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Old 6th December 2005, 03:24 AM   #1
halusha is offline halusha  United States
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Default looking for simple discrete 100W DIY amp

I've built several headphone chip-amps and would like to build a simple but good performing discrete power amp for my office speakers. They are inefficient but decent sounding, and would like to get at least 100W x2 into 8 ohms. I found the c200 at ampslab. Are there other simple, easy-to-build discrete DIY designs of similar (or higher) caliber that I could look at?

TIA,
-Jason
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Old 6th December 2005, 04:25 AM   #2
rodolfo is offline rodolfo  Mexico
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NICE
I TRY THIS ESCHEMA
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Old 6th December 2005, 05:38 AM   #3
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Hi halusha,

You might like to check out the new 'Simple Killer Amp' I designed
earlier this year, it's available as a kit (without heatsink) -

http://members.dodo.com.au/~gregball/guru_004.htm

and is discussed quite thoroughly in -

Simple Killer Amp!

This design delivers more efficiency, more performance, is 'patent pending', simple to build and, for now, low cost. You can build two simple low cost monoblocks (only needs 160VA traffo) and blow the monster amps away.


Cheers,
Greg
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Old 6th December 2005, 06:27 AM   #4
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plenty of good designs.
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Old 6th December 2005, 02:18 PM   #5
jboehle is offline jboehle  United States
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Thanks for the suggestions/links!
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Old 6th December 2005, 04:25 PM   #6
Joules is offline Joules  United States
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AKSA with out a doubt http://www.aksaonline.com/welcome.htm
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Old 6th December 2005, 08:22 PM   #7
AKSA is offline AKSA  Australia
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Halusha,

Delighted to discuss the issues by PM here.

Cheers,

Hugh
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Old 6th December 2005, 10:05 PM   #8
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hugh

i wouldnt mind some pointers on PA design...i still have trouble figuring out how much current the input stage and the VAS require, say for class A??..and how do you work out the miller cap value?
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Old 7th December 2005, 03:51 AM   #9
AKSA is offline AKSA  Australia
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Hi DW,

Hmmm. How do you describe all the parameters of a PP Class A SS amp in a few sentences? Tricky........

Stage current in the input diff pair need not be much more than 1mA. Anything up to 5mA is fine, with jfets needing more to extract the last bit of transconductance.

Stage current in the voltage amplifier depends upon:

1. Number of output pairs, their beta, and input capacitance.
2. Bipolar or Mosfet.
3. Driver parameters.
4. Required slew rate.
5. Stage current in diff pair.
6. Rail voltage.
7. Loading of the VAS output; sometimes a resistor is used here, as in the Symasym 5 from Michael Bittner.

As a general rule, for 36V rails, a 60W into 8R amp, around 7mA is sufficient with a high beta bipolar output stage. Increase this around 40% for mosfets, since they have no driver and are connected directly at the gate to the VAS output.

When you look at amp design, there are a myriad of designs and operating points out there, and some are just plain wrong, yet they work OK. You have to do your testing carefully, and move the operating points up and down until you get the optimum sonics. This takes time and effort.

Miller capacitance calculation is possible to calculate, but it's tricky because of parasitics in the circuit board and other spurious issues around the design. As a general rule, the best approach is empirical. Start around 100pF, and using standard values, move down to the minimum. The amp will become audibly fuzzy and distorted at too low a value, and may even blow up the speaker AND the output stage; go up 10pF from this point, and move forward in 5pF steps from this reference. The value will change with layout, component choice, etc. Of course, it goes without saying that you should use a conventional loudspeaker with commonplace impedance characteristics; you will find that the optimum miller cap for one speaker will be different for another. This is perhaps what makes it so difficult; you have to make the amp stable for all loads, yet too much Miller capacitance will destroy the sonics.

Hope this at least introduces you to the complexities; it's not particularly scientific, more empirical, but there is a very good reason for this; there is no standard load, and the amp is not judged on its specs but rather on its sonics - a highly subjective, market judgement which any designer ignores at his peril.

Cheers,

Hugh
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Old 7th December 2005, 09:29 AM   #10
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DW?..lol..im sure thats from a cartoon..ah yes..darkWing duck..hahahhhhaa..used to love that show..nice one hugh ..thank you so much for the paragraph you wrote..being as that i am inexperienced, i will go away and digest this stuff...may i ask more questions if needed?..thnak you once again i really appreciate it
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