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RH84SE, use as headphone amp ?

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Hi all !

I got a question;
I've got a new DAC (the super pro 707) and am now bypassing my onboard soundcard.
Now I'm not so sure of the soundquality, running my headphone directly out of the DAC.

The headphone I use is a Senn HD650, presenting an impendance of 300ohm

On my desk, there is a non-used RH84SE amp (build myself a year ago),
and I was wondering if I could use that amp as a headphone amp...

Do I need to rebuild it, or can I use some resistor L-pad on the 8ohm taps
to make it work ?

Please help me out!


I've seen Fisher receivers that have fixed resistors on the OT's to drive the headphones. I've been wanting to do this on one of my KT88 SET amps so I can have a headphone jack. I don't have any idea as to the value of the resistor, but the wattage is probably dependent upon the output power of the amp. I'd be interested in what more experienced builders have to say about this.

I've seen Fisher receivers that have fixed resistors on the OT's to drive the headphones.

Appears to be the same on my Pioneer ER420 receiver which I'm stripping at the moment. Removed 100 ohm resistors. Power output is about 15watts per channel.

There is a speaker on/off switch on my amp which appears to dump the channels into 16 ohm resistors. This would probably be required by your RH84 SE as well

Might be more fun to make a dedicated headphone amp. :2c:
That simple...

Goodmorning y'all !

Is it really that simple ?

It sure sounds the way to go, but last thing I want to do is wreck my HD650...

I found this on the internet:

HeadWize - Headphone FAQs

But I do not see how the amplifier will see a "normal" load of 8 ohms that way.

Will try the 10ohm resistor trick tonight,
I think I got some 10W resistors lying around...

thanks for your replies !!
Hi empee

Remember that almost all power produced by the amplifier will be 'wasted' in the 10 ohm load! A RH84SE will produce about 5W in an 8ohm load. That corresponds to an output voltage of around 6,3V RMS, as P = V^2 / R. Using the same formula it is easy to calculate that 6,3V RMS in the 300 ohms of the HD650 will result in a max power dissipation of about 0,113W. I can't find detailed specifications for the HD650, but I am quite sure they can handle it: probably your ears will suffer more than the cans :)

It works !!

Hi Erik,

It works !!
I only had a couple of 6.8ohm resistors
(50W mounted on heatsink, 12W unmounted)
But I figured most '8ohm' speakers are usually around 5ohms

so it works, but I now see (hear) I really need to re-work the powersupply.....


This might be the moment to decide to make a dedicated headphone amp.

Having said that,
I've got a pair of speakertransformers (originally meant for 100V speakers),
made by Philips, using the same core of the allmost legendary AD9058 output transformers.

Measuring them, I calculated that putting 300ohms on the secudairy,
I will get a reflected load of about 5.5K on the primairy.

What if I were to use these transformers in the EL84 schematic ?
Tubes in triode, to minimize output power ?

What's your verdict on this scheme ?




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100V transformers might not cope very well with DC across its primary, however if they are indeed 5.5K : 300R they must be very high wattage type so core is probably large enough to still stay in the linear region despite DC bias ? Try it out and let us know whether you're missing out on low tones (bass) or not.
Some 100V transformers I have are already built with same orientation (no spacer between both stacks though) and are welded together. "What welder hath joined together no man shall put asunder" :D I think that simplified the construction as interleaving requires more work.
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