flipper wrote
> Discussion about the headphone interface on the 6AW8 PC
> Speaker amp
> got me to thinking so I whipped up a little OTL circuit,
> just for
> headphones, using the 6CY7 (essentially a 'half power'
> 6EM7)..
>
> Why? Well, when researching the series resistor thing I
> noticed a lot
> of the so called 'tube' headphone amps have solid state
> outputs and a
> lot of the all tube ones couldn't drive 32 ohm headphones
> very well..
> The 6CY7 should take care of both those issues, and with
> the IEC
> specified 120 ohm source impedance
>
> Schematic and pics of the breadboard (I only did one
> channel) are up
> on my site http://flipperhome.dyndns.org
>
> It's not really fleshed out more than needed to breadboard
> and see the
> circuit work, but that it did.
>
> There's plenty of room for tweaking it. Spice indicated
> excellent open
> loop performance, in case that's what floats your boat,
> but I wrapped
> 13dB around it anyway for the exercise. And there's room
> for a bit
> more gain since, for the sake of simplicity, I didn't
> bypass the gain
> triode Rk.
>
> I figured some might get a few laughs at my sloppy
> breadboard so I
> included a pic of it for your amusement.
Exactly the same as my breadboard, except you use screws
where I use solder.
I started a thread a while ago about valve headphone amps,
and I don't think any different now, except thanks for the
detail of the IEC standard (regulation, guideline...what are
these things and what are they for?).
For what it's worth, my current design aim is for an
open-loop output impedance of 5 ohms, with feedback to
reduce that to a couple of ohms at most.
At the time of that thread I was looking into the consequent
desire to limit power output for safety reasons, and
wondered how. I will experiment once I've built the amp,
which will be some time after I've found some money to pay
for the transformers.
I think you will find, if you look at decent high fidelity
headphone amp designs, and transformers intended for decent
valved examples, they achieve an impedance ratio of 1:10 or
lower. One problem of traditional valved designs is that
they are intended for phones of 70 ohms or more. 32 ohms is
increasingly the norm, it seems. I can't see how driving a
non-linear voltage-controlled device of 32 ohms impedance
with a 120+ ohm source is ever going to be a good idea.
Perhaps if a high output impedance is enforced, high
impedance phones may become common, but surely that would
entrench a compromise to the detriment of fidelity?
cheers, Ian
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