mosfet biasing

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ed
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#1 mosfet biasing

Post by ed »

A few years ago when I first built the Pass F6 I was having real problems getting a stable bias. This was the mosfet version and not the jfet version which I’d started much earlier. I was keen to stick to the original circuit in order to keep comparisons with other builds straightforward. The bias kept wandering, quite significantly. At first I thought it was thermal issues so I replaced the silpads with mica and goo, and for belt and braces changed the mosfets to matched pairs.

No change, the bias was still wandering. It turns out it was mains fluctuations. My mains varies wildly between 242v and 255v. I finally set the bias in a mid point to try and compensate for the fluctuations but never really discovered what the changed do to the SQ. In the back of my mind was always the thought that the dc offset changes with bias and I was certain that was having a negative effect. Nevertheless it still remains my goto amp.

I usually follow the ‘if it aint broke then don’t fix it rule’ but I’ve decided to get a mains regenerator and set the bias with a mains voltage of 230v to see if there are any changes in SQ. This may be an extravagant itch to scratch but I’ve always wanted to play with a regenerator to feel the difference.

I know there are less than 2 people interested in my SS ramblings but I thought I’d mention it in the spirit of contribution.

Btw the build is fully documented here:
http://www.vitalstates.org/diy/amplifie ... twatt%20F6

And the trials and tribs of the bias challenge start here:
http://www.diyaudio.com/forums/pass-lab ... f6-46.html
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#2 Re: mosfet biasing

Post by Nick »

A variac would maybe cheaper. But will be interesting what you find.
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#3 Re: mosfet biasing

Post by Dave the bass »

Wouldn't the Variac just 'ride' the changes in mains fluctuation though? It'll still fluctuate wont it?
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#4 Re: mosfet biasing

Post by pre65 »

On my Pass single ended mosfet amp I used a DIY hi-fi filament module for the bias.

The module was loaded with a 10R 50W Arcol resistor, and fed from a 30va 0-6v + 0-6v toroid.

A stable bias is essential as even a tiny drift makes a noticeable difference.
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#5 Re: mosfet biasing

Post by ed »

Dave the bass wrote: Sat Aug 12, 2017 10:29 am Wouldn't the Variac just 'ride' the changes in mains fluctuation though? It'll still fluctuate wont it?
man you're on fire this morning...is it the Cornish air?

or to put it another way, yes, I think so
Last edited by ed on Sat Aug 12, 2017 10:42 am, edited 1 time in total.
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#6 Re: mosfet biasing

Post by Cressy Snr »

Is there voltage regulation anywhere in the power supply circuit?
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#7 Re: mosfet biasing

Post by ed »

Cressy Snr wrote: Sat Aug 12, 2017 10:42 am Is there voltage regulation anywhere in the power supply circuit?
no
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#8 Re: mosfet biasing

Post by Cressy Snr »

Is that for sound quality reasons?

I don't know much about solid state circuits, but couldn't the bias be stopped from wandering by regulating the power rail(s) and just for added insurance, regulate the bias supply? It might be cheaper than using a regenerator.
Just throwing ideas out there.
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#9 Re: mosfet biasing

Post by IslandPink »

Interesting stuff .
Just looking at the circuit I can't see immediately why the bias would vary more than proportionally with the power supply voltage - but obviously it does. Is it the use of the fixed 5.1v zener in setting the quite small bias from 23V ?
What's the power output of the F6 ?
Would definitely like to hear it - has a fine reputation.
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#10 Re: mosfet biasing

Post by IslandPink »

( Laptop caused repeat post )
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#11 Re: mosfet biasing

Post by pre65 »

The DIY hi-fi modules I use are regulated, at a price. I had a pair in my spares box .

https://www.hificollective.co.uk/compon ... ament.html

There are some on Ebay a LOT cheaper that might do you Ed.

http://www.ebay.co.uk/itm/Tube-Amp-Fila ... 1438.l2649

or

http://www.ebay.co.uk/itm/Tube-Amplifie ... 1438.l2649
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#12 Re: mosfet biasing

Post by Nick »

The output devices will never be perfectly matched, so a change in Vds for each will lead to a non equal change in Ids so the DC offset will move.

Phil, the problem isn;t the bias voltage changing, its the supply voltage.

Dave, I meant adjust the voltage in to 230v every time it was turned on. Not suggesting as a final fix, but maybe to find what advantage a fixed voltage gave.
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#13 Re: mosfet biasing

Post by ed »

IslandPink wrote: Sat Aug 12, 2017 10:52 am Interesting stuff .
Just looking at the circuit I can't see immediately why the bias would vary more than proportionally with the power supply voltage - but obviously it does. Is it the use of the fixed 5.1v zener in setting the quite small bias from 23V ?
What's the power output of the F6 ?
Would definitely like to hear it - has a fine reputation.
Hi Mark

Here are some early readings that are linked from the build page. They are illustrative of the movement but don't show the full range of mains variations that I experience here:

http://www.vitalstates.org/diy/amplifie ... ements.jpg

The bias range is set by the zener but without recourse to notes I can't recall if its 5.1 or 9.something, there were variations between the semisouth jfets and the mosfets.

it is 25 watts into 8 ohms......I have it running with the electrostats and the gas pedal is no more than half way to the floor..

yes it's awesome, at least it is for me.
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#14 Re: mosfet biasing

Post by ed »

Nick wrote: Sat Aug 12, 2017 11:06 am The output devices will never be perfectly matched, so a change in Vds for each will lead to a non equal change in Ids so the DC offset will move.

exactly so!...I think some sort of servo mechanism could be rigged, I even thought of an avr tracker but at the end of the day I want to have a play with a regenerator anyway...it may give up secrets with the other amps I have.

It was very interesting comparing notes with other builders of this circuit. I have deduced that the US mains is more stable thatn ours and it was interesting to find that one other UK builder had exactly the same bias problems as me...He came from lincolnshire which is not to far away, which may or may not be relevent. I naively think that the variations in mains are localised to sub stations or even street cabling and connectivity. Maybe Ali has an angle on this.

I do know that since the underground was relaid in our street(The leccy man said the previous line was getting too hot) We've had higher voltages.
Last edited by ed on Sat Aug 12, 2017 11:17 am, edited 1 time in total.
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#15 Re: mosfet biasing

Post by pre65 »

Nick wrote: Sat Aug 12, 2017 11:06 am

Phil, the problem isn;t the bias voltage changing, its the supply voltage.

Sorry, I thought the supply voltage change was affecting the bias.

My bad.
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