[sdiy] OT: Bass Guitar Preamp

[Tim Daugard]

From: "Scott Gravenhorst" <music.maker at gte.net>

Tim:
> >The redesign that I'm not happy with, adds a variable impediance matching network to
allow
> >the instrument to be loaded heavier. I play bass, I wanted to be able to load the bass
> >down to reduce the high end. This is the circuit that adds noise.

Scott:
> I'm not a bass player, so consider the source - why not just use a good variable cutoff
LPF
> designed for the bass guitar freq range to reduce the high end?  Maybe a parametric
equalizer for
> bass?

Not a problem. The FLT, FLV, and some of the other modules were created just for this
purpose. I prefer this method. If you look at the patch pages I put up, the first patch is
a straight clean path. Distribution amplifiers (ADA), switching (SWE?) and then the mixers
(AMX) allow the signals to be rerouted to filters etc. This is one of the reasons I
started building a modular system.

Many effects pedals (if you carefully analyze them) use the same method - an input buffer
transistor or opamp - the effect - an output driver. The biggest problem I see with
multiple pedals is:

Instrument => InBuf -> FX -> OutBuf => InBuf -> FX -> OutBuf => (etc.) => InBuf ->
Preamp -> Power Amp

My method is:

Instrument => InBuf -> DA -> FX -> (Buff if needed) -> FX -> Mixer -> OutBuf => Power Amp

Scott
> What is the advantage of loading the instrument to perform this function?

Some effects boxes are reported to create some of the sound quality by loading the
instrument. The claim is that even when bypassed (not a true bypass) the effect changes
the sound. I wanted to create a module with a variable impediance input to check the
effect of loading the instrument. There are noticeable effects. However, the loading has
to be heavier (less than) 100K to be really noticable. This loading has also (apparently)
increased hum and other noise problems.

A question for you other geniuses: Does a microphone have to be loaded with 600 ohms? I
use the same circuit, with more gain, for microphones with apparently acceptable (for rock
and roll) results. My mic preamp creates a 2- 300K load (I'd have to check to see if the
fixed resistor is 100k or 180K).

A further note that references a comment from Anti:

I have found that to much gain at the input buffer increase noise - especially from RF
sources. My circuit is designed to provide enough gain to get the signal up to full signal
level. The module gain can be increased to clip the signal, but this is not it's function.
The gain needed for a passive bass or guitar should be on the order of 23 db. An active
instrument will need even less gain.

Somewhere in my notes is a website that discussed using low noise opamps in effects
circuits. It pointed out that the resistive noise from a 100K pickup exceeded the nf for
the 5534 opamp. The key to noise is:

1) Keep the RF out of the signal path
2) Establish a good noise figure with the first amplifier
3) Eliminate hum (usually a problem with the bass or guitar - humbuckers are a partial
solution)


Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm