Effective OP-AMP Input Impedance, JFET vs. BiPolar

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby matt239 » 08 Aug 2011, 18:35

phatt wrote: I doubt you will ever reach **group consensus**

Perhaps not.. :lol:
- but maybe, at least, a kind of general agreement on a range of acceptable design parameters for a specific case?
One of the nice things about this forum, is not everyone's opinion is set in stone, & in general we seem open to evidence/science.

I really don't have an "opinion" on the issues I've asked about here; I knew I didn't have a complete understanding. I just want to know what will work, what the compromises & tradeoffs are, & what peoples reasoning/thought processes were..

DrNomis wrote: Well, I reckon you probably will need a buffer of some sort to provide a high enough input impedance for a guitar, the output of a guitar is usually at a high impedance (for passive pickups), a buffer will make the effect sound brighter, personally I think it's good design practice to design for a high input impedance, and a low output impedance anyway.... :hmmm:


Right, but one of the things postulated here was that an op-amp as a non-inverting amplifier, with some gain, - could have a high enough input impedance on its' own..
I don't think we've reached a definitive conclusion.. ???

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby merlinb » 08 Aug 2011, 22:13

matt239 wrote:Right, but one of the things postulated here was that an op-amp as a non-inverting amplifier, with some gain, - could have a high enough input impedance on its' own..
I don't think we've reached a definitive conclusion.. ???

Yes it has a high enough input impedance on its own. No mystery there.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby phatt » 10 Aug 2011, 04:06

matt239 wrote:
phatt wrote: I doubt you will ever reach **group consensus**

Perhaps not.. :lol:
- but maybe, at least, a kind of general agreement on a range of acceptable design parameters for a specific case?
One of the nice things about this forum, is not everyone's opinion is set in stone, & in general we seem open to evidence/science.

I really don't have an "opinion" on the issues I've asked about here; I knew I didn't have a complete understanding. I just want to know what will work, what the compromises & tradeoffs are, & what peoples reasoning/thought processes were..

DrNomis wrote: Well, I reckon you probably will need a buffer of some sort to provide a high enough input impedance for a guitar, the output of a guitar is usually at a high impedance (for passive pickups), a buffer will make the effect sound brighter, personally I think it's good design practice to design for a high input impedance, and a low output impedance anyway.... :hmmm:


Right, but one of the things postulated here was that an op-amp as a non-inverting amplifier, with some gain, - could have a high enough input impedance on its' own..
I don't think we've reached a definitive conclusion.. ???




Hi Matt,, obviously you are still a bit confused. LOL

>>Sadly the possibilities in electronics is just so vast that what might work in an **ideal perfect text book world** is more often than not,,, going to have
issues in a Real World application. Again I stress each techno freak has differing views on this but this is my way.
I tend to listen carefully when folks have built Real world equipment and are working LIVE players or at least worked a lot with stage equipment.
If max power transfer was the game then we would all be using oxygen free copper wire to join the nodes and hang 10 meg resistors off every input,,Ker??
Cricky This is an amplifier not a race car!!
Go turn up ALL the sliders on your graphic EQ and you have max tone but of course it's useless used in that manner,,There has to be loss for it to work well.
The trick is knowing where to use that loss to maximise the effect of tone shaping.<<
----
The pic should give a rough idea of how to keep the noise gremlins OUT.
Replace the opamp buffer with a transistor if you wish,,The opamp is obviously better.
Open a lot of pro gear and you will often find an opamp buffer is the very first thing the signal passes thru. If it's good enough for a lot of the better
equipment makers,,, then it's good enough for me,, wink.
With operational chips costing only a few more cents than a single transistor you may as well use opamps.

Note; I've not bothered to use exact values on the pic so up to you to work out the gain stuff.
I'd breadboard this stuff anyway so you soon get to work out what values to use.

As should be obvious, these 2 circuit have close to identical outputs,,, the first (u1) will be quite, while the gain is kept small but if you crank up R4 and R11 the circuit with the buffer (u2-u3) will be substantially quieter.

Re hi freq loss due to lower input impedance;

Consider this,
Some of the most revered Amps of yesteryear have absolutely shocking hi freq response. Those old Marshall Amps are devoid of treble above 4,000 CPS.
Yet most desire something resembling **THAT kind of TONE** and are often prepared to pay big dollars to get it.
Yet some tecks STILL constantly quote text book hifi influenced data which may well be correct for hifi but good guitar sound/tone is worlds away from flat.

As to the claim about loading of the PU's due to low imp input rolling off hifreq.

well yes if it's plugged into an old tape deck input or some other pathetic low Z input but an opamp buffer with 300k input is plenty high enough.
Yes your PU will loose a bit of hi freq content but most of it is not worth bothering with and it's DEAD easy with modern gear to have too much hi freq content,,,
Don't you guys what the famous tone ???????????????????? :popcorn:

Go work it out for yourself,, no electronics involved!!!!
FACT;
90% of the notes you can play on a guitar have a fundamental freq below 1,000 CPS
(1khZ).
For those with 24 frets the highest possible fundamental freq is a tad over 1,300hZ.
Now double it for some harmonic content, 2,600. x2 again for some sibilance, 5,200.

Now I'm the first to admit I'm crap at maths but you will find most of the Hi Freq loss that some babble on about is well above what's important for good
****LIVE Rock guitar****. (a recording studio is a whole other subject)

If heavy rock guitars where a high freq instrument the amp would have a 3 way hifi crossover setup much like a hifi speaker,,, go try it,, see how long you
can bear the sonic insult of distortion pedals through hifi speakers. Yukas-Pukass!!!!!!

Frankly It's up to your ears to work out what sounds better as we all hear differently,,but my money is on limited bandwidth.

If you still complain of tonesuck you likely have other deeper issues happening.
Try turning up treble on most modern gear and you are also turning up a lot of unwanted crap higher up the frequency bands which will mostly have a negative
impact asto much hifreq destroys good OD sound.

If You are not able to cut through,,,your problem is you have NO MID BITE.
THIS is due to The Whole circuit design NOT tone stacks or poor quality opamps
The Whole Amp has to work together and once the Amp cranks up you need to defeat the higher frequency content. Your tone stack on it's own CAN'T DO THAT.

**It's the severe lack of hi freq content that makes those old amps worth so much**

Someone once stated that a good Marshall had a mid range HONK that no other amp could deliver at that time,,, and I have to agree.
But that honk is not derived from GAIN in the mid band,,,,, confused yet? :scratch:

It is the *massive cut* above 4khZ (and also below 100hZ) that allows for the Amp to be played hard and loud and cut through. By subtraction you emphasize
the mid freq you need for good mid treble response so now when turned up the result is MID BOOST.
Bingo mid boost by ***Subtraction***

If you start building your own gear in the end you will find far greater TONES by cutting out frequencies that are not desired rather than trying to boost
the freq you do want to hear.

ADDING GAIN at freq you want will almost always add NOISE.
Tiss far better to wipe what is not used THEN turn up the gain.

PS,, if someone can give me a REAL World Example of 1 meg or 10 meg resistors delivering very low noise levels with off the shelf components while developing massive gain from one operational chip,,,,them I'm quite prepared to try it.

Until then I'll use buffers and medium input values. Winky.

A CARVIN Legacy with it's 220k grid resistor has more good guitar Mid/Treble that you will ever need. Replace it with a 1 meg and you WILL have issues.

I should know,, as I've done that. winky

Have fun,, Phil.

Cheers,, Phil.
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby mictester » 10 Aug 2011, 06:54

Phil

When working on Japanese effect designs back in the 80s, we experimented with both transistor and op-amp buffering. Transistors were chosen in almost every case because of cost constraints. You have to remember that most ex-factory pedal prices were in the range of $2 - $5, and adding an extra op-amp could almost double the component costs! Everything was always done down to a price!

It's quite instructive to build a Tubescreamer and start to substitute component values. It's also instructive to consider the low, guitar-loading input impedance of the Big Muff and the Fuzz face....
"Why is it humming?" "Because it doesn't know the words!"

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby phatt » 10 Aug 2011, 13:05

mictester wrote:Phil

When working on Japanese effect designs back in the 80s, we experimented with both transistor and op-amp buffering. Transistors were chosen in almost every case because of cost constraints. You have to remember that most ex-factory pedal prices were in the range of $2 - $5, and adding an extra op-amp could almost double the component costs! Everything was always done down to a price!

