[twodeko]

Howdy All,

Been a while since my last post but I have an idea that I would like to try and make here. I just recently bought a MacBook Pro and realized that it had an optical output within the headphone jack that I want to use for an amp. What I would like to try and make would be a semi-portable DAC with optical inputs, and a tube amplifier stage to drive my headphones (Grado SR225's). I have built a CMOY before and have a couple boards for the Alien USB DAC, but would be interested to either work off a previous project or start one myself.

Another question would be if I could/would want to drive my Yamaha stereo amplifier with a device like this, or could potentially have a switching circuit to drive a line out signal to my amplifier.

FYI - I am a third year Electrical Engineer focusing on device physics/analog design/integration of the two. My soldering experience includes the CMOY along with quite a few cables that were done for a DAW.

Thanks!

 

[dsavitsk]

Quote:

Originally Posted by twodeko /img/forum/go_quote.gif What I would like to try and make would be a semi-portable DAC with optical inputs, and a tube amplifier stage to drive my headphones (Grado SR225's).

For a tube to properly drive Grados, you will need some means of lowering the output impedance. A cathode follower won't do it, so you either need a transformer, or a solid state circuit of some sort, or a bunch of low impedance tubes in parallel (which you don't want to do).


Quote:

I have built a CMOY before

Keep in mind that tube amps worth building use high voltages which the cmoy has not prepared you for. Not hard, but it does require an extra level of care. You can also look into the various low voltage tube hybrids (Millett, soha, yaha, meha, etc.) but none will equal a true high voltage tube amp performance wise.

Quote:

Another question would be if I could/would want to drive my Yamaha stereo amplifier with a device like this, or could potentially have a switching circuit to drive a line out signal to my amplifier.

This is easier as a cathoide follower will work to drive the high input impedance of your next stage.

Quote:

FYI - I am a third year Electrical Engineer focusing on device physics/analog design/integration of the two.

I would suggest Morgan Jones' Valve Amplifier book, available at Amazon or BN. It should all be pretty straight forward

 

[twodeko]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif For a tube to properly drive Grados, you will need some means of lowering the output impedance. A cathode follower won't do it, so you either need a transformer, or a solid state circuit of some sort, or a bunch of low impedance tubes in parallel (which you don't want to do).

I will take a look into working with transformers and finding out what a cathode follower is, but I was curious with what you had mentioned about a solid state circuit. What would the typical output voltages of the tubes be? The only implementation that I have used for lowering output impedance with MOS transistors was to have a small current bias in series with an N/POS that had the signal as the gate voltage. You lose a little gain and need two since one inverts the signal, but it could maybe work. If you used an opamp without any gain, would that void the use of tubes?

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif Keep in mind that tube amps worth building use high voltages which the cmoy has not prepared you for. Not hard, but it does require an extra level of care. You can also look into the various low voltage tube hybrids (Millett, soha, yaha, meha, etc.) but none will equal a true high voltage tube amp performance wise.

My thoughts on this is that if I am going to spend money to build a nice amplifier for my headphones, why not give them the full force?

Always an option though.

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif This is easier as a cathoide follower will work to drive the high input impedance of your next stage. I would suggest Morgan Jones' Valve Amplifier book, available at Amazon or BN. It should all be pretty straight forward

I have to purchase quite a few other books on Amazon, so it can't hurt to throw another $40 on there and call it a day

Thanks for the help, but does this sound like something that has already been done and pieced together? My background in valve/tube amplifiers is very limited, but am willing to read up on it and have a listen.

 

[Uncle Erik]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif I would suggest Morgan Jones' Valve Amplifier book, available at Amazon or BN. It should all be pretty straight forward

Excellent book; I recommend it as well.

For a nice Grado amp, you might want to pick up a back issue of Vacuum Tube Valley #16. You can find VTV here:

http://www.vacuumtube.com/

In that issue, there are plans for the Dickman headphone amp. He specifically designed it to drive Grados. I built one and am quite happy with it. About $250 in parts and it's not terribly complicated. Check it out.

 

[PeterP]

Dsavitsk is right about using an output transformer to get a nice low output impedance and eliminate the need for a large output cap. At 32 ohms (Grado’s) you need 250uF just to get down to 20Hz and you should probably double that value at a minimum and IMO transformers sound better than large value caps.

If you haven’t done so yet, you should spend some time at:
http://headwize.com/projects/index.htm
http://www.audioasylum.com/forums/tubediy/bbs.html
and
http://www.audioasylum.com/forums/bottlehead/bbs.html
there’s lots of good info and advice to be had.
Also google Aikido Amp
FWIW I would suggest that you look for something simple using octal IDHT as a first project. 6SL7, 6SN7, 6BL7, 6BX7, even 6EM7 all can make great amps that shouldn’t be too hard to get up and running.

