Engineer's Thumb Compressor modify for mic input. How?

[nguitar12]

Just wondering if anyone know how to modify Engineer's Thumb Compressor for mic input while keeping the input for the guitar signal, like adding a switch for guitar/mic input.

Thanks in advanced.

[Hatredman]

Ballanced or unballanced mic input?

[nguitar12]

Ballanced or unballanced mic input?

So far I am going to make one for dynamic mic ( for live performance )
I an not quite sure whether it is Ballanced or unbalanced.

[Mark Hammer]

The critical issue here is whether the mic signal level will be able to "push" the rectifier section hard enough to actually result in gain reduction.  Remember, the circuit functions by essentially going "Oh dear, that's too much, I'd better turn down".  If the signal you feed it is well below guitar level (and a non-preamped mic signal generally will be), then the rectifier will rarely, if ever, mutter "Oh dear".

So, you'll want to find some means to boost the mic signal before it arrives at the Thumb, in order to make the Thumb behave as expected/intended.

[nguitar12]

The critical issue here is whether the mic signal level will be able to "push" the rectifier section hard enough to actually result in gain reduction.  Remember, the circuit functions by essentially going "Oh dear, that's too much, I'd better turn down".  If the signal you feed it is well below guitar level (and a non-preamped mic signal generally will be), then the rectifier will rarely, if ever, mutter "Oh dear".

So, you'll want to find some means to boost the mic signal before it arrives at the Thumb, in order to make the Thumb behave as expected/intended.

That mean I simply need to add a variable mic preamp circuit to boost the mic signal to the same as guitar signal before Engineer's Thumb circuit? Commercial compressor design for mic are using the same topology?

[Mark Hammer]

Pretty much every compressor will operate in a very similar manner: gain cell, and sidechain (tapped off the input) to detect the signal envelope and dictate gain reduction in response to peaks.

Depending on the specific compressor, it may anticipate a fairly specific input-signal amplitude (e.g., guitar level, OR line-level), or it may provide some flexibility and adapt to a wide range of input levels.  That adaptation will either come in the form of adjusting the gain of the sidechain section only, or adjusting a signal preamp on the input, to prepare the sidechain to respond appropriately.  Maybe it's just the way it is drawn, or simply a limit of my understanding, but I don't see a way in the drawing to easily adjust the gain of the input signal or the sidechain.

So, my best recommendation is to either add an input gain stage, or simply build a mic preamp box.

[samhay]

> Maybe it's just the way it is drawn, or simply a limit of my understanding, but I don't see a way in the drawing to easily adjust the gain of the input signal or the sidechain.

You can attenuate the signal hitting the sidechain by using the 'threshold' control, but if you need to boost the signal, then you will need to put something in front of the circuit. A mic preamp would be a good place to start there.

[Johan]

The easiest ( but perhaps not the best) way to interface a vocal mic with a guitar input is to go down to the music store and get a shure a85f mic to line adapter.  You get the signal up and impedance matches. . Perhaps not the most fun or diy way. . But easy..more time over for playing with more fun things

[merlinb]

or simply build a mic preamp box.

+1

Build a compressor and build (or buy) a mic preamp. It's way more flexible to have these things in separate boxes because you'll find them useful for other things, eventually.

[nguitar12]

OK so I decided to build one preamp suggested.
So far I am looking to build one of the following circuit using INA217





I find that the second one seem easier and it doesn't contain the chip OPA137 that I am not be able to find. Can someone please tell me

1. What is the different between two circuit except the Phantom Power section ?
2. What is the purpose of the chip OPA137? Can it be replaced by other chip ?
3. It is a good quality preamp that suit for home recording ?
4. It is a suitable circuit that boost the mic single in order to use the guitar pedal as discussed before?

Thanks so much

[merlinb]

The OPA is acting as a DC servo to ensure the output rests at exactly 0V. It doesn't have to be an OPA, you could use most any opamp. As it says on the diagram, it's optional. You can eliminate it and simply ground pin 5 (this seems to be missing from the second diagram), but since this may leave you with a small DC offset you may want to add a coupling capacitor at the output.