It's quite instructive to build a Tubescreamer and start to substitute component values. It's also instructive to consider the low, guitar-loading input impedance of the Big Muff and the Fuzz face....


Hi Mic, Yes point taken, Thanks for the input. :wink:

OK then if that is the direction the builder wishes to go with then why not a *bootstrapped BJT or FET input*?

That would raise the input Z.
Maybe even a FET with a BJT follower then too opamp. :hmmm:

My observation;
I read posts from a fair number of home type builders and quite a few obviously struggle with the more complex circuits and opamps can help to really simplify the layouts,, especially if they use perf board and like layouts.

So in my mind it may be a little more outlay up front but greatly raises there chance of first time success.
Phil.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby DrNomis » 10 Aug 2011, 14:03

phatt wrote:
matt239 wrote:
phatt wrote: I doubt you will ever reach **group consensus**

Perhaps not.. :lol:
- but maybe, at least, a kind of general agreement on a range of acceptable design parameters for a specific case?
One of the nice things about this forum, is not everyone's opinion is set in stone, & in general we seem open to evidence/science.

I really don't have an "opinion" on the issues I've asked about here; I knew I didn't have a complete understanding. I just want to know what will work, what the compromises & tradeoffs are, & what peoples reasoning/thought processes were..

DrNomis wrote: Well, I reckon you probably will need a buffer of some sort to provide a high enough input impedance for a guitar, the output of a guitar is usually at a high impedance (for passive pickups), a buffer will make the effect sound brighter, personally I think it's good design practice to design for a high input impedance, and a low output impedance anyway.... :hmmm:


Right, but one of the things postulated here was that an op-amp as a non-inverting amplifier, with some gain, - could have a high enough input impedance on its' own..
I don't think we've reached a definitive conclusion.. ???




Hi Matt,, obviously you are still a bit confused. LOL

>>Sadly the possibilities in electronics is just so vast that what might work in an **ideal perfect text book world** is more often than not,,, going to have
issues in a Real World application. Again I stress each techno freak has differing views on this but this is my way.
I tend to listen carefully when folks have built Real world equipment and are working LIVE players or at least worked a lot with stage equipment.
If max power transfer was the game then we would all be using oxygen free copper wire to join the nodes and hang 10 meg resistors off every input,,Ker??
Cricky This is an amplifier not a race car!!
Go turn up ALL the sliders on your graphic EQ and you have max tone but of course it's useless used in that manner,,There has to be loss for it to work well.
The trick is knowing where to use that loss to maximise the effect of tone shaping.<<
----
The pic should give a rough idea of how to keep the noise gremlins OUT.
Replace the opamp buffer with a transistor if you wish,,The opamp is obviously better.
Open a lot of pro gear and you will often find an opamp buffer is the very first thing the signal passes thru. If it's good enough for a lot of the better
equipment makers,,, then it's good enough for me,, wink.
With operational chips costing only a few more cents than a single transistor you may as well use opamps.

Note; I've not bothered to use exact values on the pic so up to you to work out the gain stuff.
I'd breadboard this stuff anyway so you soon get to work out what values to use.

As should be obvious, these 2 circuit have close to identical outputs,,, the first (u1) will be quite, while the gain is kept small but if you crank up R4 and R11 the circuit with the buffer (u2-u3) will be substantially quieter.

Re hi freq loss due to lower input impedance;

Consider this,
Some of the most revered Amps of yesteryear have absolutely shocking hi freq response. Those old Marshall Amps are devoid of treble above 4,000 CPS.
Yet most desire something resembling **THAT kind of TONE** and are often prepared to pay big dollars to get it.
Yet some tecks STILL constantly quote text book hifi influenced data which may well be correct for hifi but good guitar sound/tone is worlds away from flat.

