help me understand this circuit if you would please ...

[Derringer]

I'm referring to this article (it's not long at all):
Design Guidelines for Bipolar Transistor Audio Preamplifier Circuits
By Mike Martell
N1HFX
http://www.rason.org/Projects/bipolamp/bipolamp.htm

I have no issue with the mathematics but there's something here that I'm missing.
My question is, why when designing the second stage has the collector current changed to 10ma when it was 5ma in the first stage?

This is all stemming from what I'm trying to design/tweak here:
http://www.diystompboxes.com/smfforum/index.php?topic=68673.0

Thanks

Bill

[ClinchFX]

It would appear that the designer may have wanted a lower output impedance and therefore higher current output drive capability for the second stage.  Unfortunately, he didn't explain why he chose those collector currents.  It's possible that he wanted an output impedance close to 600 Ohms, but if that is the case, the 10K pot at the end doesn't make a lot of sense.

Another way to look at it:

In the first stage, at 5mA Ic, the 1.2K collector load resistor will drop 6V, giving a collector voltage (at the bias point) of 6V.  In the second stage, the collector load resistor is only 560 Ohms, the closest readily available value to 600 Ohms.  In order to drop 6V across this resistor Ic will have to be a little more than 10mA.

Many guitar effects are purposely biased away from half supply voltage.

Peter.

[gez]

I didn't read the article, but most designs intended for battery use would choose lower collector currents in the 0.5mA to 1mA range.

[Derringer]

thanks for the replies


So can you tell me how something like an LPB is setup/designed differently from the gain stages that are shown in the article there?

[Derringer]

I didn't read the article, but most designs intended for battery use would choose lower collector currents in the 0.5mA to 1mA range.

Gez, here's what I'm driving at ...



I understand that R1 is chosen to create a specific bias on the collector, but then how/why exactly are R2, R3, and R4 chosen for use in a guitar-related effect? What effect do R2, R3, and R4 have on the sound/feel of the circuit?

If you or anyone else has links that detail this info, I'd greatly appreciate that as well.

Thanks,

Bill

[foxfire]

try this, http://www.diystompboxes.com/smfforum/index.php?topic=47572.0. rylan

[Derringer]

perfect

thanks Rylan