# Question  about Tube Headphone Amp using 12AU7



## Tommy Thong

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


----------



## KimLaroux

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.


----------



## Avro_Arrow

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


----------



## nikongod

Quote: 





> Originally Posted by *Tommy Thong* /img/forum/go_quote.gif
> 
> 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: 





avro_arrow said:


> 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.


----------



## KimLaroux

Quote: 





nikongod said:


> 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.


----------



## Avro_Arrow

How about something like this:
   
   

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


----------



## nikongod

Quote: 





kimlaroux said:


> 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.


 
   
  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.


----------



## dsavitsk

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.


----------



## KimLaroux

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?


----------



## dsavitsk

kimlaroux said:


> 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


----------



## KimLaroux

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.


----------



## Tommy Thong

Quote: 





dsavitsk said:


> 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 interesting
	

	
	
		
		

		
		
	


	



  Quote: 





avro_arrow said:


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


 
  and how?
	

	
	
		
		

		
		
	


	



   
  Quote: 





avro_arrow said:


> 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


----------



## Avro_Arrow

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.


----------



## Tommy Thong

is it like this?


----------



## Avro_Arrow

Ya, thats the right idea.


----------



## KimLaroux

So what's the difference between a buffer and an op-amp? Isn't a buffer an op-amp on unity-gain?
   
  What difference would it make to use a buffer vs an op-amp as the output stage?


----------



## Tommy Thong

kimlaroux said:


> So what's the difference between a buffer and an op-amp? Isn't a buffer an op-amp on unity-gain?
> 
> What difference would it make to use a buffer vs an op-amp as the output stage?




if you want to use buffer as output stage, you can get 250mA @output thru your headphone but you cant do buffer roller( unchange able),
if you use opamp ic, you just get smaller output but you can do opamp ic roller(just put socket) so you can listen to different opamp chip
:rolleyes:


----------



## wakibaki

An opamp can be a buffer, but a buffer may not be capable of being used as an opamp i.e. it may have fixed unity gain. See e.g BUF634.
   
  Although an opamp *may* be capable of sourcing as much current as a buffer, it's not always the case. Buffers often have a high bandwidth, whereas an opamp used at unity gain has a bandwidth less than its gain-bandwidth product. These are generalisations though. Not all opamps are unity-gain stable.
   
  w


----------



## Tommy Thong

I wiil try this schematic,  it's more simple than the original schematic, Any comment or advice are welcome


----------



## Avro_Arrow

I think you are going to have too much offset voltage from the BUF634
  without an op amp controlling it. At best maybe 30mV, at worst as much
  as 100mV.
   
  Can you wrap it in a loop with a OPA1641 or LME49990?
  You could use the CRD I listed earlier in place of the 4k7 resistor.
   
  The only question I have to ask is...if you are going to put an IC
  after the tube, what is the point of building a tube amp?
  Just through an op amp in front of BUF634 and be done...


----------



## Tommy Thong

Quote: 





avro_arrow said:


> I think you are going to have too much offset voltage from the BUF634
> without an op amp controlling it. At best maybe 30mV, at worst as much
> as 100mV.
> 
> ...


 
  the CRD, i ordered it today, do you mean like The Wire Headphone Amp? I have The Wire SE-SE withh setup Denon CDplayer - JOB DAC (by Goldmund) - The Wire Headphone Amp - Sennheiser HD650, sounds very pleasant but i want to listen Tube Hybrid amp using 12au7 (i love the Mullard 12AU7's sound) coz'  sometimes, i  listen Classics (most piano) and vocals.


----------



## Avro_Arrow

Yes, like "The Wire".
  The stock "Wire" build has a gain of one so
  it qualifies as a buffer.  That would definitely
  let the sound of the tube come through.


----------

