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Old 24th March 2015, 11:48 AM   #1
EUVL is offline EUVL  Europe
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Default F5 Headamp ?

There are already umpteen versions of the F5, starting from the Selectronics Profet, then F5, F5X, F5T, F5 with 2SK2013/2SJ313, High-Voltage F5X for electrostatic headphones, ……. So why add another one ?

There have been various posts vaguely mentioning a downscaled version of the F5 as headphone amp. But I could not remember / find a proper schematics for precisely that. Since there have been recently some discussions about a discrete Pass type headphone amp, I thought I should add yet another one.

This is purely a lunch-time mental jogging exercise. That means, translated in English, that I have not built, nor do I intend to in the near future. But I think it is so easy and simple, that not much can really go wrong. So I leave here the Spice files as well as a Vero-board layout, so as to encourage those fearless DIY’ers to make a real attempt.

Performance in simulation is excellent. Even at 14V pk-pk into 50 ohm load, distortion is less than -80dB. Frequency response is equally impressive. Bandwidth over 1 MHz, and just a touch of overshoot with a 100n pure capacitive load (which is of course unreal).

And other than the input JFETs, the rest are easy to source components. You do need matched devices if you want low distortion.

PS straightly speaking the feedback network is more Profet than F5. Hence the "?" in the title.


Enjoy,
Patrick
Attached Files
File Type: asc F5 Headamp IRF THD.asc (4.7 KB, 60 views)
File Type: asc F5 Headamp IRF Freq.asc (4.6 KB, 25 views)
File Type: pdf F5 Headamp Vero.pdf (369.3 KB, 144 views)
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Old 24th March 2015, 02:43 PM   #2
juma is offline juma  Germany
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Let's mix it up somewhat and save those Toshiba JFETs, without comprimising SQ.
P1/P2 set the Iq of the output stage to about 50-100mA as well as DC offset at the output (start adjusting with maximum resistance of the pots).
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File Type: gif ha-F5-BJTin.gif (7.9 KB, 243 views)
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Old 24th March 2015, 03:08 PM   #3
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very nice. 2n5457/2n5460 lower gm, somewhat higher noise, but good in this ap.
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Old 24th March 2015, 04:18 PM   #4
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Well if we go BJT then there are hundreds of circuits, e.g. the JLH headphone amp.

But Nelson is known for FETs, so I considered it appropriate to pushing a FET circuit.
And you can of course use LSK170/LSJ74.

The 2nxxxx parts will cause more distortion.
So one needs to jack up the OLG somewhere else ......


Patrick
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Old 24th March 2015, 09:54 PM   #5
EUVL is offline EUVL  Europe
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For completeness here the schematics in pdf.
Also attached the schematics of the JLH headamp, based on a similar topology but using BJTs.


Patrick

.
Attached Images
File Type: gif JLH Headamp.gif (7.0 KB, 186 views)
Attached Files
File Type: pdf F5 Headamp Schematics.pdf (13.4 KB, 72 views)
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Old 25th March 2015, 12:04 PM   #6
EUVL is offline EUVL  Europe
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I have been asked by PM :

> Can i use this circuit as a Line preamp to drive my Aleph j power amplifier ?

Yes, but you probably do not need so much current driving capability for a line level preamp.

> How can i null the offset voltage and bias current for the out put devices ?

By adjusting the values of R3, R4.

> what kind of head phones can i use with this circuit ?

In principle anything from 25 ohm to 300 ohm, except for a couple with really miserably efficiencies.
Would have no problems with a HD800, HE500 or K701, to name a couple.


Patrick
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Old 25th March 2015, 12:19 PM   #7
EUVL is offline EUVL  Europe
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In response to post #3, here are Spice files for the Fairchild JFETs.
As expected higher distortion, despite the lower Idss and the correspondingly increased values of R3 & R4.

One should also note that the Fairchild TO92 devices are also now obsolete.
One can continue to use the SOT23 devices (MMBF5457/5460).
Dissipation should be OK.

And still a good, low cost project.


Patrick

.
Attached Files
File Type: asc F5 Headamp IRF FAIR THD.asc (4.6 KB, 12 views)
File Type: asc F5 Headamp IRF FAIR Freq.asc (4.5 KB, 3 views)
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