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Old 29th January 2013, 11:59 AM   #51
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I don't follow your argument here - the filament supply is floating, so it's only earthed through the 4.3K resistor, as is the signal. Am I missing something?
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Old 29th January 2013, 12:23 PM   #52
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Well, it does actually work quite well with a 4P1L. The triode curves and Dmitry's model suggest around -3.5V bias allows operation as low as 70V, which should be fine. A little more class A power dissipation, but you have your DHT.

BTW, please don't confuse this circuit with the usual "tube hybrid" circuit. The kind where a triode stage has a source follower stuck on the output.
My circuit centres on a compound-device, with the FET acting as a current-amplifying slave for the cathode follower. The FET only pumps current, at high dynamic impedance - until the cathode follower is satisfied.

The (triode) cathode current varies by only a fraction of the load current, allowing the dynamic output impedance to remain almost constant throughout the swing. This is quite different from the usual follower circuits, where output Z has a strong dependency on load current. If you like LOW and CONSTANT output impedance, this circuit takes some beating.

The FET type: yes, it's a lateral - a derived version of the Hitachi 2SJ50 and successors.
Attached Images
File Type: jpg HeadFiCompound4P1L_1.jpg (103.1 KB, 440 views)
File Type: jpg HeadFiCompound4P1L_FFT.jpg (12.1 KB, 431 views)
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Old 29th January 2013, 12:34 PM   #53
iko is online now iko  Canada
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Quote:
Originally Posted by regal View Post
were not for you and iko sharing your filament power designs
Just want to clarify that what I posted earlier is NOT Rod's design. As far as I know his schematics are not in the open domain. My circuit uses a cap multiplier, not a gyrator, and a CCS. Just sayin'!
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Old 29th January 2013, 02:28 PM   #54
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Back to the headphone amp. I really don't see the reason to a load impedances in the speaker range. There are plenty of very good power amps of low power that will cover that range. I'd say that an amp that can do 50 ohm loads and 300 ohm loads will cover lots of headphones. I'm working on a DHT amp for a 50-60 ohm load because those are the headphones that I have, and I'll post my results here. With a different output transformer a 100, 200, or 300 ohm load would be just as easy to drive.
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Old 29th January 2013, 04:49 PM   #55
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Regarding playing headphones off different impedances. I would say the best is changing arrangements of multiple secondary windings: with three windings and a rotary switch one can do a lot as shown in the links (not authored by me).
Sowter Type 8650 Headphone Output transformer (for a probably nice OPT)
AÜ_Hinweise (for changing between impedances)
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Old 29th January 2013, 07:28 PM   #56
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Here's my draft, using the 4P1L, since it was the topic of discussion lately. I have a good stash of 4P1L and plan to do something with it. Simpler circuits appeal to me, hence my preference for this. It's easy enough to replace the gyrator and coupling cap with an IT.

In reality I will implement filament bias via the attached circuit which worked very very well in my 26 preamp. I developed this circuit independently but it's just been pointed to me that it'is almost identical to Rod's older version that he posted in this forum, so credit deservedly belongs to Rod Coleman. Almost identical does not mean it is the same. Small changes can make a big difference in performance. In simulation Rod's older circuit can bring a 1V ripple at the input 32dB down to 25mV. Small changes in the circuit make it bring the 1V input ripple 92dB down, to 25uV. I will not belabour the point, I'm sure that his new circuit is a huge improvement over the circuit he posted in the link below. All I wanted to do is to offer an alternative which, based on my experience, is excellent in performance and simple to implement.

New DHT heater While we're at it, I want to say that Rod's kit is really good value.

Edit: the thd results, for what it's worth, are at 1kHz, and about 2.2V p-p into the 50R load. Also, there might be better operating points for this tube, but I didn't look too hard.
Attached Images
File Type: png amp-hp-4p1l-4p1l-1a.PNG (32.6 KB, 381 views)
File Type: png amp-hp-4p1l-4p1l-1a-freqresp.PNG (19.3 KB, 371 views)
File Type: png amp-hp-4p1l-4p1l-1a-thd.PNG (29.9 KB, 338 views)
File Type: png psu-filament-4p1l-1a.PNG (32.8 KB, 124 views)
File Type: png psu-filament-4p1l-1a-freq.PNG (22.2 KB, 126 views)
File Type: png psu-filament-rodcoleman-1a.PNG (12.1 KB, 40 views)
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Old 29th January 2013, 07:41 PM   #57
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Old 29th January 2013, 09:35 PM   #58
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Quote:
Originally Posted by Rod Coleman View Post
A compound triode - bipolar stage, with a 2nd-order dc bias controller, to give zero dc offset, no output capacitor at all.
I like that idea. One could also be completely blasphemic and use something like an LME49600 headphone buffer amp on the output with a triode on the input to provide a gain of a few V/V.

~Tom
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Old 29th January 2013, 09:39 PM   #59
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I like that idea! One could use a DHT tube to drive the LME49600.
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Old 30th January 2013, 08:51 AM   #60
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Quote:
Originally Posted by tomchr View Post
I like that idea. One could also be completely blasphemic and use something like an LME49600 headphone buffer amp on the output with a triode on the input to provide a gain of a few V/V.

~Tom
It will be fun to try out some of these 'sand-assisted triode' ideas.

This is definitely the route to follow, unless you are prepared to spend the money on a properly-specified, AND high quality transformer (in this case, the Sowter) - and a no-compromise set of power supplies. DHTs need a high-quality environment to show their worth.

Using a DHT with a cheap transformer, or power supply cost-cutting - is certain to be a disappointment, that will easily be embarrassed by all-SS solutions.
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