Simplest Vari-Mu compressor?

[Schmilsson]

I'm looking for one to gain a greater understanding of how it works and also to perhaps build one. Are the altecs it or are there even simpler designs out there?

 

[jackies]

Well, there's famous PRR vari-mu design... Specifically designed to be as simple as possible..
Do a search.

 

[Schmilsson]

Thanks!
Yeah, I know of the PRR, I'd thought I'd stay in the tube realm, and, it looks more complex than say an altec 43x.

 

[Johan]

build the PRR vari-mu!..
it is a simple build with a verified layout and it sounds just great...
since you are in sweden, BLR (bilradiolagret) has the groundloop transformers PRR suggests, LH-musik has the tubes and sockets, kjell&co has PCB blanks and etch sollution and if there is anything else needed, elfa has "everything from antenna to ground"
..only thing other than that, look at gyraf G9 for ideas how to get the voltages you need without having to customwind your transformer..
johan

 

[simonsez]

Solid state doesn't always simpler....I think Altec is the simplest one...or may be PRR with altec side chain?

 

[Schmilsson]

Agreed, I have a really hard time seeing the PRR as easier. Three opamps and pcb's and what not. I think I wanna stay with tubes and p2p for convenience.

 

[gary o]

Federal am864u is very simple and the ge ba9/collins 356e/unilevel all the same and simple & fun.

 

[3nity]

Hi Gary just wondering how easy and simple is to build a Federal Limiter?
How does it sound??

Thanks

 

[gary o]

Well few years back I made one within a few hours it wasnt pretty I didnt really know what I was doin & still dont but still learning I made a cock up with resistor values for input attenuattor weired thing was it worked and sounded great I dont think anyone had DIYed one a these at the time so I was on my own a bit I got some advice from a few people but I never really got it working right all my posts should be here still, I thimk........Since then I think a few people have gathered more info & made some nice ones tho I keep reading about problems with it.....I suppose you could say its a simple circuit & can sound great but is a bit fussy to get working right......do some searches there lots here about it I think.

I had fun and better results with the unilevel GEbA9 circuit same as gates 26J try a search gates 26j...

 

[diyfanatic]

http://waltzingbear.com/Schematics/Misc/AM-864.htm
should be easy on P2P...

http://www.gearslutz.com/board/attachments/geekslutz-forum/91611d1218778659-federal-am864-u-limiter-current-control-manual-am864u.pdf

http://www.groupdiy.com/index.php?topic=31199.0

 

[mich]

The Altec does seem to be the "simplest" in terms of parts count. easy to p2p.

Here is some pics of mine if you didnt see it allready. - (scroll down)

http://www.groupdiy.com/index.php?topic=32633.0

 

[lolo-m]

If you want to understand the way varimu comps works, build a PRR... The only difference into PRR's and Fairchild's way to work, is that the sidechain amp is solid state in PRR's and there's an additional gainstage to fit nowadays needs...

Fairchild's SCAmp explanation, as simple as possible :
http://www.groupdiy.com/index.php?topic=31856.0

 

[gary o]

Build any of them to understand how they work personally I think the all tube old ones are more fun and will give u more crush and to me a little easyer to make.

 

[rotation]

Thanks!
Yeah, I know of the PRR, I'd thought I'd stay in the tube realm, and, it looks more complex than say an altec 43x.

I don't think p2p Altec or anything similar is easyer than PRR comp on pcb. My experiences are that you first have to build psu which can be problematic if it's regulated with IC. Then you have to make p2p layout where mystake can cause hum, etc.

In case of PRR comp somebody else already thought about all this, so you simply stuff the boards, study and mod.

Good luck!

 

[Schmilsson]

Thanks all for really good advice and input! Greatly appreciated.

A proper beginners question to follow it up:
All designs are push pull, is there such a thing as a single ended vari mu compressor?

 

[gary o]

Ther is info is here somewhere may have to wade thru all vari mu stuff but that can be fun and very interesting I seem to remember PRR talking about a single ended vari mu.....as for building spose you have to build what yr comfortable with I myself prefer tubes they more forgiving unless of coarse you touch the HT psu very simple to make I knocked up a point to point LA2a in 2 days at xmas designed it as i went good luck & have fun.

 

[directdriver]

I just saw a schematic of the Western Electric KS-16617 amplifier and noticed it includes a single-ended compressor internally and just wanted to share it here.

It uses a single 6BA6 pentode cap coupled to half 12AT7 and SE transformer coupled as output. The control voltage is created by the other half 12AT7 connected as a diode to get the negative voltage inject into the input transformer winding. It's almost like half of a Federal AM864 compressor. No info on the specs of the input and output transformers though.

Since it's single-ended so it might thump but it certainly fits the description as a simple compressor. It might be worthy of a DIY project.

Compressor section.
https://i.ibb.co/d4zpddd/WE-KS-16617-compressor-schema-1600x900.jpg
Full schematic.
https://i.ibb.co/ZBMw6CG/WE-KS-16617-schematic-full-4000x2500.jpg