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In Reply to: RE: Single stage DHT headphone amp posted by hvbias on August 24, 2014 at 10:26:40
Your DAC chip basically has the voltage you need, so you need to find a tube and transformer that balance each other and act as a buffer. I built such a thing years ago with output autoformers. Ignore the stuff in there on damping factor, etc. as it ignores the copper losses, etc.
26 has a very high rp and very low current, so you really need a very high plate load, and a very high primary to secondary ratio. However, you'll lose your voltage in the process making it a poor choice.
71a, which is what I used, may work better, though it is still not quite ideal. rp is ~2K and mu is 3. The LL1930 in parafeed configuration has a voltage ratio of 5.8:1. Used that way, you get a ~10K plate impedance. With a stepup on the input (maybe a LL1674 doing 1:4), you'll be OK, probably even after copper losses.
LL1660 for single feed is 4:1 which gets you ~5K on the plate which is OK and reduces the gain necessary from an input stepup so the LL1676 may work OK. However, recordings that are not red-lined will still be low, so the 1:4 input is still probably prudent as the 71a's mu of 3 will almost entirely be eaten up by the copper.
Probably the right way to do this is with something like the Emission's Labs 20A or 30A.
Follow Ups:
I've considered using parafeed and dsavitsk's pages have been very helpful. I asked him about his 71a amp a while ago and he said with the Intact autoformers it would only be ideal for Grados.
I do like the idea of 5x 71a Rp load with the LL1930.
I would prefer to use old stock tubes (or have the option of old stock and new production), since it's an amplifier I will probably be using for a very long time, and I would be out of luck if Emission Labs went away in the future. Also potential reliability issues in the past has me vigilant about them.
Is the 71a fairly transparent?
I said that? OK. Probably a case of not quite enough voltage. (I am dsavitsk -- not sure why I have a different name here, but I think it had to do with a lost password or something.)
71a is fine, but nice examples that are both electrically and physically matched are hard to find. I am not one who thinks DHTs offer something unattainable in other technologies, however. In the end that amp got disassembled. If I were to do it again (and I may as I have all the parts sitting around to do so) I'd use an input buffer and a stepup transformer for gain (1674, 1544, etc.) to drive the tube. It always seemed like it needed a little bit more of a push than my source could give.
Sorry if I misconstrued your words or misunderstood Doug. It has been a long time since I last messaged you and I might have mentioned using a phono stage that only output ~1v with my cartridge. But I'm using only digital with headphones now. This is Deepak from that other headphone forum :-)There is one more tube I am going to look into; a DHP wired in triode, 4p1l and I see that Kevin has done some experimenting with it in a preamp, so maybe he will chime in. It also ticks off my other requirements like low filament requirement and a plate voltage that will not be too difficult to use with a CCS. I'm trying to figure out the Rp with the filaments starved. At minimum I want to use 3x Rp for the transformer primary, and ideally 5x.
I'm not dogmatic about only using DHTs for the sake of using them. But I did like the sound of the well engineered DHT headphone and speaker amps I've heard. I have a Bottlehead Crack with the CCS add on, and it's a nice sounding amp, but a bit too dry compared to the Balancing Act. And for my taste the Sennheisers need a little tonal color injected in them.
I suppose the only thing I don't want to do is build a wasteful high gain amp. Since the amp is for my personal use and it only needs to work with 2 headphones I can be specific with the design.
Edits: 08/25/14
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