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Old 25th July 2008, 07:39 AM   #201
Mooly is offline Mooly  United Kingdom
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Hi Francis,
Try this-- it will give you a feel for what happens. If you apply say 4 volts peak to peak and take the gate to 0 volts it will conduct. There will be NO dc offset across the output-just the signal. Now take the gate higher and at around 0.3 volts a DC shift will appear as the gate channel begins to conduct. You need the gates close to this value to minimize their on resistance. That 0.2 volts makes quite a difference. Change the 22 K and 1 meg for say 100 ohm ( I am guessing these values but this is what I did ) so the resistance of the FET affects the output. The voltage now across the 100 ohm output will rise significantly as the gate goes from zero to 0.3 volts. Put the original values back and now swing the gate negative. To "block" 4 volts you need -4 on he gate, to block 6 volts you need -6 and so on.
In the inverting configuration I use there is no signal ( only a few millivolt ) across the FET, so this isn't such a problem anyway but do try it, it makes it easier to understand what happens.
Again going off memory I think the resistance of the FET was around 70 to 80 ohms at zero volts gate, and fell to below 30 ohms at 0.2 volts.
Another gem of information Have you ever wondered how to identify the source and drain. You will find it dosn't matter which way round you put the FET's. And that's official -- I asked Bob Pease at National Semiconductor, as it was something that always puzzled me. Thanks again Bob.
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Old 25th July 2008, 08:59 AM   #202
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It is too brave to use class AB follower without negative feedback by voltage. Good luck!
Speaking of resistances, it is convenient to think about MOSFET as a charge controlled resistor.

I used HEXFET followers in class A amps and drivers. No additional feedback was used to linearize them, when it was used it linearized other things like class C stage working in parallel.

The best results were obtained when it was loaded on a counter-modulated CCS and drain powered from one more voltage follower. In such case it sees more constant conditions over all working region, both Vds and current are most stable, so distortions caused by variations of transconductances and capacitances are minimal.
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Old 25th July 2008, 11:38 AM   #203
lineup is offline lineup  Sweden
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Quote:
Originally posted by Wavebourn

I used HEXFET followers in class A amps and drivers.
----
The best results were obtained when it was loaded on a counter-modulated CCS
and drain powered from one more voltage follower. In such case it sees more constant conditions over all working region, both Vds and current are most stable, so distortions caused by variations of transconductances and capacitances are minimal.
I know what you mean, Wavebourn
Counter-modulated CCS
I guess this is what you have used in some of your published original an unique output stages.

There is nothing wrong with HEXFET outputs!
Otherwise Nelson Pass & Bob Cordell, two of our most experienced audio amplifier designers
were totally wrong, to the bone

Keep your good ideas coming: Mooly and Wavebourn
I am all ears


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Old 25th July 2008, 11:49 AM   #204
Mooly is offline Mooly  United Kingdom
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Hi guys,
Look at the piccys just posted for the HEXFET job

Lineup-- I used the S.I.L. relays once in a preamp, they were fine but the signal isolation wasn't perfect. Even relays need the series shunt arrangement, as I suspect the breakthrough at HF was caused by capacitive coupling -- print as well as contacts. The FET's are brilliant, you can shove anything at the inputs 5 Khz , 100 KHz squarewave 10 volts pk/pk - anything, and the isolation is total. Not a dicky bird
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Old 25th July 2008, 11:59 AM   #205
lineup is offline lineup  Sweden
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I might try your discrete JFET switching some time.
If I need some active switching.
I do not switch very much my signals.
I have most of the time one power amplifier for one specific signal source.

And if need, I will often go pure passive:
Goldplated ELMA miniature rotary switches are my favourites.
They do not come very cheap. But they are excellent for audio.

Click the image to open in full size.
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Old 25th July 2008, 12:08 PM   #206
Mooly is offline Mooly  United Kingdom
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They look nice I couldn't do without remote volume, the amp is in the opposite corner to the seat of honour you see, but as to input switching, well the FETS's were every bit as good as a mechanical switch and they lent themselves to full remote control so why not. Must admit I very rarely switch sources, 95 % listening is from CD.
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Old 25th July 2008, 12:09 PM   #207
lineup is offline lineup  Sweden
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Quote:
Originally posted by KLe

Yes Greg's GB150D is very good
Even so, it is interesting to look at other designs, such as Moolys.
I am glad you built it, your comments are always appreciated.

One very simplified topology posted by Greg Ball was the one I have attached here.
There are some unique points to consider, no doubts.
And according to the many testimonies it performs very well!
You cant go much wrong if ordering a SKA amplifier.
This is my opinion, truly is
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Old 25th July 2008, 05:14 PM   #208
rellum is offline rellum  France
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Hi Karl

Thanks for all those explanations.
Is the second inverting opamp just there to recover the phase ?
The 2SK152 has various Idss ranges that will lead to different lower Rdson. Did you make any kind of matching for both channels of same input ?

Today, I had a look at different vendors here to source this part. Unobtainium.
Source selection is not something I'm looking for, 100% of my listening is CD. Couldn't a BF245C do the job instead of your low Rdson part ? It's Vgs range is more suitable to be driven by a chip, even if I can't reach full isolation. Any drawback ?

I have a lot of BF245Cs in my drawer and guess what I can read on the Philips datasheet from 1996 :
Features -> Interchangeability of drain and source connections.
I didn't know that they all behave the same.

I won't bother you more as I can see you started something else. I will keep you informed with my progress but it will be a winter project.

The more I listen to your amp, the more I like it.

Francis
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Old 25th July 2008, 05:46 PM   #209
rellum is offline rellum  France
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Quote:
Originally posted by KLe
Hi Francis

Yes, the fuses I was referring too are the one's that are part of the output, but included in the NFB circuit? I found, when I have replaced the fuses, that they do take at least 4 hours to run in and have at leaner sound to them ... have you noticed this.

thanks

Hi KL

I burned one of them but it was a lower value rated for initial class A testings with a lot of probes connected around the module. The original ones are still in place and I don't know what to answer...


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Old 25th July 2008, 06:08 PM   #210
lineup is offline lineup  Sweden
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Quote:
Originally posted by rellum
Hi Karl

Couldn't a BF245C do the job instead of your low Rdson part ?
It's Vgs range is more suitable to be driven by a chip, even if I can't reach full isolation.
Any drawback ?
---
The more I listen to your amp, the more I like it.

Francis
Francis.
The actual choice of JFET for off/on switch, is not very critical.
He had a lot of 2SK152 .. this is why he used them.

There are many other JFET you can use for good results.
Try your BF245C
If he would have had BF245C at home, he might have used them in the first place

BF245 is not that unusual in Audio circuits ... I have seen plenty of designs with BF245.
They are not too bad for audio signals.

Neither is 2N3819, which is a similar JFET.
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