Busking amp mods

We have not done much for the last two years, but we did finally get out with the amp.  We decided to go for maximum simplicity and used a single mic and no pick ups. It all worked very well. 
The mic, a Tbone GM55 was on a stand in between us and our heads and the instruments wer all around 18 inches from the mic.  
We had tested this set up before in the garden shed, and the secret is not to play the instruments too loud.
Simplicity is the key.
I decided to simplify our set up even further.  We were using the 4 input behringer mixer to feed the power amp, and that meant two gain controls to look after, even with just one input.

I decided to ditch the mixer. When we were playing without close miking, we had to have the mixer volume close to maximum, so, without the mixer I needed an extra 40dB of gain between the mic and the power amp. 

Mic Amp

I have used Rod Elliot’s simple mic amp before, so I decided to use it again.

The circuit shows the values modified to work from 11V. (I use either 8 alkaline batteries which gives 12V or 8 rechargeable which gives 10V.  So I split the difference and set it for an 11V supply) I raised the input resistance as the Tbone has a relatively high impedance of 415ohms.

I went for a fixed gain of as close to 40dB as I could get with the resistor values I have.
The gain calculates out at about 110. R7 should have been either 270K or 300K. I have neither, so it’s 330K.

The output can swing over 9V p-p from an 11V supply. BUT for close miking, e.g. when introducing a song, the gain will far too high and clipping will occur.  C5 was added to suppress the 300KHz switching frequency from the power amp which gets everywhere.


I checked out the mic amp under operating conditions, measuring the output on my oscilloscope..

  • Talking fairly loud, close miking, I measured – 4Vp-p at the mic amp output. This was good news for the mic amp as it was not saturating but the power amp only needs 450mV at the input for max volume. But at about 2″ from the mic, the volume dropped quite a bit.
  • Singing voice around 18″ from the mic I got 400mV. At amp output this is 16Vp-p with volume high. The amp can manage 17V or 22V depending on battery – so add in another voice and the instruments at it all sounds good.


In the amp box, 

  • I removed the clipping indication circuit
  • I removed the power supply lead for the mixer
  • I removed the magnets for holding the mixer in position.
  • I replaced the jack plug input connector (which as actually inside the box) with an XLR connector mounted on the back plate.

The mic amp was copied into TinyCad, modified,  and was built on strip board, laid out with VeeCad.

So our busking kit now comprises one amplifier box, which carries the mic cable and a mic plus a stand with no boom arm.


We have three mics,

  • a Behringer XM8500 with -70db Sensitivity, and 150hm impedance
  • an ElectroVoice N/D 257A with -53dB Sensivity and 150ohm impedance
  • a Tbone GM55 with -52.4dB sensitivity and 415 ohm impedance. 

Strangely, the Behringer  does sound the loudest of three, which goes to show you can’t put too much trust in manufacturers specifications. We use the Tbone, for no other reason than it looks the best.

Interestingly, the best place for positioning the mic, according to the Tbone’s polar diagram is around 30-45 degree off axis. Not immediately behind the speaker. Of course this also depends on the speaker.

If we ever decide to switch to a mic each, I’ll use the Behringer and the EV and a simple Y cable.

Photos and tests to follow.


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