buffer input impedance

[i2k]

What is the effect of buffer impedance to tone ?
Some booster boutige builders claims that their pedal will function as an ultra high impedance buffer (up to 5 M) which can allow the guitar sounds better. Is this claim is true ? As most of the buffer schematics I saw here only has 1 M impedance.

[ubersam]

In guitar applications, a device with low input impedance will load down a guitar p-up's signal causing signal losses. The higher the input impedance of the device, the lesser the signal losses. 1M is a decent value for most applications, with guitar p-ups impedances ranging from 10K-15K. Jack Orman explains it better here: http://www.muzique.com/lab/imp.htm

[R.G.]

A guitar pickup has an impedance which is resistive, inductive, and capacitive. At low frequencies, the resistance dominates and the impedance is pretty much the impedance of the wire resistance. As frequency goes up - which includes harmonics on the strings - the inductive nature begins to predominate and the impedance rises, becoming quite large, 100's of K to M's. Finally the self capacitance kicks in and the impedance drops off, but so does the output. This happens out in the 5KHz to 10kHz region, depending on the pickup's winding details.

The rule of thumb on buffers is that you want the input impedance of the buffer to be at least 10x the impedance driving it, and the output impedance of the buffer to be no more than 1/10 the impedance of what it drives. Higher input impedance is better and lower output impedance is better in general (there are exceptions) but diminishing returns set in quickly at 100X input impedance and 1/100 output impedance.

1M is usually enough to preserve treble for a typical pickup, which is why you see a lot of 1M's. There are people who maintain that you need even higher input impedances for better tone - meaning less treble loss, probably. 5M is the biggest commercial input impedance I know of, on the SHO.

Generally getting over 1M means going to JFET or MOSFET devices, even if that's in the input of a BiFet or MOSFET opamp.

[Gus]

R.G.

  The schematic I saw of the SHO looks like the 5meg is only at min gain.  The 10meg drain to gate reduces the input Z do to feedback More gain lower input Z.

 

[Gilles C]

My Fishman G II guitar preamp has 10M input Z, but is mostly for acoustic guitar which has a higher Z by itself and are affected by low Z preamps.

Gilles

[amz-fx]

The Catalinbread Sagrado Poblano Picoso and Super Chili Picoso pedals are based on the AMZ Mosfet Boost and the input Z is 10M.  This should be enough for any guitar pickup as well as many piezos.

regards, Jack

[R.G.]

Ack! (slapping forehead)

I KNEW I should have done a search through all of the boutique builders before replying. 

Is that the same Catalinbread that cut his teeth here on the forums?

[Gus]

R.G.

I  googled it.

http://www.catalinbread.com/picoso.html

How do you get 35Db of CLEAN boost with a 9VDC supply? or did I read the copy wrong?

Also The SHO is the first effect on the market I know of that used a bs170 in a circuit somewhat like the AMZ mosfet boost  Yes the two have some differences one has a more constant input R and one changes a little with operation point and input R with gain setting.   I think the SHO is a cool circuit.

R.G. I wonder how many remember the ampage days when figuring out the rangemaster and SHO circuit was being done.  I am suprised someone did not make a mosfet treble boost like posted at amapge about that time IIRC.

[brett]

Hi
I find that about 200k to 300k is a really useful input impedance for most pedals, and especially for amps. 
IMO tone is sometimes poorer when the highest audible frequencies produced by guitars are enhanced (8 to 15kHz).  This is certainly true for effects: turning the filter control down on a Rat gives a pretty awful results.  A "raw" Rat has way too many harsh overtones.

What would explain Jimi Hendrix's use of curly, treble-sucking cables?  Could it be that he wanted to cut those highs?  I also wonder if he felt that harmonic-producing effects such as the Octavia and the Fuzzface work best when supplied with a signal that has been scalped of overtones?
sorry for going kinda off-topic

[RedHouse]

...What would explain Jimi Hendrix's use of curly, treble-sucking cables?  Could it be that he wanted to cut those highs? ...

This occurred to me also a few years ago and I've been experimenting with it since. In my tube amp builds I have been using 47pF caps on the plate resistors which seems to help in that area rolling off highs.

This certainly fly's in the face of the cable-mojo pedlers who tout their cable have very little loss of highs.

I think in Hendrix's case it was because he liked a very midrange'y sound and when he was into the feedback zone he tried very hard to get the strings to feedback and not the pickups, listening to any of his live recordings (and see his DVD's) you can see that whenever he got pickup squeal he always adjusted something failrly immediately. Taking advantage of treble-cut would be a great help there, not to mension all the RF he picked up having those FF and RM fuzz pedals maxed.
(I'd love to see a close-up pic of one of his Marshalls Presence controls, I bet it was at least 1/2 way down)

[slacker]

What would explain Jimi Hendrix's use of curly, treble-sucking cables?

He used curly cables for the same reason everyone else did, they looked cool.
Flame away 

[rockgardenlove]

^Wouldn't be suprised.  Also, they're nicer to be on stage with IMO.  I use them as tend not to get tangled up as easily.  Normal cables always end up alll twisted up.

As for actual tonal differences, I personally don't find it too major...

[Mark Hammer]

1) People have used curly cables on single-coil guitars for the same reason they have used 250k volume pots - "constructive loading".  Some folks feel that preserving too much of the inherent brightness of single-coil pickups produces a "brittle" sound, and the use of volume pots that load down the pickups is for taming that ultra-high end.  Curly cords are used in the same manner.  You will note that guys like Eric Johnson will use curly cords deliberately to get a warmer tone, although a little bit of experimentation with a small-value cap bridging input and ground on the volume pot might be a better idea if you ask me.

2) No, you read the copy right.  It said 35db.  Of course, even if you *could* get 35db of clean gain from the pedal on a 9v supply, you sure wouldn't see a clean signal from the amp with that sort of input signal slugging the input stage upside the head.  There IS a certain amount of clean headroom built into even the gainiest tube amps, but 35db headroom is generally more than what is built in.  Nope, that much boost gets you amp grind when you even THINK of of picking hard.  Of course, the gain is the gain is the gain, and is only related to actual output level, not equivalent to it.  I can see where a low-output pickup on an instrument lightly strummed could result in a a pretty clean output signal from the pedal itself, with 35db of gain.  You'd have to restrain yourself, but it'd work.

3) One of the key advantages of high input impedance is in grappling with the cable capacitance of very long cables.  What counts as a "high-enough" input impedance will depend on the length of wire used to carry the input signal and the cable capacitance per line foot.  Tube amps generally have pretty high input impedances.  Try playing your guitar, clean, through your amp, clean, using the shortest patch cable you have, then the longest cable, and maybe two of the longest cables (wired together in a true bypass stompbox), to hear what input impedance has to fight against.  A 1ft cable of any appreciable quality, compared with 40ft of the best cable you have can easily sound like a different instrument, all with the same input impedance.  Of course, that effect is the cumulative impact of all that cable capacitance, with no change in either the output impedance of the guitar, or input impednace of the amp.  The effect of that same cumulative cable capacitance is reduced when either a) the output impedance of the insrument is lower, b) the input impedance of the amp is higher, or c) both.  Of course, as #1 above illustrates, not everybody is in a hurry to preserve all that upper treble.  Some think it harsh.