Differential Amplfiers - current sources under/over the triodes

[SY]

You can use DN2540 depletion mode MOS to do a very simple self-biased CCS with outstanding performance; these devices weren't easily available when "Valve Amplifiers" was written, but I suspect they'll be included in the next edition.

Here's a figure from my upcoming (whenever they actually get around to publishing it) AX article showing how it's done. R5 sets the current, R6 and R7 are gate-stoppers to prevent oscillation. R5 will probably be 200-750R, R6 and R7 are non-critical, 300R-1k will work, but the resistor body should be as close to the gate pin as possible.

 

[exeric]

Hi Sy,
Thanks for the info and to everyone else also. I'm wondering how to adapt MOS devices specifically to MJ idea on page 136.

 

[exeric]

Hi Exeric

In my view that is not a current mirror, MJ is only using the same reference string for both MJE350. If you change one of the 16k resistors for lower/higher value you will more/less current in that leg, therefore imbalance. They will not track currents as a current mirror does.

All threads I could find referring to the theme
http://www.diyaudio.com/forums/showthread.php?s=&threadid=111099
http://www.diyaudio.com/forums/showthread.php?s=&threadid=115756&highlight=

I could not find the thread I referred to earlier, but both threads above may help you a lot as well

Erik

I read those posts. It seems there's nothing new under the sun so after reading them I have a much better idea of how to go about it. Thanks to wavebourne also for the help.

 

[SpreadSpectrum]

Wavebourn,

I am going to build your circuit and try it out. I've played around with it a little bit in LTSpice and it seems that a FET on top is the only way to go. Can you suggest a part for the JFET?

Thanks,

Heath

 

[Wavebourn]

Wavebourn,

I am going to build your circuit and try it out. I've played around with it a little bit in LTSpice and it seems that a FET on top is the only way to go.

It depends on how big dynamic resistance you need.

Can you suggest a part for the JFET?

I used 2N5115 in mic pres.

 

[316a]

Considering a differential stage in a push-pull amp consisting of two triodes, is there anything problematic about using current sources as plate loads? I realize that there is huge gain by using one current sink under the cathodes, but I'm running out of room for a negative voltage supply, so I figured this is about the best I can get. I plan to implement something close to Pete Millet's generic driver schematic, but have a regulated supply for each stage with a current source on all the plates (250v regulated for the first stage, 350 for the second).

This setup will be driving basically the equivalent of four EL84's in DPP.

TIA,
-Paul

Hello ,
I've done a diff amp with 7N7 driving 2A3 . The loads on the 7N7 have a cascode tail set for slightly over 12mA . The anode loads use a single DN2540 CCS set for 6mA , in parallel with this is a 2.2M resistor . The parallel resistor prevents 'fighting' , it's crude but stable and sounds rather good . There is also the option to drive off the lower impedence 'mu' output from the DN2540 source pin , although I've not tried this yet .

cheers

316a

 

[ErikdeBest]

Hi Wavebourn

Thanks for the explanation, I got it, I think. When time allows I will try something out and eventually come back with more questions!

Erik

 

[Hearinspace]

It depends on how big dynamic resistance you need.
I used 2N5115 in mic pres.

Just out of interest, Why is the 2N5115 almost $100? (at Mouser anyway) Is it a special part?

Thanks

 

[Wavebourn]

Well; I don't believe it happened because I mentioned it back in 2008.

 

[Hearinspace]

Wasn't there a line used to describe Chaos Theory that went something like "When a butterfly flaps it's wings in New York it leads to a typhoon in China." ? Maybe you've started more than you imagine . . . . . . . .

 

[Wavebourn]

Well; I would not object if somebody in China flaps wings, so my business benefit from it somehow a little bit...

 

[SpreadSpectrum]

Just out of interest, Why is the 2N5115 almost $100? (at Mouser anyway) Is it a special part?

Thanks

Because it is obsolete and supply is now limited.

 

[Hearinspace]

Because it is obsolete and supply is now limited.

I had that thought but I figured that parts are usually superseded by new ones that fit into the same role but with better performance.

Well; I would not object if somebody in China flaps wings, so my business benefit from it somehow a little bit...

Well, the only help I can offer is to extol the virtues of your SVCS design. I haven't heard a better SS plate load (yet ). I'm hoping this weekend to build my first differential amp and I've already built a couple of SVCS to load the driver plates which will be DC coupled to the output tube grids. . . . . . I've been wondering if an SVCS style circuit could also be made to suit the LTP of the output tubes. I think about this because I haven't heard a transistor DC current source I really liked yet - Of course some are easily better than others but something about the sound eventually makes me seek a change. After ten years of trying different circuits and fine tunings I realized the source of the irritation was the current source itself the first time I heard the SVCS.

 

[SpreadSpectrum]

Sometimes there are designs that need that particular part for whatever reason and people are willing to pay that much to make a few more. Either that or they are too dumb to find another part that will work. Anyway, the parts suppliers will gouge if they can get away with it.

 

[Hearinspace]

I hear you ! 'Saw WBT solder at $316 for a 500gm. roll a couple of days ago. I'm trying Johnson now.