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Old 24th March 2011, 08:05 PM   #1
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Default 6922 to headphones interface?

I'm building a great little line level preamp with 6922 tubes, and I thought it would be nice to add a headphone output. The trouble is, I don't want anything solidstate, or involving feedback. The output will be a 6922 cathode follower running at 15mA, with a current source below its bias R off the cathode. I'm thinking that maybe a 4:1 transformer could work to drive 32-600ohm headphones. I'm not much of an expert when it comes to transformers, but I'm pretty sure they only work well when sourced and terminated with very close to the proper impedance. 32 - 600 ohms is a pretty wide range. What if I add a SE EL84 buffer instead? Does this need to get complicated? Any thoughts?
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Old 24th March 2011, 09:17 PM   #2
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Well,

take a look at the Morgan Jones head-amp. It is also well suited as a line-amp. Chinese rebuilds exist.
HeadWize - Project: The Morgan Jones Mini Tube Headphone Amplifier by Chu Moy

LG Carsten
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Old 25th March 2011, 12:29 AM   #3
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Quote:
Originally Posted by carawu View Post
Well,

take a look at the Morgan Jones head-amp. It is also well suited as a line-amp. Chinese rebuilds exist.
HeadWize - Project: The Morgan Jones Mini Tube Headphone Amplifier by Chu Moy

LG Carsten
Thanks a bunch. It looks pretty good.
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Old 28th March 2011, 02:22 PM   #4
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I'm trying out a ss unity gain buffer for my 6922 preamp. I don't have a cathode follower attached to the single stage tube preamp.
I need to drive down to 32 ohms. The target however is to drive an AKG-701 which is 60 ohms. I'm using MOSFET's . Sim results aren't bad and I will report back after I test it out. The MOSFETS use a separate lower voltage power supply. Currently it is 15 volts because I don't have the 18 V regulator that I had designed it for. Bias was to be at 0.1 Amp. That should easily run the 701 over 115 dBspl . So it will have enough output for clean transients.
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Old 30th March 2011, 09:13 AM   #5
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hmmm, i really think a discrete fet follower would be best, like the DAO, or perhaps even a jfet circlotron, or a tube circlotron of course
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Old 30th March 2011, 09:25 AM   #6
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I'm using a discrete MOSFET follower with a current sink as load. Just designed the pcb. Will have to see if I get time to make it before I shift to my new home in a few weeks time. The urge is very strong but there isn't much time before I get caught up with the shifting nightmare.
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Old 30th March 2011, 03:35 PM   #7
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Quote:
Originally Posted by Humdinger View Post
I'm building a great little line level preamp with 6922 tubes, and I thought it would be nice to add a headphone output. The trouble is, I don't want anything solidstate, or involving feedback. The output will be a 6922 cathode follower running at 15mA, with a current source below its bias R off the cathode. I'm thinking that maybe a 4:1 transformer could work to drive 32-600ohm headphones. I'm not much of an expert when it comes to transformers, but I'm pretty sure they only work well when sourced and terminated with very close to the proper impedance. 32 - 600 ohms is a pretty wide range. What if I add a SE EL84 buffer instead? Does this need to get complicated? Any thoughts?
Hello,
The 6dj8 cathode follower work well!
Try this transformer
EDCOR Electronics Corporation. WSM10K/600
and this capacitor
Dayton Audio DMPC-30 30uF 250V Polypropylene Capacitor
The above parts are inexpensive. Depending on the headphone impedance you can pick a smaller value capacitor.
You can set up the transformer using two impedance outputs (see the center tap) that will work for the range of loads (600 to 32 ohm) that you listed.
DT
All just for fun!
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Old 30th March 2011, 07:34 PM   #8
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Default Headphone out parts

Quote:
Originally Posted by DualTriode View Post
Hello,
The 6dj8 cathode follower work well!
Try this transformer
EDCOR Electronics Corporation. WSM10K/600
and this capacitor
Dayton Audio DMPC-30 30uF 250V Polypropylene Capacitor
The above parts are inexpensive. Depending on the headphone impedance you can pick a smaller value capacitor.
You can set up the transformer using two impedance outputs (see the center tap) that will work for the range of loads (600 to 32 ohm) that you listed.
DT
All just for fun!
Thanks Dual Triode.
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Old 4th April 2011, 06:09 PM   #9
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Anybody make any progress ?
I managed to make a pcb and populate the board for a ss unity gain buffer. It runs off a 0-18 V(ac) supply with supply parts on board ( except the trafo).

Will try it out tomorrow. Right now there are no stabilising caps. I'm sure I'd need one ! The scope will tell !
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Old 5th April 2011, 02:19 PM   #10
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I got the circuit working after blowing two transistors. Had accidentally shorted out two resistors.
Anyway all's well now. The CFP input stage required a 150pF cap for RF stability.
Now it's noiseless.
Sound is good too. I compared it with a typical ss headphone amp with an OPA2134 driving a pair of TO126 type output transistors.

I think it's slightly better in the HF and bass is more powerful....not just LF extension but 'meaty'! Mid range is similar.
However I feel the unity gain buffer is slightly veiled in some sounds. The 2134 seems a bit cleaner between notes but also seems to lack 'energy'.
Better IMD ?

From memory the JLH headphone amp ( split supply) sounds better. In fact the JLH sounded better than the 2134 when I tried it sometime ago. Right now I can't do that comparison due to lack of time.
I suspect the coupling capacitor to be the cause of the slight veiling .From experience , it's presence had a similar effect in other ss headphone map designs. So a better 470uF/16V cap might make a difference.

I could post the circuit in case anyone is interested. Nothing much really.

Note that the input stage for the unity gain headphone amp was a 6922 preamp. The ss OPA2134 amp ran directly from the DAC output.
Cheers.
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