As to the claim about loading of the PU's due to low imp input rolling off hifreq.

well yes if it's plugged into an old tape deck input or some other pathetic low Z input but an opamp buffer with 300k input is plenty high enough.
Yes your PU will loose a bit of hi freq content but most of it is not worth bothering with and it's DEAD easy with modern gear to have too much hi freq content,,,
Don't you guys what the famous tone ???????????????????? :popcorn:

Go work it out for yourself,, no electronics involved!!!!
FACT;
90% of the notes you can play on a guitar have a fundamental freq below 1,000 CPS
(1khZ).
For those with 24 frets the highest possible fundamental freq is a tad over 1,300hZ.
Now double it for some harmonic content, 2,600. x2 again for some sibilance, 5,200.

Now I'm the first to admit I'm crap at maths but you will find most of the Hi Freq loss that some babble on about is well above what's important for good
****LIVE Rock guitar****. (a recording studio is a whole other subject)

If heavy rock guitars where a high freq instrument the amp would have a 3 way hifi crossover setup much like a hifi speaker,,, go try it,, see how long you
can bear the sonic insult of distortion pedals through hifi speakers. Yukas-Pukass!!!!!!

Frankly It's up to your ears to work out what sounds better as we all hear differently,,but my money is on limited bandwidth.

If you still complain of tonesuck you likely have other deeper issues happening.
Try turning up treble on most modern gear and you are also turning up a lot of unwanted crap higher up the frequency bands which will mostly have a negative
impact asto much hifreq destroys good OD sound.

If You are not able to cut through,,,your problem is you have NO MID BITE.
THIS is due to The Whole circuit design NOT tone stacks or poor quality opamps
The Whole Amp has to work together and once the Amp cranks up you need to defeat the higher frequency content. Your tone stack on it's own CAN'T DO THAT.

**It's the severe lack of hi freq content that makes those old amps worth so much**

Someone once stated that a good Marshall had a mid range HONK that no other amp could deliver at that time,,, and I have to agree.
But that honk is not derived from GAIN in the mid band,,,,, confused yet? :scratch:

It is the *massive cut* above 4khZ (and also below 100hZ) that allows for the Amp to be played hard and loud and cut through. By subtraction you emphasize
the mid freq you need for good mid treble response so now when turned up the result is MID BOOST.
Bingo mid boost by ***Subtraction***

If you start building your own gear in the end you will find far greater TONES by cutting out frequencies that are not desired rather than trying to boost
the freq you do want to hear.

ADDING GAIN at freq you want will almost always add NOISE.
Tiss far better to wipe what is not used THEN turn up the gain.

PS,, if someone can give me a REAL World Example of 1 meg or 10 meg resistors delivering very low noise levels with off the shelf components while developing massive gain from one operational chip,,,,them I'm quite prepared to try it.

Until then I'll use buffers and medium input values. Winky.

A CARVIN Legacy with it's 220k grid resistor has more good guitar Mid/Treble that you will ever need. Replace it with a 1 meg and you WILL have issues.

I should know,, as I've done that. winky

Have fun,, Phil.

Cheers,, Phil.




I like your reasoning in this Phatt, I've just been trying out some of my pedal builds through my new Legacy Valve Custom 5 Guitar Amplifier, which I bought today from Billy Hyde Music, it was normally retailing for Aus $399.99 but I got it for just Aus $150.00, I noticed that it sounded a bit dark, so I tried my Baja Real Tube Overdrive pedal through it as I knew that the pedal was a bit bright and midrangey, well guess what?, it worked really well and the combination sounded pretty good, the humbucking pickups on my Ibanez guitar are naturally a bit dark, so I think I had a bit of synergy happening there...which was basically going along the lines of what you're saying here with respect to eqing... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby phatt » 10 Aug 2011, 15:08

Hello DrNomis,
If you like that,,Then have a read of this,,, don't worry about the attenuation stuff just get the concept,, EQ>dist>EQ>Dist>

Or how to became a tone guru by using common sense LOL :blackeye

http://www.amptone.com/truesecretofamptone.htm

Main page here (you will need at least a week to read it all, LOL.
(Some of it is a bit wordy and it does tend to repeat itself but there are some real gems inside it all)

http://www.amptone.com/index.html#eqconcepts.

Here's a snippet from the main page.

