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Old 27th September 2009, 04:00 AM   #141
Bigun is offline Bigun  Canada
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Originally Posted by MJL21193 View Post
What movie?
Duplicity, staring Julia Roberts - I'm a fan.

It's not an easy movie to follow, but very enjoyable.

I hooked up my DIY sub-woofer to the TV for the first time - nice addition !
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Old 27th September 2009, 04:06 AM   #142
Bigun is offline Bigun  Canada
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Originally Posted by keantoken View Post
It doesn't seem like hum is a problem in Hugh's other amplifiers, which are not headphone amps. So it should be possible to get satisfactory performance without the regulator. So I could go either way.

There is one thing I want to make note of. A Jfet current source has somewhat poor PSRR, but it adds plenty of H2. Recall the input bias configuration I used. It actually increases H2 in the simulator. Should we try it?

- keantoken
I have played with some simulations (my Harry 77 thread) using a JFET in various locations to purposely increase H2 and/or to emulate triode-like distortion effects. It's a nice method and the JFET is a very interesting device, having a different characteristic than all other active devices. My favourite trick in the Harry 77 amp was putting the JFET into the nfb loop.

What I found in the sims was that adding H2 in a multi-stage NFB amp is quite difficult - because it invariably results in increased risk of IM distortion at higher signal levels. I did some real DIY experiments with my TGM1 amplifier (another thread) although not with the use of a JFET for this. The higher H2 gives a nice sound, but when I crank up the volume it gets muddy pretty quickly.

It would be risky to include it in this headphone amp without prior experiments simply because it's a one-shot chance at the pcb. If Hugh prefers to keep it clean all is not lost since you and I can butcher the pcb with fly-wires and try out something along the lines you have suggested
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Old 27th September 2009, 04:10 AM   #143
keantoken is offline keantoken  United States
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The Jfet is not at all in the feedback loop. I think it is perfectly safe here.

- keantoken
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File Type: png JfetBias.PNG (8.3 KB, 258 views)
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Old 27th September 2009, 04:33 AM   #144
MJL21193 is offline MJL21193  Canada
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Quote:
Originally Posted by Bigun View Post
Duplicity, staring Julia Roberts - I'm a fan.

It's not an easy movie to follow, but very enjoyable.

I hooked up my DIY sub-woofer to the TV for the first time - nice addition !


Ah, I've been meaning to see it.

Good bass is the foundation to build on - it make everything sound better.
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Old 27th September 2009, 04:55 AM   #145
Bigun is offline Bigun  Canada
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Originally Posted by keantoken View Post
The Jfet is not at all in the feedback loop. I think it is perfectly safe here.

- keantoken
sorry, I didn't explain. I was saying that in general, I like JFETs and I like the application of them to controlling H2. And that I had tried it in a NFB loop, I realize you weren't suggesting that here.

I think your suggestion for the bootstrap take-off point is the better approach. It might be worth playing with the relative values of the bootstrap divider resistors to see if we have them optimized, I haven't looked at that at all yet.

Edit: here's a nice all-on-one pcb headamp to give us some ideas about construction...

http://4.bp.blogspot.com/_ZVgJtItVgn...-Amplifier.jpg
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Last edited by Bigun; 27th September 2009 at 05:15 AM.
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Old 27th September 2009, 05:36 AM   #146
MJL21193 is offline MJL21193  Canada
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Since the PS is not finalized, let me make a suggestion.
I designed and built a very smartly working phone stage

Picture 830.jpg

that is powered by a wall wart. The wall wart has ac output, and the benefits are that the PS itself is left in your hands - you can make it any way you want; also the transformer (and mains wiring) is not close to the amp circuit. One more plus: no bulky cord to the unit itself.
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Old 27th September 2009, 05:37 AM   #147
keantoken is offline keantoken  United States
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Quote:
Originally Posted by Bigun View Post
sorry, I didn't explain. I was saying that in general, I like JFETs and I like the application of them to controlling H2. And that I had tried it in a NFB loop, I realize you weren't suggesting that here.

I think your suggestion for the bootstrap take-off point is the better approach. It might be worth playing with the relative values of the bootstrap divider resistors to see if we have them optimized, I haven't looked at that at all yet.

Edit: here's a nice all-on-one pcb headamp to give us some ideas about construction...

http://4.bp.blogspot.com/_ZVgJtItVgn...-Amplifier.jpg
In my current simulation I have the modified bootstrap point and the Jfet in place. With this combination, H3 is about 25db below H2. I was suggesting this in addition to the bootstrap.

- keantoken
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Old 27th September 2009, 05:45 PM   #148
Bigun is offline Bigun  Canada
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I have updated the model to show both channels. I doesn't show the optional NFB take-off point yet but I have added the cross-feed. It also needs an LED power-on indicator.

I've got two versions, one with cap multiplier and one with CLC. To get similar performance from the CLC I had to use 330mH on each rail. I could use larger caps.

The CLC is a bit cleaner. BUT my sims show something strange going on sub-10Hz, some kind of oscillation due to those inductors.

Model is attached...

Keantoken - I agree, nice to have H2 higher than H3, but even with the model I have attached, this seems to hold true. I really don't know how important the relative heights of H2 vs H3 are. Potentially this needs some prototyping to find out as the sims only go so far...
Attached Images
File Type: jpg stereo.jpg (238.3 KB, 251 views)
Attached Files
File Type: txt Aspen_hdph_with2.txt (30.6 KB, 9 views)
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Old 27th September 2009, 09:26 PM   #149
keantoken is offline keantoken  United States
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You should include Hugh's input bias decoupling in the schematic. (I'm sure we're all agreed on this?)

Maybe I'm just blowing steam about the Jfet. I don't think it matters that much. I wouldn't know what it sounds like, either. A resistive bias network will be better at keeping the bias voltage at 1/2 rails, anyway.

I think we're about ready to make a prototype.

I like MJL's wall wart idea, but I don't like to carry wall warts everywhere and this thing wouldn't last very long on batteries... Perhaps we can add a "low bias" mode for using batteries?

- keantoken
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Last edited by keantoken; 27th September 2009 at 09:30 PM.
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Old 27th September 2009, 10:24 PM   #150
keantoken is offline keantoken  United States
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Quote:
Originally Posted by ikoflexer View Post
Does anyone know first hand how it sounds? Has anyone built a prototype, or, is anyone planning to do it before the group buy?
We're getting there...

Unless there's a good deal for the boards, I probably won't build one, and I have my own design ideas anyways... Hehheheh...

- keantoken
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