Why was a ceramic cap choosen in this circuit?

[belwar]

... and could this be improved upon?

I'm schematicing our the pre-amp from a Wal bass. The pickups on the bass have a strange coil configuration.. Each pickup on this five string bass has 6 Wires. 5 hot leads from the coils, and one common ground. When the pickup is connected to the pre-amp, the signal goes through a n47 ceramic capacitor and 220k resistor hooked in parallel, the the output from each of the hot leads is connected together to blend it.... Like this:



So my question is, why would ceramic be used in this case, and could you improve upon the design by switching to a different type of cap?

Excuse the crudity of the drawing, I dont have a schematic drawing program at home.

[Albert Kreuzer]

Originally Posted by belwar

So my question is, why would ceramic be used in this case, and could you improve upon the design by switching to a different type of cap?

1. Size
2. Cost
3. Good enough
(in no particular order).

You could switch to silver mica (bigger, better and more expensive) and tell us how it sounds.

Cheers,
Albert

[Enzo]

I am in no way a pickup guru, but looking at your circuit it would appear the individual pickups are wired separately and the 220k resistors mix them together for the overall output. The 470pf caps then act as brightness caps, causing a little high end boost just as a bright switch on an amp.

I'd be betting that changing the ceramic to silver mica won't do much. If you do try other types of caps, measure their capacitance carefully first. You might find that the silver mica caps sound better, but it might be because the ceramic caps measure out at 468pf while the micas measure 480pf. That will have more tonal difference than the dielectric material. I of course made those numbers up. The only real test between the different dielectrics would be made at the exact same capacitances.

[belwar]

A cap for brightening? I always thought caps were full pulling brightness out... Hmm I have alot to learn!

[Phostenix]

Caps in parallel with resistors like that will let frequencies above a certain point "go around" the resistor.

[jrfrond]

Individual outputs are nothing new, but the high-frequency shelf for treble boost seems odd for a bass. Whatever.....

While silver-mica and ceramic DO exhibit different tonal qualities (ceramic is a bit "grittier"), it is only when you hit them with lots of high-frequencies and a decent amount of signal. At this low level and on a bass no less, the difference in cost and size would not justify the use of silver-mica.

[Enzo]

belwar, a capacitor, like any other part, does a job in a circuit. it doesn't do much all by itself. The context of the circuit is everything. For example, these little caps on your pickup. We could find some 5000v caps and install them, they'd work, but they wouldn;t make any sense in the context of a guitar pickup. On the other hand, a little 50v greenie cap (typical in the guitar tone control) wouldn;t make much sense in the transmitter of your local 50,000 watt radio station. Yet both could have the same value.

Like wise the function of a cap is determined by the circuit it is in. All a cap does is pass a signal, and the larger the capacitance value, the lower the frequency it can pass. If you put that cap in series with a signal, then it passes the higher frequencies, and blocks the lower. This is how the crossover for your tweeter works in a speaker. On the other hand if you put the cap from the signal to ground - across the signal in other words - then it passes those same high frequencies to ground, leaving only the lows it cannot pass. That is exactly how the tone control on a guitar works, it rolls off the highs.

SO you cannot say caps only pull brightness out. it is that always and never thing again. They can do either job.

[jrfrond]

Caps also do NOT pass DC, and in audio circuits, their function MOST times are as DC blockers, whereas the DC voltage (like plate voltage at a tube) is blocked, while audio frequencies are passed (coupling caps). Same the cathode-bypass (decoupling) caps. ANY cap that passes DC is a bad cap.

Back in the old days, when the OCD minutia of guitar amps, sweated by many players and builders nowadays (TOO many!), wasn't the subject du jour, caps were chosen for their function, cost-effectiveness and size. Silver-mica caps were close-tolerance and often seen only in RF circuits. AF circuits did just fine with ceramic, polyester and paper caps.