"John Murphy, chief engineer for Carvin Corp., wrote "the pre-clipping frequency equalization and post-clipping EQ are absolutely critical adjustments. Once you have a well-behaved clipper -- even if it's just simple diodes, as in the stomp boxes -- it is the precise combination of pre- and post-clipping EQ that mostly determines how an amp sounds. The 'secret' of the best sounding guitar amps lies in the pre-clipping EQ response curve."

Van Halen's guitar tech recommends an EQ pedal above all, as the most valuable pedal, in his book Guitar Gear 411: Guitar Tech to the Stars Answers Your Gear Questions, pp. 75-76."

So yes EQ is ***EVERYTHING*** and it's also everywhere in the signal chain.

Armed with such a plain and simple concept and with little electronic ability it is not that hard to get good tone.

Nearly every stomp box guru still does not get the plain obvious.
STOP building fuzz boxes with tone shaping in and around one opamp ,,,,you need the TONE IN FRONT,,, not in or after distortion
(Preferably an old styled HiZ circuit as the low Z or active type may not do so well.)

One only has to look at the basic structure of the famous valve era and you will see that a TONE STACK does indeed come before the distorting/ODriven power stage.
DING.
Oh and Yes I agree Some guitars do need specific tweaks to work well.
Phil.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby DrNomis » 10 Aug 2011, 15:19

phatt wrote:Hello DrNomis,
If you like that,,Then have a read of this,,, don't worry about the attenuation stuff just get the concept,, EQ>dist>EQ>Dist>

Or how to became a tone guru by using common sense LOL :blackeye

http://www.amptone.com/truesecretofamptone.htm

Main page here (you will need at least a week to read it all, LOL.
(Some of it is a bit wordy and it does tend to repeat itself but there are some real gems inside it all)

http://www.amptone.com/index.html#eqconcepts.

Here's a snippet from the main page.

"John Murphy, chief engineer for Carvin Corp., wrote "the pre-clipping frequency equalization and post-clipping EQ are absolutely critical adjustments. Once you have a well-behaved clipper -- even if it's just simple diodes, as in the stomp boxes -- it is the precise combination of pre- and post-clipping EQ that mostly determines how an amp sounds. The 'secret' of the best sounding guitar amps lies in the pre-clipping EQ response curve."

Van Halen's guitar tech recommends an EQ pedal above all, as the most valuable pedal, in his book Guitar Gear 411: Guitar Tech to the Stars Answers Your Gear Questions, pp. 75-76."

So yes EQ is ***EVERYTHING*** and it's also everywhere in the signal chain.

Armed with such a plain and simple concept and with little electronic ability it is not that hard to get good tone.

Nearly every stomp box guru still does not get the plain obvious.
STOP building fuzz boxes with tone shaping in and around one opamp ,,,,you need the TONE IN FRONT,,, not in or after distortion
(Preferably an old styled HiZ circuit as the low Z or active type may not do so well.)

One only has to look at the basic structure of the famous valve era and you will see that a TONE STACK does indeed come before the distorting/ODriven power stage.
DING.
Oh and Yes I agree Some guitars do need specific tweaks to work well.
Phil.



Cheers, I've always been going by what my ears were telling me what sounded good and what didn't sound good, I believe when I was hearing something that my ears told me was sounding good, was actually EQing at work, I used to like modding my Ibanez TS 10 to TS808 specs because for some strange reason, my ears were telling me that my modded TS 10 now sounded much smoother modded to TS808 specs, and when I replaced the stock JRC4558D with an LF353 or TLO72, my ears were telling me that it sounded even smoother..... :thumbsup


In the late 90's while I was doing a 3 year apprenticeship with my Dad as a Fitter and Machinist, I decided to try building an Amp Head using all Valve circuitry, after some though I decided to try combining the Mesa/Boogie Mk I preamp with a 50 Watt Marshall power amp, anyway while it worked the resulting amp sounded okay in clean mode, but practically characterless in hi-gain mode, I found that no matter how I adjusted the Lo Mi and Hi controls, the resulting tone was the same, harsh and unmusical, at high gain settings it tended to sound a bit flabby in the lo end, so I thought about it and came to the conclusion that the tone control stack was in the wrong position for them to be effective, they were situated before the distortion, and the distortion very heavily clipped anything coming from the tonestack.... 8)
Last edited by DrNomis on 10 Aug 2011, 15:36, edited 1 time in total.
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby mictester » 10 Aug 2011, 15:33

phatt wrote:OK then if that is the direction the builder wishes to go with then why not a *bootstrapped BJT or FET input*?


