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Old 25th January 2013, 07:50 PM   #11
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I'm thinking 4P1L > 4P1L. The operating points are quite well established by now - see the 4P1L SE thread. They can both be used in filament bias - no cathode bypass caps. I'm also thinking transformer coupled stages - I use a Hammond 126C which works fine. Alternatively some kind of direct coupling, but hopefully without a cathode bypass/ultrapath cap.
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Old 25th January 2013, 07:55 PM   #12
regal is offline regal  United States
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How about using line transformers? They may have issues with the DC plate current, but aside from that I'd think they'd be about perfect.

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The problem is too much DCR, it affects the frequency response of headphones like Grados, Denond, AT's, modern AKG's, etc. Most line output transformers have a DCR over 100 ohms.

I think we should focus on the tubes first then hopefully Bud P or one of the other few remaining OPT experts can point us in the best direction.

For the output tube I think my vote is the 4P1LE, its cheap readily available, very linear, and capable of over 1W output. It also has a lot of gain which makes it easier to reach the overall gain we need with a driver like the #26 or another 4p1L.

The driver is a bit of a tough choice. Going off Cooters design he wa very adamit about DC coupling. If we use a #26 the plate voltage is low enough to DC couple and still use an SSHV2. The with DC coupling is the large bypass cap on the output.

Now the biggest question I have is something I have pondered since I first so that schematic is how to get rid of the bypass cap and DC couple. Negative supply for the input tube?

Or with a modern Gyrator Mufollower on the driver maybe we don't need to worry about cap coupling to the output stage? Then we could filament bias both the tubes.

One thing I learned when I built the SSHV SET 5842 spud headamp was that a big part of its magic was no big caps in the signal path. I have an LED bias and the SSHV, no input cap and no big electrolytic on the cathode, it makes a very clear absolute honest sound.
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Old 25th January 2013, 09:48 PM   #13
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Originally Posted by regal View Post
Most line output transformers have a DCR over 100 ohms.
And most speaker level OPTs have secondary DCR of a few ohms.
FWIW, I was almost able to get enough volume to drive my Sennheiser cans out of the triode strapped 4P1L SE into a 32:1 OPT. An 8R across the output in parallel with the cans. Seems to me with a nominal 2.5K:8 you'd be at about twice the output voltage already. How much gain would you need to get enough voltage for headphones?

As was said earlier, you mostly want a line amp with lowish output impedance.
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Old 26th January 2013, 05:14 AM   #14
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I know enough about transformers to know it largely emperical (meaing there are a few gurus with real tribal knowledge.)

So I agree with line level sized transformer just not the 600 ohm secondary, 5k:40 would put us right where we need with over all system gain of ~80 even lower gain with a solid 2Vrsm source.

The other option we have would be two 10 ohm secondaries where the output could be "balanced" for the orthos/senns, this would be 5k:40 balanced, and then 5k:10 for singe ended to use with the sensitive phones.

The fidelity of the transformer is better with smaller cores, I think the standard 5W core is too big. And the fidelity of the transformer is better with lower turns ratio because the coupling is better.

What we want is technically feasible its just finding someoe with the background to pull it off.
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Old 26th January 2013, 05:47 AM   #15
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Quote:
Originally Posted by Pano View Post
And most speaker level OPTs have secondary DCR of a few ohms.
FWIW, I was almost able to get enough volume to drive my Sennheiser cans out of the triode strapped 4P1L SE into a 32:1 OPT. An 8R across the output in parallel with the cans. Seems to me with a nominal 2.5K:8 you'd be at about twice the output voltage already. How much gain would you need to get enough voltage for headphones?

As was said earlier, you mostly want a line amp with lowish output impedance.
You don't need to load by 8 Ohms. Just solder there 36 Ohm resistor, or even 62 Ohm, it will be enough to protect transformer from blowing by flyback voltage, while distortions much lower than when loaded on 8 Ohm.
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Old 26th January 2013, 05:57 AM   #16
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You don't need to load by 8 Ohms. Just solder there 36 Ohm resistor, or even 62 Ohm, it will be enough to protect transformer from blowing by flyback voltage, while distortions much lower than when loaded on 8 Ohm.
Under stood, but we want to take advantage of a lower winding ratio. The lower the winding ratio the higher the fidelaty, this is because of the way the secondary is wound with the primary.

It also gives better power transfer, we can deliver much more power per gived plate swing with a 5k:32 ohm transformer than a 5k:8 transformer. This off course means less distortion.


I am also kicking around parafeed, then we could use a nickel permalloy OPT, what are the thought of parafeed vs single feed for this project ?
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Old 26th January 2013, 09:20 AM   #17
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One thing I learned when I built the SSHV SET 5842 spud headamp was that a big part of its magic was no big caps in the signal path. I have an LED bias and the SSHV, no input cap and no big electrolytic on the cathode, it makes a very clear absolute honest sound.
I experienced the same so completely agree with you: LED bias and the SSHV, no input cap and no big electrolytic on the cathode.
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Old 26th January 2013, 11:37 AM   #18
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Why the solid state HV? What's wrong with a nice 9 pin tube rectifier?
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Old 26th January 2013, 11:40 AM   #19
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Why the solid state HV? What's wrong with a nice 9 pin tube rectifier?
Why not 8 or 9 pin tube rectifier + Salas shunt HV regs? I use it and sounds superb...
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Old 26th January 2013, 12:28 PM   #20
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Why not 8 or 9 pin tube rectifier + Salas shunt HV regs? I use it and sounds superb...
Yes that is what I use on my spud, 2xmitsu 5AR4's feeding a CLC choke to the 2 SSHV's.

SS regulation is a must imho for a black background with phones that have specs that read like Efficiency @ 1mW (dB SPL) 32 ohms (Beyer T5's.)

Its just too costly to get ripple so low for phones like this without SS regulation, and the SSHV is very transparent/natural sounding. Comparing it to a maidia for expample.

I got out my quad of 4P1L's, boy they are odd looking tubes. Trying to find a western equivalent, spec wise I think they are sort of a cousin of the 306A/307A ?
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