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Old 23rd April 2017, 10:55 PM   #1
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Default OT for Hybrid Headphone Amp question.

I have been exploring various headphone amp circuits and I had an idea that since I havent run across it anywhere probably wont work, but what the heck, here goes..

In a tube gain stage - FET source follower output stage, assuming a 1 ohm Zout, could one use a 1 to 1 transformer in place of a big electrolytic output cap? My thought is that the 1 to 1 transformer will reflect back the headphones Zin, and regardless of what it is, it should be a good match for the follower. This assumes enough voltage gain and quiescent current to run a wide range of headphones.
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Old 23rd April 2017, 11:19 PM   #2
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I don't see why it wouldn't work. What do you hope to gain by doing it this way?
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Old 24th April 2017, 01:30 AM   #3
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Well it would allow you to use a presumably small and better sounding off the shelf transformer for a universal headphone amplifier, without having to have an expensive custom one wound with multiple secondary taps. Some people prefer the sound of OTs over electrolytics. I think it would be fun to try all three ways of coupling: big cap, OT, and DC servo.

I havent done the research into transformer inductance and OT frequency response yet, but given how low the power output needs of most headphones are, you might be able to use a small line transformer. Which actually leads me to another question. If you look at the frequency response plots of headphones here:

Headphone Measurements | InnerFidelity

You will see that the low frequency response of many, Im tempted say most, well regarded headphones starts rolling off significantly below 100 hz. Maybe you dont need a transformer that has enough inductance to give you flat response to 20hz and instead can optimize for mid and high frequencies.
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Old 24th April 2017, 04:30 AM   #4
PRR is offline PRR  United States
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> in place of a big electrolytic output cap?

You show a cap driving the transformer. So no difference.

10K:10K will be a poor fit for 32-600r phones. DCR will be 500r-1,000R per winding, absorbing the majority of your power. 600:600 would be a better bet.

The step-down is useful. Headphones need a few V at many mA. Your tube wants many V (24V-300V) but if you run many mA at high voltage you have a heater as well as an amplifier. Note that 600r:600"split" is a standard part that can be wired 600:150, which is a good improvement.

> assuming a 1 ohm Zout

That's surely the Small Signal Zout. In a "Power Amp" (even if milli-Power) you want to know the Large Signal Zout. Since the Source Follower is a Heavy NFB stage, small Zout is much lower than the limit for large signals. Since the MOSFET will probably go to few-Ohm, the limit is that Source resistor. If it is a typical 470 Ohms, pencil "470" as your large-signal Zout.

Resistance-coupled amplifier is at-most 6% efficient. If you find a transformer which will carry DC happy you can do 50% efficient (means less heat or less THD at the same power in/out). However I don't know of any. A 5W 5K:16 OT would work with high high power supply (300V).
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Old 24th April 2017, 06:21 AM   #5
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Before the triode warms up, it's plate will be at B+ (and so will the Gate of the MOSFET).
Unless the internal MOSFET protection diodes (if there are any) can take all the current from the triode plate load resistor, the MOSFET is gone.
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Old 24th April 2017, 09:22 AM   #6
DF96 is offline DF96  England
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A transformer is worth using when you need what a transformer can do which other components cannot do: wideband impedance transformation, common-mode isolation.

Using 1:1 transformer instead of (well, actually, as well as!) an output capacitor means swapping a cheap low distortion very wideband component for an expensive higher distortion narrower band component. This does not seem to be wise engineering, except for those who prefer the distortions which transformers introduce.
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Old 24th April 2017, 02:48 PM   #7
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Thanks for all your replies. Before addressing your various objections I just want to make sure that my original question ie that using a 1 to 1 OT after a fet follower with a 1 ohm output impedance will work from an impedance matching perspective at least, with any headphones from 12 - 600 ohms as I describe, ie the FET will see whatever the Z in of the headphones are, within reason.

@6A3sUMMER The diode clamps the voltage to 12 volts. This is a standard circuit without the OT, you can read a good description of it here at ESP or in Merlin Blencowe's great book "Designing High-Fidelity Tube Preamps"

@PRR >You show a cap driving the transformer. So no difference.

