Bring down noiselevel in amz mosfet boost

[Jay Doyle]

you can bootstrap from a the source/emitter of any unity-gain stage, but not from one with gain higher than 1.  the source/emitter will only be of use as a bootstrap if it can provide a signal that is "almost equal" to the input.[/i]

The source/emitter ALWAYS provides a signal that is 'almost equal' to the input. You can bootstrap a gain stage.

[Doug_H]

Did someone say bootstrap?

[R.G.]

The source/emitter ALWAYS provides a signal that is 'almost equal' to the input.

How does that work with the source/emitter grounded?

Or effectively grounded through a big capacitor?

Did someone say bootstrap?

And this is an example. At high frequencies and high gain, the 0.68uF cap reduces the impedance at the emitter to less than the Shockley resistance, so most of the base-emtter signal voltage only shows up inside the device, not at the external emitter pin. So the bootstrap becomes much less effective at high frequencies, which is where the matter of high input impedance matters for guitar. This leaves aside the question of whether you might not want some treble rolloff at high gains, but if you did, this would not work well for that.

In this circuit, a second emitter follower used only to generate a bootstrap voltage to drive the bias network would likely provide higher input impedance at high frequencies, no matter what setting the gain pot was on.

[Jay Doyle]

The source/emitter ALWAYS provides a signal that is 'almost equal' to the input.

How does that work with the source/emitter grounded?

Or effectively grounded through a big capacitor?

It is still there, you just can't get to it for a useful purpose.

[zachary vex]

just to clarify, i was basing my comments on a single-transistor boost, like the AMZ mosfet boost, which is the original topic of this thread.  certainly by adding more stages of active circuitry all sorts of impedance magic can be done!

honestly, in my experience, bootstrap circuits are not necessarily the quietest circuits!  in fact, the lo-fi loop junky uses a bootstrap on the input, and during bypass this circuit monitors the input jack so the unit can record at any time... it doesn't add any load, and it isn't a buffer, so one would think it's invisible... but guess what?  it adds hiss to the guitar signal just by being connected to it, without being used as a buffer!  the wonders of positive feedback, huh?  the best laid plans...

btw, my new mosfet boost (the boost in the box of rock pedal) is much quieter than the SHO with the input impedance dropped to ~1M (for low gains) and the use of metal film resistors.

[zachary vex]

The source/emitter ALWAYS provides a signal that is 'almost equal' to the input.

How does that work with the source/emitter grounded?

Or effectively grounded through a big capacitor?

It is still there, you just can't get to it for a useful purpose.

wow, this is certainly spinning my head around.  8^)  it's still there, but you can't get to it?

is that something like the proverbial "you can't get there from here?"

[Jay Doyle]

wow, this is certainly spinning my head around.  8^)  it's still there, but you can't get to it?

is that something like the proverbial "you can't get there from here?"

The source/emitter is still acting the same as it is in any circuit. but there isn't a resistor to grab any voltage off... I'm being semantic at this point.

And I resemble that remark, I'm from Vermont... :-)

[billings]

you'll have to string up a bunch of high-value metal film resistors to make 10Mohms, because the highest value i've seen available is 2.2M.

I have some right in front of me....?

http://www.mouser.com/search/ProductDetail.aspx?R=MF1%2f4DCT52R1005Fvirtualkey66000000

I certainly didn't have to buy an entire reel of them, though.

[zachary vex]

yeah, that's my point.  if you  have to buy 5000, then you can't get them.  8^P

[R.G.]

It is still there, you just can't get to it for a useful purpose.

Yep.

That's why I was proposing putting another source/emitter out where you COULD get to it for useful purposes. It's almost like the devil is in the details.

just to clarify, i was basing my comments on a single-transistor boost, like the AMZ mosfet boost, which is the original topic of this thread.  certainly by adding more stages of active circuitry all sorts of impedance magic can be done!

Yep.
I guess I was too subtle with noting that one more active device, using the same biasing and one more source/emitter resistor can make an automatically correct bootstrapping signal for you. It's not quite the same as adding more stages of active circuitry, implying that there are several more cards of stuff to be glommed on.

yeah, that's my point.  if you  have to buy 5000, then you can't get them.  8^P

Mouser lists seven different part numbers for 10M metal film for sale in one-at-a-time. The most interesting is probably:
Mouser 594-5063JD10M00F  10M metal film, 0.4W, 1/8W body size, in stock, 0.12 each in ones.

It is true that they get cheaper if you buy a lot. That particular part number is $9.00 per hundred and $80.00 per thousand. The 660-MF1/4DCT52R1005F does require a 5000 unit buy, but that 5000 resistors will only cost you $75.00. I grant you that a starving college student might not want to do that, but all-growed-up people can and do swing the under-$100 kinds of expenses if they have to. It's not a serious problem.

That's not quite the same as not being able to get them.

Did you know that if you have a sufficiently sharp knife, you can split a human hair into between 40 and 120 one-micron sized slices?

