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Power Amplifier for Guitar


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Hey folks.  Am hoping to get some tips and advice regarding the next stage of a guitar amp I`m working on with a lot of help so far from this forum.  Thanks very much!
I`m getting set to build the power stage of the amplifier...the first stage is found from a schematic and design that audio guru made for me, combined with what i alread had onboard.  theoretically it should work.  great!
i should note, i would like to use all found transistors in making all stages of the amp.  currently the pre-amp is running from two bc109s i found in an old amplifier.  For the other two transistors, the power amp ones,  I was digging in a local electonic dumpster and found a seemingly rather hi-fi phonograph with two BD162s therein.  I pulled em and have designed a non-working power amp...yipee!!!....ahm..or...rather---
would love to get it up and running and plug my guitar into it to hear the lo-fi madness.
So.   my question is-- if i follow the same principles from the schematic from audioguru, swap out the transistors to the BD162s and pump in 24 volts, will this puppy be basically a working power amp?  are the principles the same for the both pre-amp and power-amp?  Or perhaps I have to reverse the biasing and front loading of the transistors...so that the 100 ohm resistor shown first in audioguru`s schematic go to the second in the series of the power stage.......I`m a little confused about the two stages and their similarities and differences.  basically the system is
guitar--pre-amp--power amp(biulding while scratching head) -- speaker (found).

can someone offer some guidance as to the similarities and differences between audioguru`s schematic for a pre-amp and what would be a good power amp with these transistors (BD162s) and advise with the schematic from audioguru as a reference?  that would be really super.  

here`s the link to the pre-amp thread.  audioguru`s schematic and my hand drawn one can be found there.  http://www.electronics-lab.com/forum/index.php?topic=22032.0

and here are the specs on the transistor
BD162 Transistor Datasheet. Parameters and Characteristics.

Name: BD162

Material of BD162 transistor: Si

Structure of BD162 transistor: npn

Maximum collector power dissipation of BD162 transistor (Pc): 15W

Maximum collector-base voltage of BD162 transistor (Ucb): 40V

Maximum collector-emitter voltage of BD162 transistor (Uce): 20V

Maximum emitter-base voltage of BD162 transistor (Ueb): 7V

Maximum collector current of BD162 transistor (Ic max): 4A

Maximum junction temperature of BD162 transistor (Tj): 175�C

Transition frequency of BD162 transistor (ft): 1MHz

Collector capacitance of BD162 transistor (Cc), Pf: -

Forward current transfer ratio of BD162 transistor (hFE), min/max: 30MIN

Manufacturer of BD162 transistor: STE

Case of BD162 transistor: TO3

Application of BD162 transistor: Power, General Purpose

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In the other thread you posted the completely wrong schematic of a microphone preamp that will not work with a guitar pickup. I fixed it so it will work as a microphone preamp. It uses two little BC109 or BC549 transistors.

I also posted the very simple schematic of a guitar pickup preamp that uses a Jfet.

A power amplifier that drives a speaker is a completely different circuit to a preamp. Look at the thousands of power amplifier circuits in Google.

A BD162 transistor is so old that I could not find a datasheet from two different datasheet websites. It was used in amplifiers 40 to 45 years ago.
I think you should buy a power amplifier kit to make.

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Thats good thinking, a chip amp could make this project much easier, lots of little cheapo unbranded guitar amps use chip amps, my marshall 30w bass combo has a TDA2030 chip amp.

Audio, wouldn't it be easier for him to understand a simple opamp preamp? I mean a simple TL071 inverting amplifier isn't too hard to understand? The jfet amp is quite nice, jfet distortion does have quite a nice sound :) Obviously not if its pushed to power supply clipping.

