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roline 16th March 2013 03:15 AM

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I've done several other versions of headphone amps and wanted to do a simpler version. In summary it is a differential tube input driving a Mosfet follower output section in class A with the output pull down current derived from the tube heater current. One could argue that the current is non-linear which would be true for large voltage swings. For the minimal output voltage swing it is not a concern.
The output is feedback to the neg input of the diff pair. The heater current of the 12AU7 is around 160mA and if you use a 12BH7 it is raised to about 300mA. With a 24V wallwart PS, there is 12 to 18 volts across the tubes for a little more class A swing, and the tubes enjoy a little more potential.
I completed stuffing it a couple nights ago and have been enjoying it every free minute. It can drive 24 ohm cans with ease and black hole quiet with no input. ( almost DC on the heaters with a little audio ripple)
Only 2 adjustments, current through the diff pair, set at 6mA, 3 each tube section and an adjustment for adjusting the heater voltage on the tubes, 12.7 volts.
It turned out to be a simple toner transfer PCB, easy to stuff and adjust. Very pleased by the dynamic range, lower end should be about -.36db at 20 hz with 24 ohm cans. I had 3,900u electo's and .47u mylar caps to use on the output coupling.
Its design is a combination from the days at National Semi in the linear group and work done by Millet on his starving student Head Phone Amp. For those purists who whine about why a hybrid, try it and then make your comments! A lot of the components can be had at the local Rat-Shack, you'll have to scrounge a few components, but they are easy to find.

ps: Last pic is a Compactron Mosfet Hybrid amp with tone section, increased complexity especially in power supply.

kasey197 16th March 2013 07:21 AM

Congratulations ! Whatta beaut :)
Looks very professional - nice pcb layout too .... Loading the output MOSFET with the heaters is a nice touch I think .... Cheers!

blackdod 17th March 2013 07:43 PM

Very nice!!


kasey197 18th March 2013 02:05 AM

what were the transistors and mosfets you chose ? Cant see them on the schem or maybe i'm blind as a bat :)

roline 18th March 2013 04:47 AM

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You can use any small signal NPN and PNP's like 2N3904, or 2N2222 and 2N3906 or 2907. At low current and low voltage levels most will work, just pick a decent Beta. I did no matching with them to try to duplicate what everyone else could do. The PNP mirror would be the most important to match if one were to take the time. Emitter resistors could be used to desensitize mismatch, say 100 ohms. The MosFet is the IRF510, 5amp and 100 V with about a 3volt Vgs. Any N-Mosfet with low Vgs should work, again low current and low voltage. I wanted to use cheap readily available product.
I've been happy with Baldwin branded 12AU7's, not sure who really made them.
Tubes, sockets, heatsink and PCB should be the only thing needed out side of a rat shack visit, They do not have the same pots, but they do cary some. A darlington NPN can be used for the output, but they are prone to be unstable, may require a little cap to tame them if used. I chose components out of what I had on hand and a trip to the shack....

ZigaR 24th March 2013 12:49 PM

Hi, would you be so kind to provide your PCB layout? I would love to build one, and this would be a real time saver :)

Also, do you think that this kind of an amp would be useful for critical listening (recording/mixing) or is it a bit too colored, being a tube/fet animal?

Antoinel 24th March 2013 01:11 PM

roline. Great design. I am also interested in such hybrids. Please visit the Everything Else Forum, and find the Thread which is entitled "Other Application of Vacuum Tubes". I show therein other hybrids with bjt and ICs which drive a full range 8 Ohm loudspeaker.

roline 25th March 2013 03:30 AM

1 Attachment(s)
JPG of the PCB attached....
I use ground stitching to minimize ground loop potential and use insulated wire on any non-ground jumpers. I'm using a 24V wall-wart power supply so no power supply on the PCB except for 1k voltage dividers across the power supply caps. This creates the V/2 ground used as signal ground and the ground plane. Another option is to add 12V zeners on PCB across the caps for a more solid reference, or go external PS.
The indicator LED's are bright on first fire-up as the heaters are still at their low impedance. as they warm up the LED's almost go out. With the 12BH7 tubes they stay on with the sense resistor chosen.
I cannot say that this amp has "tube coloration", it is pleasant to listen to without any fatigue, you just don't want to put it down.
If others choose to try it, be sure to add your experience to the link. The design concept is open to modification to tone section etc.... I just chose this as a simple low cost concept project. From my searching I found Millet's Starving Student Hybrid that is similar in double using the heaters in the signal path.

kenpeter 25th March 2013 04:11 AM

I would guess 20V on the heater string until V1B's plate begins to conduct?
Just eyeballin' the schematic, not doing any valid actual math or anything...

Does that 100K cold pull up also want for a 220K cold pull down?
Once V1A side of mirror sees current, cold bias resistors won't matter.
DC point set by the loop takes over as both plate currents increase...

kenpeter 25th March 2013 04:58 AM

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I havn't fully thought this through. Just noodling.

Elevated heaters.
Filament bias same, warm or cold.
Little extra (sandy 4x) open loop gain.
Greatly increased output swing.

Don't take these claims too seriously.
Its all very half-baked, maybe less.

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