MUFF WIGGLER Forum Index
 FAQ & Terms of UseFAQ & Terms Of Use   Wiggler RadioMW Radio   Muff Wiggler TwitterTwitter   Support the site @ PatreonPatreon 
 SearchSearch   RegisterSign up   Log inLog in 
WIGGLING 'LITE' IN GUEST MODE

Limiter / Compressor
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Limiter / Compressor
Haralds:Werk
To handle the great dynamic range of the Shakuhachi I needed a compressor for my Shakuhachi 2 Synth project. Because a limiter is not that different I added this feature as well. This comes in handy with my Vocoder project also. The structure used here is derived from "Small Signal Audio Design" by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression or limitation is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal.


Specs and features
• Switch compress or limit
• Switch Compression/Limit rate 50% or 90%
• Compression/Limit rate adjustable 0--max
• Runs on +/-15V and +/-12V (with minor resistor changes)
• Power consumption below 15mA each rail


The documantation for download can be found in my website.



The audio signal flows unaffected through IC1A/B. When the compressor - limiter kicks in the inverted signal is added (=subtracted) at the summing node of IC1A. The signal level to subtract is regulated through a Sims VCA. The CV generation for the VCA is pretty standard. Linear for the compressor and exponential for the limiter.



Precision full wave rectifier with filter to generate the control voltage for the VCA from the audio signal.





steffensen
Cool idea!
paperCUT
I'm just about to start building Rockin' Banana! I was wondering what component changes for 12v, I'm guessing around the offset voltages before U4a and U4c?
notmiserlouagain
Very nice and original circuit with love for the detail!
Haralds:Werk
paperCUT wrote:
I'm just about to start building Rockin' Banana! I was wondering what component changes for 12v, I'm guessing around the offset voltages before U4a and U4c?


It should work with +/-12V without changes. You will loos some headroom and the SNR might be a bit lower.

The offset voltages are not the problem. The slightly negative offset for IC4A R13/R14 ensures that the VCA is really off with zero voltage input in compress mode. If not you can increase R13 slightly. The offset for IC4C is easily trimmed with TR2.

Generally when moving from 15V to 12V with OTA most problems are caused by the change in Iabc. If you run into problems here lower R27 to 24k and R25/R26 to 12k or 11k. But first try the original values. You have to adjust R10 anyway for the different Gm. This should be sufficient to compensate for the lower Iabc as well.

If I find the time I can pull mine out of the rack and put it on the bench to test with 12V.
Haralds:Werk
Just checked the module with +/- 12V. It works with no problem. You will loose a bit of resolution with the rate pot. There is no effect within the last 20%. You should be able to resolve this by lowering the Iabc limiting resistors from 13k to 11k.
paperCUT
Sorry I got sidetracked and had to debug a bit. I seem to have the same issue on both my pcbs where the inverted signal is being amplitude modulated to the point of waveshapping. Both units are outputting the full rectified input signal with no smoothing before hitting the vca, at least that's what it looks like.

Just to clarify IC6B is supposed to be smoothing out the rectified signal correct? hmmm.....
Haralds:Werk
paperCUT wrote:


Just to clarify IC6B is supposed to be smoothing out the rectified signal correct? hmmm.....


Yes, that's right. Check the values of the capacitors. It should be 470n. You should see the rectified signal at pin 7 and the smoothed signal at pin 8 of IC6
BugBrand
Having a quick look as I wanted to check the Doug Self reference -- my copy doesn't have page 682! (the subtractive stuff is mentioned around page 505)

Looking at the schematics, I wondered whether IC4 stages are somewhat redundant? I mean they're effectively inverting twice, scaling a little and adding offset - so I wondered if that couldn't be achieved direct into IC5B and Q3?

Curious to hear how it functions - is no makeup gain required?
Haralds:Werk
BugBrand wrote:
Having a quick look as I wanted to check the Doug Self reference -- my copy doesn't have page 682! (the subtractive stuff is mentioned around page 505)

Sorry, I should have mentioned it is in the second edition.

