Guitar / Microphone amplifier for live performance

[Jester]

Hi,

I hope someone can point me in the right direction regarding the basics of "Guitar" type amplifiers used by live bands to amplify microphones, electric guitars etc (Marshal, Peavy etc).

I have been asked to design an audio synthesizer of sorts and will need to understand the basics of interfacing to this style of amplifier, for example: Signal level, Impedance, etc. This device will be battery powered (that should help with ground loops) and connected via whatever standard cable/connectors are used for these types of devices.

Any suggested op amp interface circuits for the last stage to the amplifier?

Useful links?

Words of wisdom?

Thanks in advance.

 

[chuckey]

Guitar amplifiers use a 1/4" mono jack input. This means that one side of the signal is earthed, unlike proper PA/studio equipment which uses a balanced signal to break the earth path and so reduce the chances of hum loops. The only way to truly break hum loops is to use transformers.
The magnetic pick ups for guitars only produce 10 mV or so, so the amplifiers are very sensitive. most microphones will produce more then this, other then the moving ribbon type which is a very expensive studio type , these produce 1mV.
The input impedance of a guitar amp would normally be about 10K.
So driving the input of a guitar amp is easy, the only area up for discussion is the cable capacitance, so the pre-amp should have a low output impedance to deal with long cables.
Words of wisdom?, think about anti thump circuits, so switching on the pre-amp does not cause a 100Watt thump in the loudspeakers of the main amplifier.
Frank

 

[FvM]

I would give my design an output voltage around 100 mV. High enough to reduce unnecessary hum and noise, low enough for the amplifier input range. An OP output should have a series resistor to isolate cable capacitance, e.g. 200 to 600 ohm.

 

[Jester]

Thank you gentleman.

 

[Audioguru]

The magnetic pick ups for guitars only produce 10 mV or so, so the amplifiers are very sensitive.

No.
Look in Google.
Many tests show an output of 424mV (1.2Vp-p) from a moderately hot pickup. Here is a Jfet preamp with a voltage gain of only 3dB to avoid clipping:

 

[chuckey]

Perhaps things have moved on in the last 50 years, but a 3 M input impedance, looks more like a crystal pick up.
Frank

 

[Audioguru]

The very high input impedance for an electric "geetar" amplifier is used so that the magnetic pickup resonates with its cable at 2kHz to 6kHz to make it have a harsh sound with more "bite". Look at it in Google.
Most guitar speakers also have a peak at 2kHz to 5kHz.
Here is a graph that shows how much is the peak amplitude of the resonance when the amplifier input impedance is from 47k ohms (no peak) to 10M ohms: