Fet Compressor idea!!!!

[Kipper4]

So following TejfolvonDanone recent thread about his compressor.
I found this and wondered if it could be adjusted for guitar use.
And if possible with common parts to our "normal" stock as diypedal builders.

I'm not sure, but I think its a feedback instead of feed forward arrangement. Correct me if I'm wrong.

Enough drivel
heres the page
Thanks in advance for any input
Rich

http://www.swtpc.com/mholley/PopularElectronics/Feb1968/PE_Feb_1968_pg48.jpg

[R.G.]

Yes, it can be adapted for guitar use. In fact, I did that - in 1969. This compressor was one of my first guitar effects of any kind. Go for it.

[Kipper4]

Thanks RG.
I like it. Leave me hanging.
So as far as I see it the above will be expecting a much smaller (100~1000 x smaller)
signal.
The way I see it. It probably wont be a good idea to make the input 1000 x smaller or it will cause problems re the introduction of noise when making up the level at the output.
I plan to breadboard it as per the above. Subs 2n5088 (to comply with Keens second law)
I have other bjts.
I have some low gain Ac152 (IIRC) germs
Now I could go looking for RG compressors on google, but what will I learn.
Wish me luck.
Rich

[Kipper4]

Ho Ho Ho...
The first breadboard attempt turned out to be a great booster but no compression to speak of.
The second a noise maker.
I have to go to attempt 3 but more spread out on the breadboard.
so i can check my connections better and ammend values easier too.
for the pnp im going with a 2N5087.

[Kipper4]

Ding ding!!!!!!!!!!!!!
Q2 is a concertina splitter.
Attemp3 i cant tell if its compressing. but its passing signal with no noise. Bonus.

[Kipper4]

It strikes me that I can hear distortion and the pnp switching on and off.
Subs
Input 500k pot R1
Q5 2N3906
Q1 2N3819 not a sub
rest of Q's 2N5088

Q1
D 8V8
G 3V
S 0V01

Q2
C 6V3
B 4V4
E 3V8

Q3
C 2V9
B 1V
E 0V3

Q4
C 8V8
B 0V8
E 0V8

Q5 PNP
C 0V
B 0V8
E 0V7

[Kipper4]

I neglected to include the voltage divider on Q5 E for the first couple of attempt. (R13,R14)
this is now rectified.

[Kipper4]

It's better with a germanium Q5 (in my case ac153 hfe~60)
It compresses well but I can hear it pumping when the resistance changes.
What's what here.

[Kipper4]

I'll remeasure the Q5 (ac153) volts asap.
And see what's your opinions are.
I need more control over the Q5 on off thing, to stop it pumping. Or a much shorter off time so as only the attack is effected and compressed.

Chip in guys.
Even RG wants me to work this out 'cause he knows it can be done.

[R.G.]

Sorry - I didn't realize you were so far along.

Q1 is just a source follower

Q2 isn't a phase splitter, it's a variable gain amplifier. It's gain varies from below one to "high" depending on the value of the **AC** impedance at the emitter. That impedance is as high as 15K when Q5 is off to "low" when Q5 is turned fully on. Changing the conductance of Q5 is what modulates the gain of Q2 and makes the compressor work.

Q3 is a post-amplifier to bring the signal level up, and Q4 and the diode, etc. around it sense the signal level and modify the conduction of Q5 to make the output signal be fairly constant.

[Kipper4]

Thanks RG I think I got most of that.
I feel like my best bet for amending it would be to alter the values of R11 and R12.
I'll get back to the breadboard in a few hours.
The input signal amplitude is still bugging me though. I'd like to get it to where a chord or a single note will compress equally.

[PRR]

Well, I think it is over-complicated and mis-scaled for guitar.

And not a "FET limiter". An FET is stuck on front of a BJT limiter.

The "guts" is the trick of using BJT C-E path as "linear resistance". This is just-barely true IF the voltage excursion is Very Small. There was an early transistor broadcast limiter worked with this idea (but reversed). The performance limit is the AC across that transistor can not be over about 20mV or distortion rises badly. OTOH the hiss level tends to be a few mV, so the total dynamic range is not huge.

The pot in front is a very rude thing to do to weak signals.

As you note, the signal levels are far smaller than guitar. Made for quiet speech through a low-cost dynamic or crystal mike.

The real killer design flaw, at least for stronger signals of wide dynamic range, is: that variable element Q5 does not control its own input level! It always has to accept what you put in. Which is why the Max In is given as 40mV. (I said 20mV max for "linear" BJT C-E... I'll accept 40mV as "tolerable" for communications speech.)

You can do better.

[Rob Strand]

It's better with a germanium Q5 (in my case ac153 hfe~60)
It compresses well but I can hear it pumping when the resistance changes.
What's what here.

Try playing around with the voltage on the emitter with R13 and R14.

Sometimes you can make small improvements by swapping the E and C of Q5. 
This makes the transistor run with reverse beta.  This has a lower saturation voltage and may stop "clicks".
Put the silicon transistor back in when you do this.

Another trick is to add a small resistance in series with C5; not sure how useful it will be.

[Kipper4]

Thanks Paul.
I feel like my school reports are coming back to haunt me.
"You can do better."

Compressor wise I'm sure I could do a lot better. What can I say I like a challenge.
I feel like I put the gauntlet down now it's up to me to pick it up and slay this dragon.

Maybe changing the biasing or Rc and Re of Q2 is a better way to go.....
Just thinking out loud.

Cheers Paul

[Rob Strand]

Yes, it can be adapted for guitar use. In fact, I did that - in 1969.

Pretty cool RG.   Anyone that grew-up in the hobby electronics era got where they are because of articles in those magazines.  In the day, they were *the* information channel.

[Kipper4]

Cheers Rob

Lots more options there.
I'll look into those.

You gotta love this place. I do.

[Rob Strand]

Before installing lights under your car, there was:
http://www.swtpc.com/mholley/PopularElectronics/Feb1966/PE_Feb1966.htm

[jubal81]

The Bear Hug is an excellent DIY FET compressor for guitar.

You can buy a PCB here:
http://1776effects.com/product/bear-hug-compressor-new-version/

[Tony Forestiere]

The pot in front is a very rude thing to do to weak signals.

Following the theory with great interest, but this made me smile. Thanks  PRR. 

[Kipper4]

R10 =3k3 thats the only big change.
ac153 pnp
5088s
plus input 4n7 input cap

Its a bit noisy. could be the breadboard.
I could probably do away with the rude pot