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Old 3rd August 2010, 08:47 PM   #8581
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Quote:
Originally Posted by Franz Gysi View Post

Maybe I switch to 15VAC trannies later (more VA...).

Soundwise, it seems that I am on the right track to increase the voltage for the Mini F5 version.
Franz, you are on the way back to the original F5. Are you still making a Mini F5 ?
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Old 4th August 2010, 07:04 AM   #8582
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Yes, still a Mini F5!

The restriction to Mini is due to limited dissipation of the heathsinks and the limited power of the transformers.

The listening tests are quite satisfying now. Maybe still a little bit glare in the highs missing.

I will freeze it now for some days as I am moving my workshop. As soon as I will have the testequipment accessible, I will check the Mini F5.

Franz
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Old 4th August 2010, 06:50 PM   #8583
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I've read a little bit more about mosfets and hexfets. I also think to understand the F5 circuit better than ever before :-)

Now, I am quite sure, in my Mini F5 the voltage gain stage has to drive high gate capacitances of the IRFP's. So, I could try 2SK1529/SJ200 like mentionned in this thread to optain reduced capacitances

But I could also try to get higher current and lower output impedance from the gain stage.

So, I decided to reduce R1 and R2 by paralleling 15R resistors, resulting in 6 Ohm.

For my understanding, this does result in a slightly higher current through the input stage and a higher gain.

It also reduces the feedback by some dB's.

And it seems to "open" the sound.

Now my question:

I got 2SK170/2SJ74 with Idss of 6.8mA.

Is it recommended to use higher Idss devices?

Franz
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Old 4th August 2010, 09:57 PM   #8584
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Franz, HEXFETs are MOSFETs (it's just a specific production process that is responsible for the name).

Both stages in F5 provide voltage gain.

Higher current through first stage means lower gain of that stage and less global feedback and more total distortion.
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Old 4th August 2010, 10:38 PM   #8585
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Default Preamp for F5

juma:

I have built your J310 JFET preamp(from the JFET BOZ thread) for the F5. It is good. The gain is about 2+ times, which is what I want for F5 and other amps. Id of both J310 are about 10mA. I've also used your capacitance multiplier and LM317 for regualtion. However, I can hear a little high pitch hiss noise when I press my ear close to the speakers. Is that normal?

Elec.

Last edited by Electrolastic; 4th August 2010 at 10:42 PM.
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Old 4th August 2010, 10:42 PM   #8586
juma is offline juma  Germany
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Elec, let's not hijack the thread.
That hiss can come from LM317. Try Salas shunt reg.
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Old 4th August 2010, 10:47 PM   #8587
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Originally Posted by juma View Post
Elec, let's not hijack the thread.
That hiss can come from LM317. Try Salas shunt reg.
Sorry. Thanks. I will look into it. No more thread hijack.
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Old 5th August 2010, 06:25 AM   #8588
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Quote:
Both stages in F5 provide voltage gain.
Oh yes, I have overseen that. It is not a source follower.

So, my conclusion:

Driving the F5 as Mini F5 (reduced voltage and idle current) does result in high capacitances at the gates of the power transistors.

Too high to be driven correctly by the first stage.

Is the only solution to use 2SK1529/2SJ200 ?

Franz
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Old 5th August 2010, 08:49 AM   #8589
juma is offline juma  Germany
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Quote:
Originally Posted by Franz Gysi View Post
...Is the only solution to use 2SK1529/2SJ200 ? ...
If I were you, I'd run mini-F5 at +/-15V with two pairs of 2sk2013/2sj313 in output (each channel) biased at 400-500mA through each pair. The capacitancies are much lower and the linearity of those parts is significantly better. The resulting sound is noticeably better than standard F5.

Last edited by juma; 5th August 2010 at 08:54 AM.
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Old 5th August 2010, 05:45 PM   #8590
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Quote:
If I were you, I'd run mini-F5 at +/-15V with two pairs of 2sk2013/2sj313 in output (each channel) biased at 400-500mA through each pair.
Seems indeed to be a good suggestion! The datasheet looks very promising.

I've just read your thread about your F5 version with 3 pairs of 2sk2012/2sj313.

I think, I will order some of this FET's and give it a try.

Thanks
Franz
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