Just for my technical interest I asked what the weight would be for a 1 mOhm choke. 40 kg and price just below $1000 each (incl Danish tax). That would be the ultimate MoFo choke. I think not many chokes of this size exists in private homes.
The 20 mV DC I get (2.5A bias) with the 20 kg choke will probably not harm the performance of 15" woofer which handles frequencies below 180 Hz. But only 2-3 mV DC using the 40 kg choke is tempting.
I think the series resistance of the choke helps in preventing thermal runaway condition when %$#@! happens. Smallest series resistance choke may not be the first choice for all Mofo builds.
A woofer is long-throw and should handle significant DC without de-centering enough to matter. (Anyway they are never centered perfectly as built.) If DC is much-much less than audio it should be fine. Assuming you can throw lots of 20V bass in, then 0.2V (200mV) of DC should be no-problem.
And as indra1 notes, Michael's elegant plan USES the "unavoidable" DCR as bias sense and stability. If you did aim for 20mV DC, then you need <<20mV stability/tracking of MOSFET and bias-supply voltages. I dunno if that is practical? Alternatively a servo to detect mV drift in the DCR and amplify it to the many-mV change needed at MOSFET Gate.
I am going for the 20 kg chokes with 8 mOhm DCR. I can see if I can get a stable bias. If not I always will have the possibility to insert a serie resistor? I understood some threads back that there was not a requirement for a specific amount of DCR. I will try it out and see how it goes…...at least I will learn something…...
So you move from a $100 choke toward a $500-$1000 choke, "for low DCR", then a dollar resistor to UN-DO several hundred dollars of added investment?
Michael's original plan and component choices were, IMO, well-picked and balanced.
The JLH servo is not exactly right. (It aims for zero V out, you want to aim for a few mV of DC hidden in a rather large audio signal.) But it is indeed a TL072 and a few R and C job.
Michael's original plan and component choices were, IMO, well-picked and balanced.
The JLH servo is not exactly right. (It aims for zero V out, you want to aim for a few mV of DC hidden in a rather large audio signal.) But it is indeed a TL072 and a few R and C job.
Yes, the primary goal was to avoid the output cap (DC < 50 mV). Så I move from a $100 choke to a $400 choke. Will try the circuit with only that one. If I get problems with bias stability I will insert a fine wire wound resistor from choke to mosfet but stilll take the output signal from the choke to have low DC at the output. Is that a good plan? ….I will not do the servo circuit. The MoFo should be simple. If everything fails I switch to the Hammond 193......and have then learned a lot…..I hope. I will use a big heatsink so mosfet will not be fried if bias run away and I will test using a lap supply with current limiter. It will also show the current so I can see what happens during warm-up.
for the heat sink look this:
DB/B/150 Large Heatsink, 0.38K/W, 150 x 125 x 135mm | eBay
or this:
Dissipatori
DB/B/150 Large Heatsink, 0.38K/W, 150 x 125 x 135mm | eBay
or this:
Dissipatori
Last edited:
besides I have introduced on my supply a snubber that I would recommend in any case.
look this for your design:
http://www.cde.com/resources/technical-papers/design.pdf
look this for your design:
http://www.cde.com/resources/technical-papers/design.pdf
That question I have also asked myself.......but some threads back it seems one was using the MoFo without an output cap with about 0.2 V DC at the output. Using "my" choke with 8 mOhm RDC then with no input signal the DC will be 20 mV at 2.5A bias. Then if someone could draw how the voltage change at the source (relative to gnd) of the mosfet during a sinus period I may understand the circuit better. If DC raises to about V+/2 during AC operation......then of course I can't use the MoFo without an output cap.
OK......thank you! I have looked at a similar at RC Components. Think the one I looked at was a 200 mm version with a little lower C/W....but now I can see that price level is about the same......pr. mm.....quite expensive those heat sinks......
The plan is to re-use the PSUs I made for the ACA which are two LCLC filter based PSUs. The voltage will drop a bit and will be about 20V for 2.5 A bias. Ripple will still be low.....about 1 mV.
Well, if the money is already spent, all is not lost as you will still likely see some linearity benefit. And, you can compensate somewhat by ensuring your heat sink is adequate, or better still, over-sized. Inserting a .47 (ish) ohm resistor at the Source of the MOSFET alters the output somewhat but is do-able if needed and I wouldn't lose a lot of sleep over it.
It's a rather extreme solution to eliminate the cap (which adds very little distortion in the first place) but extreme solutions to minuscule problems is what we do, right?
It's a rather extreme solution to eliminate the cap (which adds very little distortion in the first place) but extreme solutions to minuscule problems is what we do, right?