Selecting OPT for headphone amp

[itsikhefez]

Hey all,
I recently built the Mainline (which was my first tube amp) and it got me hooked into the tubes!
I built a 300B Elekit and that furthered the addiction.. the sound is heavenly with some of my headphones.

I'm looking to build a single-ended 45 amp. It's going to be based on the Tubelab SE PCB.
What I'm having trouble with is selecting the OPT secondaries.
2 of my headphones are 300ohm so ideally optimized for that.
I would like to have a LoZ and HiZ option for flexibility.

I've asked alot in the diyAudio threads but majority build the amp for speakers and have less experience with headphones.... so I'm asking the experts.

I am thinking of ordering Electra-print OPT's and was recommended to go with single secondary, as one with 8 and 300 (for example) will be dis-banded due to the highly different values.

Standard OPT's for 45's come in 5K pri and 8ohm secondary.

One suggestion was that I can use that and put a ~10ohm resistor in parallel to the load, which will present an almost ideal 5K. (with 300ohm headphones).
Another suggestion was to add another 8:300ohm transformer for the highZ output.

So to summarize... I am a bit lost on which to order.
Should I go with a single 8 and put a resistor to match impedances?
Or the 2 transformer (per channel) approach?

If the former, which is ideal value? Would it be 8, 32, 120, 300??

Thanks for sharing your expertise

[Paul Birkeland]

How much power do your 300 ohm headphones require?  The 300 ohm tap is poor recommendation, it will be too noisy for any headphones I have ever used, and when you come to the same conclusion and just use the 8 ohm tap, you'll leave a lot of unused copper on the secondary (not great for HF performance).

Our Iron Upgrade Kit is very suitable for a #45 headphone amp.  I would leave it set for 8 Ohms and if you have really sensitive headphones, you can switch in a resistive voltage divider to pad down the output. 

[itsikhefez]

Thank you for the response Paul.

>you'll leave a lot of unused copper on the secondary (not great for HF performance).
Not sure I understood this.
I was recommended not to got with a dual 8 and 300 on the same transformer because of bandwidth issues.
The recommendation for a 300ohm tap was to add an additional, smaller 8 to 300 transformer.
Will the 300ohm tap always be noisy, even if it were a single secondary?

My HP's are HD650 and ZMF Aeolus, neither are too power hungry.
The HD650 max rating is 500mW.
The 45 tube outputs 1.5W and would prefer that as much is possible is available, if I ever try an Audeze for example.

If using a single 8ohm tap, putting a 12ohm resistor across the secondary would present a load of 5.4K-7.2K for heapdhones ranging 32-300ohm.
This is close to the ideal 5K load for the 45 tube, so good in terms of impedance matching (I think?)

What I don't understand is how do I calculate how much power loss is due to adding the dummy load and how much will remain to drive the headphones.

> you can switch in a resistive voltage divider to pad down the output.
Can you elaborate a bit more on this?

[Paul Birkeland]

I suspect 100mW would be plenty for either headphone.  1500mW will be way too much and the price you'll pay for having all that extra headroom will be noise and too much gain.  If you have a 5K:16 transformer and you use a 300 ohm headphone on an amp that would otherwise make 2W, you'll get the 100mW you're looking for.  I don't believe that the loading resistors will be necessary for you.

I personally would not recommend using two transformers in the manner you're describing. 

An LCD-3 on that same 5k:16 setup will give you 300mW, which is well over the recommended power for them.  Even if you had a 300 ohm secondary tapped at 8 ohms, neither is a good choice for the LCD-3.

The 5K:300 transformer would be useful in an amp that looked a lot different than what you're suggesting, but it's completely unsuitable for your proposal.

[itsikhefez]

Thanks for your time in helping with this. I think I can take it from here