Transistor gain bandwidth

[CHRISKELLY]

Hi

I've been tinkering with the drums from the Korg Monotribe (See attached schematic).

For the snare drum (SD) there is a mod position 'SD decay' where a pot to ground alters the gain of the 'body' part of the sound. I've been altering R58 and R59 which changes the pitch, but the gain varies alot depending on the pitch.

I've been trying alot of different BJT's and so far the best candidate is the BC550. But it still loses gain at higher frequencies.

Does anyone know of a BJT which has a very broad / relatively 'flat' gain that I could try?

Just to make this a more annoying question (sorry guys) I don't know the exact frequency range. But it's somewhere between a kick drum and a high tom

Cheers

Chris

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[JovianPyx]

You might get more gain by raising R56. No idea how much it should or could be raised. I'd try adding a 100K pot in series with R56 and see what happens. The gain of a common emitter amplifier is controlled by the size of the collector resistor, larger giving more gain. Could also possible lower R53 a bit especially if it's feeding a summing node.

[Grumble]

With a bandwidth of 500MHz I don't think the BC550 is the problem here...

[CHRISKELLY]

[JovianPyx]

The gain of the transistor does not fall off at audio frequencies. It falls off in the hundreds of megahertz, so the transistor's gain bandwidth has nothing to do with what is happening in the circuit. What I see is that the filter is doing what a filter should do, after all, he's changing resistors that affect pitch (which is done by changing the Fc of the filter).

However, gain can externally be changed if the circuit's output level is too weak.

[CHRISKELLY]

Ah OK I follow you - thanks for confirming

[elektrouwe]

monotribe drums were designed for low cost, not for high hackability
The disadvantage of such a low component count filter circuit is, that cuttoff freq. ,Q-factor (resonance decay time) and output amplitude can not be adjusted seperatly.
If you rise gain, decay time will increase and the circuit will soon oscillate.
If you rise pitch by lowering R58,59 , decay time will drop very fast.
This has not much to do with the transistor.
If you are modding a monotribe, you have to live with this; if you think about using the circuit for your own drum design, I suggest to invest 1$ more and use a better drum circuit ( with indep. parameter adjustment)

[JovianPyx]

This is a very good point and good advice to folks who want to modify these circuits.

I see a continuous desire here for "simple", "cheap", "easy", "low parts count" stuff and what you've said describes the situation perfectly.

Those projects are nice because they can be done in a night or two, but then one needs to understand that mods may not be possible or mods that can be done have limited effect, sometimes more limited than we desire.

That might help explain why when you shop for modules, the one that has the features you want is more expensive than the one that doesn't. C'est la vie.

[CHRISKELLY]

Thanks for the advice guys I'll definitely take it on board