Simple Transistor Compressor

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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teleK
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Post by teleK »

would applying negative feedback between the two amplifing transistors help offset the distortion given by the transistor controlling the compression?

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mictester
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Post by mictester »

Hotrats wrote:just a few ideas to try here ... (some already mentioned)

- replace the two 22k resistors by 1Meg
- stick a 1k/22uF parallel combo in the emitter circuit of the front PNP device - the stage could be NPN version, no diff
- try Ge diodes instead of Si ... will give lower threshold
- if you replace the 15k resistor by 100k pot that will give you a rough "ratio" control
- the 680r emitter resistor in the output buffer could be a little higher, 4k7 would be fine there (less noisy)
- adjust/reduce input cap to taste (0.02u perhaps ?)

good effort !
HR
You'll notice that the original circuit schematic had a note suggesting the use of 330k base bias resistors for the first stage.

I built one last night with Schottky diodes for the rectification (which give a similar threshold to germanium diodes) and found that the onset of the "squeeze" was earlier. It also had an interesting side effect in that there was a fairly pleasant second harmonic distortion added when playing hard. There are also some other useful tweaks that can be done to the values (like giving it a "ratio" control as you suggest), and I used 2k2 in the output buffer emitter - a good compromise for lowered noise.

It wasn't ever meant to be the last word in compressors, and I prefer optical or FET-based ones, but this has a place in the pantheon of guitar effect designs!
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pedalgrinder
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Post by pedalgrinder »

i've found over time the transistor compressor's when there basic designs that is to be sound better than the jfet designs simple designs that is. I mean something the like the urei 1176 just can't be beat but thats a bit more involved than a pedal.
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mictester
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Post by mictester »

I needed a quick compressor earlier today, and this is what I threw together with parts from the "Junk Box":

SimpleCompressor.jpeg
SimpleCompressor.jpeg (76.41 KiB) Viewed 2963 times

You'll see that it has an input buffer stage to raise the input impedance (and give a bit of gain). I used 1N5817 Schottky diodes in the diode pump (which gave great sensitivity), and I used 2N3904 transistors all the way through. I was powered from a 9V battery, and was thrown into a blank diecast box, with the output level control handled by an internal preset - it's a "set-and-forget" control. It gives a little 2nd harmonic distortion, but it's not objectionable (and not noticeable when driving a slightly distorting valve amp!). When engaged, it just makes the guitar "livelier" and evens out my highly variable picking!

I put it together on a scrap of Veroboard (layout to follow when I've cleaned up my scribbles). The 10k resistor in the collector of the first transistor was OK for my Strat, but can be reduced for use with Humbucker-equipped instruments.
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Post by 287m »

Waiting someone, so killing time draw this.
TransComp not born yet.
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PierreBPSFSB
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Post by PierreBPSFSB »

Unfortunatly, no THD improvement...

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