• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Depletion mode mosfet cascoded constant current source.

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Can anyone tell me if this would work.

Copied if from Gary Pimm and drew the schematic as described...but want some extra verification


Cheers,
Bas
 

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Should work

No problems here but do you really need such a high impedence current source ? A single mosfet should do the trick in most cases . Check the Supertex datasheet to calculate the value of 'R' , just like reading tube curves , simple as that . Advise fitting a multiturn pot , will make adjustment far easier

316a
 
where are you getting them from? The few places I've contacted in the US are out of stock

Hi Brett,

Got them from Mouser!

If you don't want 50 of them..I'll sell you a few for $2 a piece. That way I can recover my shipping costs :)

Kevin from K&K audio will also sell them for $3 a piece. And Paul Barker in the UK has also got a few to sell.

Cheers,
Bas
 
316a wrote:
No problems here but do you really need such a high impedence current source ? A single mosfet should do the trick in most cases .

From my point of view it is not the low frequency impedance number that should be looked at. It is the shunt capacitance that is most audable and un-desireable.

You can use the formula 1/2pi*F*R using the impedance number at 10K for "R" to calculate the shunt capacitance.

battery is the wrong way round !

Nope. The idea is to increase the voltage across the current set resistor. The higher the voltage across the current set resistor, the less that thermal drift and noise from the mosfet effect the performance of the circuit. Also improves the local feedback. You can see this in the graph by compairing the DN2540 cascode (purple) to the DN2540 cascode 1 +6 (yellow).

Gary
 
I don't see how this will work properly unless the top MOSFET is a depletion mode device, BUT I don't have data for that device. For a MOSFET, you need some VGS for drain current to flow.

mlloyd1

Bas Horneman said:
Can anyone tell me if this would work.

Copied if from Gary Pimm and drew the schematic as described...but want some extra verification


Cheers,
Bas
 
Hi Frank,

first question:
Not using this particular ccs yet. Still design phase. Dunno yet where or when I will use it. But a guy like me had better do his homework
The first time I used the depleted mosfet...single was as a constant current sink in my differential amp

second question:
No idea.
..it is supposed to sound better than a plate resistor or plate choke.

Cheers,
Bas

Don't forget I am new to diy audio...got to explore..
 
If you are gonna abuse a battery, I don't see the point of limiting
yourself to depletion mode devices. The little 12v used for alarm
fobs seems a convenient package for that sort of duty.

IGBTs seem to have the highest Gm for the least capacitance.
You might investigate some of the latest devices. Check IXGT
6N170A, cheap but might be unobtainium in hobby quantities.
Do sneak a peek at the spec sheet anyways...

If you can live with a mere 600V equivalent part, obtaining a
handful of sand to smokeout is a lot less challenging.

Nelson Pass once built a very successful IGBT amp back when
the one and only complimentary P-Channel IGBT ever offered
was briefly available.

Like sweep tubes, they are good for more than one thing.
Don't let a switching characterization limit your thinking.
 
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