First Watt F4 reduce VDC?

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Michael L
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#1 First Watt F4 reduce VDC?

Post by Michael L »

No time to build much so I am soon to take delivery of a used Pass F4 from an enthusiast in Portugal.
He has used a 5U case 450x400x210 mm and tx with 24-0-24 secondaries which gives Vdc +/-30v. This allows some 40W in class A.

My concern is that with continental mains voltage, Vdc is already close to max for the circuit. My mains when I measured was 242v.

Should I buy a new transformer or could I drop the voltage with the existing - if so how best to do it. Could I use rectifiers if it's just a few volts?
As far as I know, running the output devices closer to max using VDC of 30V isn't any problem with appropriate heat-sinking and this case has plenty.

In time however I would like to try other First Watt incarnations with this chassis, particularly a SE design. Perhaps having an 18-0-18 tx more commonly used in Nelson's First Watt amps would make life easier, especially considering how difficult some of the semiconductors are to come by if I should let the smoke out.

Finally, I do worry regardless of Vdc about any failure giving me a very hefty bill to replace my woofers (Ref 3A De Capo). No op trannies or caps here. If I were to add speaker protection using relays, would the contacts degrade the audio?
Last edited by Michael L on Fri Oct 12, 2018 2:31 pm, edited 1 time in total.
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IslandPink
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#2 Re: First Watt F4 reduce VDC?

Post by IslandPink »

Well, you could take the cellophane off the mains transformer and unwind a few turns to drop the voltage then tape it up again. Won't look so neat, but would save you buying a different Tx.
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Michael L
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#3 Re: First Watt F4 reduce VDC?

Post by Michael L »

IslandPink wrote: Fri Oct 12, 2018 2:10 pm Well, you could take the cellophane off the mains transformer and unwind a few turns to drop the voltage then tape it up again. Won't look so neat, but would save you buying a different Tx.
Unfortunately it's got an expoxy filled centre.
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Nick
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#4 Re: First Watt F4 reduce VDC?

Post by Nick »

I would just get a 18v TX.
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Michael L
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#5 Re: First Watt F4 reduce VDC?

Post by Michael L »

Yes that's what I'll do Nick
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Nick
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#6 Re: First Watt F4 reduce VDC?

Post by Nick »

Just what rating is going to be over? Vds for irfp(9)240 is 200v
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#7 Re: First Watt F4 reduce VDC?

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Michael L
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#8 Re: First Watt F4 reduce VDC?

Post by Michael L »

Michael L wrote: Fri Oct 12, 2018 10:26 pm I believe it's the input stage https://www.diyaudio.com/forums/pass-la ... ifier.html

Edit - I have found the info I read previously in the F4 build thread: Quote - you can scale the F4 up to about 30-32V. Much past that and the Jfet J74 start to let the smoke out.

No point in pushing things for the sake of a new mains transformer.
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ed
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#9 Re: First Watt F4 reduce VDC?

Post by ed »

Michael L wrote: Sat Oct 13, 2018 10:34 am
Edit - I have found the info I read previously in the F4 build thread: Quote - you can scale the F4 up to about 30-32V. Much past that and the Jfet J74 start to let the smoke out.

No point in pushing things for the sake of a new mains transformer.
scratching my head here....the input pair on mine is k370/j108...

however notwithstanding, I advise against letting the magic white smoke out...a new input pair will cost you more today than my whole amp cost in 2007, not including the heatsinks.
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#10 Re: First Watt F4 reduce VDC?

Post by Michael L »

18V transformer ordered.

I would be interested to hear people's experience with speaker protection circuits.
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