API 525 compressor

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ChrisA

Well-known member
Joined
Jun 3, 2004
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86
Location
Los Angeles
was just thinking that this would be way cool as a clone project.. Feedback compressor,very cool sound, small and compact. Should be very cheap to make.

I am not experienced enough to embark on trying to reverse engineer this by myself, but would be willing to help in any way I can if someone else is interested in taking the lead...

Just a thought...

thanks guys,

Chris
 
I posted the suggestion on the old forum... The schematics are at www.danalexanderaudio.com. However, a cleaner copy so that we could read the component values would be nice.. I think peter C's melcor's would work well for the 2520's in there! There's also schematic for the 550A EQ, but the switches on that one look tough! :mad:

-Jay
 
This:

http://www.danalexanderaudio.com./ApiInfo/525Cschem.jpg

It seems to use the Vishay VCR2 JFET for control element, but - correct me if I'm wrong - it looks like it uses the FET to control opamp gain by shunting the inv-input to ground - not by attenuating a low-level input.

Actually a pretty good idea, if you have FET's behaving well enough..

I think this has too many switches and stuff going on to be a good clone-project - also the sidechain driving transformer could be a problem to source..

Jakob E.
 
they are really cool sounding compressors...I just love them on distorted guitars.

now looking at the schematic I see what you mean...They are tiny little units though with few controls, so I thought they might be a good candidate for cloning.
 
The advantage of this topology, is you don't need a separate bias voltage source for the FET as it works in the reverse manner of a FET as attenuator;

With no control voltage, the channel resistance is low, and the gain of the amp is high. As control voltage is applied the channel resistance increases and the gain of the amp decreases.

While ok for the first 5-7dB or so of gain reduction, distortion increases beyond this point. Also, and this is FET dependant, with the VCR2N, the Vgs off is typically 5-8v, so you need a lot of loop gain in the sidechain (weird rhyme, eh?), which can cause dynamic instability in the device (been here, done that).

It's actually a fairly easy circuit to build and align, but it is more of an effect, than a device I would use for serious compression/limiting (I have used it, so I know first hand).
 
These are great on certain things...I work on an api console quite often that has a pair. My favorite thing about them is that when used on distored guitars, they have a really cool launching attack effect when everything comes back in after a break. All compressors kind of do this with a slow attack, but these really have the sound I'm after for that. Sounds like everything breaths up and then breaks through with a cool "crack" at the head of the attack. Cool effect.
 
Maybe the sidechain transformer is the same used on the output??? :grin:

Anyone got inside pics?
 
the main reason why I never bought one of these is the fixed attack, however good it may sound, it would nice to have flexability with the attack, something simple liek that makes a fixed attack compressor useable in situaitons where it would be normally useless... If we do something based off that topology, the addition of a variable attack would be beyond worthy.

my two cents

dave
 
that 550 EQ also look like a good project. anybody used one?
parden my ignorance but is the 2520 that opamp API had specially made? how readily available are they, and how much are they?
 
The API comps are way up there on my list of underrated but way cool audio toys (just underneath the early Quad8 discrete comps - great on kik & snare) and would be an extremely cool DIY project...

Ohh and yes the 2520 is the discrete opamp specifically made by API, you can order them from API direct, or Brent Averill sells a version they remake (can't quite remember the number, but check out the website), sometimes Dan Alexander also has API bits & bobs. Also worth checking out is the Melcor discrete Opamp - an early version of the 2520 - see the discrete op-amp Meta in the "Meta-Meta" for diy instructions...

Justin.
 
Because the FET sees the pre-limiting signal voltage, it overloads on strong signal. To make it work on typical line levels you have to attenuate the input, forcing a compromise with noise. It also is highly sensitive to FET parameters.

What caught my eye is the "smart" release. Q2 Q4 get a (low-passed) integration of the audio, and cut-out a release resistor if recent input levels have been high.

Cloning seems unlikely. Too many 1970s parts. It could be re-engineered with a pair of op-amps instead of the rectifier transformer etc. But you need an original to reference while working on a re-design. A lot of work too.
 
API still makes these new, so its possible, although I think diminishing returns on the amount of work involved, I would much prefer to buy a new one and mod it with an attack control. They are expensive, but IMO, not really expensive enough to justify the trouble of building one yourself AND they are rock solid on the resale too... Looking at that schem, is there a way to implement an attack control? Pot or even a fast/slow switch? I dont even know where to begin thinking about that...

Just as a tease, I have a pair of quad eight AM2B compressors which are vactrol based, fully discrete, silly low parts count and I have two original units and a full documentation package for them and plan on putting up a page with full details on building a similar one for yourself, totally great candidate for a clone, stay tuned, will have this together in the fall if Im lucky. These are really cool though, uses an am4 opamp and has de-essing side chain, very cool little box.

dave
 
Hi folks!
Some time ago, I fixed old issue 525 and elsewhere must be
scanned pcb (it is littlebit different from 'c' so I was need to redraw sch.)
Also, it use other fet than vcr2n and had different fet's distortion cancellation circuit.
too many switches and stuff going on to be a good clone-project - also the sidechain driving transformer could be a problem to source..
switches is not a problem-3x4 lorlin will work.(com-lim, stopped to 2 pos.)
Bypass (off) can be done with different switch/relays.
Also, 'c' version sidechain must be done with using some
kinda 2142 shit ;-)) -just a mind; but old ver. used ct of sidechain trafo
for moving up dc threshold in limiter pos., so, there's some fun artefacts caused with this kinda circuit.
 
I have one of these. I'd be willing to provide pictures and any info that would help.

I bought this one DOA. It has a bad transformer. Someone modded/hacked it to work in a 500 series rack and apparently allowed DC to get to the output.

CJ, if you think you can glean any useful info from this transformer, I'll gladly send it to you. It's the API 2157 line transformer.

I need a replacement for that 2157, BTW if anyone could point me to one.
 
The Transformer in the side chain of the 525 comp converts the unbalanced signal to a balanced signal with its centre tap grounded. This way two diodes can be used to obtain the rectifier DC control signal. The meter is driven by the positive going DC control signal and the fet is driven by the negative going DC control signal. The side chain 2520 A3 and T2 transformer could be replaced with one NE5532 easily.
To add attack control just cut into the track or wire that joins to the join of R5 & C3 and add a pot wired as a variable resistor. A 10k pot will give the same attack range as a 1176 has.

The Quad8 AM2B are great and one of the simplest comps around. They have the original VC2 opto's in metal cans which can still be bought in the modern plastic vactrol package. The one thing to note with these compressors when trying to link them for stereo is that there is no negative feedback in the side chain and to get them to track well you need to match the gain of each transistor. Q1 left = Q1 right etc

QE%20Compressor%20Tim%20Whitten%20240.jpg


Joe

www.jlmaudio.com
 
Hey, speaking of sidechains - would it be feasible to use the Forssell sidchain ckt but with the 525 i/o sections?
 
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