Downscaling Zen v.9 for use as headamp - diyAudio
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Old 6th March 2009, 05:24 AM   #1
kenev is offline kenev  Greece
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Default Downscaling Zen v.9 for use as headamp

Lately, I completed the Zen v.9 (without fb) and I was amazed by its sound, when played through a pair of Fostex FE167 in Voigt pipes.

I was thinking of using the same amplifier topology to drive my Sennheiser HD600 headphones. I have already seen the "Zen Headphone Amplifier" project in Headwize, but I thought of using the LU1014 power JFET as an output device, since I have a couple of them. Power requirement for the headphones is minimal (a couple of hundreds mWs), so what is needed is downscaling the zv9 circuit, with lower voltage and current and, probably, lower power MOSFETs.

Since my knowledge doesn't extend so far, could I get a little help from someone more expertized? I know there are a few here.

Thank you.
Regards,
Evangelos
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Old 6th March 2009, 08:50 AM   #2
h_a is offline h_a  Europe
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Headphones generally do not need voltage gain, so maybe you want to consider this cute little headphone amp using these power jfets

http://www.diyaudio.com/forums/showt...05#post1130805

Have fun, Hannes
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Old 6th March 2009, 03:33 PM   #3
kenev is offline kenev  Greece
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Hi Hannes,

Thank you for your reply.

Actually, I think this is not a current (trasnconductance) amplifier with its low output Z of just 4 ohms. Zen v.9 (wired as a current amp without feedback) has an output Z of 80 ohms and that's what make the Fostexes sing (at least I think so and so Mr Pass says).

Any other thoughts on rearranging zv9 for lower voltage/current?

Regards,
Evangelos
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Old 6th March 2009, 04:22 PM   #4
h_a is offline h_a  Europe
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Hi,

sorry I see you mean something really different

There's not much you can do then; reduce rails to +-15V maybe.

By the way, if an output impedance of 80 Ohms makes your Fostex sing, then maybe that does not happen with your headphones as they usually have impedances like 100 Ohm (speakers 4, 8 or 16 Ohms) - so that would be entirely different.

However, no need to stop if you like it

Have fun, Hannes
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Old 6th March 2009, 09:14 PM   #5
mjf is offline mjf  Austria
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hello.
if i remember correctly the hd 600 has an impedance of 300 ohm.
i think for listening to loud music you need an output voltage of 15 to 20v pkpk or so (clean,undistorted)..........
perhaps you can reduce the rails as far as possible for that output.
reducing the bias current is not so easily done because i think this will give you increasing distortion..............i would try it a little bit.
the output cap (10000uf) can decrease to 470uf or 1000uf.......
greetings........
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Old 9th March 2009, 11:46 AM   #6
kenev is offline kenev  Greece
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Quote:
Originally posted by mjf
hello.
if i remember correctly the hd 600 has an impedance of 300 ohm.
i think for listening to loud music you need an output voltage of 15 to 20v pkpk or so (clean,undistorted)..........
perhaps you can reduce the rails as far as possible for that output.
reducing the bias current is not so easily done because i think this will give you increasing distortion..............i would try it a little bit.
the output cap (10000uf) can decrease to 470uf or 1000uf.......
greetings........

Indeed, HD600 has an impedance of 300 ohms.

I will try lowering supply voltage and see whether is worthy.

My other question is whether I can raise the output impedance of the amp. Maybe by increasing the value of the output to ground resistor (R7 - 100 ohms in the original schematic)?

Regards,
Evangelos
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Old 10th March 2009, 12:22 PM   #7
Nrik is offline Nrik  Denmark
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Kenev
Dont reduce R7 it is only used for pulling the other side of the cap to be at ground level. Instead you should increase it to something like 2,2k-4,7k.
The 100 ohms was probably chosen to be adequately quick pull-down effect, and not loading the output too much, when driving an 8 ohm impedance. When you go for 300 ohms, the 100 ohms are way too small and would steal most of the output wattage and just turn it into heat.
So reduce output cap by times 10-30 (1.000uF is fine) and increase R7 by the same factor.

It is 'comme il fait' to have 100-120 ohms output impedance on headphone amps, primarely for short circuit protection of the amp, when inserting/pulling out the Jack.
You should basically just insert a 100/0.5W ohm resistor i series with the output. With the very linear impedance of Sennheiser HD600, it should not degrade the sound quality too much.

However: If you are a daredevil and know that you are not gonna do live-connect/disconnect, or if you mute the input before doing so, you can actually omit adding this 100 ohm resistor to the signal path.

To reduce the Idle current increase R4/R5/R6 and R1/R2/R3. There should be 0.65 volts over R4, and between 1.1 and 1 volts over R1 - choose a standing current and use ohms law to calulate the resistor values. Tell us if you need help.

I have a Sennheiser HD600 too and sorry Hanes, but these do need some voltage amplification. I would say 4 times amplification of CD level output is adequate, just to have a decent range on the input volume pot. So just keep R16 and R17 on the same level.

I too think that transistors generally sounds at their best at collector or drain, but be aware that the signal is inverted. With the Sennheisers I think there is an easy way around this though: The small plugs that go into each 'can' should be able to connect in reverse ( + to - and vice versa) ...I didn't try this on mine so I am not 100% sure.
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Old 10th March 2009, 05:19 PM   #8
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Default What about this options for JFET headphone amp?

I was wondering about this as a headphone amp? I am sure it could be scaled to the work for headphones by someone.....

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Old 10th March 2009, 06:48 PM   #9
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.. time ago I made a very very simple design as headphone amplifier with a V-channel mosfet and a lamp ad drain load...
It works fine, even without cascoding...

Simple and cheap!

Marcello
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Old 11th March 2009, 07:05 AM   #10
kenev is offline kenev  Greece
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Nrik,

Thank you for your recommendations. Actually that was exactly what I was thinking about, increasing,not decreasing the value of R7, in order to increase the output impedance.

Regarding lowering idle current, I'm not sure if this will be correct, since this will alter the operating point of the JFET and, I think this will reduce the transconductance of the JFET, thus lowering its output impedance. I think that Papa has carefully chosen this operating point for this JFET. Anyway, I can try this and see what comes out.

Regards,
Evangelos
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