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Question about Tube Headphone Amp using 12AU7

post #1 of 22
Thread Starter 

Hi,

 

I found this schematic Tube headphone  Amplifier using 1 pc  12AU7 

but  can i replace the mosfet IRF510 with BUF634 or diamond-buffer?

any comment or  suggestion are welcome

Tommy

post #2 of 22

I'm also interested in this. I would like to do something similar with my Starving Student some day. I'm just not impressed with the resolution of the MOSFET. Another option I've read that seemed interesting is a JFET output, but I've never seen a JFET circuit used to drive headphones.

post #3 of 22

You can put pretty much "anything" after the tube if you strip off the DC first...

post #4 of 22
Quote:

Originally Posted by Tommy Thong View Post

 

Can I replace the mosfet IRF510 with BUF634 or diamond-buffer?

any comment or  suggestion are welcome

Tommy

 

Yes.

I think you will find great inspiration looking at the original millet hybrid schematics. 

It should not be hard to make something along those lines with a 12au7 tube. 

12V is kinda weak. Use at least 24V or however much your buffers will allow (~30V for buf634) and sort out the heater supply with a fancy resistor. Moar volts moar betterer.

 

Quote:
Originally Posted by Avro_Arrow View Post

You can put pretty much "anything" after the tube if you strip off the DC first...

 

There is no need to strip the DC if it is within the limits of whatever you have after the tube.

 

There are certainly some advantages to doing this, but they come with added cost and complexity. 

post #5 of 22
Quote:
Originally Posted by nikongod View Post

 

Yes.

I think you will find great inspiration looking at the original millet hybrid schematics. 

It should not be hard to make something along those lines with a 12au7 tube. 

12V is kinda weak. Use at least 24V or however much your buffers will allow (~30V for buf634) and sort out the heater supply with a fancy resistor. Moar volts moar betterer.

 

What do you mean by "sort out the heater supply with a fancy resistor"? To use a resistor to drop the voltage to 13 volts for the heaters?

 

There are many hybrid designs out there, which uses different output stages. The original Millett Hybrid uses an Op-Amp as unity gain, powered on a single rail supply. Then the DIY community created the MAX millett hybrid, which replaced the Op-Amp with a Diamond buffer, with choices between BJT, JFET and MOSFET. The SOHA also used an Op-Amp but with a dual rail supply, which eliminated the need for an output capacitor. The SOHA II replaces this Op-Amp with a discrete single-ended buffer...

 

And that's just naming a few. I suppose any of these can be used with a 12AU7? The real question though is which is best, which is actually an upgrade from a single MOSFET... I read for hours on each of these designs, but I can't make up my mind as to which to try next. confused_face_2.gif

post #6 of 22

How about something like this:

 

 

I don"t have a link to the original source.

post #7 of 22
Quote:
Originally Posted by KimLaroux View Post

 

What do you mean by "sort out the heater supply with a fancy resistor"? To use a resistor to drop the voltage to 13 volts for the heaters?

 

There are many hybrid designs out there, which uses different output stages. The original Millett Hybrid uses an Op-Amp as unity gain, powered on a single rail supply. Then the DIY community created the MAX millett hybrid, which replaced the Op-Amp with a Diamond buffer, with choices between BJT, JFET and MOSFET. The SOHA also used an Op-Amp but with a dual rail supply, which eliminated the need for an output capacitor. The SOHA II replaces this Op-Amp with a discrete single-ended buffer...

 

And that's just naming a few. I suppose any of these can be used with a 12AU7? The real question though is which is best, which is actually an upgrade from a single MOSFET... I read for hours on each of these designs, but I can't make up my mind as to which to try next. confused_face_2.gif

 

Yepp, just a big fat resistor. Or I guess you could use a LM317 to regulate it or something. But there is no reason to shy away from power supply voltages that don't line up perfectly with your chosen tube heaters. 

 

The original Millet hybrid used a BUF634 buffer, not an op amp. 

 

With a few quick changes any of the output stages could be used with a 12au7. I guess the question becomes one of "how well does the 12au7 work with lowish on the plate"? IMO this is the strength of the 12ae6 (and its brothers designed to operate at similar voltage) in the classic millet - the tube is being used exactly how it was designed to be used. None of the "12V across a tube that really likes at least 70V" that we have with other hybrids. 

 

The MHSS does not suffer because of the mosfet, it suffers because the tube has only ~20V across it. Mosfets are pimp.

Totally tangentially to this thread, but have you tried to use a CCS on the tube in the MHSS? I wonder how much it would tighten things up & if you would like the change. IME CCS loading has a startling effect on other designs. 

post #8 of 22
I'd agree with everything Ari said. It is the tube, not the FET, that is the problem. Another tube to look at in addition to the 12AE6 is the 12U7, which is basically a low voltage version of the 12AU7. Just do a web search for space charge tubes.
post #9 of 22

My MSSH has a 30-15-0-15-30 transformer, and I use the TL783 to regulate the 48v for the whole amp. This allows me to experiment a lot with different voltages and power rails. One possibility is to raise the plate voltage. You say it's 20v, but isn't it 48 in the Starving Student?

 

I must admit I don't know much about tubes. I don't even know how they actually work. Anyone has documentation I should read first? If you guys say the tubes are actually the bottleneck in the MSSH, I'll study this part of the amplifier then. With my build, I can raise the voltage for the tubes to a few hundred volts if I wanted to. I can create different power rails for the tubes and the MOSFETs. Before I do that though I should learn how tubes actually work...

 

I did notice that some hybrid amplifier had CCS on the tubes, while the MSSH does not. Since I don't even know how tubes work, I could not rationalise this choice. I'm totally open to learn about it and try it out though! Where should I start?

post #10 of 22
Quote:
Originally Posted by KimLaroux View Post

Where should I start?

For the very basics on how a tube works: http://electriciantraining.tpub.com/14178/css/14178_13.htm

For a basic intro on how to incorporate one into a circuit: http://www.ecpaudio.com/pdf/parafeed_basics.pdf
post #11 of 22

Thanks! I am currently reading the Wikipedia pages about vacuum tube. I don't know why I haven't read these before... considering how many other useless pages I read there. 

 

Triodes are a lot simpler than I expected! I was always scared by the drawing of a tube in a circuit, but now that I know what each internal parts are, it's quite simple. I'll read those links next.

post #12 of 22
Thread Starter 
Quote:
Originally Posted by dsavitsk View Post


For the very basics on how a tube works: http://electriciantraining.tpub.com/14178/css/14178_13.htm
For a basic intro on how to incorporate one into a circuit: http://www.ecpaudio.com/pdf/parafeed_basics.pdf

Thank you for the link....it's very interestingrolleyes.gif

Quote:
Originally Posted by Avro_Arrow View Post

You can put pretty much "anything" after the tube if you strip off the DC first...

and how?confused.gif

 

Quote:
Originally Posted by Avro_Arrow View Post

How about something like this:

 

 

I don"t have a link to the original source.

 

i saw from this schematic, they put 330nF between tube and diamond-buffer, is it gonna strip-off the DC?

is it necessary to use fet LND150?

i will probably try this schematic. 

I'm  sorry..i'm just rookie @ tube's thing

post #13 of 22

Yes, the 330n blocks the DC from the tube.

The LND150, 1k and 360R form a current source.

You can use any JFet that will give the correct

current even if you have to adjust the resistor values.

You could use anything from a simple resistor

to a "ring of two" or LED current source as well.

 

Here is an easy substitute.

post #14 of 22
Thread Starter 

is it like this?

post #15 of 22

Ya, thats the right idea.

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