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Old 5th August 2007, 05:40 PM   #1
red is offline red  Romania
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Default Help Modifying an amplifier schematic

Hi guys,

Four years ago I have made myself an amplifier using a schematic from 1986 (at least in the book I found). I have posted it here that time but only to ask if it might work, and it did after having changed all the transistors to newer models. Anyway I made a mess of a wiring, used an old pcb from another amplifier and very old capacitors for rectification. I sounds pretty good but makes a bit of noise. I decided to rebuild it. I am currently using it for a subwoofer because it reproduces the bass really good and the noise doesn't seem to be noticeable. BUT being such an old schematic, and having that troubeling noise, I was wondering if there is any way I can improve it. I have to say that I have some electronic knowledge but I am no specialist so if you have the time to give me some advice please do so. I have no characteristics of the amplifier I only know that it has a gain of about 59 dB but I have momentarily no equipment for testing it.

So my plan is to first improve the schematic (if possible) than create the proper pcb an I hope by that time to have a signal generator and an oscilloscope and see if my work was worth or not.

Here is the schematic
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Old 5th August 2007, 08:15 PM   #2
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first of all, i'd increase C2,3 to 470uf each to improve bass response. i'd also increase C8 to 1uf and reduce R31 to about 200-300 ohms to improve turn-off of the drivers. same for the bias circuit, increase C4 to 1uf or more, reduce the values of R24,25 by about 90% (1k for R24, 200 for R25).

reduce R1 to about 10k, increase R4 to 100k. your input impedance is determined by R4 mostly, since the input impedance of a diff amp is near infinity. add "flyback" diodes across the C-E terminals of the output devices to protect the devices from reverse voltage spikes.

also, you might check and see (with an oscilloscope) if this amp is oscillating, since there are no miller caps in this amp. that might be where your noise is coming from.
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Old 6th August 2007, 02:18 AM   #3
anatech is offline anatech  Canada
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Hi Red,
The base of Q13 is connected to the emitter of Q11, right? You show it shorted to the supply. R24 in your bias circuit is in a position where if it fails, your output stage will go high bias and burn out possibly. The variable element should be in the emitter-base leg wit ha resistor to define the maximum bias limit (so you can't short the base to the emitter of Q8)

You could replace C2,3 with one non-polar 220 ~470 uF, bypassed with a film cap (you could probably use a polarized electrolytic here as this voltage will be very low). C8 should be increased to 1 ~ 10 uF (as unclejed613 suggested), bypass again or use a film cap. R31 is normally around 220 ohms, just on average. No need to waste power here. R32, 33 should be non-inductive types, as should R34.

I see you are using R1 and R4 as a pad. That's okay, although BJT differential pairs sound better driven with a low impedance source. Your call and it's a "tweak" after you have the basic circuit running. In that case you would install your pad at the input of the amp before the buffer, R1 should then be decreased to 100 ohms and R4 may not be required. I would use a DC blocking cap on the input if I were you.

Unclejed613's suggestion of flyback diodes is always a good idea. 1N5402 should be okay, 1N5404 would be better (just voltage breakdown).

Also, as Unclejed613 stated, you should always check the circuit with an oscilloscope to make sure it isn't oscillating. Also check the frequency response for peaks that would encourage it to "ring". This normally occurs somewhere from 50 KHz to 1 MHz depending on many things.

-Chris
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Old 6th August 2007, 12:26 PM   #4
red is offline red  Romania
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Thank you for your prompt answers.

The first thing i have to mention is the fact that I have connected Q13 wrong in the schematics. It is connected to the emitter of Q11 and not to the rails.
I was thinking to change C2 and C3 with a single non-polarised capacitor and I will increase it. I will also change R1 and R4. I will try both your advices and see which one works better. Also I will add a capacitor on the input I was thinking at a 1uF but I don't know if that is large enough.

I was thinking to change somehow the output transistors because I want to be able to use it on 4 Ohms load also.
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Old 6th August 2007, 12:35 PM   #5
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I forgot to mention the fact that I was thinking to use a small inductor in parallel with a 2 ohms resistor on the output terminal (that means in series with the load) to increase the stability of the amp and of course the flyback diodes.
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Old 6th August 2007, 03:34 PM   #6
anatech is offline anatech  Canada
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Hi Red,
Get it working first. Change one thing at a time so you know what the effects are.

-Chris
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Old 6th August 2007, 11:53 PM   #7
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that's always a good idea..... make one change at a time and test it. same thing applies to config files in software. if you make a bunch of changes at once and it crashes, it;s much harder to find out what caused the crash. i occasionally get equipment that has been "modded". usually has something ugly it's doing to the sound as a result of the modifications, but since several mods were applied simultaneously, it's sometimes difficult to find out exactly which one caused the equipment to go south......
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Old 7th August 2007, 02:28 AM   #8
anatech is offline anatech  Canada
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Wink That's when charges go higher ....

Hi unclejed613,
Quote:
but since several mods were applied simultaneously, it's sometimes difficult to find out exactly which one caused the equipment to go south
At this point you would ignore the mods that do not impact the operation, but check the solder joints. The customer is then charged to return the unit to a stock condition leaving the non-harmful mods.

This may make the repair job too expensive to proceed, but that is better than a technician losing hours upon hours of work for nothing.

Time to withdraw and find a battle you can win.

-Chris
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Old 7th August 2007, 09:38 AM   #9
Bonsai is online now Bonsai  Taiwan
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Red,

some good suggestions here. You should defineltry check the output once running to make sure there is no oscillation. If you are hearing shh sound out of the speaker, the amp probably is.

Make the changes suggested, and the check for oscillation. If you have this problem, let us know and we can make some proposals to tame it.
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Old 7th August 2007, 10:29 AM   #10
red is offline red  Romania
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So I took your advice and these days I worked on the PCB. I am not so sure I am doing the right job but here it is. Please comment anything that doesn't look good...

After I will finish the PCB I see the results and then make the modifications.
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