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Old 25th November 2009, 06:26 AM   #1
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Default Solid Solid State Power Amplifier

This amplifier was inpsired by conversation with John Curl in a ‘Blowtorch thread’, as well as by his thoughts ( www.parasound.com/pdfs/JCinterview.pdf ).

The aim was to build a reliable, solid, good sounding amplifier.

Topology is complementary differential, with JFET input, MOSFET cascode and VAS, and BJT multiple pair output stage. The circuit shown might be an inspiration for DIYers in their designs.

A functional sample was build, the circuit is very stable and rugged.

The project is not a commercial project, no PCBs or kits will be sold.
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Old 25th November 2009, 07:05 AM   #2
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Hi Pavel , before being submerged from requests of PCB and kits that you stated won't be made, can I ask you for completeness the values that are not written of some resistors in the differential stage? And the type of mosfets in the VAS stage.Eventually the supply rails voltage.
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Old 25th November 2009, 07:17 AM   #3
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Hi pico,

well - there are good reasons not to show them.
2SK170BL/2SJ74BL are matched to within 0.5mA Idss, and Idss is about 9.5 - 10mA. You make a choice of R1 by measurement then, to get Id = 8mA approx. The R5 and R7 are chosen again by measurement, to get appropriate current through VAS Mosfets (check voltage drop across R6, voltage corresponds to Id of input stage + VAS current). You may get something like R1=7R and R5, R7 = 750R for 2SK413/2SJ118 mosfets. Anyway, measurement is necessary. The DIYer should be at least average experienced building this amp. Rails - start with +/-30V and may increase to +/-50V if operating safely.

Last, but not least - voltage at gates of M3,M4 and M5,M6 is set to 13V approx by dividers R75/R76 and R77/R78. Do not forget this, not to thermally burn input JFETs. Before increasing supply rails, consider this.
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Last edited by PMA; 25th November 2009 at 07:24 AM.
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Old 25th November 2009, 07:24 AM   #4
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Quote:
Originally Posted by picowallspeaker View Post
Hi Pavel , before being submerged from requests of PCB and kits that you stated won't be made, can I ask you for completeness the values that are not written of some resistors in the differential stage? And the type of mosfets in the VAS stage.Eventually the supply rails voltage.
I thought that would be the first question

Nice link to the JC interview.
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Old 25th November 2009, 10:12 AM   #5
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Pavel any listening impression yet or are you still tweaking?
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Old 25th November 2009, 10:15 AM   #6
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I want to make some more measurements, and I plan for serious listening tests in a week.
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Old 25th November 2009, 11:17 PM   #7
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Looks very simple and elegant PMA - looking forward to seeing the results
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Old 26th November 2009, 07:25 AM   #8
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Thanks Bonsai, I will let know if there are any interesting results.
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Old 26th November 2009, 04:14 PM   #9
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Quote:
Originally Posted by PMA View Post
Hi pico,

well - there are good reasons not to show them.
2SK170BL/2SJ74BL are matched to within 0.5mA Idss, and Idss is about 9.5 - 10mA. You make a choice of R1 by measurement then, to get Id = 8mA approx. The R5 and R7 are chosen again by measurement, to get appropriate current through VAS Mosfets (check voltage drop across R6, voltage corresponds to Id of input stage + VAS current). You may get something like R1=7R and R5, R7 = 750R for 2SK413/2SJ118 mosfets. Anyway, measurement is necessary. The DIYer should be at least average experienced building this amp. Rails - start with +/-30V and may increase to +/-50V if operating safely.

Last, but not least - voltage at gates of M3,M4 and M5,M6 is set to 13V approx by dividers R75/R76 and R77/R78. Do not forget this, not to thermally burn input JFETs. Before increasing supply rails, consider this.
this circuit is similar to Lender's circuit - go to
The most linear stage for large voltage swings - Lender's Circuit
An additional similar circuit I have seen from JLH, but I forget the title and the location here in this forum
Is there are a possible to work without selecting? I. e., I will use jFETs with a very wide range of Idss and a wide range of Vgs threshold voltage.
As I know, it is possible by tube amplifiers, i. e. each tube has in this case their individual bias spindle potentiometer.
Then I can selected, too. But now I select only to identical curve shape (curve passage) of the transfer curve, much more important as Idss and Vgs.

Last edited by tiefbassuebertr; 26th November 2009 at 04:26 PM.
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Old 26th November 2009, 04:44 PM   #10
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Maximum output voltage will be less than 10 Volt power. This will have a very low efficiency.
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