adding impedance and what is a servo balanced output?

[sbm]

Greetings folks,

I have a two-part question related to some literature I've seen in regards to stompboxes, and I wonder if they really play any conscious part in building stomp boxes.

1)  I've seen some equipment state that the input you plug into has 1M impedance.  I understand that in many (most, all) cases, the point is to pull voltage in preference over the current in order to try and capture as much of the signal as possible, without stripping any highs.  Is the addition of a 1M resistor all that's required between the input signal and ground to accomplish this?

2)  I've also seen some stuff in the high-end audio world (and probably nothing to do with pedal building and such) state that the outputs are servo balanced.  What does this mean exactly, and what effect would this have on plugging the output of this to a pedal's input?  What kind of circuit would create this?

Thanks for any insight.

[R.G.]

1)  I've seen some equipment state that the input you plug into has 1M impedance.  I understand that in many (most, all) cases, the point is to pull voltage in preference over the current in order to try and capture as much of the signal as possible, without stripping any highs.  Is the addition of a 1M resistor all that's required between the input signal and ground to accomplish this?

No. The addition of a 1M resistor between input and ground would cut the input impedance that the signal sees to 500K. If the input of the effect has a 1M input impedance, adding another 1M to ground puts the new 1M in parallel with the effect input, and so eats twice as much signal current. 1M between input and ground is used for other things, notably stopping popping in "true bypass" pedals.

2)  I've also seen some stuff in the high-end audio world (and probably nothing to do with pedal building and such) state that the outputs are servo balanced.  What does this mean exactly, and what effect would this have on plugging the output of this to a pedal's input?  What kind of circuit would create this?

The high end audio world is very much like Disneyland on acid. 

Outputs being servo balanced means that (1) the outputs are DC coupled, and so have a frequency response all the way down to DC; (2) the device has a circuit that senses any DC on the output and drives the input of the device to force the output voltage to be very close to zero; (3) this will have substantially no effect whatsoever on a pedal's input, as almost all pedals are NOT DC coupled on their inputs, so they mostly don't care, with the exception of switch popping on bypass if there is any DC level on the input. (4) the circuit used is an integrator which has the integration tied to the output, the reference to ground, and the output of the integrator to a DC coupled point inside the unit. The integrator will sense any DC voltage other than ground/0V and drive the DC coupled point on its output in a direction that nulls out the offset.  In general, this is just one integrated circuit opamp set up as an integrator.

[PRR]

"1 Meg" is the customary limit for a vacuum tube grid resistor. Until the 1980s, that was the highest input impedance a mike/pickup designer could work to (without great excess cost).

1947-1976 Fenders Gibsons Ampegs run about 1Meg input.

This is NOT terribly critical for wound pickups on the end of a cable. A ten-foot cable is 300pFd. 300pFd is similar to 100K ohms at 5KHz, 35K ohms at 15KHz..... anything above 500Hz is dominated by cable capacitance loading, NOT the 1Meg. With a wound pickup there is also a resonance in the 2KHz-4KHz range, where the effective impedance can be higher than capacitive impedance (but lower than the 1Meg grid resistor). But if it were a lot higher it would sound ringy and unpleasant. There may be a peak to 400K around 3KHz. As long as the grid resistor is higher, loading is small. (Some later Ampegs designed for a 2.7Meg input, perhaps in hopes of "more string-zing; these amps did not take over the market nor did the idea of very hi-Z inputs catch on with other designers.)

> Is the addition of a 1M resistor all that's required between the input signal and ground to accomplish this?

WHY? You feed the input with whatever you got. Don't tack stuff on, that's just loss.

As R.G. says, another 1Meg to ground gives 500K. Your pickup is 5K bass and mids, say 400K at the top useful overtone. The 5K bass-mid will drive 1Meg, 500K, even 100K fine; but the 400K top ring is mildly loaded by 1Meg and significantly loaded by 500K. Net result: same bass-mid, less top-ring, "highs reduced".

You may be thinking of "impedance match". You have amplifiers. You NEVER want a true impedance match.

> as much of the signal as possible, without stripping any highs

Well, if there were an "ideal" pickup for electric guitar, you would WANT to "strip highs". All strings have inharmonic overtones; even if they didn't, the overtone series turns sour past the 11th or 13th or so.

But if you want "more highs" from e-guitar, use VERY short cable and a wide-range (which will be low-output) pickup winding. With typical inputs, a 1-foot cable will shift the pickup's top-ring an octave higher. (This is part of what onboard electronics do; but they or the amp have to limit the extreme top end to be musical.)

You can also boost highs with the amplifier. If the treble control is not enough, there's various top-boost and screamer add-ons.

Note that all guitar speakers rise to a peak at 2KHz-6KHz and then DROP. You may add a piezo tweeter to carry the top two octaves. With a hot cone, you may need a serious array of piezo tweets to be heard. And once heard, you may find it irritating.

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> outputs are servo balanced

We do not listen to DC. We do not want stray DC on our audio. Amplifiers tend to have stray DC. The direct cheap way to fix this is a Blocking Capacitor. However in many situations this must be a physically large cap. Or a less-large cap with dubious perfection. "Servo" is an amplifier (yes, anOTHER amplifier) levering a small (and perhaps more-perfect) cap to do nearly the same thing as a large Blocking Cap (if everything is well).

[sbm]

Wow!  RG and PPR, what excellent replies, thanks very much.

One other view I am coming from is not just to add to an existing circuit, but more so the motives to design 1M impedance and servo balanced outputs in the first place.  (In other words, what do you get out of that?) But I think I've got the answers to that as well.  Thanks!