There are a few good reasons - bootstrapped stages tend to hiss (though your 4 kHz roll-off would go a long way to mitigate this!), and you need a fairly large capacitor in an electrically vulnerable spot in the circuit. I found that most of the cheap electrolytics (and remember - we were trying to keep things cheap) were really microphonic. I first noticed this with an experimental pedal with a bootstrapped input stage that would suffer acoustic feedback when used at "stage" levels, and would transmit a huge "bang" when the footswitch was operated!

FETs are OK for input stages, but it's too easy to punch a hole through the gate with the slightest ESD. Again, this leads to unreliability. This is why it isn't sensible to put CMOS switches on the inputs of effects (something I've seen lately in some high-priced Boutique rubbish). The most reliable topology was a BJT used as an emitter follower feeding into either an FET or CMOS switch to feed either the effect or the bypass. It's also a good idea to have something similar on the way out of the circuit, and you can also have a nice low output impedance to drive cables if you need to.

The basic Tubescreamer is a design classic, and is probably one of the most reliable music circuits ever made. It causes me great amusement when I see the clueless Boutique Boobs remove the BJT input "for greater clarity" or "more transparency". Looking at some of the Boutique stuff around today, I frequently wonder how much of it breaks, and whether the BBs have the ability to repair under warranty, or do they just strap in another gooped board?
"Why is it humming?" "Because it doesn't know the words!"

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby earthtonesaudio » 10 Aug 2011, 17:23

mictester wrote:
phatt wrote:OK then if that is the direction the builder wishes to go with then why not a *bootstrapped BJT or FET input*?


There are a few good reasons - bootstrapped stages tend to hiss (though your 4 kHz roll-off would go a long way to mitigate this!), and you need a fairly large capacitor in an electrically vulnerable spot in the circuit. I found that most of the cheap electrolytics (and remember - we were trying to keep things cheap) were really microphonic. I first noticed this with an experimental pedal with a bootstrapped input stage that would suffer acoustic feedback when used at "stage" levels, and would transmit a huge "bang" when the footswitch was operated!



The only reason for including a bootstrap cap is if the DC potentials are different. If the offset is negligible you can bootstrap with just a resistor. Example here.


FETs are OK for input stages, but it's too easy to punch a hole through the gate with the slightest ESD. Again, this leads to unreliability. This is why it isn't sensible to put CMOS switches on the inputs of effects (something I've seen lately in some high-priced Boutique rubbish). The most reliable topology was a BJT used as an emitter follower feeding into either an FET or CMOS switch to feed either the effect or the bypass. It's also a good idea to have something similar on the way out of the circuit, and you can also have a nice low output impedance to drive cables if you need to.


Totally agree with you for MOSFETs (including CMOS), but both JFETs and BJTs are equally robust to ESD.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby mictester » 10 Aug 2011, 21:56

earthtonesaudio wrote:Totally agree with you for MOSFETs (including CMOS), but both JFETs and BJTs are equally robust to ESD.


JFETs are, I agree, more robust than MOSFETs and CMOS gates, but experience and experiment shows that JFETs are not quite as robust as BJTs. We did lots of tests back in the early 80s. We found that a 2N5551 was probably the ideal buffer transistor because of its high operating voltage (160V Vce).
"Why is it humming?" "Because it doesn't know the words!"

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby matt239 » 11 Aug 2011, 02:50

matt239 wrote: ... one of the things postulated here was that an op-amp as a non-inverting amplifier, with some gain, - could have a high enough input impedance on its' own..
I don't think we've reached a definitive conclusion.. ???

I misspoke a bit here; We clearly could conclude that an o-amp has enough input-Z,
- What I meant was we hadn't come to a definitive conclusion on whether to use an op-amp buffer, or discrete FET, or BJT... all factors considered; cost, reliability, input Z.

