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Old 28th May 2011, 08:04 AM   #431
lineup is offline lineup  Sweden
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Quote:
Originally Posted by fab View Post
Just some thoughts:
I think that jfet sounds best when biased at more than 70% of their IDss so they say...
This is why I recommend at least 5mA for a 2SK170BL.
John Curl always says that JFETs runs better with high current.

Quote:
Originally Posted by AKSA View Post
There are some impressive Supertex vertical N mosfets, the DN2530, which cost about a dollar from Mouser and which can be run at higher current. These are typically 0.3S, or 300 mmho, so much more loop gain is possible with these. I will see if I can obtain a Spice model from an engineer friend.
However, there are other factors to the bass. It might not all be DF.....
Hugh
Hugh.
Supertex has got SPICE models at their website.
Supertex - SPICE Models



swordfishy
I would like to see a schematic of that circuit you use.
You said you would post a circuit.

There are many factors that could make you perceive less BASS.
For example if the upper of the frequencies are more dominant, bright
then the BASS, eventhough not changed, would seem less.

Ideally you should setup 2 circuits. One with MOSFET input and one with JFET.
And then you can switch to make a better A-B compare.
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Old 28th May 2011, 08:09 AM   #432
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Quote:
Originally Posted by Fcrawley View Post
Anybody going to construct a PCB?
Fcrawley

There is still experimenting with different circuits.
But I am sure someone will make a PCB in the end.
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Old 28th May 2011, 09:14 AM   #433
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Hi all,
last night I came across this thread and read it in a rush. Thanks to all contributers, it was fun and instructive!

I have a few Q's, though.

Greg, which are your main speakers? Which is your daily poweramp?

All, I never activley biased a 2sk170. The datasheet makes clear that this FET at or near IDSS exhibits a very low Vgs. I am not sure that you are in a good operational mode if you bias the gate to 0mV if the source sits at nearly 2V, but maybe I'm missing something?

Or perhaps we learn here that a mosfet input stage could sound better than pure data and common sense would imply?

Nice project!
Rüdiger
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Old 28th May 2011, 09:32 AM   #434
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Quote:
Originally Posted by AKSA View Post
The input device is a FET, yes, but the Vgs of the high gm devices like the DN2530 is high also, around 2V6, and being depletion mode, the gate potential is BElOW the source.

Hugh
Oops, I thought Greg used a 2sk170 here. With a high Id it would sit aprox. 50mV below the source when left self biased.

Rüdiger

EDIT: Has someone done a LT-Spice sim and would share it here?
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Last edited by Onvinyl; 28th May 2011 at 09:42 AM.
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Old 28th May 2011, 11:33 AM   #435
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My simulations at 5mA through 2SK170BL
gives the gate is like 0.100 - 0.150 V below the source.
(The exact value is -129mV)

I also did find swordfishy telling several Volts a bit questionable.
He must have done some bad measurmentt.
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Old 28th May 2011, 12:17 PM   #436
GregH2 is offline GregH2  Australia
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lineup, onvinyl, mikelm,

Maybe a FET VAS will improve things...but we are going to a lot of effort to improve a circuit to make a jfet work...when it was already so good without it

The circuit I said was at 2 volts was the one with a 500R drain resistor (1-2mA fet current), and 1k/100R feedback network. I think if you simulate that circuit you will see that my measuements were absolutely correct.

Moving on, my current circuit has a 250R drain resistor and 200R/20R feedback network (about 4mA current). Bias voltage is at very close to zero volts now (a few hundred mV).

onvinyl,

To answer you question, which I assume you're asking to see whether my opinion means anything Well, I don't have a Krell and B&Ws, that's for sure, and I don't have golden ears either.

The speakers I do most of my mod to mod testing with are a small set of Jamo bookshelves and when I think it's worth the effort I drag in my Soavo 1 Floorstanders. These are not well known, but they are high end Yamaha, and on par with their famous efforts from the 1970s. They have a flat frequency response and sound very bland with most amps - hence my looking for a bit of low order distortion.

Here is the AVHUB summary for them:

Quote:
The frequency response of Yamaha’s Soavo-1 as shown in Graph 1 extends from 50Hz to 10kHz ±3dB when using pink noise as the test stimulus. This is an excellent result. This trace is the averaged result of nine individual frequency sweeps measured at three metres, with the central grid point on-axis with the tweeter. Although the capture is unsmoothed, some smoothing is inevitable as a part of this averaging process. Overall, the graph shows superb flatness right across the midrange, so that it’s within just ±1dB from around 300Hz all the way up to 2kHz, which is the note ‘C7’ in the top octave of a piano keyboard. Below this you can see there is a slight ‘lift’ in the bass between 80 and 240Hz that peaks at around +3dB at 150Hz. There’s also a very slight prominence in the high treble, between 4kHz and 10kHz (the latter frequency being the upper measurement limit for this particular test), but since it’s only around +1dB and could affect only the harmonics of instruments, and not their fundamentals, I don’t think it would be audible.