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audioguru, could you explain what you mean by input impedance?  i`m a bit unclear about it.  sound as an ac wave enters the signal path (input of the amp) and from that point where is the other point(s) where you are measuring the resistance along the signal path to get the measurement of input impedance?  is that the whole system?  just to and through the first transistor?  i`m a bit confused.  impeding means hindering...de-energizing a bit, or slowing down...doesn`t it?  how does that relate to ohms and a weak AC signal from a guitar microphone?  i read you are a master of theory...perhaps you could enlighten me.  where is the impedance being measured, what are the units, what happens when the impedance is low compared to when the impedance is high with a weak AC signal traveling through from one end to the other? 
is resistance the same as impedance? 
also, thanks everyone for the helpful and surely much easier solution to making a guitar amplifier.  i`m dead set on making a transistor amp though.  reconstructed a 50`s tube amp earlier this year and was a great experience.  now i`m in the sixties.  the specs for the transistors i found are above in my first post.  if no one wants to look at them and audioguru`s schematic for the pre-amp...then ho-hum.  but i`m going to do it anyways.  tis a learning experience.  at this rate i will have caught up with the modern technology for amplifiers (perhaps the ideas you folks are suggesting would be easier for me to implement) i reckon i`ll be in my 80`s.    here`s to learning! (chink) sound of glass against monitor.  glug glug.

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audioguru, could you explain what you mean by input impedance?  i`m a bit unclear about it.  sound as an ac wave enters the signal path (input of the amp) and from that point where is the other point(s) where you are measuring the resistance along the signal path to get the measurement of input impedance?  is that the whole system?  just to and through the first transistor?  i`m a bit confused.   impeding means hindering...de-energizing a bit, or slowing down...doesn`t it?  how does that relate to ohms and a weak AC signal from a guitar microphone?  i read you are a master of theory...perhaps you could enlighten me.  where is the impedance being measured, what are the units, what happens when the impedance is low compared to when the impedance is high with a weak AC signal traveling through from one end to the other? 
is resistance the same as impedance? 

There's a good article on impedance on Wikipedia.

also, thanks everyone for the helpful and surely much easier solution to making a guitar amplifier.  i`m dead set on making a transistor amp though.  reconstructed a 50`s tube amp earlier this year and was a great experience.  now i`m in the sixties.  the specs for the transistors i found are above in my first post.  if no one wants to look at them and audioguru`s schematic for the pre-amp...then ho-hum.  but i`m going to do it anyways.  tis a learning experience.  at this rate i will have caught up with the modern technology for amplifiers (perhaps the ideas you folks are suggesting would be easier for me to implement) i reckon i`ll be in my 80`s.    here`s to learning! (chink) sound of glass against monitor.  glug glug.


Oh I see, you think simply putting a power transistor into a low power pre-amplifier design and increasing the voltage will make a power amplifier? Well no it doesn't work like that, you need a class A-B design for a power amplifier which is a puch-pull amplifier. Why not simply solder the transistors back in place and use the hifi amplfier?

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audioguru, could you explain what you mean by input impedance?

Resistance reduces DC current. Impedance basically reduces AC current from an AC signal. The guitar pickup with the Jfet has an output resistance of 6.8k ohms. If it drives an amplifier with an input impedance as low as 6.8k ohms then the signal is divided so that half of it is thrown away. You need a power amplifier that has an input impedance of at least (6.8k x 5=) 34k ohms. Its input impedance includes its volume control, input biasing resistors and the input of the first transistor. The 51k resistor R4 in the guitar pickup preamp schematic can be removed.

..... a weak AC signal from a guitar microphone?

I have been talking about the fairly high level high impedance signal from a guitar pickup and now you are talking about the completely different low level low impedance signal from a microphone. What is it??

The specs for the(output) transistors I found are above in my first post.

But all the important details are missing. You don't even have a photo that shows which pin is which. You don't know how hot they will get because the case specs are not mentioned.

at this rate i will have caught up with the modern technology for amplifiers

You will be trying to make a power amplifier with transistors and a circuit that has not been used for 46 years. Maybe none of us can remember the very old circuit (it had a quasi-complementary output stage).

perhaps the ideas you folks are suggesting would be easier for me to implement?

I suggest that you purchase a modern power amplifier kit.
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