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Old 1st September 2012, 02:17 PM   #12581
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run dual matched pair of input Jfet's or drop 2 pairs of outputs.
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Old 1st September 2012, 02:56 PM   #12582
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Quote:
Originally Posted by lhquam View Post
Does each FET have its own gate stopper resistor? Perhaps you should show the exact schematic and a photo of the layout.
I have each FET with its own 100 ohms on the gate pins.
schematic is like my original F5 version and I just quadrupled the output.
I use a PCB from Ebay.
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Old 1st September 2012, 03:09 PM   #12583
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Default turbo headache

Quote:
Originally Posted by jackinnj View Post
You mean "each JFET" gatestopper -- not absolutely necessary, but it can be done with slight improvement of measurements.

Of course, the gatestopper for the MOSFETs has to be as near the gate as possible.

Using multiple output devices? -- the input stage might be running out of steam due to the paralleled input capacitance of the output devices.
The 2SK175/2SJ55 has about half the input capacitance as the IRF250, so four of them should work, even the F5 Turbo V1 uses two hefty IRF's or similar [=like four of my devices].
So the lack of steam should show up with low frequency (which it did, having a high roll-off of still 650 KHz instead of 1-2 MHz in my single pair version). The PCB by the way don't like an extra pair

I made a F5-turbo style feedback reducer (now 102 ohms/10 ohms); added an input filter 80 kHz.
I did some testing: connected just one pair again instead of 4 pairs in the right channel.
  • right channel driven, OK, driving both channels gave oscillation.
  • I noted the left channel with 4 pairs could not stand zero load, it immediately oscillated.
Still this interference.
  • Can it come from the common power supply? It is easy for me to split the final capacitor cell as I have two per rail.
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Last edited by triode_al; 1st September 2012 at 03:38 PM. Reason: corrected F5T info
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Old 1st September 2012, 03:30 PM   #12584
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turbo v1 only run dual pair outputs.
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Old 1st September 2012, 03:31 PM   #12585
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Quote:
Originally Posted by AudioSan View Post
turbo v1 only runs a dual pair outputs.
correctly corrected . .
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Old 1st September 2012, 03:46 PM   #12586
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Quote:
Originally Posted by jameshillj View Post
-

There was some mention a long time back about a small Gate/souce cap on the power fets to solve oscillation problems of higher capacitive loads, I think ....
thanks james
This is a good idea as my drain connection is based on a pin with a semi-star halfway the heatsink (not four separate wires to the PCB). Same as with gates. The PS rail connection is also based on one long wire dropping down.
This could indeed give spurious interference.

Local decoupling is a good id, using some 20 pF drain-gate if I am correct. My TO-3 devices are flat on the heat sink, so impossible to add a cap across the pins now.
- - [sorry next week] I will try to work out some layout improvements.
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Old 1st September 2012, 05:13 PM   #12587
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Yeah, it's a "funny bugger" at times and it surprising how component changes alter the sound - I've not had any of your problems but just use the single pair of fets, altho the one I built with the triple units of the smaller 2013/313s didn't exhibit any particular problems apart from some jfet trimming - I began using jachinj's input jfet gate stopper/filter input arrangement on this one and it was a lot "smoother" but a bit touchy of resistor quality - used some Rhopoints, not cheap!

Maybe problems with your wiring - it sounds a bit casual, altho the amp is surprisingly pretty tolerant of this. I'm still a bit surprised at the way Nelson's F series production amps have the central 0volt point on the end of the pcb next to both the I/P & O/P sockets and no problems whatsoever.
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Old 3rd September 2012, 10:56 AM   #12588
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Default All OK now

Quote:
Originally Posted by jameshillj View Post
Maybe problems with your wiring - it sounds a bit casual, altho the amp is surprisingly pretty tolerant of this. I'm still a bit surprised at the way Nelson's F series production amps have the central 0volt point on the end of the pcb next to both the I/P & O/P sockets and no problems whatsoever.
Indeed it seems to have been a wiring issue.

F5 heatsink flush mounted 2SK175-2SJ55.jpg
  • I had one pair, the top ones, directly connected to the board placed on the bottom of my amplifier, not on the heatsink with the resistor on the board still in its proper place
  • three parallel pairs distributed lower on the heatsink, all with the source resistors directly connected to the pins and one wire from the board to these fets.
This indeed meant the architecture, the layout, was messy, with a unequal sharing of wires, easily leading to hogging between devices.

To check my difficulties, I tried the single pair (as before) this was OK; next I connected the other three parallel pairs only. [And will add the fourth pair later]

So: Now I have my F5-turbo V1,
I did need the input jfet gate stopper/input filter to not excite the ringing in VHF.
The difference is now 0,5 dB between 8 Ω and 5 Ω - a very low DF like I had expected. [My very good sounding 300B has a low DF].

I have 21,5 volts and 1,4 A bias each side. [I have to restrict the voltage due to my speakers]

Maybe next year I will go towards a turbo version V2 with omitting the source resistors all together.
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Old 3rd September 2012, 11:27 AM   #12589
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I'm not too sure what to make of your build, Al -

It seems that you have 4 pairs of the TO3 devices grouped in complimentary pairs and this will make all the wiring much longer than having all the P type together, and the same with the 'N' ones -
If you don't mind me asking, why do you need 4 pairs to operate a 5 - 8R load with a 1.4A current and a 21 volt rail? - seems a bit excessive and, unfortunately, not guaranteed to give better results - Perhaps I'm missing something obvious here, sorry....

I do like the big heatsink and having the gate stoppers resistors close to the transistor pins and a neat way of holding the power resistors down.
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Old 3rd September 2012, 01:26 PM   #12590
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Actually it is one set top to bottom N-fet and the other side the complementary ones. Each set of devices is now 'fed' from a pin in the middle, obviously making it very balanced in capacity and inductance (even if both are low).
Yes, with these devices the lines get long.
I originally wanted to mount the PCB's in the center of the heat sink, that would have given neat layout and proximity almost like IRF devices - but I noticed then my capacitors would not fit (width of 4).

audio L1013073-- 35, 90 mm 548.jpg

And yes, a bit overdone, one pair is OK too, has a lower DF even what brings out the bass a little bit more, and it works without any artifices like input filters . But on the other hand, the high is better handled now.

And for the rest, shifting into turbo-mode is just curiosity .
albert
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Last edited by triode_al; 3rd September 2012 at 01:28 PM.
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