Should you decide to chase tube amps just remember to respect that high voltage, work on live equipment with just one hand, and always discharge the high voltage before touch anything with your hand.

And above all, HAVE FUN.

 

[dsavitsk]

Quote:

Originally Posted by PeterP /img/forum/go_quote.gif Dsavitsk is right about using an output transformer to get a nice low output impedance and eliminate the need for a large output cap. At 32 ohms (Grado’s) you need 250uF just to get down to 20Hz and you should probably double that value at a minimum and IMO transformers sound better than large value caps.

The cap does not set the output impedance. The cap sets the point where the frequency starts to roll off, assuming that the output impedance is low enough. The low output impedance ensures that there is a proper transfer of signal to the headphones. The best you can do with a cathode follower, even with something like a 6as7 and a huge cap, is still not an ideal way to drive headphones.

That said, output transformers tend to be very expensive. However, some you might try that are more affordable are the Edcors, the hammond 125ese, or there is a hammond that the Mappletree Ear uses that is something like a 600:8 ratio that people say works well with the grados'. You basically use it to couple a cathode follower to the phones.

Aonther very nice option is something like rjm's 3 channel "Szekeres" mosfet driver used after a tube stage. http://www.geocities.com/rjm003.geo/...o/diy_sze.html Very high quality for a lot less money than transformers.

 

[AndrewFischer]

Quote:

Originally Posted by PeterP /img/forum/go_quote.gif Also google Aikido Amp

http://www.tubecad.com/2004/blog0011.htm
http://www.tubecad.com/2005/January/blog0030.htm
http://www.tubecad.com/2006/09/blog0080.htm
http://www.tubecad.com/2006/09/18/Ai...%20A%20PCB.pdf

The Aikido looks like a great project for Hi-Z headphones. Building one is next on my list. Although 30 ohms is tough to drive with tubes, John Broskie does give component values for 30 ohm headphones. I'd get a pair of Sennheiser's rather then pay for two >300µF capacitors. HD-580s are around $130. That is less than decent output transformers or audiophile output caps.

If you get boards, be sure to use the Mono ones. The stereo board can't be configured as a headphone amplifier.

 

[PeterP]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif The cap does not set the output impedance. The cap sets the point where the frequency starts to roll off, assuming that the output impedance is low enough. The low output impedance ensures that there is a proper transfer of signal to the headphones. The best you can do with a cathode follower, even with something like a 6as7 and a huge cap, is still not an ideal way to drive headphones. That said, output transformers tend to be very expensive. However, some you might try that are more affordable are the Edcors, the hammond 125ese, or there is a hammond that the Mappletree Ear uses that is something like a 600:8 ratio that people say works well with the grados'. You basically use it to couple a cathode follower to the phones. Aonther very nice option is something like rjm's 3 channel "Szekeres" mosfet driver used after a tube stage. http://www.geocities.com/rjm003.geo/...o/diy_sze.html Very high quality for a lot less money than transformers.

As you state the output cap will, for the most part, set the low frequency performance. When using OTL (output transformer-less) designs, that cap will be quite large and good sounding, large value capacitors tend to be very much a personal taste (and some folks will say that they don't exist). This applies to all OTL designs using cathode followers, White followers, SRPP etc. Can they be made to work, sure and there are lots of examples out there. Just not my pref right now.
The output transformer is simply a way of achieving an impedance match from the amp to the headphones. And, I seem to simply prefer the sound of an output transformer. IMO a very nice compromise (and something that I could live with for a long time) would be the Aikido built as a Hi-Z headphone amp driving an output transformer. I prefer the sound of a small film cap and output transformer to the sound 470uF of Cerafine or BG caps. Plus you get the added safety of the headphones being isolated from the high voltage by both the output cap (probably a film type) AND the output transformer. Double isolation, from a safety stand point, is always a good thing

 

[twodeko]

Is the book of interest Building Valve Amplifiers or just Valve Amplifiers?

 

[dsavitsk]

Quote:

Originally Posted by twodeko /img/forum/go_quote.gif Is the book of interest Building Valve Amplifiers or just Valve Amplifiers?

I've not read it, but it has been recommended to me on several occasions by people that ought to know. It deals with the practical aspects of actually assembling a valve amp: part orientation, selection, etc. It will likely keep you from learning these things the hard way which can be quite expensive and dangerous.

Also, check out the tube section at diyaudio.com as there are a lot of very knowledgeable people (including the author of the afore mentioned books, under a pseudonym) who hang out there.