Optimizing output transformer for high impedance headphones

[Natural Sound]

I own 3 generations of Bottlehead S.E.X. amps and have been enjoying what each has to offer for the last 16 years or so. I also own a slightly modified Crack/Speedball amp. I’ve always preferred the Crack with my 300 ohm Sennheiser headphones for some reason. I’m guessing it has something to do with a better impedance match.

My question is in regards to the recommendation discrepancy between the S.E.X. 2.1 and 3.0 manuals.

2.1
“32 Ohm tap. This configuration is recommended for dedicated headphone use with all headphones of 32 ohms or greater impedance.”

3.0
“Using the 4Ω configuration with headphones of any sensitivity or any impedance will result in the lowest possible noise floor and the highest possible sound quality.”

It sounds like you learned something along the way. But to me the logic seems backwards. Of course I believe you guys. You probably have better ears than I do. But I was wondering if you could explain this phenomenon to me. The only thing I can think of is that the primary impedance of the OT-2 is somehow affected and matches the power output triode in a more desirable manner.

 

[Paul Birkeland]

It sounds like you learned something along the way.

We did indeed!  Most 32 ohm headphones are incredibly sensitive and wiring the output transformers to 32 ohms leads to a very high noise floor that is completely undesirable with these HPs.

But to me the logic seems backwards.

How the OTs are wired is a balance of available power, output impedance, and noise floor.  If you drive 32 ohm headphones with the S3X amp wired for 4 ohms, you are increasing the damping, decreasing the noise, and decreasing the available power.  For most 32 ohm headphones, power is not so much of an issue.

 

[Natural Sound]

That makes sense. Thanks Paul.

 

[adydula]

Paul,

After building the Crack and Mainline, and wanting to build something I am thinking of the SEX amp.
I will be running 420 ohm Audio Technica dynamic headphones with 100 dB/mW sensitivity.
Also a HD 650 at 300 ohms
Also Final Audio 8000 Pros at 60 ohms @ 98dB/mW.

Wondering how to wire the transformer for this set of headphones?

Also read about an impedance switching arrangement??

Thanks for you help!
Alex

 

[Paul Birkeland]

I would start with the amp wired for 4 ohms and see how you like it!