On board Elec. buffers & noise reduction

[Phorhas]

Hello, I have a couple of questions about the subjects mentioned:

1. should an on-board buffer be set pre-volume pot or post-volume (like an external one)? what are each placement's benifits over the other, if any?

2. If I wish to have a noise reducing buffer, should I split the input to both inverting and non-inverting inputs of an OA, setting the non-inverting to a gain of 2 and the inverting to a gain of 1, so not to thin out the tone, or will this set up not work?

Thanks,
Dan

[R.G.]

1. should an on-board buffer be set pre-volume pot or post-volume (like an external one)? what are each placement's benifits over the other, if any?

For lowest noise, amplification should happen before any attenuation or extra resistance. So for lowest noise, amplify/buffer before any other resistances. The difference may be small with as large a signal as a pickup makes. However, getting the signal before any other components also minimizes the loss of treble to any parasitic capacitances, as well, so you may do a better job keeping treble if you buffer right after the pickup.

2. If I wish to have a noise reducing buffer, should I split the input to both inverting and non-inverting inputs of an OA, setting the non-inverting to a gain of 2 and the inverting to a gain of 1, so not to thin out the tone, or will this set up not work?

Buffers never reduce noise, they can only add noise. However, if this lets you avoid other noise sources later, it may be a net noise advantage.

For the lowest noise in *any* opamp circuit, the input should always be inverting. This works directly against you in keeping treble, because an inverting opamp generally has a low input impedance. The noise increase in a non-inverting amplifier is a minimum of 1.414 times the noise in an inverting amp; but this may be worthwhile because of the increase in input impedance and the amount of treble you keep.

Guitar pickups are quite difficult to design low noise inputs for because the source impedance starts low (5-20K ohms) and steadily increases because of the inductive nature of the pickup winding. The treble end impedance is quite high, maybe half to two megs, and so needs an even higher impedance amplifier input to avoid treble loss. Very high impedance inputs also pick up noise badly, so it might take considerable reworking and shielding to get truly high end results consistent with your questions.

[Phorhas]

Gotcha

So more Q's...

1. For a pre-volume buffer, what uotput impedance should I set, is the lowest possiable - always better?
2. What reworking, apart of cavity shielding, should I make when designing?
3. Basically, an external, post-volume buffer is better than nothing, right?
(oh, and should I use the lowest output impedance possible as well)?

Thanks,
Dan

[R.G.]

Before I start into this, remember that the law of diminishing returns is always with us. You typically get the anecdotal 80% of the return for the first 20% of the effort, and expend the next 80% of the effort to get the last 20% of results.

1. For a pre-volume buffer, what uotput impedance should I set, is the lowest possiable - always better?

It has to be lower than 10% of the lowest impedance that the buffer sees. In this case, it happens with the volume control full up and the cable capcitance in parallel with the volume control and the input of the amp (or effect!) that the buffer drives. Cable is often 30pF per foot, can be 100pF per foot. For a 20ft cable, that's 600pF - 2000pF, and the impedance of a 2000pF cap at, say 7kHz, is Xc = 1/2*pi*7k*2000pF = 11.4K. So to overwhelm that loading at that frequency, you need an output impedance of 1/10 of that, or 1.14K. An opamp can do that nicely, a simple transistor or JFET buffer has a harder time at low currents. Getting to 100 ohms or 10 ohms output impedance doesn't improve things much over that. So any output impedance around 1K is pretty darned good for this purpose.

2. What reworking, apart of cavity shielding, should I make when designing?

Since the buffer input impedance is **HIGH** and it's out waving around in the air, you need it shielded from the rest of the world, and particulary you need the buffer as physically close to the pickup as you can get it. ON the pickup is best. That may be hard to do. But again, you get most of the reward from cavity shielding. Putting the buffer on the pickup may make wiring power to/from it hard and may make pickup switching difficult.

3. Basically, an external, post-volume buffer is better than nothing, right?
(oh, and should I use the lowest output impedance possible as well)?

Very much so. A post volume buffer is much better than nothing. Again, the thing that kills you is the cable capacitance, and the same numbers apply.

[Phorhas]

First of all R.G, I wanna thank you.

now, one more... when putting a pre-volume buffer I can use a low resistence volume put with out it damaging my pickup power, yes?

Cheers,
Dan

[waldo041]

I use a pre-volume buffer and i use a 25k audio volume pot with no problem. hope this helps

peace,
waldo

[Phorhas]

thnks for all replies.

BTW, should I shield the buffer apart of the cavity shielding? something like using a shrinktube over the buffer board, copper sheet aroud it and then again - shrink tube over that?

[petemoore]

  YOu probably cant tell any difference shielding short wires inside of shielding..
  I've shaped copper sheet into a 'short u', insulated and ground wired it, then taped it as a ground plane under/to the circuitboard between very high gain circuits to good result.
  Buffer has a gain of ~1.

[Phorhas]

So let's design

Input from PUs going to 0.1uf cap -> 4M7 resistor to 1/2+v -> non-inverting input of OA (TL072 for example) w/o any resistors in the FBL -> 820r load resitor to buffer internal ground -> 10uf output cap -> 25k log pot to guitar round.

all of the grounding in side the buffer including the batt ar going to it's own GND plane, with onw line connected to the guitar ground.

stereo jack wired to save batt. life.

Vref is made by two 10k resistors through the 2nd half of th OA.

will this be a good on-board pre-amp?

[Phorhas]

Bump... ?