DHT Headphone amp for HifiMan Edition XS

[merlin el mago]

I'm looking for ideas/suggestions to make a diy DHT Headphone amp for HifiMan Edition XS.

Specs
Frequency response : 8Hz-50kHz
Impedance : 18Ω
Sensitivity : 92dB

Salas advice: "Needs custom opt ratio and 1W audio power at least", I guess after the output transformer.

To make a budget proto I want to use not too expensive tubes so russian 6C4C/6S4S (2A3 equivalent) or 4P1L or similars

TIA
Felipe

 

[euro21]

Hmmm... 1W ... a lot of headroom...

https://www.headphonesty.com/headphone-power-calculator/
92 dB, 18R, 110dB loudness ... 63mW

 

[merlin el mago]

Could be OPT 1W?

 

[euro21]

If you want to use OPT, some calculating necessary.

Assume, that expected power is 100mW.
On 18R this power requiring 1.34V RMS voltage.
If you have 5k:8 -small- transformer, on the primary this is 33.5V RMS signal.
If the input is 2V RMS, needed 16.75 gain.

The load on secondary is 11k25, so tube output impedance must be lesser, than one third-one quarter of this, 3-4k.

So at least 17x gain, 3-4k output impedance tube requiring.

 

[merlin el mago]

Thanks Bela, any suggested schematic to start?

Attached Ale's schematic using 4P1L + 6C4C

I have the Rod Coleman regs. & the Ale's hybrid mu-follower for 4P1L.

 

[martyh]

10K/32 transformer with 6C4C will work as long as it will take the standing current. Making the amp quiet enough for headphones will be the hard part.

 

[merlin el mago]

What about to use output transformer with secundary tap of 16 ohms?

 

[euro21]

If you have small 5k:8 (at least 30-40mA capable) OPT, even one stage (Spud) is enough for 70-100mW.
5687, ECC99, 6N6P is appropriate (at little different gain).
Sample:


If the transformer is gapless (no DC parafeed), gyrator loaded tube via 30-60uF capacitor can drive it parafeed mode.

 

[martyh]

What about to use output transformer with secundary tap of 16 ohms?

Should work fine. I try to use transformers at or below their rated primary Z but 16 to 18 is close enough to not really matter.

 

[merlin el mago]

If you have small 5k:8 (at least 30-40mA capable) OPT, even one stage (Spud) is enough for 70-100mW.
5687, ECC99, 6N6P is appropriate (at little different gain).
Sample:
View attachment 1277781

If the transformer is gapless (no DC parafeed), gyrator loaded tube via 30-60uF capacitor can drive it parafeed mode.

I'm looking DHT, now I am using 12B4 SE SPUD with Ale's hybrid mu-follower.

 

[euro21]

If you want to use direct heated tube for one stage spud, you will be in trouble.
The majority of them hasn't enough gain ... which has (for example 841, EML20/30), is rare or very expensive or practically unusable (high voltage, high output impedance==high impedance OPT) for this purpose.

BTW are you calculating values as I wrote in #4 post?
"So at least 17x gain, 3-4k output impedance tube requiring."

 

[merlin el mago]

Yes I know DHT can be SPUD thanks Bela, that's the reason I posted Ale's schematic with two stages of amplification, if I understood correctly I have to look for a DHT with a Rp of 3-4K, right?

 

[merlin el mago]

Ale's Moglia aka Bartola info about 4P1L:

"Some thoughts

If we have a tranformer gapped between 5 and 10mA then we can bias the 4P1L at 100-113V. The anode resistance will be between 2K9 and 3K7 approximately. This is even less than half of what the 26 anode resistance is. In fact, the 4P1L in this mode has higher anode current and power capacity as well as three times of the transconductance. All in all, is a real winner on paper at least. Haven’t heard the 4P1L in this mode, but am sure it will sound really well after using 4P1L in pre-amps for many years.

I’d probably aim to run it at 8mA if possible to reduce anode resistance and operate the 4P1L out of the knee."

https://www.bartola.co.uk/valves/2014/08/04/4p1l-with-screen-as-anode/

 

[merlin el mago]

Looking for DHT with Rp 3-4K candidates:

Tube Rp Pmax
31 4K 375mW
211 4K 18W (hig voltage needed ~1.000V)

Near 3-4K
10Y 5K 1.6W
801A 4.5K 3.5W
842 2.4K 3W
12A 5K NA

 

[45]

You still need a 2-stage amplifier. The only way to make a spud amp is to use a step-up input transformer with ratio at least in the 1:3 to 1:4 range if the target is 2 Vrms input for 1W output. That might appear appealing but more often it is looking for troubles because the input transformer will load the source. So you need a beefy source that can drive low impedance + large capacitance. I have seen many attempts in this direction over time, more often with poor results. Someone might have been able to get good results but in the end it will never match a proper 2-stage amp in my opinion.

The 2-stage amp instead could be a DC coupled amp. For example 2P29L+4P1L without need of input transformer (or if you like it and want isolation it can be 1:1). The OPT for the 4P1L can be a small 5K/16R one.

 

[merlin el mago]

Thanks 45, I'm not looking for a SPUD amp so agree 2 stages headphone amp no problem, I'm a little be worried about 3rd harmonic distorsion 4P1L to use as finals.

 

[45]

I am not aware of any problem with 3H distortion of 4P1L. Don't take individual experiences as a rule. If you have a few you can select even better. For example, search back the 3x4P1L SET amp by Michi and will see the usual 2H dominant behaviour and lower THD overall compared to 99% of SET amps.

 

[merlin el mago]

Thanks for input 45, also I can't see schematics SE amps with only one 4P1L tube as finals....

 

[45]

That is because output power would be in the region of 1.5W max for class A1. Because 4P1L is cheap, parallel or triple SE is convenient for driving loudspeakers. For normal SE you need 5K/16R in your case. That would result in about 5.6K for 18R load. For 1W output power you have quite some flexibility on quiescent point of 4P1L and can get THD figure you like most....

 

[45]

https://edcorusa.com/products/cxse5...1&_fid=2d957ae06&_ss=c&variant=41131379654843

This built to order so you can specify 16R and 30mA working DC current for the 4P1L. I think 25-30mA for 4P1L is a good bet for low THD.