BugBrand wrote:
Looking at the schematics, I wondered whether IC4 stages are somewhat redundant? I mean they're effectively inverting twice, scaling a little and adding offset - so I wondered if that couldn't be achieved direct into IC5B and Q3?

R25 (13k) sets the maximum Iabc and R23 (5k1) sets the saturation to avoid clipping of Iabc. So you have a input resistance of 5k1 for the constant current source. Usually this Ri is not sufficient for general CV inputs. So I just took my often used circuitry for CV inputs with high Ri (100K). I didn't want to mess up the constant current source. In this case here your suggestion might work because the input is derived direct from the PFWR and Ri is not an issue here.
BugBrand wrote:
Curious to hear how it functions - is no makeup gain required?

The input signal is inverted and routed in parallel through the VCA. Then added back to the original in dependence of the signal voltage which steer the VCA.
This device is build for my Shak 2 Synth project. So no makeup gain is required here. It could be easily added. Just make the gain of the last stage variable.
ersatzplanet
I almost made a Eurorack module based on a old Compressor/Limiter a company I used to work for made (the Symetrix 501), and sell it as a "Speaker Saver" module for modular users. Something that would be the last module on the chain and sit there and do nothing, until the accidental speaker killing bleep or gronk happened, and stopped it from doing damage.

I'm amazed nobody makes a good hard limiter for this purpose.
TOPLEL
ersatzplanet wrote:
I almost made a Eurorack module based on a old Compressor/Limiter a company I used to work for made (the Symetrix 501), and sell it as a "Speaker Saver" module for modular users. Something that would be the last module on the chain and sit there and do nothing, until the accidental speaker killing bleep or gronk happened, and stopped it from doing damage.

I'm amazed nobody makes a good hard limiter for this purpose.


I thought about putting back-to-back clipping diodes (probably LEDs) on the output jack of my output module. As a hard limiter to save my hearing.
Ok maybe not LEDs because i would need a lot of them, maybe a zener with a breakdown voltage around 7-8V?
Would that be enough? (since my oscillators are around 5V plus i added some extra voltage for resonance and a little gain here and there)
ersatzplanet
TOPLEL wrote:
ersatzplanet wrote:
I almost made a Eurorack module based on a old Compressor/Limiter a company I used to work for made (the Symetrix 501), and sell it as a "Speaker Saver" module for modular users. Something that would be the last module on the chain and sit there and do nothing, until the accidental speaker killing bleep or gronk happened, and stopped it from doing damage.

I'm amazed nobody makes a good hard limiter for this purpose.


I thought about putting back-to-back clipping diodes (probably LEDs) on the output jack of my output module. As a hard limiter to save my hearing.
Ok maybe not LEDs because i would need a lot of them, maybe a zener with a breakdown voltage around 7-8V?
Would that be enough? (since my oscillators are around 5V plus i added some extra voltage for resonance and a little gain here and there)


Good hard limiting (and compression) is not easy to do and requires much circuitry to deal with different signal content. It is not voltage limiting/clipping but very fast and very accurate level correction. Limiting is Compression with a very large ratio. The heart of a good limiter/compressor is built around a good RMS converter that can convert the varying levels of the input signal into a decent control voltage to run the internal VCA. Think of a REALLY good envelope follower that can react fast to transients like drum hits, yet doesn't "flutter" with low frequency bass signals. The RMS converter is the core difference between compressors. How this output signal is conditioned makes the other differences like "soft knee" and others.

To do a limiter right, it has to react fast and clamp down the VCA the right amount (not just turn it off) and go away just as fast as it came on so the results sound like the signal just never got louder than the output level you set. This can be abused of course by setting the input/output levels very low so all the dynamics are removed from the signal, but is set correctly, the listener will not even know it was even there.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Page 1 of 1
Powered by phpBB © phpBB Group