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Old 15th February 2005, 11:32 PM   #1
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Default Could someone review these?

I took a few schemes, from a couple of places(4) and assembled them, but there are TONS of errors(from what people told).

Here's what have been told:

1. Ditch the LED bias scheme at the input. The input should be biased with an equal-value potential divider. As you have it the bias will be unsymmetrical and as battery voltage changes you will get level shifts.

2. The 100k after the 741 is doing nothing as the next stage is loading the filter more than the 100k ever will.

3. Q1 has no DC bias. Look in a text book to see how to bias transistors -- it's quite simple.

4. 1N4148 will be OK for D1 and D2.

5. LM386 has no DC bias when the switch is changing between the circuit and ext amp. This will cause massive clicks and pops. See answer to 3.

6. Your switch for clean/fx is in the wrong place as well. It should switch between those 2 pots. As you have it the output of the fx cct is being force-fed with the input.


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Old 16th February 2005, 12:22 AM   #2
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Old 16th February 2005, 09:29 AM   #3
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[list=1][*]The LED biasing scheme is OK. It's a bit better than a voltage divider if the 9 volts com from a poorly filtered wall wart.[*]Yep, the 100K is unnecessary, but harmless.[*]Q1 is biased alright. Bias comes from the 100k resistor between Q1-s base and Q2-s emitter. It's quite simple.[*]OK, I suppose.[*]As I understand it, th LM386 has no power at all when routing to an external amp, so yes there might be clicks, just like when you turn it on. Big deal. However, switching the power to the effect circuit might cause a more annoying clicking. I'd let the transistors be on with the rest of the circuit. It'll only use about 0.5 mA[*]What you have is a classic instrument amp circuit. It's not exactly a true additive mixer, but it does the job. By moving the switch (if I understand the suggestion) you wouldn't be able to mix seamlessly between clean and dirty signals.[/list=1]
What I would do is to increase the output cap to at least 1000F. With only 220F and an 8ohm loudspeaker you'll have a cut off frequency of about 90 Hz, which is a little high. With a 4ohm speaker it will definitely be inadequate.

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Old 16th February 2005, 04:30 PM   #4
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Revised the circuit a bit, but I think I created more problems.
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Old 16th February 2005, 09:36 PM   #5
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Looks OK to me. What kind of problems do you think you've created? Maybe you should put a 100 k resistor from the switched end of the capacitor on the input of the FX circuit to ground. Otherwise you may get clicks when you switch on the FX circuit.

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Old 20th February 2005, 03:19 PM   #6
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Don't know, when I change something in a scheme, it's generally worse.
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