[puretube]

10M1%SMD

10M1%0.6Wthru-hole

22M cermet

100M cermet

this is from europes largest electronix mailorder
(available for less than half price from others)

[zachary vex]

i stand corrected.  they aren't made in the brand i've been using for years... the Xicons peter out at 2.2M in 1/8w.

i guess they snuck them into the Vishay line last August!  Xicon's limit was 1M until this year... i wonder what's driving the demand for these high-value resistors?  certainly not guitar pedals.  8^)

[zachary vex]

100M cermet

this is from europes largest electronix mailorder
(available for less than half price from others)

wow, Ton, if those were available in high-kilovolt ratings like we used to be able to get in the 70's, i could build one of those huge voltage multipliers i always wanted, now that i can afford the parts!  8^)  as a kid i always had dreams of an enormous lightning machine... now i'm worried if the stereo's too loud for the neighbors.

someone give me an example of a use for a 100Mohm resistor in modern SMT circuitry.  those contacts are so close together you could breathe on it and significantly affect the resistance!  8^)

[R.G.]

someone give me an example of a use for a 100Mohm resistor in modern SMT circuitry.  those contacts are so close together you could breathe on it and significantly affect the resistance!  8^)

OK.

How about as a breath detector?

Here's another: a 9nA current source for a poor resistor substitute for the reverse diode leakage in a Millenium-style bypass.

Hmmm... calibrator for a pH cell?

Smoke detector ionization chamber load?

Besides, if you were serious about using it, you'd probably use a conformal coating on the board.

And if you really wanted to do high voltages, you'd dip the whole thing in a jar of mineral oil to keep Mother Nature's nasty coronas from forming anyway. Standard practice for HV stuff back in the day, and much, much easier with small components.

[zachary vex]

the high-voltage 100M resistors i remember were sealed in glass tubes with metal end caps, and were several inches long.  we'd wash them with alcohol after installing them.

another solution to leakage would be to machine a slot through the board between the solder pads.  i've done this to prevent/eliminate carbon tracking in my iMPAMP design, but now i use Q-Dope (polystyrene in MEK) after scrubbing the rosin off the board.

[Doug_H]

Did someone say bootstrap?

And this is an example. At high frequencies and high gain, the 0.68uF cap reduces the impedance at the emitter to less than the Shockley resistance, so most of the base-emtter signal voltage only shows up inside the device, not at the external emitter pin. So the bootstrap becomes much less effective at high frequencies, which is where the matter of high input impedance matters for guitar. This leaves aside the question of whether you might not want some treble rolloff at high gains, but if you did, this would not work well for that.

Right. Actually the bass rolls off at high gain (relatively speaking) which is very desirable for me. The high Zin from the bootstrap makes a nice "transparent" boost yada yada... at low gain. Whereas at high gain the Zin drops which keeps the bass from getting flabby, also smoothing the highs etc - very desirable although I suppose that's subjective in the end.

For me this is a good example of how from a cursory "EE glance" at a circuit it can first appear to be a poorly designed piece of electronics. But when you consider the real requirements ("smooth high gain- transparent low gain with a passive pickup as an input" vs. "ridiculously high Zin consistent over the whole gain spectrum") it's really a very well designed circuit that meets the reqts. The "EE" stuff is not directed at your comment btw, it's just a general observation. I'm an EE too fwiw.

[Doug_H]

honestly, in my experience, bootstrap circuits are not necessarily the quietest circuits! 

I wonder if that is partially due to the unbypassed emitter resistor?

Not that any of this has anything to do with the original question...

[GFR]

honestly, in my experience, bootstrap circuits are not necessarily the quietest circuits! 

Perhaps because bootstrapping is actually positive feedback?

[Jay Doyle]

Poor hilbli, asks a question and gets a bunch of dorks swinging our EE (or in my case non-EE) weiners around while completely avoiding an answer to his question:

To sum up for less noise in your AMZ MOSFET Boost (as is):

- Make R3 a metal film
- Make R4 a metal film
[These are the only two that can remotely effect the noise of the signal.]
ALSO: audition many BS170s to find one that has a low noise rating.

If you feel like adding/messing with the circuit:

You can bootstrap the input by adding a high value cap from the source node to the Vr junction. You will have to get rid of the power filter cap, but this will allow you to lower the value of R3 to a more reasonable amount so that the thermal noise of the resistor isn't an issue any longer. The inputZ WILL get lower as the gain is turned up.

I honestly would need a better description from R.G. to visualize his bootstrap slave method.

BUT, and this is something to consider if you feel like switching all of your power connections around, an option is to try a P-Channel MOSFET like the BS270. Joe Davisson has reported a lower noise in these units.

Hope you got this far to get an answer.

Good luck,

Jay

[petemoore]

I recently built a 2nd amz foost for my bandmate, he wants to use the amz mosfet boost to bring up his strat to the same volume as his les paul.
  Have you tried replacing the 2nd MB for the 1rst MB in that position?
The volume level is working fine only he complaints that his signal has become more noisy when using strat with amz mosfet boost turned on, escpeccially when he is also using his overdrive.
When he is using the boss eq to boost up the volume of his les paul his signal has much less noise.
Is there a way to bring down the noise in the amz mosfet boost?
  I'm not certain this is a problem inherent with other Mosfet Boosts into Overdrives scenarios [ie maybe there is something 'unique' to this 2nd Mosfet Boost or the OD...]...er, I think we agree it 'shouldn't do that'...and taking it one step further, maybe it doesn't do it except under the exact conditions in your bandmates case.
  Where I'm going with this is...perhaps 10meg resistors of varying tolerances [high/low] won't adress the noise issue directly, nor will other noise reduction methods, the Mosfet Boost circuit is generally known as being ~quiet and well behaved.
  Not that 'quiet' mods can't reduce low noise even further, but more along the lines of what you're experiencing isn't necessarily caused by resistor noise...the description leads me to believe the noise you're experiencing is greater than what the MB usually makes, and may [or may not...for all I know about the problem and scenario] be related to resistor noise.