Any Good Limiter Schematics? (no clipping)

[waspclothes]

So I shot my mouth off a bit.. Let me explain.

My bass player friend just got a new amp, but said he misses the Limiter that's built into his old Ampeg head (SVT-350H I believe). I said that it would be "very easy" to build a similar limiter into a pedal for him that he can use in his new amp.

So far I've been unable to find any schematic for the 350H (let alone one with the compressor section circle and labelled "copy this part!") and other limiter circuits I've been finding online seem to just clip the signal to hard-limit it, or some have adjustments that the SVT limiter does not have. From what I could tell, the SVT limiter just allows a signal through without issues until it reaches a certain threshold and then attenuates it to match that. It seems to do it very transparently as well. Anyone have any schematics for this?

[Justin Thomas]

I didn't scour the schematics, but do any of the ones Enzo put here help?

http://music-electronics-forum.com/t13334/

Justin

[darkfenriz]

Opto? That might be a bit slow for a bass limiter in my view...

[J M Fahey]

Well,that´s what actually is inside the SVT350 and what waspclothes´ friend is missing.

But not sure about a pedal version; if anything a Rack one ... or at least some box standing on top of his current head (which we IGNORE, Mr Waspclothes ... ) inerted in his loop .
You need +/-15V for this limiter to work.

IF you want to build this one, I´ll suggest what schematic section to crop and build.

9V single supply is a very limiting factor so foot pedal compressor/limiters are more toys or gimmicks than serious stuff.

EDIT: and for transparency and good effect limiters need to be at an amp point where signal level is relatively high (definitely not instrument level) and after most gain and EQ so it only cares about peak level and nothing else.

[MarkusBass]

I'm not sure what exactly is the problem. If you search in google for "bass compressor schematic", you get hundreds of good bass compressor schematics supplied from +9V, usually they are build using opto. One of the best is Demeter Compulator but there are also many others. Demeter Compulator is very simple but it is used in many recording studios to get vintage-type compression.
If you set minimal compression, it will work as limiter. Or, you are looking for something else?

Mark

[Enzo]

Opto? That might be a bit slow for a bass limiter in my view...

Isn't the holy grail of compressors, the LA-2A based on optos?

[Leo_Gnardo]

Isn't the holy grail of compressors, the LA-2A based on optos?

True, and the light element in the original LA-2A is a swatch of electroluminescent panel (tape). In reissue versions I don't know what they use, I haven't been inside one.

One problem with bass and compressors/limiters is if the attack time constant is too short, the sensing circuit can mistake the waveform for the envelope and do some very strange things to the sound, so approaching the comp/limiter as a "leveling amp" is appropriate. Same story for bass from synthesizers, not just bass guitars.

[darkfenriz]

LDRs (optos) tend to have very fast attack and slow release, say a couple of ms attack to hundreds of ms or more than a second release. That's fine for many compressors.
For limiters in strict sense you'd expect a relatively fast attack (yet 20ms is still fine and does not distort bass freqs as much) with a fast release, say <100ms.
Just my 2c, not saying it is a very big deal.

[Mick Bailey]

The 350H limiter has a number of issues that need to be considered in relation to a pedal build. Firstly, the 'sense' line is taken from the power amp output (labelled 'output' on the schematic). So it needs a high-level signal to drive it and isn't the usual signal-in/signal-out pedal arrangement. Secondly, there's no level adjustment so it's pretty much fixed around the 350H circuit levels.

For my money, if I was taking this approach I'd use a Rod Elliot type of design, which is powered off the amp's output and requires no additional supply. I've built this using an NSL32SR2 optocoupler with no issues whatsoever - 2Kv isolation and fast response time.

[Mark Hammer]

The SSM2166 chip is still available from Small Bear electronics and probably a few other places. It is the engine of a high-quality control strip, that can be used as a compressor or limiter. It also includes a downward expansion function for use as a sort of noise gate. IN short an extremely quite, and quiet-capable, chip, that requires very little in the way of external components to be a full-fledged dynamics processor.

[waspclothes]

The SSM2166 chip is still available from Small Bear electronics and probably a few other places. It is the engine of a high-quality control strip, that can be used as a compressor or limiter. It also includes a downward expansion function for use as a sort of noise gate. IN short an extremely quite, and quiet-capable, chip, that requires very little in the way of external components to be a full-fledged dynamics processor.

Thanks Mark! I see the LM13700 datasheet also shows a schematic for an "Automatic Gain Control" But I don't think it can be tweaked to ensure it works great with bass. I might breadboard it though since I have a few of those chips around.