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Old 18th May 2009, 06:45 PM   #1
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Default Tube headphone amp designs?

Howdy folks,

I am looking for a simple design for a tube headphone amp for use with some Grado's. I am a bit overwhelmed by the designs I am finding online, does anyone know of a proven design that would be very DIYable with proven results? Say, something with an EL34? I would be down for a triode/pentode switch and definitely something without OT's. I would be willing to give up tube rec in the interest of cost (college student) Single ended would be ideal. If anyone has done something similar, I would be very interested. I also have a stash of 6L6's is I could do anything with those worth listening to...

Thanks!
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Old 18th May 2009, 06:52 PM   #2
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EL34's and 6L6's are absolutely too powerful for headphones.

I've had excellent results with the Bijou which can be gotten in kit or PCB board only form.

Headwize has a nice collection of projects.

Since headphones don't need a lot of juice, little preamp tubes are sufficient for providing power for the job. At least in most cases.
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Old 18th May 2009, 07:02 PM   #3
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Hi!

I assume your grado's have about 32 ohms impedance (like most Grado's I have seen...) This is a really low impedance for tubes, even 'power tubes' as the EL34.

You state that you don't want an output transformer, which I assume is due to the costs associated with such a device? One alternative would be to use a mains toroid... I am thinking about something along the lines of the headphone amp made by member dsavitsk...

http://www.ecp.cc/less-pressivo-plus-plus.html

One could substitute the autoformer at the output for a small toroidal (also in parafeed mode), something like 2x115V and 2x9V/ Put all windings in series, obtaining one 248V winding, tapped at 18V. Connect the Grado's across this winding, and the autoformer action will convert the 32 ohms impedance to a more tube friendly 5k (approximately). Load the tube with a CCS or resistor, select a nice coupling cap and a potentiometer at the input and it should sound nice.
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Old 18th May 2009, 07:03 PM   #4
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Sounds pretty good, I like the looks of really simple circuits. I am still on the search for a very pure circuit with minimal components. Could I essentially also just build one of the tube preamp circuits I am finding and just tie the output to my phones?
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Old 18th May 2009, 07:07 PM   #5
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Quote:
Originally posted by whitelabrat
EL34's and 6L6's are absolutely too powerful for headphones.
This is simply not true. They are capable of more power than you want, but you don't have to use it. They would not be my first choice in tubes, but they will work fine.

As for designs, to my ear, Grados sound best with a low Zout and high damping factor. There are basically four ways to accomplish this -- output transformers, a solid state output stage, brute force (lots of tubes in parallel), or negative feedback. That list is in decreasing order of preference.

Additionally, you likely don't need any gain. Instead, all you need to do is attenuate and provide current. For details on some measurements of this, see http://www.ecp.cc/less-pressivo-plus-plus.html
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Old 18th May 2009, 07:08 PM   #6
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ErikdeBest,

I would actually spend the money for an OT if it would make my circuit simpler, but I really like the looks of the amp in the link. I have built tube amps but have not gotten into CCS or more advances parafeed style designs yet. I would really like to optimize this circuit for the Grado's too, and yes, you are right about the impedance.
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Old 18th May 2009, 07:09 PM   #7
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dsavitsk ,

do you know of a good circuit using OT's? What tube would you use?
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Old 18th May 2009, 07:11 PM   #8
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Quote:
Originally posted by ambience exists
Sounds pretty good, I like the looks of really simple circuits. I am still on the search for a very pure circuit with minimal components. Could I essentially also just build one of the tube preamp circuits I am finding and just tie the output to my phones?
It depends on the design, but the answer is likely no.

For what it's worth, a basic mosfet source follower will be hard to beat for sound quality and it will cost a ton less than a tube amp with similar driving ability. Here's an example of how to do it: http://www.headwize.com/projects/szeke1_prj.htm

It won't light up like a tube, but maybe you could use Nelson Pass' trick and use a light bulb as the load.
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Old 18th May 2009, 07:16 PM   #9
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Quote:
Originally posted by ambience exists
dsavitsk ,

do you know of a good circuit using OT's? What tube would you use?

6H30, 6N6p, 5687, 12B4 could all be decent choices depending on power/gain options.

Here is one example of how to do it: http://www.ecp.cc/less-pressivo-build.html

As Erik says, some of the small power toroids can make really good and inexpensive parafeed transformers. For single feed, Edcor makes some good and cheap options that can work. For what it's worth, I am agnostic as to whether single feed or parafeed is a "better" technology -- both have advantages and disadvantages.
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Old 18th May 2009, 07:16 PM   #10
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Wow man,

That circuit looks real simple, I bet it could be built really pure. I guess sometimes I think about the high voltage and transformers and whatnot associated with tubes and just want to do a nice SS project. I have never done one before though, any advice?
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