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Old 30th January 2013, 10:12 AM   #61
regal is offline regal  United States
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Quote:
Originally Posted by Rod Coleman View Post
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.
Yes but I think headphones sound good transformer coupled. Its not at all uncommon to spend near $1k on headphoes so why not get a good OPT to go with it?

That parafeed sowter with mu metal hybrid core looks very tempting. Its 2W at 50 hz and .5 w at 20 hz.


Running a 4P1L at 70V's is not "Fun" its a big compromise, not what this thread is about honestly.

Last edited by regal; 30th January 2013 at 10:23 AM.
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Old 30th January 2013, 10:28 AM   #62
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Transformer coupled DHT is a high-quality solution to the problem, there is no doubt.

I am trying to emphasise the point that it works ONLY if the component and power supply quality is high. Driving $K headphones with a cheap transformer, or skimped power - makes no sense at all.

For any DIYers without the cash to acquire transformers at the Sowter level, the solution is not to use a cheap alternative transformer - the solution is to use FET-assisted triodes, or some variation on it, IMO. The capability of this kind of architecture will surprise some DIYers, I am sure.
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Old 30th January 2013, 10:47 AM   #63
regal is offline regal  United States
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Quote:
Originally Posted by Rod Coleman View Post
Transformer coupled DHT is a high-quality solution to the problem, there is no doubt.

I am trying to emphasise the point that it works ONLY if the component and power supply quality is high. Driving $K headphones with a cheap transformer, or skimped power - makes no sense at all.

For any DIYers without the cash to acquire transformers at the Sowter level, the solution is not to use a cheap alternative transformer - the solution is to use FET-assisted triodes, or some variation on it, IMO. The capability of this kind of architecture will surprise some DIYers, I am sure.
I am open to the hybrid solution, but not at a 70V plate no way.

The question is what is a good quality output transformer? Of those mentioned is Sowter the only way to go ?
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Old 30th January 2013, 12:03 PM   #64
12B4A is offline 12B4A  United States
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Are these the design goals?
-Must provide 100-500mW power for low and high efficiency 600 ohm impedance range headphones
-Must provide 1.5 watts of power for low efficiency 32 ohm range headphones
-Must provide xx power for xxxx headphones acquired at a later time based on whim
-Must be transformer coupled DHT tube

The Sowter headphone transformer line seems to be the only pre-designed transformers to fit this bill if all signal line transformers are to be completely disregarded because of the DCR assumption.
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Old 30th January 2013, 12:07 PM   #65
regal is offline regal  United States
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Originally Posted by andyjevans View Post
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?
The signal is in the filament bias resistor loop, it doesn't dip to the 4.3K resistor which is just elevating the ground, just like biasing a DC heater with an indirect heater tube.
So the big cap is not in the signal path, just there as noise filter. This design is filament bias (no cathode bypass cap) with an elevated filament. It is means to have DC coupling without a cathode bypass cap. The #26 was chosen over the 4p1L for the first stage in order to keep the B+ lower so that the SSHV2 can be used.



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Originally Posted by ikoflexer View Post
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'!
Yes I know they are different but you reported silent background with a #26 preamp so that it good enough I think. I believe Rod's designs have been posted, the secret sauce is the high performance transistors ( I have a couple of his kits that I've been itching to try on an all DHT SET.)
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Old 30th January 2013, 12:43 PM   #66
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Quote:
Originally Posted by regal View Post
I am open to the hybrid solution, but not at a 70V plate no way.

The question is what is a good quality output transformer? Of those mentioned is Sowter the only way to go ?
The supply voltage in my design was carefully considered. Just like you, my first thought was that it won't work so low. But 70V is fine for a 4P1L cathode follower. Please look at the curves - the anode current is almost constant in this design (thanks to the FET), so you can get away with only 3.5-4V of bias, without problems. The 4P1L only operates in a tiny curve around the operating point, so high voltage is not needed at all.

This is completely different from the usual common-cathode circuit, where higher voltage will be needed.

Other trafo makers can match Sowter quality, but none of them are willing to make the customised trafo we need. Sowter's catalogue is full of curious designs - some of which no doubt stem from this kind of project.


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Last edited by Rod Coleman; 30th January 2013 at 12:56 PM.
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Old 30th January 2013, 01:18 PM   #67
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If not is to much expensive what about an OPT with several taps to fit all kind of headphones impedances?
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Old 30th January 2013, 01:37 PM   #68
iko is online now iko  Canada
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Quote:
Originally Posted by regal View Post
The #26 was chosen over the 4p1L for the first stage in order to keep the B+ lower so that the SSHV2 can be used.
I would be surprised if SSHV2 cannot do 260V.

On the topic of transformers. For 32 ohm headphones, is the Edcor 25W, 1250 to 8 ohm not good enough?

http://www.edcorusa.com/p/465/cxse25-8-1_25k

For 50 or 64 ohm headphones, how about the Edcor 25W, 1250 to 16 ohm?

http://www.edcorusa.com/p/467/cxse25-16-1_25k
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Old 30th January 2013, 03:48 PM   #69
TheGimp is offline TheGimp  United States
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Rod, Drop the input amplitude down to 1mV and increase the time of the capture. I think there is a 25Hz 0.75mV oscillation due to the feedback loop.

I don't know if sensitive headphones would pick this up or not.
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Old 30th January 2013, 03:59 PM   #70
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I'll take a look....

Is it some breakthrough from the power supply noise, that I included in the B+ supply (the Sine component)?
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