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?
First Watt F4 reduce VDC?
#1 First Watt F4 reduce VDC?
Last edited by Michael L on Fri Oct 12, 2018 2:31 pm, edited 1 time in total.
- IslandPink
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#2 Re: First Watt F4 reduce VDC?
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.
"Once you find out ... the Circumstances ; then you can go out"
#3 Re: First Watt F4 reduce VDC?
Unfortunately it's got an expoxy filled centre.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.
#4 Re: First Watt F4 reduce VDC?
I would just get a 18v TX.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#5 Re: First Watt F4 reduce VDC?
Yes that's what I'll do Nick
#6 Re: First Watt F4 reduce VDC?
Just what rating is going to be over? Vds for irfp(9)240 is 200v
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#7 Re: First Watt F4 reduce VDC?
I believe it's the input stage https://www.diyaudio.com/forums/pass-la ... ifier.html
#8 Re: First Watt F4 reduce VDC?
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.
#9 Re: First Watt F4 reduce VDC?
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.
There's nowhere you can be that isn't where you're meant to be
#10 Re: First Watt F4 reduce VDC?
18V transformer ordered.
I would be interested to hear people's experience with speaker protection circuits.
I would be interested to hear people's experience with speaker protection circuits.