Graph 2, which shows the high-frequency response in far greater detail, thanks to the use of the gated sine wave technique, shows that the rise in output level above 4kHz increases to about 4dB at 6–7kHz, after which it rolls off, shelves slightly, then rolls off to end in a sharp –8dB dip at 15kHz after which there’s a rise to a +10dB peak at 30kHz. This 30kHz peak would be the fundamental resonance of the aluminium dome, but at this frequency it’s high enough to be well out of harm’s way, where it would be completely inaudible to the human ear. And severe though that suck-out at 15kHz seems to the human eye, it has such a high ‘Q’—occurs over such a narrow bandwidth—that it, too, would be inaudible to the human ear, not least because few—if any—people over the age of 40 can hear a 15kHz tone.

Low-frequency performance is shown in Graph 3, which was measured using the standard near-field microphone technique. You can see that the Soavo-1 is tuned to a very low frequency (32Hz) and that the woofers (the single trace is the summed version of both bass drivers) roll off very smoothly from around 150Hz down to this frequency. The port’s output peaks a little higher than I might have expected given this tuning, at around 40Hz. The port nonetheless produces significant energy over quite an extended bandwidth, as its output is 6dB down at 23Hz and 80Hz, so it’s able to substantially reinforce the output from the bass drivers.

Impedance was controlled, except that the lower resonant bass peak is moderately high at well over 40Ω and the upper resonant peak also comes in at a fairly high 34Ω (at 54Hz). You can see the ‘saddle’ between the two is at 32Hz, electrically confirming the system tuning. The variation in impedance between the two peaks is confirmation that Yamaha’s engineers have deliberately tuned the Soavo-1’s port a little high. The impedance of the Soavo-1 drops to around 3.2Ω at 160Hz, and there’s a difficult point in the impedance around 85Hz, where the impedance is 5.4Ω and the phase angle is –63°, which means any amplifier used with the Soavo-1 will need to be capable of delivering its rated power output into a 4Ω load. However, it does mean that the Soavo-1 should be rated as ‘nominally’ 4Ω rather than the 6Ω specified by Yamaha.

The amplifier you use to drive the Soavo-1s won’t have to be overly powerful in order to achieve high sound pressure levels in your listening room, because Newport Test Labs measured the Soavo-1’s sensitivity at 89.5dBSPL at one metre, for a 2.83Veq input, which is both very good (being significantly higher than the average for all speakers) and also a fraction higher than Yamaha’s own specification. It’s also a good result because Newport Test Labs’ methodology for testing loudspeaker efficiency is particularly stringent.

The last graph in this series is a composite plot showing how the various drivers interact with each other to produce the complete ‘system’ response as measured by Newport Test Labs of 40Hz to 20kHz ±6dB. Looking at the left-hand side of this graph, it’s apparent that the particular loading and alignment used by Yamaha means that although the frequency response is –3dB at 50Hz, the roll-off below this point is a little shallower than I might have expected, so you can expect to hear significant bass output down to around 30Hz, particularly if you position the speakers effectively in your room.

Overall, this is an excellent set of test results, showing that Yamaha’s engineers have created a very musically balanced loudspeaker in the Soavo-1.
My daily amplifier is a meticulously constructed set of Peter Daniel Gainclone PCBs, which sound better than they have a right to. I also have a Cambridge Audio Azur 840W which is their flagship multi $k product designed by none other than Douglas Self. It is currently on EBAY as it is so sterile it could disinfect a hospital. Amazing figures but no soul whatsoever, and definitely not a good match to my already bland speakers. I hate it.

I have also built and owned a Sansui 500 Valve Amp from the 1960s, Musical Fidelity A1, JLH class A, aleph 3, Aleph 30 and several zero feedback designs of my own design as well as a heap of high end consumer audio - pioneer, sony, etc, etc.

So now you have an idea of where my opinions come from.

BTW, I think you might be on to something with your comment about the mosfet input...

As this is all getting so confusing I will post a new schematic later tonight so we all know where things are at!

Last edited by GregH2; 28th May 2011 at 12:21 PM.
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Old 28th May 2011, 12:48 PM   #437
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Now I understand your measurement, swordfishy.

Say you compare MOS and JFET in a good way (switch A/B)
and you prefer MOS for input, then of course you should go for MOSFET.
For lower THD I think JFET will win.
But THD is not everything.
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Old 28th May 2011, 01:11 PM   #438
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I do believe that the Jfet version will benefit significantly for inserting a Hawksford cascode...You raise gain quite a bit..overcome the input capacitance and lower the output impedance...
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Old 28th May 2011, 01:14 PM   #439
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Can you link to a Hawksford cascode?
What is it like?
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Old 28th May 2011, 01:19 PM   #440
MiiB is offline MiiB  Denmark
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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...

Last edited by MiiB; 28th May 2011 at 01:23 PM.
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