@ PHATT
You make many good points, & on a lot of it you are "preaching to the converted" :D
-I don't feel the need to hang 10Meg resistors off every input.
-I don't expect or want a guitar rig to perform like a hi-fi.
-Most guitar speakers roll off steeply above about 6k, & I don't really require any response above about 10k in a guitar rig.
-And, I would never, for instance, raise all the sliders on a graphic EQ! :lol:

phatt wrote: ADDING GAIN at freq you want will almost always add NOISE.
Cheers,, Phil.


Right. - A corollary to this in the real world is: Once treble is gone, you can't get it back. (Without adding a lot of noise..) Therefore it's important to PRESERVE treble in cables, & inputs, & pre-amps. - You can always roll-off unwanted treble content in the later stages..

Of course it depends on the type of guitar tone your after.. for jazz or classic rock sounds, losing some treble @ the input is OK, some folks even used to plug in to the "Lo" input! :shock:
but, if you want any "twang" or "jangle" then some "treble" or "high-mids" -depending how you call it, has to make it into the amp.. -of course "very high" freqs will still be rolled off @ the speaker..

So, I'm getting closer, but I'm still not sure what to use!
I'm pretty convinced that anything >1M is overkill, & I'm pretty convinced that anything <500k will roll off too much treble for my purposes.. So there is a range @ least.. -but:

- is there some OTHER reason a discrete buffer would be quieter than going straight into the op-amp?
- how serious is the threat of spikes/ESD damaging an op-amp in circuit? even a BJT op-amp?
(Many of our boutique friends make "bufferless" units, do they have a high failure rate?)(honestly, I really don,t know..)
- how serious is the threat of spikes/ESD damaging a discrete FET input?
- is going straight into a LOW GAIN (<5x) op-amp stage much worse than going into an op-amp buffer?

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby matt239 » 30 Aug 2011, 05:59

Wow. we lost several posts there.. :shock:
I dunno if any of that can be recovered??

Anyway, you guys gave me some answers to my previous post ^ above.

And then I said something like:
OK. -- Seems like op-amp is the way to go...

This kind of brings me back to my original question: Won't the LM833 have;
- lower noise,
- plenty-high input Z

- possibly more resistant to ESD & large input transients than a FET input op-amp
- good overall performance/reasonable price..

Welcome back! :D

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby kleuck » 30 Aug 2011, 08:05

merlinb wrote:
phatt wrote: you are bound to end up with noise issues if you just keep hanging 1meg resistors of everything thinking you are adding some mojo.

Not true. Basic noise theory tells us that shunt resistors need to be as *large* as possible for maximum signal-noise ratio (and series resistors should be as small as possible). High *node* impedances are susceptible to hum and other capacitive interference, but in any normal setup the source resistance will shunt the 1Meg resistance, thereby reducing the node impedance.

A 220k input impedance will cause noticeable damping of pickup resonance and treble, especially with humbuckers. 400k is pretty much the bare minimum you want to aim for.

Indeed : http://terrydownsmusic.com/technotes/gu ... cables.htm
Here the variable is the pot.
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Q: Is there any advantage to using solder with a 2% silver content?
A: Yes. Silver solder keeps werewolves away from your amp.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby earthtonesaudio » 31 Aug 2011, 02:13

mictester wrote:
earthtonesaudio wrote:Totally agree with you for MOSFETs (including CMOS), but both JFETs and BJTs are equally robust to ESD.


JFETs are, I agree, more robust than MOSFETs and CMOS gates, but experience and experiment shows that JFETs are not quite as robust as BJTs. We did lots of tests back in the early 80s. We found that a 2N5551 was probably the ideal buffer transistor because of its high operating voltage (160V Vce).


Probably a bit much to ask but do you happen to recall the failure modes you encountered? I haven't done any serious reliability testing but from a pure theory standpoint I wouldn't expect there to be much difference. Both JFETs and BJTs have a PN junction at their input, so why would one be more fragile than the other? :scratch:
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby phatt » 01 Sep 2011, 15:50

matt239 wrote:
Right. - A corollary to this in the real world is: Once treble is gone, you can't get it back. (Without adding a lot of noise..) Therefore it's important to PRESERVE treble in cables, & inputs, & pre-amps. - You can always roll-off unwanted treble content in the later stages..

