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Old 18th February 2013, 05:56 PM   #11
tinitus is offline tinitus  Europe
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some suggest to bootstrap the jfet buffer ?
(don't know if it makes sense)

can I ask...what is the 1M from drain(power) to base(input) doing ?
I know other curcuits use it, but is it needed with these jfets ?
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Old 18th February 2013, 06:20 PM   #12
JMFahey is offline JMFahey  Argentina
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In this case (split supply) it's not needed.
Its function is to raise the Source voltage (by raising the gate voltage) in single supply circuits, to raise the input signal handling.

Bootstrapping?
Maybe, if you want to raise input impedance.
Not really needed here, because 1M from Gate to ground is fine.
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Old 18th February 2013, 06:27 PM   #13
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Quote:
Originally Posted by tinitus View Post
some suggest to bootstrap the jfet buffer ?
(don't know if it makes sense)
No it doesn't, pretty pointless with an FET.

Quote:

can I ask...what is the 1M from drain(power) to base(input) doing ?
I know other curcuits use it, but is it needed with these jfets ?
Again, seems pretty pointless, just the single resistor to ground is all that's required - and it could be increased to increase input impedance, meaning there's no need for bootstrapping (which is what you'd do with a bipolar input in order to give a higher input impedance).
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Old 18th February 2013, 06:50 PM   #14
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Quote:
Originally Posted by tinitus View Post
some suggest to bootstrap the jfet buffer ?
(don't know if it makes sense)

can I ask...what is the 1M from drain(power) to base(input) doing ?
I know other curcuits use it, but is it needed with these jfets ?
I don't know about bootstrapping, but the two 1M resistors between ground and V+ create a reference voltage. The V+ to base resistor wouldn't be needed in a dual supply configuration (I could also skip it and connect the base to ground resistor to the opamp's voltage divider).

Good info about fet-buffers: Basic Buffers
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Old 18th February 2013, 07:02 PM   #15
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Originally Posted by Nigel Goodwin View Post
Again, seems pretty pointless, just the single resistor to ground is all that's required - and it could be increased to increase input impedance, meaning there's no need for bootstrapping (which is what you'd do with a bipolar input in order to give a higher input impedance).
Isn't a vref bias better than ground? It might be pointless in this case, but that's beyond my knowledge. I've made buffers like this before and they've worked fine, so I didn't see any point in modifying the circuit.

That buffer is actually the thing I'm the least unsure of about this design.

EDIT: Just noticed a typo in the schematic, the 3k3 source resistor should of course go to ground, there's no negative supply in the design anymore (but the 1M resistors still stand).

Last edited by Fredde79; 18th February 2013 at 07:09 PM. Reason: typo
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Old 18th February 2013, 08:00 PM   #16
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Originally Posted by Fredde79 View Post
Isn't a vref bias better than ground? It might be pointless in this case, but that's beyond my knowledge. I've made buffers like this before and they've worked fine, so I didn't see any point in modifying the circuit.
It just seems a waste of a resistor?, you're wanting a negative bias on the gate - achieved by the voltage drop across the source resistor - biasing the gate positive doesn't seem a very good idea?.

It's not something you normally see used commercially.

Quote:

That buffer is actually the thing I'm the least unsure of about this design.

EDIT: Just noticed a typo in the schematic, the 3k3 source resistor should of course go to ground, there's no negative supply in the design anymore (but the 1M resistors still stand).
Try taking the top resistor out, and see if you can tell any difference

According to the webpage you linked it's supposed to improve large signal handling, but it could just as well make it worse

Either way shouldn't make a great deal of difference.
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Old 18th February 2013, 08:17 PM   #17
JMFahey is offline JMFahey  Argentina
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Quote:
It just seems a waste of a resistor?, you're wanting a negative bias on the gate - achieved by the voltage drop across the source resistor - biasing the gate positive doesn't seem a very good idea?.

It's not something you normally see used commercially.
Sorry but it's very much used commercially.

And the point is, as I said before, to improve input signal handling.

Another important point is to make biasing and signal symmetry more predictable, without time wasting individual tweaking or obsessive Fet matching.

DIYers, of course, add a so called "biasing" preset to every Fet stage
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Old 18th February 2013, 08:47 PM   #18
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The impact of the existance or removal of this resistor goes a bit beyond my scope, but I have another question:

Since this amp is going to be used live on stage, it would be very nice to have some kind of limiter so that I don't become deaf when the singer drops his microphone, or when the sound guy accidentally bumps the monitor feed.

One way to achieve this seems to be using a diode clipper. (A collection of circuits here: Designing A Limiter For Headphone Amplifiers | HeadWize)

Is this a good idea? One obvious problem is the difference in efficiency amonst headphones, it would clip too early on some and too late on others if the clipper is placed at the output, right?. If it's placed at the input, it would work only relative to the input signal, not in absolute terms.

I'm open for suggestions here, but the design cannot be anything too fancy, because I need the design to fit in a small enclosure.
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Old 20th February 2013, 08:57 PM   #19
tinitus is offline tinitus  Europe
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don't know
but mighty glad I finally got the compressor out of my bass system
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Old 27th February 2013, 08:10 AM   #20
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I would reconsider using the LM386 for a headphone amp, it's a total crap house IMHO.

Why not either go for LME49600/ BUF634 or an opamp stage with follower transistors (for instance BC327/BC337, or BD139/BD140 if you need more clout)?
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