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Old 2nd April 2004, 05:47 PM   #21
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Default Re: bias at choke current handling

Quote:
is there any issue with choke saturation running the bias so close to the choke rated current?
Yes, usually ready-made PSU chokes are designed to operate not so much below saturation. At low frequencies the max. peak current through the choke may raise and the choke must be capable of handling this. So it wise to stay lower than the maximum current rated to have some headroom. Also distortion raises a lot at low frequencies when you bias close to the maximum rating of the choke.

Cheers
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Old 2nd April 2004, 08:47 PM   #22
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The chokes I have are specified to 3 A DC and I run them at 3 A bias.
No problems even at full signal and 20Hz.
Those 14lbs Hammond behemoths should be by far enough for this kind of operation.

On the output Iīd use 2x4700uF or 4x2200uF.
Paralleling caps lowers the ESR and inductance.
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Old 3rd April 2004, 09:16 AM   #23
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Quote:
Originally posted by AudioGeek
what kind of power supply do you use?
so I would only need 25v BG caps??? or 50v???
My power supply is a 500VA 22vac (IIRC) feeding 4x30 amp 300v ultra-fast ST diodes then a choke of approx 1mH then 104 paralled 2200uF 35v electros. Ripple at 7 amps load is about 100mV p/p but it is a sinewave shape (much lower audibility) thanks to the filter choke, not a sawtooth like in a cap-only filter.

The output coupling cap voltage needs to be only as great as the voltage drop across the choke.

One great feature not mentioned yet about choke loaded amps is that if it all of a sudden decides to go belly-up then if your choke is sufficiently low resistance it prevents too much dc voltage being developed across the speaker until such time as the fuse blows.
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Old 3rd April 2004, 11:11 AM   #24
Fuling is offline Fuling  Sweden
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Yes, chokes in the PSU is a very good idea.
In my amp I measured a couple of volts p-p sawtooth ripple before the choke and 100mV p-p sinewave shaped ripple after it.
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Old 5th April 2004, 02:15 AM   #25
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Default source follower PSRR and input cap.

for a IRFP240N running at around 28vdc 3A or so... what is the PSRR with a 100mH choke as an inductor??

trying to figure out minimum ripple I should have on PS.

Also, I want to use a IT and a tube to drive the gate. should I use a 1:1 or a 2:1 for some current drive???

what current is required for decent slew rate at 30Khz?

Thanks!!!!
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Old 5th April 2004, 11:29 AM   #26
Fuling is offline Fuling  Sweden
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Canīt answer the PSRR question except for the obvious: As low ripple as possible. As i mentioned before I had a similar amp (very similar, actually) that had about 0,1Vp-p ripple out from the PSU but nothing audiable on the output.

A stepdown IT seems to be a great idea. Youīll hardly have any use for all the voltage swing that a decent tube stage can deliver, so trading some swing/gain against better drive capability must be a good thing.
I had similar plans for my balanced project, but I changed my mind towards a balansed Zen linestage. Might try tube/IT in the future, though.

A bit of current though the driver tube canīt hurt, Iīd say a few tens of mA. What tube are you going to use?
And while weīre at it, what IT?

BTW: A resistor across the IT secondary migh be useful to damp out resonances and stuff.
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Old 5th April 2004, 04:03 PM   #27
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Default IT and tube

well the IT could be a EP 2:1 (4K:1K) step down.

driver tube.... hmm... haven't got that far yet.

deciding whether I should build an integrated or just an amp.
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Old 5th April 2004, 05:06 PM   #28
Fuling is offline Fuling  Sweden
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EP?

There should be plenty of good tubes to use in this circuit.
6C45, EC86, ECC88, E55L, E81L, plus probably a few hundred that I canīt remember right here and now

Integratad or plain power amp, well, thatīs up to you alone.
Separate chassis are way less messy, though.

May I suggest that you put all tube circuitry (preamp + driver) in one chassis and the power follower in another?
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Old 5th April 2004, 06:34 PM   #29
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Hi,

Depends, there are several options regarding tube configurations: SRPP, Mu-stage and interstage transformer. For a Mu stage and 1:1 interstage transformer you need a mu of app. 20 for a gain of 20. So unless you are intending to use feedback, look for tubes with a mu of around 20. The EC86 has a too high mu of 68. The new ECC99 looks promising as well.

Also have a look here for some ideas of driving stages:

http://www.wdehaan.demon.nl/hybrid.html

Cheers
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Old 9th April 2004, 09:05 PM   #30
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Default trying to decide on ouput coupling caps.

ok... BG's are waaayyyy to expensive for me.

so looking at the Nichion Muse caps. Michael Percy has some 2200 and 4700uf caps.

any recommendations for caps and what to bypass them with??

also what should I bypass my supply caps with??

supply is 68,000uf - 2.2mH choke - 68,000uf cap per channel.

probably a 22vac tranny at 5A per channel for supply.

comments???

Thanks!! you have been a great help.
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