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woodrough 18th January 2013 06:28 PM

tube amp designs for low impedance headphones?
Hi, I'm enbarking on my next project and would like some opinions from people on decisions of OTL tube designs. Specifically with low impedance headphones such as these Grados I have that are at a 32ohm level.

The two designs I am toying between are:
An OTL Tube Headphone Amplifier | HeadWize
No-Compromise OTL Tube Headphone Amplifier | HeadWize

I really want to just do the no-compromise design but it doesn't really seem to address the low impedance phones thing too much. The other design talks about modifying the the output resistor to stabilize the amp with low impedance phones, but I have some questions on gain an current draw with the amps ability.

Also i am not entirely adversely to transformer output designs but I am a little skepticle about accidental unplugs and if that would damage the output transformer. Also linearity and base response.

Does anyone have tips on successful all-tube headphone amp designs for low impedance phones?

cotdt 18th January 2013 06:57 PM

An OTL design would not be a "no-compromise" design for low impedance phones. Need transformer.

For reference, I have Denon D2000 (25 ohms) and have owned Grados (32 ohms).

You would use an 8 ohm speaker transformer. Everything would be the same as a speaker tube amp, except the power supply needs to be extremely quiet. You need DC heater supply for DHTs, though AC is fine for IDHTs.

No compromise and easy to build? Try the Tubelab SE 45 with the transformers in a separate enclosure. I can confirm this works really well. I've used the Salas High Voltage Shunt Regulator with it as well, but it is already quiet stock.

woodrough 18th January 2013 07:35 PM

No way... Even with the fact that the output transformer is at 8ohms, it drives 32ohm phone drivers?

I also want to keep it strictly tube driven, I don't really want to use current regulators etc.

My example would be this:
Hi-end headphone amplifier

So with this design, I just need to source a plain old 8ohm output transformer?

cotdt 18th January 2013 07:44 PM

Yes, an 8 ohm output transformer. Use the lowest impedance you can get away with, that still provides plenty of voltage swing. Headphones are so efficient that if you use a 32 ohm output transformer, most tubes will have too much gain. I have even driven 600 ohm Sennheiser headphones and there was enough gain (out of a 300B amp).

I have made the amp that you referenced, with both the 5842 and D3a. It is a good amp. I used the 16 ohm tap out of my transformers to drive headphones. Note that the transformers referenced on his site is for 600 ohm headphones. You want 16 ohm secondaries for this design. My transformers costed $600 by themselves.

woodrough 18th January 2013 10:10 PM

Fascinating! Learn something new everyday. So when shopping for output transformers, 8 ohm secondary of course, but what about primary impedance? Lower the better? Primary inductance?
What about wattage, any correlation of size / safety margin to freq response? What do I need to look out for to obtain the sweet spot?

Also, like full on speakers, if something happens that cause the headphones to get unplugged and the secondaries see no load while under power, is it true it would damage the transformer?

cotdt 18th January 2013 10:43 PM

It is possible to damage the transformers with no load, because it can arc, but I tested this before and in the real world for audio use it is fine. To be safe, you would always have the headphones connected. Short circuit is fine, too. So it's not easy to damage the transformer.

Primary impedance would be 5k or 7k for 5842/D3a. You want 16 or 32 ohm secondaries if building this amp. The 8 ohm can be used for designs that can put out more power, but here we are talking about using driver tubes as power tubes. The transformers should be able to handle at least 30mA, which any SE transformer can do.

woodrough 19th January 2013 12:17 AM

ooh man, this is interesting info for sure! Is there a nice way to transition the transformer to a dummy load via switch as a mode for swapping of headphones? Or does everyone just power down the whole system every time something goofy like that needs to happen. I may have remembered wrong but I thought that no load on a transformers output saturates the core? Or is arcing a result of just that.

So how would I derive the 5k or 7k by the tube data sheet? That may be an elementary question but I didnt see any values on the data sheet that would relate to primary impedance ratings. (I saw plate resistance, but thats different)

I am perusing through Edcor's site for output transformers and im finding them all to have a screen grid tap. 40% or so. If a transformer has one, (although the schematic doesn't define using a screen tap) will utilizing it in this specific diagram improve linearity? Or will it just mess with me. and if yes, the #2 grid gets hooked up to that right?

Sorry if this sounds like amateur hour, I just don't want to mix up concepts with full sized speaker drivers with tiny headphone drivers. This is tremendous help!

cotdt 19th January 2013 01:49 AM

A general rule of thumb for tube amps is that you want a damping factor of 3 to 5. It affects things like frequency response, and lowers distortion as it is easier for tubes to drive higher impedance. Less damping factor will sound looser in the bass, while more will sound tighter (SS amps have very high damping factors). The 5842 has an output impedance of around 1.7k. So we multiply that by 3-5x to get 5.1k-8.5k. My choice would be 5k:16 ohm or 7k:32 ohm, whichever is easier to obtain. I would recommend the Hashimoto H-507S that I use myself at 5k:16 setting.

On transformers with a screen grid, you will just not connect it. It is for pentode tubes to behave like something in between triode mode and pentode mode.

woodrough 19th January 2013 03:18 AM

Ah! that brings a lot of sense to the table.
So I want to deal with Edcor for best quality bang for buck. I want to use their CXSE line to hit the 20 to 20k freq response. Also at the same time, I am interested in using the D3A tube upgrade specified on that site. On the data sheet, it is specified that the plate resistance is 1.9k. Edcor goes up to 16ohms secondary so if I multiply 1.9k x 3 I get 5.7k. However in the 16ohm catagory, the primary impedance is only 5k.

My question is: would having 700ohms difference below be an audible degradation of the bass? This would mean looser base, but is it within a tolerable/permissible range?

Or am I better off working with a 5842 tube where the dampening resistance is closer matched appropriately with Edcors products.

What have you found in sound between the 5842 vs D3A? With these factors, which rout is more pleasing?

cotdt 19th January 2013 03:25 AM

Sounds like a plan. It will make a superb headphone amp.

You get an additional damping factor of 2x using 16 ohm primary to drive 32 ohm, so I wouldn't worry about it. 5842 and D3a sounds very similar. Go for D3a since it has more gain. The Russian 6c45p is another good alternative.

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