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Old 28th May 2011, 03:04 AM   #421
GregH2 is offline GregH2  Australia
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Default JFET listening impressions - you may be surprised.

I have spent the morning listening to the JFET front end with dfferent types of music and now feel I can make some solid observations. First up, it is not all good.

The midrange clarity has definitely improved. More air around voices and the amp just makes everything sound easy, like it's lazily following the source with no real effort required to do so. It is a nice sound.

However, and this is a big factor, the wonderful bass performance from before has disappeared. Completely gone. It is possible that the old version was exaggerating the bass and that this one is more faithfully following the level of bass in the recording, but I'm not sure I like it. The difference between the MOSFET and JFET front end is night and day.

I thought maybe things would improve if I made the feedback capacitor larger, so I doubled it to 440uF. No difference.

This circuit is identical to the last one except I have use polypropylene capacitors this time rather than polyester. I have a hard time believing that the capacitors could make this much of a difference to the bass performance so I'm going to blame the JFET.

The VAS transistors are from the same tube and the output fets are the same ones I used last time.

The difference is so obvious that were I to make a final version to put in a box, I would be leaning towards the MOSFET version.

So, any ideas why this is so? I badly wanted the JFET to be better, and in some ways it is, but that oh so important bass performance just isn't there and this is very disappointing.

Is it possible that the circuit really needs a device with some decent transconductance in the front end to perform? The MOSFET's transconductance is something like 6x higher.

Last edited by GregH2; 28th May 2011 at 03:06 AM.
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Old 28th May 2011, 03:53 AM   #422
MiiB is offline MiiB  Denmark
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How much current are you passing through the jfet..???

the 170Bl needs to run close to Idss to sound good....5-6 mA
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Old 28th May 2011, 03:53 AM   #423
fab is offline fab  Canada
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Quote:
Originally Posted by swordfishy View Post
I have spent the morning listening to the JFET front end with dfferent types of music and now feel I can make some solid observations. First up, it is not all good.

The midrange clarity has definitely improved. More air around voices and the amp just makes everything sound easy, like it's lazily following the source with no real effort required to do so. It is a nice sound.

However, and this is a big factor, the wonderful bass performance from before has disappeared. Completely gone. It is possible that the old version was exaggerating the bass and that this one is more faithfully following the level of bass in the recording, but I'm not sure I like it. The difference between the MOSFET and JFET front end is night and day.

I thought maybe things would improve if I made the feedback capacitor larger, so I doubled it to 440uF. No difference.

This circuit is identical to the last one except I have use polypropylene capacitors this time rather than polyester. I have a hard time believing that the capacitors could make this much of a difference to the bass performance so I'm going to blame the JFET.

The VAS transistors are from the same tube and the output fets are the same ones I used last time.

The difference is so obvious that were I to make a final version to put in a box, I would be leaning towards the MOSFET version.

So, any ideas why this is so? I badly wanted the JFET to be better, and in some ways it is, but that oh so important bass performance just isn't there and this is very disappointing.

Is it possible that the circuit really needs a device with some decent transconductance in the front end to perform? The MOSFET's transconductance is something like 6x higher.
swordfishy,

Just some thoughts:
I think that jfet sounds best when biased at more than 70% of their IDss so they say...

Also, I wonder if paralleling jfet (same IDss) would increase the transconductance...

Fab
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Old 28th May 2011, 04:08 AM   #424
AKSA is offline AKSA  Australia
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SWF,

Yes, I assume you use a 1K gate stopper as well, my bad.

Fab, I think you are right.

A mosfet has typically 5-10 times the transconductance of the jfet, which is measured in tens of millisiemens.

This will affect loop gain, greatly decreasing it. It should also make the amp easier to stabilise of course. However, low loop gain will increase output impedance and seriously impact the damping factor, a big factor in rendition of bass.

