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AudioGeek 1st April 2004 03:39 PM

load on source follower choke or mosfet?
thinking of a source follower.... running a IRFP240n at around 3A.


A) at 32v with another IRFP240n as a CCS. 16v center... cap coupled to speaker load.

B) at 19v or so, choke (100mH, 1ohm) loaded. 3v at top of choke. cap coupled to speaker load.

what are the advantages of both? Disadvantages?

Can I get a show of hands as to which one is "better" ?

Planning on using a BG as the cap.... maybe 5000uf or so. probably get away with a lower voltage cap in 'B'.

I know a lot of talk has been said about this...

thanks in advance.

Fuling 1st April 2004 03:56 PM

If you have the chokes, use them!
The efficiency goes up from ~20 to ~40%, which pays off when it comes to PSU and heatsinks.

Well, biasing requires some degree of care, but the DC resistance in the choke is high enough to serve well as source resistor.
Not a problem really.

One thing that might become a problem (of not taken care of)
is slow "thumps" from the prior stage during startup.
Since there is no constant current "mechanism" in the choke
you´ll need some kind of protection during these "thumps".

A timer circuit controlling a relay that clamps the "grid leak" (tube term...) for some tens of seconds solves everything.

AudioGeek 1st April 2004 04:57 PM

well I am going to IT couple to the gate of the MOSFET.

maybe run a ECC99 or 6SN7.

what size choke should I use? and what should the current rating be?

thinking of 50mH - 100mH 1ohm choke. 3A bias.

would this be adequate for 8 and 4 ohms?

Z= 2(pi)fL

so the -3 for 4 ohms is around 15Hz for 50mH. should go to 100mH if I can.

what should my power supply voltage and my source voltage be to have symmetrical clipping from a choke load?????

or is this relavent as the speaker load will dominate over the choke load? (AC wise).

AudioGeek 1st April 2004 05:07 PM

just thought of something
would the coupling cap in 'B' have to be a Non-polarized cap??? This would seriously be a problem.

perhaps I put 2 polarized caps in series and bias them in the middle?

Pjotr 1st April 2004 06:24 PM


Originally posted by Fuling

If you have the chokes, use them!
The efficiency goes up from ~20 to ~40%, which pays off when it comes to PSU and heatsinks.


Well a choke is best regarding efficiency indeed. But it raises from 10% to 20% for SE topology, not to 40%! 100 mH is a good choice. I would take the other end of the choke to ground then but then you have DC across the choke. To get rid of that you need a coupling cap. A better option IMHO would be to use a 1:1 transformer in stead of the choke. The primary and secondary can be wound bifilar then which allows for a very tight coupling between the windings. This way you also avoid switch on thumps if you let the bias current come up slowly but anyway you need some kind of servo to control the bias current.

Cheers ;)

AudioGeek 1st April 2004 06:34 PM

hey theres an idea.

do you have any recommendations for this transformer?? I am biasing at 3A or so...

probably need a shunt circuit on the primary side to lessen any frequency peaks.

AND I can get rid of that nasty coupling cap.



Pjotr 1st April 2004 07:08 PM


For such a transformer you need basically the same core and amount of turns as for the choke. You will need a gapped core to handle the large DC current and a primary inductance of the same 100 mH. I wouldn’t go higher than 300 - 500 mT for the standing induction to keep distortion low. If you are not be able to calculate such a transformer yourself a good winding house for sure can.

Cheers ;)

Fuling 1st April 2004 08:48 PM


But it raises from 10% to 20% for SE topology, not to 40%!
I don´t agree. Maximum teorethical efficiency for an CCS loaded SE amp is 25% versus 50% for a chokle loaded design.
When I had my "choked" follower running I got 30W out @ 80W power consumption.

AudioGeek: 50mH is enough for 4 ohms load and 100mH for 8 ohms. 3A bias is enough for 36W into 8R and 18W into 4R, teorethically.
A 1:1 output transformer is a very interresting idea, but there is another way to get rid of the output cap: Balanced operation.
Two low voltage/high current followers driven by a a balanced signal, speaker connected between the outputs.
That´s the concept I´m working on right now.

AudioGeek 1st April 2004 09:08 PM

i had similar idea.
yeah... i thought about that too.

cancels some 2nd harmonic though. hmmm..

what to do.

i loved the sound of me SE SS amp... so I think thats what I will do.

could do split supplies and dc couple the outputs... then there is the potential of dc on output.

Fuling 1st April 2004 09:43 PM

I don´t know if a dual rail supply is the way to go here, since you´ll need +19 and -3V. IF you´re going for the choke, that is.

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