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Very simple quasi complimentary MOSFET amplifier
Very simple quasi complimentary MOSFET amplifier
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Old 23rd February 2019, 02:37 AM   #2421
manniraj is offline manniraj  India
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Hello Hugh, I am using this version of bandol layout. Both the trimmers are placed side by side in this layout.

thanks
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Last edited by manniraj; 23rd February 2019 at 02:43 AM.
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Old 23rd February 2019, 03:03 AM   #2422
avtech23 is offline avtech23  Australia
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Very simple quasi complimentary MOSFET amplifier
Are you using the schema from Very simple quasi complimentary MOSFET amplifier

If so, VR2 should be the bias pot and value of 1k was recommended as a correction on the BOM.
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Old 23rd February 2019, 03:33 AM   #2423
manniraj is offline manniraj  India
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Quote:
Originally Posted by avtech23 View Post
Are you using the schema from Very simple quasi complimentary MOSFET amplifier

If so, VR2 should be the bias pot and value of 1k was recommended as a correction on the BOM.
Yes thats the one I am using and you are correct I have used 1K from the latest BOM with the correct trimmer value.

Thanks
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Old 23rd February 2019, 05:42 AM   #2424
bandol83 is offline bandol83  France
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Hi manniraj,
Sorry for the late response but I am in holidays in Mauricius and I have no access to my files! Hopefully l have seen that Hugh and avtech23 answered your question
I will be back next week...
Rgds,
Marc
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Old 23rd February 2019, 06:56 AM   #2425
AKSA is offline AKSA  Australia
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Thank you Marc, lucky fellow......!!

VR1 is the offset.
VR2 is the bias (the pot which sits across the BD139 Vbe bias generator).

That is all you need.......

As AV23 mentions, 500R may be insufficient for range, 1K permits you to bring down the bias current to much lower values. However, around 125mA is correct as we mentioned across the RCR 0.33R resistor (45mV).

Hugh

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Old 23rd February 2019, 08:17 PM   #2426
manniraj is offline manniraj  India
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Quote:
Originally Posted by AKSA View Post
Thank you Marc, lucky fellow......!!

VR1 is the offset.
VR2 is the bias (the pot which sits across the BD139 Vbe bias generator).

That is all you need.......

As AV23 mentions, 500R may be insufficient for range, 1K permits you to bring down the bias current to much lower values. However, around 125mA is correct as we mentioned across the RCR 0.33R resistor (45mV).

Hugh

Hugh
Thanks Hugh, yes VR2 is for the bias and I could dial it to 45mV across one of the 0.33R resistor using extended pins underneath the board to test the voltage. The offset is damn good on both the boards with drifting between -0.2mV - 5mV. Pretty surprised that without any changes to the VR1 the offset is so good on both the amp boards

But the bias is not stable sometimes drifting downwards or upwards of 45mV even after running it for more than an hour. Does my heat sinks not sufficient as they run quite hot after an hour with 45mV bias? I thought bandol was running his with smaller heat sinks than mine. Yet to hear the amp will set the bias/offset again and hear the music by tomorrow.
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Old 23rd February 2019, 08:32 PM   #2427
AKSA is offline AKSA  Australia
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Offset is not tightly controlled on this amplifier and the reason many users like a servo.
A servo is NOT needed however.
Wait until the amp is warmed up, half an hour idling, then set the offset to 0mV -/+15mV. It will move up and down regularly according to thermal effects.

Furthermore, the bias (quiescent) is dynamic and will change a little. Set it up warm, half an hour after idling, and accept that it will go up and down a little. Be sure that the Vbe multiplier, the BD139, is in thermal contact with one of the output devices. These changes are normal, transistors are extremely sensitive to temperature effects, but as it warms up the quiescent should drop a little. This is intended to cool off the amp should it become very hot under heavy use.

Congratulations, you have a new amplifier to test now!

Cheers,

Hugh
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Last edited by AKSA; 23rd February 2019 at 08:34 PM.
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Old 24th February 2019, 03:05 AM   #2428
manniraj is offline manniraj  India
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Quote:
Originally Posted by AKSA View Post
Wait until the amp is warmed up, half an hour idling, then set the offset to 0mV -/+15mV. It will move up and down regularly according to thermal effects.

Be sure that the Vbe multiplier, the BD139, is in thermal contact with one of the output devices. These changes are normal, transistors are extremely sensitive to temperature effects, but as it warms up the quiescent should drop a little. This is intended to cool off the amp should it become very hot under heavy use.

Hugh
I did not get what you meant by thermal contact of BD139 with the output devices? Right now I have mounted the BD139 beside the mosfet output device on the heat sinks. But using a thermal insulation pad with slight amount of thermal paste. Yes you are right regarding the temperature increases/decreases the bias also changes a bit drifting along. But I could set the bias to around 45mV (usually what I can get best is around +/-5mv after warm up for 30 mins and resetting it back). There is output ground, so what I have done is use couple of wires on one of the heat sink mounting holes to take as grounds for both the channel outputs to my speaker protection boards. As bandol suggested I have connected the both PSU output grounds together. But do you want me to take this PSU ground where I have joined to the chassis ground the same place where I have connected the safety ground from IEC to the chassis?

Thanks
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Old 24th February 2019, 10:04 AM   #2429
AKSA is offline AKSA  Australia
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No, your system will be fine. But the big question is this:
'Do you have any hum or intrusive noise?'
Of course, you will have to connect your speakers to find this. Then, your most sensitive measurement, your ears, will tell you the truth.

If yes, you have some work to do with the earthing system. Otherwise, you are fine.

Hugh
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Old 26th February 2019, 09:19 AM   #2430
bandol83 is offline bandol83  France
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Hello Hugh,

Finally I am back from holidays Thank you for taking care of the support during my trip

While I was on the beach with nothing to do, I had a question in my mind about the Quasi (that is very good by the way and I still need to make some more listening test now that I have more time...) :

- Why we are using a MOSFET instead of two NPN bipolar transistors like the 2SC5200 or the NJW3281 for the output stage ?

Thanks for advise and best regards,
Marc
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