Most Transparent Buffer?

[merlinb]

I wonder if i could make the Impedence setting resistors switchable  and find say 3 different resistors with 3 different values?

You could use something like a 2Meg pot. Then you would have an infinity of input impedances to play with!

I am wondering if it is necessary at all to filter out the inaudible frequencies.

It's certainly good practice to have an RF filter at the input of any opamp circuit that connects to the outside world.

[karbomusic]

You could use something like a 2Meg pot. Then you would have an infinity of input impedances to play with!

I have some 11 detent position pots I had considered using for that exact purpose.  

[upspoon12]

You could use something like a 2Meg pot. Then you would have an infinity of input impedances to play with!

I have some 11 detent position pots I had considered using for that exact purpose.  

I have been looking for some of those 11 knobs. Havent found em on ebay for a while >=|

[ashcat_lt]

While being both facetious and serious, I use the tone knob for that; it's built in already.

That.

Keep in mind that the resistor which "sets the impedance" is actually there in order to bias the guitar signal up to the middle of the power supply so the buffer can reproduce the full swing.  Any noise that comes with that bias voltage will go right on through to the amplifier, and any system with wires going off the board to a switch and back will add at least some noise.  You can't filter that noise, because you'll also end up filtering the guitar signal.  It's only a problem if it's a problem, but in the one I built it was a problem.

The other thing to keep in mind is that it's kind of diminishing returns as the impedance goes up.  The difference between 100K and 500K is immediately noticeable.  The difference between 500K and 1M (which is actually more since 500 > 400) is pretty subtle.  The difference between 1M and 10M...well, it's hard to say that it's not just placebo effect.  

For the most part, the difference is not in the actual cutoff frequency (until the in-Z gets way low) but in the height of the resonant peak right at cutoff.  You can't really get higher frequencies out of the thing without changing at least the cable (and/or the pickups), but a higher in-Z will give you a little bit more of the highest frequencies that you are already getting.

Really, I don't think you'll ever miss anything if you just set the in-Z at 1M and go.  Back to the quote.  The T pot on your guitar sits parallel to the buffer input.  The load impedance seen by the pickups can never be bigger than whatever that pot is set to.  Most of the action of that pot (like the top half of its rotation) comes more from this change of load, with the capacitor itself doing almost nothing.  Turning the T pot on your (passive) guitar is exactly the same thing as changing the input-Z of the first active device after it.

[ashcat_lt]

...tried to modify, quoted instead...

[upspoon12]

Really, I don't think you'll ever miss anything if you just set the in-Z at 1M and go.  Back to the quote.  The T pot on your guitar sits parallel to the buffer input.  The load impedance seen by the pickups can never be bigger than whatever that pot is set to.  Most of the action of that pot (like the top half of its rotation) comes more from this change of load, with the capacitor itself doing almost nothing.  Turning the T pot on your (passive) guitar is exactly the same thing as changing the input-Z of the first active device after it.

While everything in your post made a lot of relevant sense to me, This line was the AHA! moment for me in regards to this question!


THANK YOU!
haha much new understanding these past couple days.

[PRR]

> partner who is a session guitar player.

I'd think his goal would be crisp and clean (including any intended distortion) and *consistent*.

Whatever he plays, the engineer and Producer will twist and sculpt to match *their* vision. If he plays bright, but that doesn't sit-in-the-mix with a bright (or brightENED) vocal, they will darken-down his guitar to taste (their taste, not his taste).

And if he's delivering different tone on different takes, which have to be inter-cut to get a complete track, they will go mad trying to get a unified sound. And may call someone else next time.

Buffer at guitar could be very good. One day the set-up is so tight he's practically sitting on his amp with a 3-foot cord. Another day the producer calls for naked guitar but the amp in another room (isolation) and a 50-foot cord. A buffer at the "naked" guitar could make both set-ups sound similar (+/- room effects and loss of self-feedback).

You would think any recording studio would have low-low radio interference. Some are quite careful. Others are clueless. And I've had watchmen with 5-Watt walkie-talkies walk-through in the middle of a take. My feeling is that anything over 20KHz ought to be cut early.

[upspoon12]

You would think any recording studio would have low-low radio interference. Some are quite careful. Others are clueless. And I've had watchmen with 5-Watt walkie-talkies walk-through in the middle of a take. My feeling is that anything over 20KHz ought to be cut early.

This is my thought as well. He owns a recording studio and we are both audio engineers so i understand all of that. and His studio is well insulated as well. However he does piece work for other sessions and many different players/artists who demand him to be completely versatile with his tone, with each of his guitars (which is  upwards of 12-14 in regular use)So my major goal is to make the looper and anything that i make for him with much versatility.