Of course it depends on the type of guitar tone your after.. for jazz or classic rock sounds, losing some treble @ the input is OK, some folks even used to plug in to the "Lo" input! :shock:
but, if you want any "twang" or "jangle" then some "treble" or "high-mids" -depending how you call it, has to make it into the amp.. -of course "very high" freqs will still be rolled off @ the speaker..



Hi Matt,
Re the use of Low Input; I would not hassen to stake my life on that assumption.

Contry to popular reading,,There is more USEABLE top boost available if you do a simple tweak on the low input. :secret:
This might help get your head around how we assume so much only to find out it is far from accurate.

The missing link for Texas tone.
viewtopic.php?f=2&t=4316
(My circuit Drawing is halfway down)
Not my great mind that worked it out just years of searching out how Amps really work and I stumbled on this trick. :wink:

There is this blanket assumption that ALL guitar inputs must be at least 1meg to get some kind of mojo happening.
IME it's more likely the cause of many frustrations.
Sure 1 meg on a triode gives max sensitivity for mag PU's but it's not the only way to get great things to happen.

Go have a look at the input of a *Carvin Legacy*,,, first triode has 220k Grid R.
That Amp has more treble than anyone would ever need.
1 meg and it would probably all turn to crap an sludge.
Of course opamps are not triodes and things are a lot different but just giving you some food for thought.
Phil.
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby kleuck » 01 Sep 2011, 16:04

That's not a "treble" issue, but a "characteristic resonance peak" which gives the character and liveliness of a specific guitar/Pu/cable issue.
And yes, when it's gone, it's gone, you can boost the treble, and what you have is only the almost exact same sound as anyone's guitar, which would sound pretty differently in a more classical amp.
I had an amp like that, indeed worst : 100K on the output, a Tele sounded almost the same as LP Junior !
Eventually, i modified that first.
Read this page, it's obvious :http://terrydownsmusic.com/technotes/guitarcables/guitarcables.htm
And this one too : http://buildyourguitar.com/resources/lemme/
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby matt239 » 04 Sep 2011, 06:58

The RLC network of your pickup, controls, cable, and input Z determines both the resonance peak, & treble roll-off above that peak.

The modification to the Lo input trims bass so that it appears to lose less treble.. Why not just not lose treble in the first place??
I also don't see the point of using the Lo input to pad the signal/lower the level. The pre-amp section in Fender amps isn't going to distort with any normal guitar signal.
If we seek more power tube saturation from Fender amps, use smaller amps, or power-soak.

The relationship between the phase splitter and the power tubes is fixed. If you reduce your level @ the amp input, & then crank the volume on the amp, you're still driving the power tubes hard. (or if you make the guitar signal quiet enough, then the amp would just get quiet; no power tube saturation.)
To change that would require changing the gain structure in the amp; less drive, & more inherent power-amp gain.

Low noise requires getting as much level as possible into early stages of a device (short of distortion, & with a little safety margin..) & using less gain.

I'm not really seeing a compelling reason NOT to use a 1M pull-down resistor, 150 Ohm series resistor, & an LM833.

Right? :D
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby marsens » 04 Sep 2011, 09:10

How about controlling input impedance to get interesting result in the output signal?
For example:

> In high gain circuit (fuzz, distortion, etc), I find it's too sharp and harsh to my ear if frequency above 4kHz is equally boosted as frequency below 4kHz. And also not really good if low frequency is also boosted equal than the rest (muddy?).
> In clean boost circuit, high impedance may give sparkle and crisp that is, musically, good to my ear.
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Re: Effective OP-AMP Input Impedance, JFET vs. BiPolar

Postby matt239 » 04 Sep 2011, 20:41

Right. The Lo input Z is supposed to be part of the magic with the Fuzz Face, though I think a fuzz with a buffer in front can also sound good.
Why not just shape the pre-fuzz tone with filtering? That way you can choose the precise frequency response desired, & even make it switchable or adjustable.. :)

So for a general purpose device:
I'm not really seeing a compelling reason NOT to use a 1M pull-down resistor, 150 Ohm series resistor, & an LM833.

Right? :D
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