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Old 19th January 2010, 03:08 AM   #1
jol50 is offline jol50  United States
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Default JFET amplifier? Is there one...

I read a thing saying JFET amplifiers can sound like a tube amp. I assume that is the soft clipping a tube does. Found some stuff on guitar effects using them but little else. Is there a car amp built with them....are they in the input section only like a preamp, I don't find any high power ones?

I see First Watt had this Zen amp but I don't see one, but I'm not into home stuff.

Or is this a pointless comment? I would make a car amp clip softer if I could, though nobody seems to care about it. There must be more to this I am not finding.
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Old 19th January 2010, 03:25 AM   #2
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Why not simply buy an amp with enough power so that it never gets driven into clipping?

You could probably build a device to reduce the input level when the output reached a certain level (before clipping).
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Old 19th January 2010, 06:33 AM   #3
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Proton and NAD used a "soft clipping" or "anti-clipping" in their home amps and Proton's car amps.

The home amps had an LED to indicate when it was working, but the ones I used had 6dB headroom with variable rails for ridiculous short term power. I only got my ACC to show up a couple of times even on the 40Wx2 amp, and blinks were so brief that it wouldn't have made much difference if it was clipping.

As far as JFETs go, yeah it's pointless until power JFETs become commonly available. Idss is typically too low for output device use. There was a surge of popularity here when the Lovoltechs were introduced, but AFAIK they are no longer around except as buyouts. What seems to be available otherwise isn't priced for car audio or DIY use.

If you're curious, go to PassDIY.com and look for the Zen variants. ZenV8 was an exercise with these Lovoltechs and Nelson briefly explains the similarities. In brief, the I/V curve resembles that of a triode at certain Vgs, and they are more linear than MOSFETs.
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Old 19th January 2010, 07:17 AM   #4
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Years ago a few power jfet amps were built they are now highly sought after and
verry verry pricey. Power jfets are expensive. Recently Simisouth has started making
some silicon carbide based power jfets and Nelson offers an amp the J2 that uses this device. In the group buy forum we even have a group buy for this part I believe
the buy is closed now and believe the price was ~$60 per device! Yes $60 per device.
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Old 19th January 2010, 06:49 PM   #5
jol50 is offline jol50  United States
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Quote:
Originally Posted by Perry Babin View Post
Why not simply buy an amp with enough power so that it never gets driven into clipping?

You could probably build a device to reduce the input level when the output reached a certain level (before clipping).
I know that is the right way, I was looking at some tube stuff and figured someone would have built a SS amp that acted like a tube at clipping. With a price like that I can see why not.

That is another thing I don't quite understand, not knowing that much about amp design, why not build an amp that limits at clipping....I've never seen one anyway just the LED for clipping. I don't know what feedback does at clipping, I was looking for that when I got into the tube stuff. I do have some amps that seem to stop after clipping some, like an old PPI I have. It clips some on a sub and can't be driven harder while other amps can be clipped to massive distortion (and those tend to be cheaper amps). Have an Infinity to try with the clipping lights but not got to that yet.


Quote:
Originally Posted by tsmith1315 View Post
Proton and NAD used a "soft clipping" or "anti-clipping" in their home amps and Proton's car amps.

The home amps had an LED to indicate when it was working, but the ones I used had 6dB headroom with variable rails for ridiculous short term power. I only got my ACC to show up a couple of times even on the 40Wx2 amp, and blinks were so brief that it wouldn't have made much difference if it was clipping.

As far as JFETs go, yeah it's pointless until power JFETs become commonly available. Idss is typically too low for output device use. There was a surge of popularity here when the Lovoltechs were introduced, but AFAIK they are no longer around except as buyouts. What seems to be available otherwise isn't priced for car audio or DIY use.

If you're curious, go to PassDIY.com and look for the Zen variants. ZenV8 was an exercise with these Lovoltechs and Nelson briefly explains the similarities. In brief, the I/V curve resembles that of a triode at certain Vgs, and they are more linear than MOSFETs.
I actually have a NAD 3140 I use once in a while. It is not that impressive compared to the larger HK (2x60) I used until the volume control got strange. I can't find a manual and can't figure out what the buttons on the rear do, the clipping thing does not seem to work though everything else does. Thanks for the info I'll look that stuff up.

Quote:
Originally Posted by woody View Post
Years ago a few power jfet amps were built they are now highly sought after and
verry verry pricey. Power jfets are expensive. Recently Simisouth has started making
some silicon carbide based power jfets and Nelson offers an amp the J2 that uses this device. In the group buy forum we even have a group buy for this part I believe
the buy is closed now and believe the price was ~$60 per device! Yes $60 per device.
All I can say is ouch. The most expensive output I've seen was in a sony mobile ES, some special toshiba I didn't have to buy thankfully.

I'm certain people like the soft clipping much better than typical SS, I'm surprised nobody has tried to make a car amp like that. Things may be going the way of DSP in amps, but it is hard to find something someone has not tried in the past. Some amps clip more like a tube, IMO Linear Power amps do and I think that is why some like them so much. Seems like that would be an innovation in a car amp given how people like to abuse them, and it is not like you can make a sonically cleaner amp really.

For me I'd rather use the smallest amp that will fit better in my install, and also prefer it is not capable of damaging volume. People say well just set your gain with a big amp, but I can't do it because the level of inputs is always different. I have to allow clipping on high level in order to hear low level input just between two CDs. I'd need huge amps I really don't want, I try to use multiple IB subs when I can and they leave no room for amps. An amp that communicates is so much easier to live with and get the most out of.

I have seen on my scope how some amps clip to a wave flat and generate more harmonics, while other amps the wave is cut off on an angle on the back side like it lost power....or maybe that is the PS giving up I don't yet know. But they do sound different.

What bothers me the most about the clipping, is many amps gradually go into it. The kicker I have now it starts to sound dirty and it is clipping a little. My favorite amps break up fast and clip a lot at lower Hz, back off just a little and they sound as clean as ever without ever screaming at you. But maybe there are no set reasons why one amp does it differently than another, or it is dependent on too many factors?
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Old 24th January 2010, 09:34 PM   #6
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The Brax X2000 uses JFET's on the input side of things....Bipolar for output.

-Nate
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Old 15th February 2010, 09:03 AM   #7
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Question power jfets

I just purchased some LU1014D power jfets from an ebay source. The spec sheets on these, apparently old stock, says 50 amps at 24 volts! I havent tried these in audio application yet, but I will. I also note increasing interest in power jfets as power switches, such as mentioned in ad from SemiSouth Labs in MS.
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