JFET input, MOSFET VAS, LATERAL output = Perfect!! - Page 45 - diyAudio
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Old 28th May 2011, 01:38 PM   #441
fab is offline fab  Canada
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Quote:
Originally Posted by swordfishy View Post
...
FAB, paralleling input devices might work, but it would be nice if we could avoid it I think. Do you think that sound wise paralled jets would still be better than a single MOSFET?
It was only a suggestion but I have never tried paralleling jfets neither a mosfet as input. My latest experience with a single jfet as input with about 70% IDss of bias, mosfet VAS and Lateral mosfet in class A (1.1A) gives plenty of bass but I have a big power supply too...

Low source resistor for the jfet as you finally did is the way to go.
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Old 28th May 2011, 01:49 PM   #442
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Quote:
Originally Posted by MiiB View Post
It's a cascode that follows the signal...it can be set by a string of 3 Red LED-diodes from the buttom of the Jfet and with 10 Kohm to positive rail... Transistor can be the complementary of the one used the VAS...
I think mikelm suggested a cascode like that in a previous post...:
JFET input, MOSFET VAS, LATERAL output = Perfect!!
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Old 28th May 2011, 02:00 PM   #443
lineup is offline lineup  Sweden
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I know now what is a Hawksford Cascode. Thanks.
What happens if we have not clean power rails to feed Cascode?
I want to keep Fetzilla somewhat SIMPLE.
I do not like idea to complicate. Just for some small improvement.
But let me try to simulate and see how much improvement with Hawksford Cascode.
And anyone is of course free to test and use it.


Regarding MOSFET input vs. JFET input.
I was surprised to see ZVN3310A had a little less THD than 2SK170BL.
MOSFET ZVN3310A are surely nice small mosfets.
I was sure the JFET had less THD.
But this is not a fact in my simulation.
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Old 28th May 2011, 02:14 PM   #444
lineup is offline lineup  Sweden
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With MOSFET ZVN3310A input.
And AKSA suggested bias of the input.
And BJT BD140 VAS.
Feedback divider 330/22 Ohm.
Output biased 100mA.
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Old 28th May 2011, 02:58 PM   #445
GregH2 is offline GregH2  Australia
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Yes lineup,

My simulations show the same thing but I have been nervous to mention it. The MOSFET, at least in simulation, has lower THD then the JFET and similar distortion character. Actually, the FET had half the distortion if I recall correctly (though both were very low). The models may not be accurate though so we need to be cautious.

If they are accurate, whether this translates to better sound I do not know yet, but I am leaning that way. We need to make sure we aren't trying to make a square shaped tool fit a round hole. The mosfet just might be better in this application - at least with the circuit kept simple with no cascodes, etc, and we need to be willing to accept that possibility. But I'll give the JFET a very good go before deciding. I suspect the improved distortion figures are purely due to the much higher transconductance and if that is the case there are probably even better devices to use. The depletion mode device hugh posted had 300mS (3x the zvn3310a and 15x the 2sk170)!

Here is the current circuit I am using with the JFET. Please recall that it was originally the same as the mosfet version in post 336, but had limited bass performance. Dropping the feedback network from 1k/100R to 200R/20R to get more gain in the input stage has improved the bass slightly, but my feedback capacitor is now too small - as Hugh suggested.

I think by making the feedback capacitor even bigger, maybe 2000uF as Hugh suggested, we may be back on par with the mosfet. Currently with 470uF, in simulation we are losing a lot of gain at 50Hz, about 1V on a 15V sinewave. I will try this tomorrow. It's been a big week and I badly need sleep - this is just too much fun

With the mosfet version (see post 336) and 1k/100R, 220uF still had good bass because the current through the feedback shunt resistor was much smaller. 220uF was adequate, though still a little small.

Also, decreasig the VAS degeneration to get some more OLG might help bass too, but then we also increase distortion and instability.

Or, as mikelm suggested, we could revert back to a fet VAS to get some more OLG that way.

For a simple circuit - single ended input with no cascode, single ended VAS, lateral P-P output and no compensation, the mosfet version is looking pretty nice though.

Lineup, you are right, I need to be able to A/B test these things a bit more easily than I can now, as I currently have a minimum of 10 minutes between tests.

However, I can say with absolute certainty that the mosfet version does indeed have more bass as certain things in the room shake with it that don't with the jfet version
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Last edited by GregH2; 28th May 2011 at 03:16 PM.
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Old 28th May 2011, 03:16 PM   #446
lineup is offline lineup  Sweden
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Maybe we should try a BJT input, too.
BC550C might give those FET a good game
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Old 28th May 2011, 03:24 PM   #447
GregH2 is offline GregH2  Australia
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Quote:
Originally Posted by lineup View Post
Maybe we should try a BJT input, too.
BC550C might give those FET a good game
Yes please do! I'm off to bed but will try models tomorrow.

BC550 = cheap, low noise, plentiful.

Pinout is the same as the ZVN3310A and 2SK170. I can drop it right in if your models suggest it may be worthwhile.

Isn't it interesting how we have gone all over the place with this design - singled ended, LTP, jfets, mosfets, BJTs, cascodes, compensation etc. How educational it has been for me.

When we have a final design I promise to produce a PDF complete with schematic, DC offset plot, stability test plots and FFT, because now we are up to over 400 posts and all the data will be all over the place and difficult to follow. I will borrow some instruments to get a distortion spectrum and THD.

I hope everyone partaking or even just reading along is enjoying the ride. I wish someone else was making one too because I hope my listening tastes aren't taking us in the wrong direction!

At least most of the pinouts are the same, so when we finally make PCBs we can swap parts easily. I know mikelm is going to want to give the FET VAS a go, and others will need to try the JFET input

Last edited by GregH2; 28th May 2011 at 03:31 PM.
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Old 28th May 2011, 03:36 PM   #448
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Swordfishy,
thanks for your elaborate answer, that does indeed shine some light on things!

I myself seem to have a faible for yamaha, both my vinyl rig and my drum kit are made by them and both I admire a lot.

Hugh,
earlier in this thread you mentioned you didn't like cascodes soundwise. I have not much experience with poweramps, in low level high gain phono amps I experimented a lot with casc. and every time I added one atop a 2sk170, 2sj74 or a 2sk369, things improved a lot regarding clarity especially in the lower registers. Cascodes can oscillate nastily, and that screws sound quality. The way you bias them at the bases makes an difference as well, soundwise.
Would you mind to elaborate your findings re cascodes in power amps?


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Old 28th May 2011, 03:51 PM   #449
lineup is offline lineup  Sweden
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Quote:
Originally Posted by swordfishy View Post
BC550 = cheap, low noise, plentiful.

Pinout is the same as the ZVN3310A and 2SK170. I can drop it right in if your models suggest it may be worthwhile.
Here are simulation THD at 1kHz 1 Watt 8 Ohm.
All other things equal for each transistor
Code:
THD % - Transistor input
0.003170 - BC550C (BJT) Winner!
0.003965 - ZVN3310A (MOSFET)
0.004620 - 2SK170BL (JFET)
Comment:
Differences in THD are so small, that other qualities like Harmonics spectrum and result at other frequencies and/or power levels can be used to decide what input transistor you use.
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Old 28th May 2011, 04:23 PM   #450
GEirin is offline GEirin  Argentina
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Hi all.
OK. The winner is BC550C (BJT) Transistor input (THD % 0.003170)
And what is the winner of the best sound?
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