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Old 12th September 2017, 02:01 PM   #441
cats squirrel is offline cats squirrel  United Kingdom
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Hi Shaan, a question:

I am interested in separating the power rails into just FET's and rest of circuit. I think I can do this without much effort. However, I have two 30-0-30 volt toroids, 160VA, which I want to use as the 'rest of the circuit' and buy some new toroids with a liitle more current for the output FET's. But my FET's are limited to just 8 amps each, these are the ALF of the original VSSA design, which I bought at the time. They are unused. Would the sensible thing to do be to use a 300VA toroid at 25-0-25v (35v rails) at a high current or 30-0-30v (42v rails) at a slightly lower current?

Please note, I am using two amplifiers to feed one loudspeaker (lower mid range to super tweeters, not bass <100Hz)) so the total power output will not be more than 100 watts into 4 Ohms for the two amplifiers, I guess. So each 'channel' will contain two amplifier boards, one 160VA toroid, one 300 VA toroid (and a 12V toroid for switch on and output monitoring).

ps I have used a polypropylene capacitor across the inputs to the FETs as I think polyprop sounds much better than polyesters, but I haven't finished the amplifiers to test this, yet!
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Last edited by cats squirrel; 12th September 2017 at 02:16 PM.
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Old 12th September 2017, 11:25 PM   #442
shaan is offline shaan  India
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Quote:
Originally Posted by cats squirrel View Post
Hi Shaan, a question:

I am interested in separating the power rails into just FET's and rest of circuit. I think I can do this without much effort. However, I have two 30-0-30 volt toroids, 160VA, which I want to use as the 'rest of the circuit' and buy some new toroids with a liitle more current for the output FET's. But my FET's are limited to just 8 amps each, these are the ALF of the original VSSA design, which I bought at the time. They are unused. Would the sensible thing to do be to use a 300VA toroid at 25-0-25v (35v rails) at a high current or 30-0-30v (42v rails) at a slightly lower current?
You can run the MOSFETs from 35V while the previous stages run from 42V (with maximum power available being 15-20W more than if running the whole amp from 35V alone i.e. about 70W/8R with the option to run it into 4R load if needed). So I'd suggest a 25-0-25 300VA toroid.

Quote:
Please note, I am using two amplifiers to feed one loudspeaker (lower mid range to super tweeters, not bass <100Hz)) so the total power output will not be more than 100 watts into 4 Ohms for the two amplifiers, I guess. So each 'channel' will contain two amplifier boards, one 160VA toroid, one 300 VA toroid (and a 12V toroid for switch on and output monitoring).


It's gonna get loud.

Quote:
ps I have used a polypropylene capacitor across the inputs to the FETs as I think polyprop sounds much better than polyesters, but I haven't finished the amplifiers to test this, yet!
Yes a better quality capacitor is always welcome anywhere. However, this capacitor (C2) doesn't affect the sonics as much as C3 and C4. So if space allows make these polypropylene too, or if space doesn't, the best polyester you can get.
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Old 13th September 2017, 02:34 PM   #443
cats squirrel is offline cats squirrel  United Kingdom
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thanks, Shaan. Polyprop 100nF ordered for all those on board. Toroids to get!
As regards loudness. I have pondered this, and have come to the conclusion that two amplifiers covering different parts of the audio spectrum won't sound any louder than one covering all of it. I'm doing it to ease the burden, especially as I have a low impedance for the tweeters (a soft dome and a true ribbon above 4.5kHz) in parallel! Impedance may dip below 4 Ohms, but as classical music slopes off quite rapidly above 1kHz, I think I should be OK.

Again, many thanks.
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Old 14th September 2017, 01:00 AM   #444
asuslover is offline asuslover  Romania
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I'm not sure, why would be so big difference if C3 and C4 are polypropylene?
What impact would that have?
C6 and C5 are suppose to be audio caps? I did not see that anywhere.
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Old 14th September 2017, 03:36 AM   #445
gtose is offline gtose  United States
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Old 14th September 2017, 07:57 AM   #446
shaan is offline shaan  India
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Quote:
Originally Posted by asuslover View Post
I'm not sure, why would be so big difference if C3 and C4 are polypropylene?
What impact would that have?
C6 and C5 are suppose to be audio caps? I did not see that anywhere.
C3 to C6 are feedback caps. They provide a path for AC signal to be attenuated accordingly and reach the error correction node, But their own non-linearities are not corrected by an amplifier, instead they are amplified. So the best quality (i.e. most linear) components should be used in here.

A capacitor being marked "for audio" doesn't automatically make it superior. All the test THD measurements of V4 were done with industrial grade capacitors and the sonics never seemed to complain.
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Old 14th September 2017, 01:31 PM   #447
atupi is offline atupi  Romania
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Really good news Shaan, it looks like my Mosfets survived. I managed to adjust both channels on DMM, next is testing with cheap speakers.
I noticed the total consumption (aka bias) is increasing bit by bit in 10-15 minutes, when does it stopes?
How long i need to wait before i can say my bias is stable?
Thanks alot for your help!

PS. Practical advice: for me it was much easier to use two DMM's in order to achieve VAS biasing and 0mV DC out in the same time. So one DMM on R29/30 and another one on SPK out and target 450mV on one and 0mv one the other one

Last edited by atupi; 14th September 2017 at 01:45 PM.
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Old 14th September 2017, 01:59 PM   #448
shaan is offline shaan  India
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Quote:
Originally Posted by atupi View Post
Really good news Shaan, it looks like my Mosfets survived. I managed to adjust both channels on DMM, next is testing with cheap speakers.
I noticed the total consumption (aka bias) is increasing bit by bit in 10-15 minutes, when does it stopes?
How long i need to wait before i can say my bias is stable?
Thanks alot for your help!
It depends a bit on cooling of the VAS+MOSFET stage and thermal coefficient of the MOSFETs at a particular bias.

Usually the process of optimum biasing is to let the amp warm up for half an hour and then re-adjusting the bias to normal value. After cooling when you start the amp again it will start with lower than set bias and after half hour of warming up (or less time if loaded) it will reach the previously set bias.

Quote:
PS. Practical advice: for me it was much easier to use two DMM's in order to achieve VAS biasing and 0mV DC out in the same time. So one DMM on R29/30 and another one on SPK out and target 450mV on one and 0mv one the other one
Yup two is better than one.
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Old 14th September 2017, 02:04 PM   #449
shaan is offline shaan  India
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Old 14th September 2017, 02:23 PM   #450
atupi is offline atupi  Romania
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Can you expplain a little the need of J1, J2 jumpers? Thanks!
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