED problem, but one thing you should double check is the FF, FB jumpers/connections... you have to be careful with this, it seemed like an easy place to make a mistake.

BTW I used your etch pattern, so rest assured you got it right and it will work barring any assembly/wiring mistakes

Kelly

 

[Ptownkid]

yeah, I followed the schematic and directions very carefully, and I did do that jumper right.

It passes audio, it's just really quit, i mean really.

I'll go over it again with a fine tooth comb, maybe I have some resitors messed up as the picture of component placement is hard to read at best.

Worst case scenario, I'll build another and use it to check the first one if it turns out right, haha.

If anyone else has any input as to why my led is fully lit at all times, that'd be great.

 

[Sleeper]

I looked at the schematic and had a think.
here's a couple of things I did.

In the make-up gain part of the schemo, I tested the jfets and changed (i think) R8... better compare witht the jfet sorter below to see what the correct resistor is.
check out PRRs sorter, that one will do it.
http://www.groupdiy.com/index.php?topic=1766&highlight=jfet+matching

it makes me think you might be compressing all the time if the LEd is lit, that will give you LOW output level. I guess if you were to lift the FF/FB jumper, the compressor would be out of circuit and you could be sure the makeup gain section is working correctly.

The optos are kind of heat sensitive, if you look at their datasheets they sometimes have special instructions for keeping heat off of the component during soldering.

Kelly

 

[Ptownkid]

Ok all is well, got it working. I'm not really sure what the problem was, but I suspect some dirt was playing tricks somewhere.

Sweet

 

[Sleeper]

Isn't it always something like that.
Mine worked and tested 100% and the first session I tried to use them on, one of them started going all intermittent.

I opened it up and found that I had completely forgotten to solder one leg of a resistor :roll:
Kelly

 

[enthalpystudios]

[quote author="kiira"]
Have you seen this?

http://www.paia.com/fracrak.htm

The rack is 33$ and the front module plates are $3.50. Paia also sells bipolar 15V amd 18 V. PSs for them too which don't take up a rack space. They are alos anadized already. I dunno... they are a lot cheaper than anything else Iv'e seen though not as zoot with full enclosed boxes and all that.

Kiira[/quote]

That is indeed extremely cool.... I wonder if it will also fit the boards elco is working on.... Might make a very inexpensive way to build a box of utility's

and the 'power wing' for it also comes in 12v..... very cool stuff.

billy

 

[Ptownkid]

can someone quickly explain or show me how to wire a bypass into this?

 

[Sleeper]

what do you mean exactly? you want to keep make up in the circuit and bypass the compressor?
If this is the case I THINK you could just do what I mentioned for testing and put in a SP switch that lifts-opens the FF/FB jumper and you would be good to go.

from looking at the schematic the only thing I'm not totally sure about is whether the vactrol might push a small amount of resistance when i'ts not getting any control voltage signal... which it wouldn't when the FF/FB jumper is open/OFF. even if it did it would be a pretty small amount of input voltage drop.

If you mean like a bypass for the whole thing, search for relay bypass, put in a relay and bypass the whole circuit. roger foote has a pcb for this, and you could buy 24volt relays. pretty easy, and once you suss out how that works you'll want to put it on everything. It's cool cause if you do it right, the bypass will be bypassing when there is no power on. Then you can put this normalled on a patchbay and just leave the power off when you arent using it and signal will still pass thru.

Kelly

 

[Baltimore]

Would it be possible to sub in a 1731 for the makeup gain stage (with a +/- supply of course) and have this circuit still perform well? I don't see why it wouldn't, just want to be sure before I order all the parts.

 

[Sleeper]

I think it will work, you really ought to compare this to the forssell opto before you do the subs. check the meta for that

pretty sure it should work fine.

sleeper

 

[Ptownkid]

me thinks that the forsell opto is in a whole other league than this guy. This one is super simple and basic as hell.

 

[bluezplaya]

So I am curious how easy a build this is. And also how nice it sounds. It looks as though no one has posted here in a while and just wondering if anyone has had much success with this build.

Adam

 

[Ptownkid]

sounds pretty good and it's really simple. I posted a home etch file in this thread somewhere.

 

[bluezplaya]

Yes i saw it. Where can I get a "home etch" kit? :green:

 

[Ptownkid]

Well...you need

-Press and peel blue (smallbearelec.com)
-access to a photocopier of laser printer
-An iron
-Copper clad board
-Ammonium persulphate crystals (or ferric chloride, but it's nastier)

There's tons of info on etching on the net.

http://www.tonepad.com/photoessay.asp?photoEssayID=10&sequenceNo=1
http://www.tonepad.com/photoessay.asp?photoEssayID=11&sequenceNo=1

 

[jrfred453]

Does anyone know where to get a couple of OPA251PA or OPA251PAG4 for this project? Mouser and Digikey don't have any available until December 2008! I looked around other places online and had no luck. Is there a replacement that suits this project well?

 

[jrfred453]

Does anybody have any advise on powering 2 of these compressors without the wall wart. Was thinking I could use mnats power supply board with the output wired +24vdc-0-+24vdc(since I have it leftover) and get a transformer to power that.
How big of a transformer is needed? The wallwart was 400ma so is it 400ma + 400ma = 800ma then? Will the Amveco te62065 or te62075 work? Or is that overkill? Please Help!

 

[bluesman714]

What would be typical applications for this type compressor?