He is excellent with delivering consistent tone and everything when he does a session and his recall for rerecording is phenominal. Its more on the different session to different session front where he needs to sound different on any given day.

Buffer at guitar could be very good. One day the set-up is so tight he's practically sitting on his amp with a 3-foot cord. Another day the producer calls for naked guitar but the amp in another room (isolation) and a 50-foot cord. A buffer at the "naked" guitar could make both set-ups sound similar (+/- room effects and loss of self-feedback).

so from the sounds of this you think it would be beneficial to have a buffer box alone close to him so that it can run the length of a long cable - this makes total sense. Having said that, would it be advisable to have a buffer in his looper on top of that?

[Gus]

So is it a buffer or a gain stage set higher than X1?

Do  search for "buffer" at this site.  Check R.G.s site for a cool little buffer

If this person is a pro session player
I would use the best mechanical parts
Add RF rejection like PRR posted
Make sure the buffer is stable in worse case environments
things like
a long cable with high capacitance on the output
protection for the output if is was connected to phantom power by mistake in a studio
input protection from a possible large input signal
as posted 1 meg might be a good compromise for input resistance
if you are going to make something switchable the input cap might be better for a changeable input highpass frequency control

[bool]

I have used buffer-ed basses as sessionist in late 80's and during 90's and early 00's (technically it wasn't a true buffer because it also had 2dB gain to match "active" basses, so a booster). There's more than meets the eye wrt a "good" design. The most important thing to notice is that while you may design a circuit that on paper (or in simulator) looks "perfect", there will be interactions between gear that will be used so you have to design around that. To be useful in a real situation, even a simplistic design like a buffer will have to be designed to behave with what it will be plugged into. A 1Hz-100kHz bandwidth, f.e, is a waste of everybody's time. You will need to reject subsonics and RF and provide high-enough output impedance to make it well behaved but not too high to loose the "grip" on the cable. If you want to plug into a fuzzbox, try a 20kOhm potentiometer (as rheostat) wired in series with the output.

[upspoon12]

So is it a buffer or a gain stage set higher than X1?

Do  search for "buffer" at this site.  Check R.G.s site for a cool little buffer

No it will be a buffer, i do not want to boost signal past x1 i will want it to be as close to 1 as possible.

I looked all over the geofx site and couldn't find this buffer you were refering too, i'm sure its there, it'll likely be a case of not looking past my nose.

If this person is a pro session player
I would use the best mechanical parts
Add RF rejection like PRR posted
Make sure the buffer is stable in worse case environments
things like
a long cable with high capacitance on the output
protection for the output if is was connected to phantom power by mistake in a studio
input protection from a possible large input signal
as posted 1 meg might be a good compromise for input resistance
if you are going to make something switchable the input cap might be better for a changeable input highpass frequency control

This is excellent information, a couple things here that i now realise were quite the oversite in what is required. rf rejection filters and subsonics etc i have a fair grasp on. However not sure what might protect the circuit from an accidental application of +48v. The 1 meg does sound like a nominal starting point, i'll likely try a couple of other points abocve and below but that will definitely be the starting point.

I certainly like the idea of the switchable input cap. Thanks for the suggestion!

I have used buffer-ed basses as sessionist in late 80's and during 90's and early 00's (technically it wasn't a true buffer because it also had 2dB gain to match "active" basses, so a booster). There's more than meets the eye wrt a "good" design. The most important thing to notice is that while you may design a circuit that on paper (or in simulator) looks "perfect", there will be interactions between gear that will be used so you have to design around that. To be useful in a real situation, even a simplistic design like a buffer will have to be designed to behave with what it will be plugged into. A 1Hz-100kHz bandwidth, f.e, is a waste of everybody's time. You will need to reject subsonics and RF and provide high-enough output impedance to make it well behaved but not too high to loose the "grip" on the cable. If you want to plug into a fuzzbox, try a 20kOhm potentiometer (as rheostat) wired in series with the output.

awesome. good stuff to know here. Thanks!

[R.G.]

I looked all over the geofx site and couldn't find this buffer you were refering too, i'm sure its there, it'll likely be a case of not looking past my nose.

Try here:
http://www.geofex.com/FX_images/Onboard_Preamp.pdf

It's specifically optimized for being inside the guitar/bass. It has low current, long battery life, flat frequency response, and low distortion.

[upspoon12]

Thats awesome, thanks R.G.

much appreciated! This is an interesting circuit!