[Tingc222]

Hi All!
 
I'm in the process of building a speaker terminal adapter for the HE-6 (~50-65Ohm, purely resistive). Here's what I've gathered so far.
 
This is for anticipated use with 25 to 75 W per channel speaker amps (with one tube amp at 75W/ch) with output impedance of approximately 0.5Ohm.
 
Per channel:
 
Source +  -------------- R1 ----------------------------------- HE-6 +
                                            |
                                            |
                                           R2
                                            |
                                            |
Source - ------------------------------------------------------ HE-6 -
 
 
This is based on L-pad design. Simple impedance match calculation revealed that an ~10 Ohm load can be achieved with R1 and R2 = 5 Ohm. I should also see some signal attenuation with this, likely on the order of a few db.
 
I went ahead and ordered 4 non-inductive audio grade resistors at 5Ohm, 10W each for this purpose.
 
I'd like to use this primarily to protect my tube amps which may not do well with a native ~65ohm load that the HE-6 provides.
 
Need I worry about anything? Did I overlook something?
 
Thanks!!

 

[Tingc222]

I'm sorry I totally posted this in the wrong forum section. Can Mods move this to DIY? Thank you!!

 

[nikongod]

These values work well
R1=0ohms.
R2=12ohms.

In the unlikely event you actually need attenuation start with 2&10ohms, then 4&8 if that is not enough.

 

[nikongod]

5&5 ohms may eat up too much gain. With smaller r1 and larger r2 more power goes to the headphone, which means the amp is putting out less power and hopefully running more linearly.

 

[Tingc222]

With smaller r1 and larger r2 more power goes to the headphone, which means the amp is putting out less power and hopefully running more linearly.

 
Thanks for your help nikongod. That's what I came to realize in my sleep last night
 
Interestingly, the HE-adapter is not built using the typical L-pad configuration from what I can tell. It uses a 10R in parallel but a 25R in series, coming AFTER the parallel connection for the 10R...Not sure how well that plays with the sound of the headphones. In my thinking, any resistors in series should be seriously scrutinized for and be of signal-carrying quality, and be of as low resistance as possible. It's an unfortunate trade-off with attenuation, however, which may be why Hifiman went with their less-than-ideal setup for the HE adapter:
 
http://www.head-fi.org/t/603433/lightbox/post/8268211/id/545984.
 
Also, resistors in an impedance matching/attenuating circuit should all be non-inducting, in order to prevent frequency-dependent changes in resistance.
 
I found this gem of a thing, for anyone else in my position. It sums up everything and gives a nice estimation of output impedance seen by the headphones too (kudos to robrob):
 
http://www.head-fi.org/a/headphone-resistor-network-calculator
 
Using an 8R in parallel and 4R in series gives an attenuation of -3.9dB, which might be ok. I'll start with just a 8R in parallel (24W total dissipation) and see where that takes me.

 

[Planar_head]

The attenuation network used by Hifiman allows the amp to work at its preferred load impedance.
 
If you do the math, the attenuation network with a 50 ohm headphone (i.e. HE-6) gives a load impedance of 8.8 ohms. The attenuation is -3.5 dB, neglecting amplifier loading.
Though, compared to just a parallel 10 ohm resistor, the load impedance doesn't change that much (it goes lower), though the attenuation probably decreases (that is, it gets less negative).
 
And to clear up any doubt, both resistors are important for "hi-fi" applications. The parallel connected resistor just has less influence on the phase and frequency response compared to the series connect resistor.
 

 

[Tingc222]

 
If you do the math, the attenuation network with a 50 ohm headphone (i.e. HE-6) gives a load impedance of 8.8 ohms. The attenuation is -3.5 dB, neglecting amplifier loading.
Though, compared to just a parallel 10 ohm resistor, the load impedance doesn't change that much (it goes lower), though the attenuation probably decreases (that is, it gets less negative).
 

 
Interesting...can you explain why the parallel resistor would not decrease load impedance?
 
From what I understand, a 50hm headphone and a 8-ohm parallel resistor should give a 7ohm total load impedance, hence matching it to the 8ohm sepaker out...

 

[Planar_head]

Oops, I neglected to say that I used the values that they used for the HE-adapter that you mentioned in your previous post. The simulation, on the other hand, was done with a typical L-pad network.