Jfets don't have a lot of power handling either. 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
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Last edited by AKSA; 28th May 2011 at 04:33 AM.
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Old 28th May 2011, 05:34 AM   #425
GregH2 is offline GregH2  Australia
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All, danspy especially. I have told you a lie and I apologise. I always thought the source of the input fet was at nearly zero volts and thus for using a jfet you would need a negative gate voltage. This is not the case. Depending on drain resistance the source is always at a positive voltage of at least a few volts, and therefore even with a JFET you need a positive bias voltage. Therefore the bias arrangement I posted in post 336 will be fine.

The good news about this is that the layout for a jfet or MOSFET version is identical making pcb construction easy. My bias voltage is current somewhere around 5v, needing 3 LEDs to get enough voltage. Maybe a single Zener would do better here Hugh?

Anyway, onto the bass side of things.

Hugh, glad to know my theory about needing more transconductance in the input stage wasn't completely stupid! I must be learning something.

I have improved it slightly. As you guys suggested I have increased the jfet current from 1-2mA to 5mA or so by dropping the drain resistor to 250R. I also added the input filter and added more PS bypassing. While the bass is still a little weak, the overall sound is improved. More separation and transparency. Very good.

However, even the bass issue is not too bad. While there is definitely much less than the MOSFET version, there is still quite a lot compared to other amps I have heard. So overall it's in pretty good shape now.

Hugh, perhaps dropping the source resistor of thie input stage from 100R to 20R or something might help the OLG and improve damping factor?

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?
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Old 28th May 2011, 05:50 AM   #426
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by AKSA View Post
SWF,

Personally, I feel that people who design with more than 10 transistors for a simple power amp should be heavily fined to teach them the error of their ways.....

Hugh
Hi Hugh.

That would be me. Then i am sentenced to forced labor, until i by my heart can say i will not use more than ten active devices in an amp.

Does this include the currentsources...

- Sonny

Last edited by sonnya; 28th May 2011 at 05:55 AM. Reason: spelling errors
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Old 28th May 2011, 08:28 AM   #427
GregH2 is offline GregH2  Australia
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Hi all,

Well I think I have improved things a bit by dropping the feedback network to 200R/20R.

Next thing to try is dropping the VAS degeneration to 20R and see if that helps too.

Just hope it stays stable!
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Old 28th May 2011, 08:30 AM   #428
sonnya is offline sonnya  Denmark
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Why don't you run it through LT spice (Not OLD SPICE :O) ), this way you can very quickly check up on phase margin and gain.
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Old 28th May 2011, 08:45 AM   #429
AKSA is offline AKSA  Australia
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SWF,

I've tried your model but can't get it to work using Notepad and LTSpice, any tips?

The entire 1st stage source current passes through the series fb resistor, therefore to keep voltage drop across this resistor to the minimum, a low value should be used. This mandates an even lower value for the shunt fb resistor, forcing use of a very large shunt capacitor. If we assume that 22R is about as low as you can go on the shunt resistor, then the 6dB corner frequency would be reached when the Xc of this cap is at 22R. That corresponds to 3.3Hz for a 2200uF cap, which is surely close enough for government work.

If we now set a gain of 18 for the amplifier, the series feedback resistor becomes 375R. 5mA passing through this resistor would give a voltage drop of 1V875, which means the source will be around 1V9 ABOVE the output, which we set to 0mV by adjusting the bias on the gate of the input device. The 2,200uF feedback cap will thus be polarised with just under 2V, should be rated to 5V or 6.3V, and should be protected with a 4V7 zener to prevent damage should the output stage blow.

Hope this explains the fb network clearly,

Cheers,

Hugh
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Old 28th May 2011, 08:58 AM   #430
mikelm is offline mikelm  England
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Quote:
Originally Posted by swordfishy View Post
Hi all,

Well I think I have improved things a bit by dropping the feedback network to 200R/20R.

Next thing to try is dropping the VAS degeneration to 20R and see if that helps too.

Just hope it stays stable!

Perhaps you now need a FET VAS

hehe
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