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Loxjie P20 OP AMPS Ref Voltage +-18V too high for bursons V6(16.5v)?
Loxjie P20 OP AMPS Ref Voltage +-18V too high for bursons V6(16.5v)?
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Old 15th July 2021, 10:03 PM   #11
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Loxjie P20 OP AMPS Ref Voltage +-18V too high for bursons V6(16.5v)?
I was looking at the specs and thinking: it needs 140mA. And maybe 18V (I see 12V). But chips are cheap. Simple enough to series/parallel. In my head I figured 8 opamps, four chips.

Then I found this gut-shot. The power stage is indeed four chips. The tubes do seem to be wired as something (maybe just cathode-follower?). Then there is a small opamp chip at the input jacks which may do the balancing chores. And logic display or power chips.
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Old 15th July 2021, 10:21 PM   #12
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Okay, well that makes more sense Ö having the opamps in parallel.

The small IC at the input is the digital volume control I suspect.

The tubes do have high voltage , 100 volts , it uses DC to DC converter. It runes hot. I read it is a cathode follower at the input - just a comment on some forum. Anyway, youíre likely correct about it.
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Old 20th July 2021, 06:05 AM   #13
Johnloudb is offline Johnloudb  United States
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Here is what Iíve come up with so far for the output stage, after some probing.

Not positive about the cap, but would work as a high pass filter. Wasnít able to get a good reading on the ? resistor because of capacitive coupling. Anyone want to defend this circuit?

The way I see it Ö I know the input , output and the power supplies. I can remove this opamps and put in my own amplifier design. Probably using the OPA1612.
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Old 20th July 2021, 06:45 AM   #14
Johnloudb is offline Johnloudb  United States
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The JRC op amp has big voltage droop with current above 10ma. Thatís one reason I prefer the OPA1612. Under load the circuit posted above would multiply the droop issue Ö with the input of the second op amp buffer being fed by output of the gain op amp which drives the output as well.
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Old 21st July 2021, 01:46 AM   #15
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Loxjie P20 OP AMPS Ref Voltage +-18V too high for bursons V6(16.5v)?
I strongly suggest your circuit is more like this than your drawing.

> big voltage droop with current above 10ma.

I would call 10% drop "not big", even small. 1dB. The JRC55320 at 10% drop (14V to 12.6V) is flowing 45mA. And 12.6V at 45mA is 280mW at 280 Ohms, VERY LOUD. OTOH, at 100mA we have 5V which supports 250mW in 250 Ohms, not a small sound. Doubled-up we may have 5V 200mA for 500mW in 25 Ohms, and slightly less in 32 Ohms. (The 47 Ohm resistors will modify this especially for low-Z cases.)

Are you actually having trouble getting loud-and-clear? Or just fretting?
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Old 21st July 2021, 08:00 AM   #16
Johnloudb is offline Johnloudb  United States
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Yes, youíre correct about the wiring, thanks. I was wondering why the circuit looked different than an application note I saw. I did some more probing tonight and filled in more of the missing pieces.

Iím guessing the question mark is for global feedback to the amp input? Havenít found where that goes to yet. Any thoughts?

As for what I hear from the stock unit Ö not very dynamic sounding and lacking some detail. But it is nice sounding and pleasant Ö certainly well worth the $100 USD spent on it. I bought it with the idea of modding it and have higher expectations for it. Iíll see what I can do with it Ö I donít think what I hear is only attributable to the tubes.
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Old 22nd July 2021, 02:50 AM   #17
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Okay, well I think the next step is to hook this up to a scope and see what the gain is Ö open and closed loop. Still not sure about that other 5k resistor, and what it is connected to. I have some time this weekend to play around with it.
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