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Author Topic: Quick question about this circuit  (Read 3525 times)

Fossilshark

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Quick question about this circuit
« on: June 12, 2016, 01:45:27 AM »
I built this amplifier circuit and i plan on using it for a (very) low power guitar power amplifier. It worked ok but there was some problems (bad quality at low volumes, buzzing, ect) wich was expected im pretty sure i can work those out.

My question is about the two diodes in series. I built it using one because that's all i had, how do those effect the circuit?
« Last Edit: June 12, 2016, 01:48:11 AM by Fossilshark »

DrGonz78

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Re: Quick question about this circuit
« Reply #1 on: June 12, 2016, 04:27:54 AM »
Those two diodes bias Q1 and Q2. So it could be that eliminating one from the circuit makes for a colder bias, and the result is cross over distortion.
“A person who never made a mistake never tried anything new.” -Albert Einstein

phatt

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Re: Quick question about this circuit
« Reply #2 on: June 12, 2016, 08:22:35 AM »
Also try connecting R1 to the 9 volt terminal instead of the speaker,,, or am I missing something? ???
Phil.

incurably_optimistic

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Re: Quick question about this circuit
« Reply #3 on: June 12, 2016, 09:53:15 AM »
I was just about to ask that question - I thought that this resistor was supposed be connected to +9v to provide enough current to bring the diodes into conduction, but then I thought "hell, maybe it's some smart trick that I just don't understand" so I didn't say anything  :loco

Fossilshark

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Re: Quick question about this circuit
« Reply #4 on: June 12, 2016, 11:06:08 AM »
Thanks, ill try adding another diode. I will try connecting R1 to + just to see what happens. Thanks!

Enzo

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Re: Quick question about this circuit
« Reply #5 on: June 12, 2016, 11:58:43 PM »
Just move R1 to the other speaker terminal.

Agree, you need both diodes to eliminate crossover distortion.

tonyharker

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Re: Quick question about this circuit
« Reply #6 on: June 13, 2016, 05:30:31 AM »
R1 is connected the way it is - is to 'Bootstrap' it via C1 to raise its AC impedance load on Q3 thus increasing the gain without upsetting the DC conditions.  Leave it where it is.

Fossilshark

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Re: Quick question about this circuit
« Reply #7 on: June 13, 2016, 06:38:09 AM »
I gave up on this circuit. Know where i can find a circuit for a 2 watt power amp using 3904/3906 or will i have to buy more powerful trannies (in wich case ill go like 10-15 watts)

DrGonz78

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Re: Quick question about this circuit
« Reply #8 on: June 13, 2016, 05:21:59 PM »
Honestly that first 3 transistor amp schematic was just awful really. I saw this one and thought this is a way better way to do it. At least build this one with what you have and analyze how it all works. Then at that point build a 15 watt guy.

http://www.deeptronic.com/small-audio-power-amplifier-using-3-transistors/
« Last Edit: June 13, 2016, 05:25:51 PM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

gbono

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Re: Quick question about this circuit
« Reply #9 on: June 15, 2016, 03:01:52 AM »
The class A amp on the same site uses even fewer components.

http://www.deeptronic.com/audio-amplifier-circuit-types-and-classes/

Loudthud

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Re: Quick question about this circuit
« Reply #10 on: June 15, 2016, 05:27:04 PM »
The 3904/3906 transistors are not intended to handle very much power, only about 0.35Watt. They are just marginal in the 9V battery circuit. They will work OK in a preamp though. You really need some real power transistors, ones that will bolt up to some kind of heat sink.

The first thing you need is a power transformer. Probably the easiest thing to find is a 24VAC "wall wart", one rated at 40VA is about as big as you can find. This will work well to make a power amp good for about 12 Watts at 8 Ohms. The 24VAC transformer will make about 33VDC after rectification and filtering.

The circuit in this thread: http://www.ssguitar.com/index.php?topic=3747.0 will work OK on 33V. The transistors in the original article will be impossible to find and the layout will need to be modified for most current production power transistors. You can use the 2N3904 for Q1 and Q3 and a 2N3906 for Q2.

See if you can find MJE3055T for Q5 and MJE2955T for Q4.
« Last Edit: June 15, 2016, 05:40:50 PM by Loudthud »

exztinct01

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Re: Quick question about this circuit
« Reply #11 on: June 16, 2016, 05:52:38 PM »
how about that tda8932 amp in a thread in diystompboxes http://www.diystompboxes.com/smfforum/index.php?topic=114335.0?
~ Stephen

Loudthud

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Re: Quick question about this circuit
« Reply #12 on: June 16, 2016, 08:24:11 PM »
how about that tda8932 amp in a thread in diystompboxes http://www.diystompboxes.com/smfforum/index.php?topic=114335.0?

Class D amps can sound pretty bad when they clip like they do frequently in guitar amps. What the OP said in the diystompbox thread is not too encouraging but it won't cost much to buy one and experiment.

J M Fahey

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Re: Quick question about this circuit
« Reply #13 on: June 19, 2016, 01:56:09 AM »
1) tonyharker said it right: the resistor goes to the output cap>speaker junction to be bootstrapped and be able to swing to full 9V, otherwise it will never reach it.
2) the circuit is crude, but if you want to learn is fine.
3) you will never get 2W out of a 9V supply, not enough available voltage, plus lots of losses which add up.
And if it´s a 9V battery, forget it, because you can´t pull much power out of them.
Popular LM386 amps do not even reach 1W from such a battery, yet they are much used (Little Gem, Smokey, etc.)
I bet some modern Class D chipamps can get some interesting power, after all many are designed to work from meager 5V available at USB connectors.