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sizing up power output for diy headphone amp

post #1 of 7
Thread Starter 
Looking for help from diy tube amp designers.

Provided that the headphones can handle the power, what dB level should be considered a design maximum SPL?
(90 dB? 95 dB? 100 dB? other?)
post #2 of 7

I usually aim for about 110db. Not that I listen that loud, far from it. But since headphones require so little power, it doesn't hurt to make sure that even the worst transients won't clip. It also guarantee that the amp in normal operation is always cruising in its most linear operating area.

post #3 of 7
Here is my take on the issue.

Dimensioning a tube amp for headphones is problematic if a wide range of phones is to be accommodated.

Headphones range from 16 to 600 ohms and perhaps beyond. They have a wide range of efficiencies. You can discover these easily. An amplifier capable of 8vrms and 250mA should be capable of driving the large majority of phones and capable of destroying many.

Dimensioning a SS headphone amp is straightforward. It must deliver sufficient current, generally of more concern with low impedance phones. It must be capable of swinging sufficient voltage, more generally of concern with high impedance phones. Issues such as damping factor more or less take care of themselves, and both constraints can readily be met in the same design. The primary limitation on versatility of SS is the available voltage, which may be from batteries. The amplifier will not complain if the output is left O/C.

At one end of the strategies we can adopt with a tube amp, is to decide what maximum voltage swing our phones might require. Say +/-12V. Then we design a tube amp to deliver +/-12V into 8 ohms (9W) and operate the system into a dummy load, plugging in the phones in parallel.

Or we can take an off-the-shelf O/P transformer with a range of output impedances and attempt to design something around it. We can, of course, accept some limitations on the range of phones driven.

It's worth bearing in mind in this context that:-

1. Single-ended transformer-coupled class A tube amps will tolerate an O/C at the output (to the best of my knowledge).

2. Just because max. power out is at a given load, it doesn't mean that there's no power out at a higher load, and triode distortion decreases with load.

Or at the other end of the scale we can commission a custom transformer with whatever range of output impedances we think will accommodate our desired range of phones.

w
post #4 of 7
Thread Starter 

Thanks both for the responses.

 

I built a SE tube amp (kit) for speakers a couple of years ago, and now have a desire to go through the design process for a SE tube amp for headphones.  My headphones are the Sennheiser HD600's with 300 ohm nominal impedance.

 

I recently finished building a voltage divider circuit to be used with the tube amp.  As you noted, it provides a steady 8-ohm load to the amp, even with no headphones in the jack, and it attenuates the input to the headphones nicely.  Very effective, and actually retains nice sound quality.  But, I still want to go through the design process for a SE amp.

 

My goal is to design an amp with enough versatility to drive a fair variety of headphones in addition to my own, but understand that it may not be suitable for many others.

 

Of course, this project will be accomplished in my "spare time".  ;)

 

Any other relevant comments or tips are welcomed.  This is a learning adventure for me.

post #5 of 7
Ugh, I typed a lengthy answer on my phone and then it bugged. Short answer then.

Your life gets much easier with transformers with multiple secondaries. Have a look at Sowter headphones transform. They are not cheap but they will give you a good idea of the kind of specs you're after.

Most dynamic headphones are fairly sensitive. Have a look at drivers tubes. 32-300 secondaries and 250mw will cover a lot of headphones.
post #6 of 7
Quote:
Originally Posted by dave r View Post
 

Thanks both for the responses.

 

I built a SE tube amp (kit) for speakers a couple of years ago, and now have a desire to go through the design process for a SE tube amp for headphones.  My headphones are the Sennheiser HD600's with 300 ohm nominal impedance.

 

I recently finished building a voltage divider circuit to be used with the tube amp.  As you noted, it provides a steady 8-ohm load to the amp, even with no headphones in the jack, and it attenuates the input to the headphones nicely.  

 

 

Just use an 8-10ohm (however many watts) resistor in parallel with the headphones. 

 

By running the amp into a resistor divider you are running it at higher power output and increasing distortion. 

At least try both. 

 

For the new amp - Another vote for a transformer with multiple secondaries, or a single multi-tapped secondary. So nice. Lundahl also makes some multiple-secondary transformers that work nicely with headphones. 

 

You should design your amp to run your headphones 30-40dB louder than you listen. 

It is not hard to design any amp that will drive any headphones to krakatoa-at-actual-SPL or a good approximation of "The Bloop" but if you only listen at 65dB, who cares? 

 

By designing for 30-40dB overhead you simultaneously cover ANY differences in recording levels, and any unexpected peaks. Easy! 

With the amp set up this way you will never have to park the volume control in the ugly "low volume with poor balance" spot. 

post #7 of 7
Thread Starter 
I typically listen at around 70 - 75 dB, and it's probably about that with my HD600's. That gives me some chance of hearing the telephone, or somebody calling me (yes, dear?). Occasionally maybe 85 dB, so the 100-110 max spl sounds like a good target.

ONetics is available to me locally, so custom winding is definitely an option, but I'll look at Lundahl and Sowter. On the other hand, I might be able to scrounge up some existing iron from one of my audio accomplices.

I'll try the single 8-ohm in parallel for the headphones and see how that works. (my OPT has 4, 8, and 16 ohm taps, so I could try different values on those taps accordingly.)

Thanks again for the responses. Even ball park ideas help me get on the playing field. ;^)

Dave
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