Preamp to Headphone transformers?

In short, I have an excellent preamp (ARC Ref 6). Rated voltage is 2V RMS (balanced) to a 200 ohm load with a max of 70V RMS. This is just way too much for headphones, but it wasn’t designed for them.

First, the output voltage rating is based on a load of 200,000 ohms ("200K"), not 200 ohms.

Second, the 70 volt number is irrelevant, as the output voltage will be a function of the gain of the preamp, which is not particularly high; the volume control setting of the preamp; the output voltage rating of the source component; and of course the volume of the source material at any given time.

And while the combination of those factors may or may not result in too much voltage into the headphones, depending on the particular headphones, the much more significant problem is that the output impedance of the preamp (nominally 300 ohms single-ended and 600 ohms balanced, and probably significantly higher than that at deep bass frequencies) is much too high to drive almost all headphones with decent sonic results.

What you would need to accomplish what you are asking is some sort of active buffer stage that would provide a very high input impedance and a very low output impedance. Which might be doable, but it would probably be simpler and better to just buy a headphone amp and connect it to the tape outputs/record outputs of the preamp, as Clio proposed.

If you decide to do that, while I don’t know what the unspecified output impedance of the preamp’s tape outputs is, to minimize the possibility of impedance incompatibilities I would suggest choosing a headphone amp having an input impedance of at least 47,000 ohms (47K).

Regards,
-- Al

Thank you. I missed the k on the output impedance!

So that’s what been confusing me...my sennheisers are supposedly relatively high impedance at a whole 300ohms, but that’s not much compared to what the ref 6 is rated for.  I like the idea of the buffer stage that you discussed, because then I could just use the preamp to change volume. 

Hopefully something like that exists?

Huh. Found something interesting...it’s a high impedance to low impedance transformer but designed for older radios to output to headphones. If nothing exists, I might be able to adapt it pretty easily to make a homemade rig.

Regarding the buffer stage possibility, you might want to contact a gentleman named Tom Tutay, of Transition Audio Design in Florida. I don’t think he has a website, but his contact info can probably be found via a Google search.

I recall that he has custom made active buffer stages for some others here at modest cost (hundreds of dollars, not thousands), which provided excellent results in high quality systems. Although those cases involved impedance issues resulting from some preamps (including ARC models) having to drive power amps and subwoofers simultaneously. So I don’t know whether or not the output impedances of the buffer stages he can supply would be low enough to be suitable for use with headphones, even high impedance models such as your Sennheisers.

Also, keep in mind that the output stage of the Ref 6 would be loaded by the parallel combination of the input impedances of the buffer stage and your active speakers, which will be significantly lower than the input impedance of either of them. And it will also be loaded by the sum of the capacitances of the cables to both destinations. So depending on the particular numbers that are involved connection of a buffer stage could impact the sonics you hear from your speakers.

Regarding the transformer possibility, first, headphones that were used with older radios typically had much higher impedances than modern audiophile-oriented headphones, e.g. 2000 ohms, and also required considerably higher voltages. Also, if the turns ratio of a transformer is chosen such that it would step up the impedance of the phones to a value that is a suitable load for the preamp, in combination with the input impedance of the active speakers, it may step down the output voltage provided by the preamp to too low a level. I see that the Sennheiser requires 1 volt to produce an SPL of 103 db, for example, which is relatively low sensitivity for a modern pair of headphones. And if the Ether Flow C has a much lower impedance while having greater sensitivity, like many other headphones, the higher turns ratio (and correspondingly greater voltage step-down) that would be needed to provide impedance compatibility between the preamp and the phones would probably leave you in the same boat, despite the greater sensitivity and the need for less voltage.

And all of this is not to mention that the bandwidth and sonic quality of a transformer intended for use with older radios would probably be severely deficient for use in a modern audiophile-oriented system.

As I said, it would probably be simpler and better to just buy a headphone amp.

Regards,
-- Al

Thank you very much for the info. 

Yeah, the minimum impedance on the old radio/headphones transformers was 600-20k, which is massive...and as you noted, the output voltage might be too weak. 

I’ll contact Mr. Tutay.  Much appreciated!