Sansamp GT2 / Behringer TM300 to SansAmp Classic conversion


Click to enlarge

TLDR: SansAmp make circuits to emulate vacuum tube amplifiers. Starting at $20. You can customize the circuit with a soldering iron to get you own custom sounds. If you already know how to make electronic things, this article will get you started.

The Behringer TM300 is the $20 (Sweetwater $27.82 USD shipped to Canada) version of the $300 SanAmp GT2, and is therefore something of a bargain, although the components are presumably cheaper. There is no 20$ copy of the SansAmp Classic, but the circuitry is similar.  Tweed/Brit/Calif on the GT-2 corresponds to Bass/Lead/Normal on the Classic. 

The Classic has an interface of 8 on/off switches, which is uniquely binary and nerdy. Each switch changes the values of 1 or 2 capacitors or resistors in the circuit, which lends itself to further customization by the user.

The switches on the Classic can be approximately duplicated on the GT2/TM300, you could drill a hole through the plastic on the TM300 and add an array of DIP switches, perhaps to the left of the on/off LED.

Here is the GT2 schematic (from TonePad), changes in red are the Classic switches:

Click to enlarge


The first 6 switches are implemented as shown above. There are 2 switches for the "Clean Amp", both switches together give you full "Clean Amp", but engaging 1 switch gives you a halfway effect. "Low Drive" adds a 1 meg resistor and 2.2n capacitor in-between 2 gain stages, so use a metal film resistor to reduce noise. The Mid Boost and Vintage Tubes settings are simple, "Bright" is the same as the bright switch on a lot of amplifiers.

Since the TM300/GT2 already has a 3-way microphone placement switch, I didn't implement the Speaker Edge and Close Miking, partly because the circuit on the Classic is less like the GT2 in the speaker sim section.

You could just pick the effect that most appeals to you and install a single toggle switch. On my original GT2 there was too much hiss on the Tweed setting at low gain. The "Clean Amp" switch would fix this problem. The 2 mid boosts look useful. Presence Drive would be like the "Hot" setting with the 1k resistor (top left) replaced by a 10k pot and always on.

To locate the components on the board, you will have to trace from the switches. Resistor code translator is here. So there is something of a puzzle for you, to be enjoyed by anyone who thinks that 8 DIP switches would be a great user interface.

(Simplified: Less gain, reduce the two 330k resistors to 100k. Vary the cap marked "2.2" at the top to vary low frequencies.)

The Bass setting on the Classic is scaled from the GT2 Tweed, but I think they both give the same mid cut notch. If you replace the 100k resistor with a 1M pot, you can vary the center frequency of the notch.

The other obvious mod would be 5 pairs of red LEDs on the output of IC2b for symmetrical clipping, or 5 LEDs in series on the output of IC2a for asymmetrical clipping. That would give a different distortion quality. You could replace the 1k resistor on S2a with a 10k "Presence Drive" pot to vary the gain on the Brit and Cali settings..

So, if you want a weekend project there you have it. There is a stripboard layout for the Classic, but that is a lot of work.

The surprising thing is that "British" is "California" with one capacitor changed. Just a bass cut goes from Boogie to Plexi. "Tweed" is "California" with less gain and a mid frequency notch. So you do a lot with very little using the SansAmp system. It is like a comedian who does 10 celebrity impersonations in 2 minutes, none of them are exact, but close enough to get the general effect.

Geddy Lee of Rush. No schematic for this though.

1 comment:

Anonymous said...

Are there Mid Boost 1 and 2 good on the schematics?