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Old 3rd September 2005, 05:40 PM   #1
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Default MOSFET current source as triode load

Hello,

I have a Mcintosh clone tube preamp which I am going to optimise.
At the moment it sounds fine (thanks to some help from ppl on old threads) although i have only had it running into a Hitachi HA-3 power amp but i know i can improve its crosstalk and lower its distortion.

The source of the crosstalk is the tube regulated supply which can allowa 0.2V pk-pk sine wave onto the tubes HT at signal inputs of 1Khz which gives a poor crosstalk of -50.46dB at 5V pk-pk out.
I have been looking at ways to improve PSRR of anode followers and it seems that a good way is to replace the anode resistor with a current source, MOSFET's is easy to work with due to their very low gate current.

My questions are:
is this the correct way of using a Mosfet as a current source?

do i set the Drain current using the voltage accross the 1K resistor?

do i apply the same voltage to the gate from my resistor divider as i wish to obtain at the source?

Thanks a lot
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Old 3rd September 2005, 05:44 PM   #2
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Huh. -50dB is pretty significant, as I recall. Sure you ain't been smokin'?

Tim
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Old 3rd September 2005, 05:49 PM   #3
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Tim, yea i know its bad but i am sure the 200mV supply ripple is responsible for it, this figure improves with increasing frequency. It is -64.4dB at 20Khz, I was hoping that adding CCS to each gain stage would help it.
Maybe one to the output buffer too.

Craig
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Old 3rd September 2005, 05:54 PM   #4
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Well, a MOSFET current source will absolutely help the power supply rejection. But while you're at it, don't let your grounding off the hook until you're sure it's optimal. That's often a major source of crosstalk.

Gary Pimm has a very nice-looking IRF820-based CCS on his website. I'd start with his design.
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Old 3rd September 2005, 06:05 PM   #5
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Hi SY

Yes i have been careful with my grounds after i experienced AC hum on power up months ago.

I used star grounding and measure 0.01-0.016 ohms to ground from every point. so i should get very minimal noise from my grounds this time

p.s. in my diagram i have put no cap on my potential divider, i will put a 15uf cap there / / with 1uf or so, this will take care of my PS noise on the gate.

Craig
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Old 3rd September 2005, 06:11 PM   #6
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It will be much easier to build CCS with the Depletion type of MOSFET. There is one already designed for this application - IXYS10M45S from IXYS Co. Digi-Key sell it.
Check this site FYI:
http://www.pacifier.com/~gpimm/ccs_performance.htm
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Old 3rd September 2005, 06:42 PM   #7
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StevenOH,
Thats a good idea, they can be used like an N channel Jfet current source, is that correct?.

Any suggestions as to which one would be suitable
VDS>300V, ID>1mA
?

Thanks
Craig
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Old 3rd September 2005, 07:04 PM   #8
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Yes, you are right, because both are the depletion type and need just one resistor to set the current. The one I have mentioned is good up to 450 V, 300 mA, 40 W.
http://www.ixys.com/98704.pdf
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Old 3rd September 2005, 07:42 PM   #9
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Hello,

In another thread I've raised a question on a ccs and no one seemes to respond yet. As I notice this thread shows competence on the function of a ccs I'd like to invite the contribuants to comment also on my question in the next thread:

Power Supply Design >Shunt-regulated 24V DC Power Supply for Preamp Modules

Don't want to abuse this thread, merely trying to stimulate curiosity.

Thanks for the bandwidth, Arjen.
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Old 3rd September 2005, 08:36 PM   #10
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Thanks StevenOH

I will definatly use a depletion Nmos device now for the simplicity.
I cant find the device you recommend on my electronics source www.rswww.com, but im sure there are loads of other suitable parts i can find.

Cheers
Craig
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