You may have seen my other thread, on modding my amp, but I think I've come round to starting over as I should be able to find the dosh to do something sensible.
I'll just preface this section by mentioning that while I would really like to buy and own a torpedo, the amount postage now costs between the US and France, it's pretty much impossible to justify.
Parafeed is a topology that really appeals to me, due mainly to it's simplicity in exchange to what it provides. This sums up my audio philosophy (not that I don't like a bit of complexity and originality here and there :))
The classic parafeed headphone amp on a budget is of course Dsavitsk's L'espressivo which evolved into the Torpedo, however Mr Millett's ECC99 parafeed amp has also caught my eye: http://www.pmillett.com/ecc99_srpp_headphone_amp.htm
However the transformers are the difficult part of such a design particularily for headphones (I can't even get myself some cheap specos to try as neither they or equivalents seem to exist over here in Europe). P Millett used a very nice pair of Sowters, which I'm sure sound lovely but also come in at £180 for the pair, which is more than I'd like to spend on the total project...
I've read a fair amount of info on diyaudio about using mains toroids as parafeed transformers, and wondered what your feelings were on this. (In particular there is a guy named Shoog on diyaudio who's used toroids for about 10 different amps, some headphones and some not, all to very good effect).
I happen to have a spare Talema mains toroid hanging around (well it's part of my valve amp supply). It's a 230V:2*12V toroid. I hooked it up to my CKKIII as I couldn't connect it to valves as they need it to power them... I hooked it up so that it was taking the sound out of the amp on the primary and outputting into both channels of my DT880s effectively loading the secondary with 125R nominal (I had the secondaries hooked up in series for this process). SO what load was this putting on the amp?
If my reasoning is correct, then with an ideal transformer the amp would have seen a load of:
(230^2)*125/(24^2)=11480R with a step down ration of 9.6.
I then listened to a couple of tracks, and it was certainly listenable but the top end seemed bloated and the bottom end lacks (as well as being unsurprisingly fairly quiet; the CKKIII can output 7.07VRMS before clipping giving us a max output of 4.35mA for both channels, so about 2.17mA per channel assuming an ideal transformer.)
To get an idea of what I was actually hearing I fired up RMAA and took a few measurements:
I RMAAd with a 110R resistor across the transformer secondary and as the channels were still bridged obviously the crosstalk figure is meaningless. I also couldn't quite get to 0dBs, but it wasn't too far and it was able to take a reading.
For reference here is an RMAA of my CKKIII with a 220R load per channel (taken on another day):
This transformer certainly didn't play nicely at all, but I don't know if the tiny Zo of the CKKIII plays properly with the transformer or not. This is where I'm out of my depth.
Do any of you have experience with mains toroids as parafeed OPTs, and if so, how was said experience. And does anyone have any advice for this project while considering that my main cans are 250R but I'll also be getting some Grados fairly soon... can't have this design being too easy now, can we?
Valve wise I'll almost certainly be going with 12AU7s as I have some already with one triode as preamp feeding into a parallel pair driving the OPT.
Any advice on this subject would be very welcome as I'm out of my depth when it comes to using transformers for things other than power supplies!
Edit: I forgot to mention that beyond the THD coming from using the transformer, the other thing that worried me was the ramp up in higher frequencies, but I don't know if this is me abusing the transfrormer by putting it on my transistor amp or just how the frequency response of this sort of transformer is...
Edited by Goobley - 6/12/13 at 3:25pm