Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Low-cost Parafeed ideas/help please!
New Posts  All Forums:Forum Nav:

Low-cost Parafeed ideas/help please!

post #1 of 16
Thread Starter 

Hi all,

 

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.

https://www.dropbox.com/s/pcheyv5wyh2lwf9/Ckwtrans.pdf

 

For reference here is an RMAA of my CKKIII with a 220R load per channel (taken on another day):

https://www.dropbox.com/s/auo88tts0iib60u/ckalone.pdf

 

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!

 

Cheers,

Chris 

 

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
post #2 of 16

For Grado, there are the Hammond 119da which can be found in Europe at around 20€ a piece. But they're not really suited to high impedance headphones. Another option would be to use those in a Mapletree Ear+ kind of amplifier (see two versions: http://hollowstate.netfirms.com/Super2manual.pdf and http://hollowstate.netfirms.com/mapletreebranches1.pdf ).

 

I've had a look on these for high impedance cans: http://www.don-audio.com/Edcor-xSM15K-600 but dcr of the secondary seems high.

 

As an equivalent to the speco there might be the monacor tr-1005, which can be found quite cheap: http://www.ebay.fr/itm/TR-1005-TRANSFORMER-100V-LINE-10W-/200791644729?pt=UK_Sound_Vision_Other&hash=item2ec01d5a39  But I didn't tested it those.

 

As for toroidals, their suitability as output transformers will vary a lot... it can get a bit expensive to buy a few just to test which are the ones that work.

post #3 of 16

Wait for dsavitsk biggrin.gif

 

In the meantime you can have a look there : http://www.head-fi.org/t/625157/lespressivo-tube-amp I understand we have basically the same goal ...

post #4 of 16
Thread Starter 

Thanks for both your replies, I'm amazed that anyone made it all the way through my post!

 

Those mapletree amps are particularily interesting, I'm currently having a look at an LTP driving an EL84 CF like they use. 

Thanks for making this project appear doable :)

 

Cheers!

 

Of course, I'm always open to more ideas...

post #5 of 16
From your RMAA, something is clipping -- either the circuit, or the soundcard, or you are saturating the transformers.

For transformers, there is nothing magical (or really even good) about those Specos. Line matchers are a dime a dozen and should be available from any electronics supplier. For toroids, the Amvecos seem to work well.
post #6 of 16
Thread Starter 
I'm going to look into the transformers clipping but I won't be able to put it onto my scope for a good week as I have exams next week. Thanks for pointing that out though.

I'm currently looking at getting a 'proper' output tube to drive a transformer (either in CF like the mapletrees, or parallel with Ra). This is the transformer I'm looking at: http://www.don-audio.com/Edcor-XSM10K-600

Thanks for pointing me in the directin of their site. The prices aren't too bad.

My question is related to what valve I should attach it to. Current candidates are with the EL84 or the 6AS7/6080, any thought on either of these?

And does anyone see an issue with using a transformer like this for the HT? http://www.tme.eu/en/details/tmbz30_022m/transformers-with-fastening/indel/tmbz-30022m/#

Cheers!
post #7 of 16

You seem to be having fun with this whole learning exercise. Hopefully I can learn a few things too by following your progress.

 

popcorn.gif

post #8 of 16
I'd take a EL84 over a 6080 any day. 6080 is a garbage tube.

Keep in mind that you are stepping down the voltage by a factor of more than 9, so if the amp can output 7V, you are getting less than 0.8V of output. I'd wager that the amp is what is clipping.
post #9 of 16
Thread Starter 

I'd certainly agree with that, but for the price of those Edcors (I can get them here for about 50€) either in 0.5W or 2.5W variants, I think that they're probably less risky than toroids. I'm certainly quite keen on running triode strapped EL84s but I now have to work out how to drive them with the AU7s that I already own, as taking into account the transformer a single gain stage from an AU7 (mu=17) is not going to be sufficient so I'm thinking I may Cascode and then add variable NFB. I'll have to see what numbers the sims chuck out.

 

Cheers!

post #10 of 16

Quick question : are the edcors xsm10K-600 built to be used in SET amps with DC accros the primary ?

post #11 of 16
Thread Starter 

Nope, they're interstage and/or parafeed output transformers so they won't tolerate any DC in them

post #12 of 16
Quote:
Originally Posted by bidoux View Post

Quick question : are the edcors xsm10K-600 built to be used in SET amps with DC accros the primary ?

 

No. You'd have to get another serie. In the xse serie, you have a 8k-50 that would be fine for Grado and in the gxse serie, there's a 15k-600 that would be fine for high impedance headphones (and a 5k-600 too).

 

Those are quite cheap in the US; once you add shipping and customs...


Edited by 00940 - 6/15/13 at 2:03pm
post #13 of 16

They look good and would be perfect for the job but shipping only is twice or three times the price of a pair so it looks like a dead end ... I think I will be trying a pair of cheap toroids from Mouser.

post #14 of 16
Thread Starter 

I'm prepared to spend the money once to do a proper job, the question is whether the edcors are actually better than standard toroids... and I don't really have the experience to know. If anyone has experience of mains toroids versus other types of transformers I'd be keen to hear it.

post #15 of 16

@bidoux: if you're aiming for SET, you might try to ask on cinetson.org for example. There are some people in there who know quite a lot about what's available as output transformers in France.

New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Low-cost Parafeed ideas/help please!