X5HV - the F5 principle applied to a headphone amp - diyAudio
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Old 5th February 2015, 10:08 PM   #1
hesener is offline hesener  Germany
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Default X5HV - the F5 principle applied to a headphone amp

Have been working on this for a while, but here it is - a headphone amplifier based on the F5. In this case, with a few twists:

Designed for (Stax) electrostatic headphones, and will swing +/-250V at the output
Symmetric construction, inspired (but probably not as elegant) as the X5 from EUVL
No thermal compensation, but simple servos
The boards have voltage regulators on there as well

This picture shows the testrun I did tonight - I just had to try it, once it was up and running (and not smoking ) (well, almost no smoke )

The two boards with heatsinks are the amplifiers, the other board is the power supply for the amps and bias, powered by a custom toroid.

Hope you like it, more details later....
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File Type: jpg full amplifier testrun.jpg (134.7 KB, 489 views)
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Old 5th February 2015, 10:10 PM   #2
hesener is offline hesener  Germany
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... and here is a measurement result: This picture shows a 10kHz square wave with the load connected (a headphone, that is), and this channel swings 250Vp-p with a slew rate of 46V/us.
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File Type: bmp X5HV 10khz square.BMP (12.8 KB, 81 views)
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Old 5th February 2015, 10:10 PM   #3
hesener is offline hesener  Germany
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Will provide schematics and all the rest in the coming days, for those interested.....
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Old 5th February 2015, 10:56 PM   #4
ramallo is offline ramallo  Europe
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I'm interested :-)
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Old 6th February 2015, 04:34 AM   #5
EUVL is offline EUVL  Europe
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I hope you will forgive me.
Could not resist the challenge.
Let's see how close I get ..........

PS
I don't think you are using 600V P-FETs.
I can only find them in TO247 from IXYS.
But I guess you are using Fairchild, so rails limited to +/-250V ??



Patrick

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Attached Files
File Type: asc F5XP HV V1a THD.asc (11.5 KB, 26 views)
File Type: asc F5XP HV V1a Freq.asc (11.2 KB, 13 views)
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Last edited by EUVL; 6th February 2015 at 04:37 AM.
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Old 6th February 2015, 04:54 AM   #6
EUVL is offline EUVL  Europe
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Actually if one is outputting +/-250V differential, one might even get away with +/-130V rails.
Then plenty more choices for FETs.

The lower rail voltages also make life easier for the cascode FETs.
One can then use the smaller Zetex ones, such as ZVN4424 / ZVP4424.

The one I post above will do +/-400V differential and still have decent performance.
Only simulation of course.




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Old 6th February 2015, 09:27 AM   #7
hesener is offline hesener  Germany
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Hi Patrick, I was hoping you would show up

Here is the amplifier schematic. It will simulate, but the results will not be real, reason being that I couldnt find the original sim on my hard drive and so I did not include the models for the output MOSFETs. But if you plug them in, it should work.

I used the combo of PF5102 / 2xJ270 since they have comparable gm and capacitances, same for the FQP3P50/FQP5N50NZ. Well, the match is not superb, but at least for the output devices, due to the heavy source degeneration, it doesnt matter that much. Certainly the circuit can be optimized, but it does work pretty well!

This also doesnt have the servos and power supplies, this will come in a later post....

enjoy!
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File Type: asc X5HV.asc (10.1 KB, 45 views)
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Old 6th February 2015, 09:32 AM   #8
hesener is offline hesener  Germany
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Forgot to mention - R13/R14 as well as R30/R31 are trimpots, so I can play with the source degen of the input stage. And the base voltages of the cascode bipolars are also adjustable so I can finetune the DC offset on the output (this explains the 4 trimpots you can see on the picture of the PCB).

As it stands, it does have a couple volts DC at the output, which is completely ignored by the headphone. Eventually I will finetune and get it to zero.....
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Old 6th February 2015, 09:44 AM   #9
hesener is offline hesener  Germany
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Here is the regulated bias for the headphone. Stax "Pro" types require 580V, standard types 230V (I believe), and there are various other models on the market. R2/R3 are in fact a 10k trimpot to adjust the bias voltage from 200V to 600V. It is powered by a 600V/10mA winding on that custom toroid transformer, followed by a rectifier (1N4007 works fine here) and a cap (1uF or so).

It is using the good old '431 in a rather unusual configuration, so I had to invert the output to be able to use a n-channel MOSFET. You may want to use a 800V power MOSFET in this configuration, I just had no other simulation model.... But in principle it works fine. In fact, on my board I used a 1200V IGBT - had to increase R9 to 10k since the threshold voltage of that device is much higher.

The headphone bias pin is connected to the drain of the power mosfet, through a 1M resistor, followed by a 1nF cap, and another 1M resistor.

enjoy
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File Type: jpg ELS regulated bias.jpg (106.2 KB, 427 views)
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Last edited by hesener; 6th February 2015 at 09:59 AM.
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Old 6th February 2015, 09:45 AM   #10
hesener is offline hesener  Germany
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Very nice, and very close!! With regulated 250V rails, I can use 2SC3503/2SA1381 as cascode transistors with sufficient margin.

Quote:
Originally Posted by EUVL View Post
I hope you will forgive me.
Could not resist the challenge.
Let's see how close I get ..........

PS
I don't think you are using 600V P-FETs.
I can only find them in TO247 from IXYS.
But I guess you are using Fairchild, so rails limited to +/-250V ??



Patrick

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