Posted by & filed under BBDs+Delays, Chorus, Flanger, Projects, Stompbox stuff.

What’s a “compander”? It’s a simple way to make your circuit quieter! So how does it work? Let’s say we’ve got a fairly noisy effect, like a BBD. The basic idea is that you make quiet signals louder before you put them through the effect, and then you make them quiet again after the effect, and thereby make the noise quieter too. And that’s the clever bit!

“Compander” is a compound of “Compressor” and “Expander”, since those are the two parts of that process. The compressor is the part that makes quiet signals louder (“compresses the dynamic range” in the jargon) and then the expander reverses the process and makes them quiet again (expands the dynamic range). Obviously it’s important that the two stages cancel each other out if we want the overall dynamics of the signal not to get mangled.

SA571 PinoutThe SA571 chip provides two sets of the building blocks you need to build compressors and expanders, so you can build a companding circuit with just one chip. The two halves of the chip are identical copies of each other (apart from the power pins on pin 4 and pin 13, obviously) so you can use either half for either job.

Compander

The C1 and C7 1μF values are a compromise between fast response and low ripple. If you want a faster response, try 470nF. If you want less ripple, try 2.2μF.

The technique is used in the PT-80 delay to reduce the noise added by the PT2399 delay chip. You can see on the PT-80 schematic the two halves of the SA571, one before the delay and the other after. The circuit values have been tweaked a little compared to my example above, but it’s otherwise identical.

It’s useful in any BBD-based circuit to help reduce noise from the BBD. The process will help reduce both noise from the BBD itself, and also clock feedthrough. This can make for a quieter flanger, chorus, or analog delay. The  compressor stage goes in front of the BBD pre-filter (the anti-aliasing filter), and the expander stage goes after the BBD post-filter (the reconstruction/clock filter). The diagram below shows the layout of a typical BBD-based pedal. This could be a delay, or it could be (with the optional LFO) a chorus or a flanger. They only differ in detail, not in structure.

BBDWithCompanderDiagram

This diagram makes it quite clear why good quality analog delays or flanger circuits are so complicated! That’s a lot of stages! Still, they’re all there for a reason, and if you want to try and reduce noise, you probably need to add a compander.

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  • V571 Compander / Dual VCA

    The CoolAudio V571 is a compander noise reduction chip, but it contains two VCAs which can be used independently. It is a modern replacement for the SA571 and NE571. The chip is very useful to squeeze a bit of extra performance out of BBD-based circuits like delays, chorus, or flangers. It can also help reduce noise in PT2399-based delay circuits. Download CoolAudio V571 Compander datasheet There is a short article about compander noise reduction with schematics for the compressor and expander... Read more »...

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8 Responses to “Noise Reduction with Companders”

    • Tom Wiltshire

      The compressor/expander stages have to derive a signal representing the overall volume level from the input. This is usually done by rectifying and lowpass filtering the waveform with a fixed filter. If the lowpass filter’s cutoff frequency is too high, some of the original audio will still be present in the volume-level-signal (the “envelope”). This is “ripple”.
      If you reduce the cutoff frequency of the filter to remove the ripple, you can easily make the filter so severe that it cannot respond to quick transients and fast note attacks. The envelope response becomes slow. So there is always a trade-off between high cutoff/faster response and low cutoff/less ripple. No single solution fits all situations, and it is best to tweak a design for your intended purpose.

      HTH,
      Tom

      Reply
    • Tom Wiltshire

      Probably nothing good! The purpose of C6 is to provide a path to ground for the AC signal. R2+R3 provide DC feedback around the output op-amp, but AC feedback is handled by the path from pin 7 back to pins 2 and 3. That path is AC coupled, so can’t affect the DC feedback.
      HTH,
      Tom

      Reply
    • Tom Wiltshire

      Technically, I don’t know. Practically, no there isn’t much difference. It’s possible that an AGC would reduce peaks as well as boost quiet sounds, whereas a compressor only boosts quiet sounds (at least as far as I understand it). But I’m not certain. Certainly they use very similar techniques and technology, and the differences are only in how it is set up.

      Reply
  1. Sascha

    Hi Tom,
    I’m currently trying to figure out how the Expander side is biased correctly. The PT80 uses a 10k on pin 12, yet in your example it is disconnected. I’ve seen 8k2 been used in the compander cookbook and sometimes up to 20k. What does this resistance do?

    The other question is there are some SA571 with 15k/25k internal resistances R3/R4, how can those be biased?

    Reply
    • Tom Wiltshire

      The stages are designed so that you can modify the internal resistance values by using external resistors in series or in parallel with them. In the expander case, it uses the internal resistances, and doesn’t need anything connected to pin 12. Notice pin 11 is connected to the output, to provide feedback around the op-amp.

      Firstly, 15k/25k isn’t very different as a ratio from 20k/30k (0.6 vs 0.6667) so it’s not a huge difference. Since you can adjust the R3/R4 values, you can fine-tune it in any case. The R3 value can be increased by adding a resistor in series on pin 6/11, and the R4 can be reduced by adding a resistor in parallel to ground from pin 5/12. That gives a lot of design freedom.

      HTH,
      Tom

      Reply

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