Im showing a cap because I was thinking about the possibility of using a line transformer, in which case it would a 10uf or less polypropylene. However thinking about it maybe Edcor would provide a 600:600 XSE or GXSE SE OT either of which can handle the 10 - 50ma of quiescent current which I think is all you would need in this situation. I will find out today.

>That's surely the Small Signal Zout. In a "Power Amp" (even if milli-Power) you want to know the Large Signal Zout. Since the Source Follower is a Heavy NFB stage, small Zout is much lower than the limit for large signals. Since the MOSFET will probably go to few-Ohm, the limit is that Source resistor. If it is a typical 470 Ohms, pencil "470" as your large-signal Zout.

Ah, I didn't know this and maybe this kills the idea. Is there some way to keep the Zout down across the range of likely signal strengths? Across a broad range of headphone impedances/sensitivity combinations, the power needed for 110db peaks is less than 25mW. What if use a CCS instead of a resistor?

@DF96 Probably not wise engineering, neither is using tubes, ; ). Fun to explore unusual possibilities.
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Old 25th April 2017, 03:34 AM   #8
PRR is offline PRR  United States
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> handle the 10 - 50ma of quiescent current

Ah, your image in post #1 shows no DC in the OT. So not sure what your plans are.

> @DF96 Probably not wise engineering, neither is using tubes, ; ).

In headphone driving it is not absurd to use a dump-truck to fill a flower-pot. So yeah: use the lowest efficiency topology and don't quantify device and load V and I, you can probably be happy without breaking the bank.
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Old 25th April 2017, 11:30 AM   #9
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>In headphone driving it is not absurd to use a dump-truck to fill a flower-pot. So yeah: use the lowest efficiency topology and don't quantify device and load V and I, you can probably be happy without breaking the bank.

Oh come on. I'm not showing a more complete circuit because i'm looking for help on whether or not the basic idea will work at all and whether or not one uses a gapped OT or cap coupled one is irrelevant to the basic question. I'm simply trying to learn more, experiment, and have some fun. Questions of efficiency at the level of headphone amplifier power are absurd on a forum where people help each other build high wattage Class A OTL tube power amplifiers and 1000 watt plus volts SE transmitting tube amps.

One of the things that make headphone amplifiers interesting to play with is that the power levels are so low you can explore all kinds of crazy stuff for the Sheer Hell of It, that would be completely impractical at loudspeaker levels. If this kind of thing interests you great, if not that's fine too.

As it stands based on the responses, it looks like what I want to do will work and the remaining issues I have to explore are:

What are my options for setting up a source follower output stage so that Zout stays close to 1 ohm at signal peaks?

What is the optimal impedance of the OT primary and secondary, given a range of 12 to 600 ohm headphones. PRR has suggested 600 ohms as a good place to start because these are readily available. Id like to know what the theoretical optimum might be in this application and how to calculate it.

What minimum inductance is needed in the OT for 3db down low frequency response to 20hz and given real world headphone response maybe closer to 100hz and how to calculate this? How low a response can I expect from off the shelf line transformers? Are any potentially usable?

For a voltage stage I now feel inclined to do something equally wasteful and eccentric. How about the Salas 6V6 linestage. As I recall it has a gain of around 12db which should be on the plenty side of reasonable and a wonderfully inappropriate use of a device.

If anyone has any positive suggestions or show stoppers (especially as in "Stop!, this ain't going to work"), related to all this I would appreciate them, otherwise I will post more after further research.

I think I will name this the Totally Inappropriate Least Efficient Topology Eccentric Headphone Amplifier. TILETEHP for short.
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Old 25th April 2017, 12:08 PM   #10
TheGimp is offline TheGimp  United States
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How about using a pair of these matching transformers:

EDCOR - PCW10K-7K/300-32

The circuit is called a Para-feed output, or Parallel Feed output.

The topology is sound and there are numerous implementations if you search for them.
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Last edited by TheGimp; 25th April 2017 at 12:11 PM.
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