active/passive instrument input

[moosapotamus]

I did some searching on this and while I didn't have much luck, I did find out that this seems to be something that a lot of folks are interested in figuring out how to do. So, I tore apart one of my aphex xciter pedals (you know, the ones with the active/passive push button switch), traced out the input and found this...



While I don't actually own a guitar or bass with active electronics, I recently built a pedal for someone who plays an active bass and they were having some issues. So, I built this up with everything soldered directly on to a DPDT toggle switch. My initial tests feeding it with a really hot, preamped signal seem to be very good. It drops the level and prevents overloading and distortion from occuring.

Any educated thoughts on how good of a design this really is for a switchable active/passive instrument input?

~ Charlie

[alteredsounds]

Dredging up bit of an oldie here but my bass player has asked me to build him a fuzz and i'm having the usual issues with an active bass.  Did anyone try the above idea or have any thoughts about it?  Thanks.

[petemoore]

  The 5k/diodes look like a distortion fragment, hopefull signal level is below their turn on point, if signal purity is to be maintained, looks like a limiting config.
  The 30k/10k looks like a 1/3 divider, and would probably attenuate the amplitude of relevant frequencies fairly evenly provided a low impedance source [ie active] is seen.

[zyxwyvu]

It seems to me those diodes must have been zeners. 1N914's have a reverse breakdown voltage of around 100V, so you'd have to have a pretty hot signal to get those to conduct! Choosing a couple zeners around the maximum input voltage of the pedal would work better.

[petemoore]

  I'm used to looking at diodes in that position, not that orientation.
  Seems the signal would pass through the Gnd. diode, and be attenuated whenever above the FT of the top diode..a clipper...if the pickup output ever gets that high.
  Perhaps there simply put there as a limiter for when the active pickups are used when switch is in passive state.

[Ben N]

Except for the odd diode thingie, it is basically a riff on the old Fender hi/lo amp input jacks, just with a switch instead of switching jacks, i.e. a pad.

[alteredsounds]

Cheers for the input guys, the diode setup was what was confusing me as i couldnt really see what was being done.  I'll give it a try and see how it works as Colin sounds like he had success with it.

[cpm]

the zeners threshold thing can be seen here (#5):
http://www.muzique.com/lab/zenmos.htm

[George Giblet]

The divider for "active inputs" is very common.  The divider typically provides a -10dB pad.  You will see many devices with -10dB pads.

It usually works fine  however, while active basses have low output impedance sometimes the designs use pre-emphasis and de-emphasis to help combat noise.  Unfortunately in order to save opamps and battery power the output stage has the RC network used to provide the de-emphasis.  The 40k load the -10dB pad presents to the output of the bass is right on the limit of modifying the frequency response.

For the protection network (it's not a distortion network) I suspect:
- the diodes are zeners
- the  output is taken from the across the zeners not the "otherside" of the 5k resistor as shown.

Typically the zeners don't conduct.  However if the zener voltage is too low then an active bass might clip them.  Typically 9.1V zeners are safe.

These zener protection networks are fine.  The only thing I don't like about that circuit is the zeners are DC coupled to the input.  If you have a preceeding device which pushes out DC (and some do) it can cause the zeners to conduct this causing havoc.  The fix is to add an input cap.   Figure 3 shows a typical set-up for the zeners:

http://www.passdiy.com/pdf/balzenpre.pdf


The bottom line is that input stage usually works but there could be weird cases where things go wrong.

[TELEFUNKON]

High time to repair your preceeding DC-outputting pedals? (or is that DC wanted)?

[Gus]

alteredsounds
What fuzz circuit? if it is a FF type the series resistor to the input can help.

Charle

   The 30K 10K is a preset volume control for 1/4 of the input signal, tap at 10K out of a total of 40K.  So one could use a volume pot in place of the 30k and 10K.

The diodes being zeners and the 5K resistor being in series with the signal and the output from the end connected to the diodes of it makes more sense like George posted(I don't think the 5K would be a current limit for the zeners as drawn because there is the 30K in circuit).  Clip at a diode drop plus the zener voltage each way.

Also George posted about the loading of the 40K and that if the preamp uses internal not buffered RC based filter(s) at the out of the preamp the 40K might "Move" the filter frequency.  George what active bass preamps do this?  And one can move the total loading up to 100K or whatever would a total 500K load be a better pick?

[snap]

the 30k is only in the circuit in passive mode! (now connect a high-out instrument by accident)