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Old 10th December 2008, 03:50 PM   #1
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Default Krill - The little amp that might...

This new thread is a split from the thread at:

I have found a special amplifier, am trying to contact the designer

I'm doing this to eliminate the 90% or so off-topic posts and to get a better to feel for any interest in this project.

This schematic was drawn by the same program I use for simulations. It simulates very much like it measures in the real world so I am reasonably certain that it is correct. No doubt there will be some problems found that I can correct later.

I both simulated and built this with the parts shown. You may substitute other brands of transistor and it should work well. Use what is available to you. Please use a decent power supply. I use a 120 V to 24 V times 2 at amp 4 transformer and 20,000 uF of filter cap per rail per channel.

Please pay attention to the odd values of R6 and R12. In the real world R6 will be a 330 ohm parallel with a 5K trim pot. I prefer to use multi-turn pots for the precision they give. R12 will be a 33K with a 10K pot in parallel. When the amp is first powered up adjust R12 to give 25 to 30 milliamps of current through the two output devices. R6 will set the DC offset. Run the Amp into an 8 ohm load at 10W for approximately 30 minutes and then readjust both resisters. Remember the adjustments are made with no load on the amplifier. If you have a distortion meter, choose an appropriate output power (I like to use 10W) and adjust R12 for minimum distortion.

This basic and simple amplifier sounds very good just as drawn. If there is enough interest I will show improvements to the design as well as higher powered units.
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Old 10th December 2008, 03:53 PM   #2
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Here is the schematic with all voltages shown.
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Old 10th December 2008, 04:04 PM   #3
Zen Mod is offline Zen Mod  Serbia
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thank you for sharing .

respect .
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Old 10th December 2008, 04:53 PM   #4
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This is actually 2 amplifiers.

One voltage amplifier x21. This has been seen a 1000 times before.
I like that you have added 1 mA current source (D3,D4,Q6) to bias the input pair current source.
This should give very good PSRR.

One special Dunlap output follower (buffer).
This one is the new and interesting part

Both amplifiers share the same supply.
This is the thing I would change. First.
Because a low level voltage amplifier and a 50 Watt output stage
works at very different power levels.
Any possible interference from Output Supply rails to input stages should be avoided!

Thank you
Great with some new and good ideas.
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Old 10th December 2008, 06:24 PM   #5
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Quote:
Originally posted by lineup
This is actually 2 amplifiers.

One voltage amplifier x21. This has been seen a 1000 times before.
I like that you have added 1 mA current source (D3,D4,Q6) to bias the input pair current source.
This should give very good PSRR.

One special Dunlap output follower (buffer).
This one is the new and interesting part

Both amplifiers share the same supply.
This is the thing I would change. First.
Because a low level voltage amplifier and a 50 Watt output stage
works at very different power levels.
Any possible interference from Output Supply rails to input stages should be avoided!

Thank you
Great with some new and good ideas.

You're correct that the voltage amplifier is nothing special. I stated that in the thread that I referenced. I also stated that you could use any voltage gain stage that you prefer. The only thing I'm claiming here is the design of the output stage. I did design the voltage gain stage also but I realize that this is in the public domain and can hardly be considered mine.

Yes, both amplifiers do share the same supply although a different one for each channel. I'd mentioned improvements I would show if there is sufficient interest. A separate power supply for the front end would be obvious. In my production units I used a voltage doubler with a regulated supply for the voltage gain stage.

The amplifier just as presented has exceptionally good sound and very low noise. I wanted to present the amp in it's simplest possible working form. We can make things complicated and more expensive later.
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Old 10th December 2008, 07:21 PM   #6
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Default Very good...thanks.


regards,

Carlos
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Old 10th December 2008, 07:29 PM   #7
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Quote:
Originally posted by Steve Dunlap

You're correct that the voltage amplifier is nothing special.

Very cool. A single cap/transformer coupled triode would make a great hybrid amp.
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Old 10th December 2008, 07:55 PM   #8
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Old 10th December 2008, 08:12 PM   #9
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We need more of good power buffers.
With good sound and good quality.

Nelson Pass has done some. PMA has one old.
Steve Dunlap's is a very welcome contribution.

As said, voltage amplifier can be our own choice. Many ways.
It is not difficult to find very hi-fi alternnatives.
Something like the discrete one Steve Dunlap posted, a good op-amp or a nice tube amp.
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Old 10th December 2008, 08:26 PM   #10
AKSA is offline AKSA  Australia
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Thank you Steve,

Very clever circuit, shows great insight and experience.

I agree strongly with analog_sa; this would be absolutely wonderful with a triode input stage, cap coupled. A very good triode to use would be a 6SL7, t-barred for a rp of 22K and mu of 70. With a 50K load, that would have a gain around 43. That would drive it very nicely indeed.....

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