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Old 16th July 2015, 10:51 AM   #1
bobma is offline bobma  Australia
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Default based on Graham Dicker DIGI amp

I want to make a small amp to put in my bedroom that is as simple as possible since it will be built on proto-board. will it work?
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Old 16th July 2015, 11:01 AM   #2
Mooly is offline Mooly  United Kingdom
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The basic topology looks OK. I would say that emitter resistors on the output devices are madatory for thermal stability. 0.22 or 0.33 ohm would be suitable. Also consider adding resistors across the base/emitter junction of each output device. Something in the region of 220 ohm. The feedback return resistor could do with being AC coupled too to improve DC stability and offset. A small cap (say 10uf) across Q3 would be beneficial too.

As with anything, you can tweak tweak tweak but the resistors mentioned are essential, the AC coupling of feedback desirable.

Also add a zobel network at the output consisting of a 0.1uf and 10 ohm in series to ground. You might well need a small (say 15pf) cap across R5.

Why not simulate it ?
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Old 16th July 2015, 11:15 AM   #3
bobma is offline bobma  Australia
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OK thanks mooly. The high power Dicker amp does not use emitter resistors on the outputs so not sure they are required?

The input is AC coupled to stop base currents going through the volume pot that will increase DC imbalance and cause a nasty crackle as the pot is turned. The tail current is very low @ 1/2 mA and the ground resistor small @ 10k - so there should be very little DC offset as long as the LTP transistors are well matched which I will do. that is why I think I can get away DC grounding the feedback node. I really am looking to save every part I can.

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Old 16th July 2015, 11:16 AM   #4
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Also consider adding resistors across the base/emitter junction of each output device. Something in the region of 220 ohm.
That will definitely improve performance, but then the drivers will probably be running too hot.

The amp is indeed about as simple as possible. It will sound fair, much room for improvement.
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Old 16th July 2015, 11:28 AM   #5
Mooly is offline Mooly  United Kingdom
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Emitter resistors greatly aid thermal stability. If you run the amp at a deliberately low current and thermally couple Q3 closely to the outputs then you may get away with it.

You would have to see just how unbalanced the input stage is by building it.
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Old 16th July 2015, 11:41 AM   #6
bobma is offline bobma  Australia
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That is what I'm counting on. I will mount the bias spreader to the backside of an output transistor and hope for the best. Remember the original Dicker design used just a simple diode string with no part of it in thermal contact! No emitter resistors on the high power version either!! 100's built here in Australia...
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Old 16th July 2015, 11:42 AM   #7
Mooly is offline Mooly  United Kingdom
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I've just run a quick simulation on your circuit. The offset comes out at -1.7 volts, AC coupled and its still nearly -400mv.

Edit... make that -1.3 and -0.25. I had the supply voltage higher than you show. Offset should really be supply independent.
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Old 16th July 2015, 12:54 PM   #8
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Increasing both feedback resistors considerably would help the offset. You may also be able to improve matters by tweaking the LTP tail resistor and/or collector resistor.
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Old 16th July 2015, 01:51 PM   #9
Mooly is offline Mooly  United Kingdom
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I think there are just to many compromises being made here in the quest for low component count. Why not build a chip amp if simplicity is the main requirement.
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Old 16th July 2015, 02:32 PM   #10
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Why not build a chip amp if simplicity is the main requirement.
My thoughts exactly.
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