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12AU7 Opulent Student amp ideas - Page 2

post #16 of 18
Thread Starter 

Cheers waki,


I've filled in the info like you suggested, had been meaning to get around to it. I've been following your thread for quite some time now and it was a large part of what convinced me to start trying to do my own design. I have been looking into a fully tube design too using dissimilar tubes and speaker output transformers (with a 10R resistor in parallel with the load à la Jonokuchi). Any thoughts on this? The main advantage to this for me is that it's very easy to power speakers to about 5W.


With regards to this amp I have been mulling over a few technical issues and getting ready to order some bits for a proof of concept prototype.


I'll keep this thread up to date,

Cheers :)

post #17 of 18

There's a live thread over on DiyAudio about a DHT SET headphone amp:- http://www.diyaudio.com/forums/tubes-valves/228625-all-dht-set-headphone-amp.html


The stated object in the DiyA thread is ~9vrms into 600R (AKG Sextet), 7vrms into 38R (Hifiman HE-500) and hum <-80dB @ 0.4vrms.


It's wandered all over the place without really producing a design yet, however. It's got an ideological bias too because they want to go with DHTs, which everybody is insisting is driven by the sound, but I think it's just another artificial (mad) constraint. I mean, if you want a tube amp, you want a tube amp, with a tube sound, otherwise build a solid-state one, it's a damn sight easier to meet the specs.


I can get 9vrms into 600R from the 6N6P no problem, I dunno about 7rms into 38R though, @ 7pk-pk THD is up around 8%, but I'd be happy to be driving most headphones, I don't need to cover absolutely all. One thing, 600R might cause problems with the OPT primary impedance getting very high. You could slug the O/P with a resistor, but it will constrain the voltage swing. That's when you start to think about more capable OP tubes.


I think dissimilar tubes is a good idea, I'd be thinking 6SN7 for the gain stage, they have a good reputation in terms of distortion and 6AS7 (6080) or even 6528 (expensive) for the O/P. For the moment I can get away with 1.5A for the heaters. If I went to 6AS7 O/P I'd have to rethink the whole transformer issue, not just the PT, which is not a big problem, but the OPT, since the tube has a radically lower Ra.You could get 5~6W with a 6528, I would think, if you went for a bit less you might still get away with one of the Hammond Universal series underdriven using the 32 ohm tap, or there's the 1627SEA, 30W, only 144 euros each. That'd start to be a real luxury item. 



post #18 of 18
Thread Starter 

It'd certainly start to be luxury with transformers like the expensive hammonds. There's just the small matter of me needing to keep my entire budget well under the price of two of those. I'm looking quite hard at the impedance matching route at the moment. With an EL84 output it looks fairly promising along with one of the cheaper OPTs from here http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrse


I see no real point in 'perfect' valve circuit like those DHT ones, as far as I'm concerned it defeats the object. I've built a CK2III and TBH I don't like it, as my Dad said when comparing it to my linear regulated dual PS MSSH, "There's just one that sounds HiFi". And it wasn't the technically better one on paper... Like yourself I'm not aiming at being able to power all 'phones, as I don't own all the different headphones that exist. I have a battered pair of SRH440s and my special DT880s. These are fairly standard anyway so just being able to power standard headphones (50-300R and no crazy orthos) would be more than sufficient and my main aim.


I was thinking early, have never heard of this done, but how odd would it be to have a  5k:8 traffo in normal positioning and then a parafeed style or similar 8:32 or 8:300 traffo providing a headphone output? I'd imagine these'd have to be custom wound, but it's not like they'd have any DC in them or anything and I'd guess could be done quite cheap.


Hmm pondering will ensure... but I really should sleep now and let my subconscious have a go for a bit.


Edited by Goobley - 2/9/13 at 4:31pm
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