Mounting a preamp in a pedal case

Discussion in 'Pickups & Electronics [BG]' started by awilkie84, Jan 28, 2014.

  1. I want to take a 2-band preamp and wire it into a pedal case.

    So, I figure the basic wiring for this is REALLY easy. Just change the pickup leads to a stereo jack & move the battery lead to the input jack instead of the output.

    Can anyone help me with where/how to wire in the 9V AC adapter lead & the on/off switch?

    Here's the pre-amp I plan on using:

  2. worshiprocker

    worshiprocker Tonepump Junkie

    Aug 12, 2011
    Mesa, Arizona
    Endorsing Artist: Spector Basses
    I am very interested in how this turns out.
  3. evermelon


    Apr 20, 2010
    Phoenix, AZ
    on/off switch would essentially be like an active/passive switch because you still want the signal to pass through when the pedal is off. Now an adapter jack, no clue (since the battery lead needs to be engaged when there is no adapter cable plugged in, and then needs to disconnect when there is...)
  4. Firstly, switch the input and output of the volume pot around. I don't know why they would show it that way, when it is not necessary to have that compromised behavior.

    The bypass switch can be done with standard active/passive switching. Add an LED and resistor on the third pole of a 3PDT switch to get your indicator light.

    There should be diagrams out there for pedals with DC inputs. Simply copy the wiring from those.
  5. RobbieK


    Jun 14, 2003
    Personally, I'd run the vol on the output. I'd also wire the bypass switch slightly different.

    Do the spector basses with this pre OEM, have active-passive switching? If not there's a chance you'll need a load/bleed resistor on the output (or input if you shift the vol to the output) to minimise clicks in switching. 1-2 meg on the input or 100K for the output.

    Oh, and I'd run a reverse polarity protection of some sort. A diode on the batt +ve is nice and simple but will drop the voltage slightly.

    Are you planning to run it on a pedal board with a 9V adaptor? That link has the DC jack wired the reverse of standard (boss) wiring where the centre pin is actually negative. You'll need a 2.1mm DC plug with a plastic chassis or some isolation bushings if you use a metal plug.

    And you should have one jack that's isolated from the metal box, and then run all the earths to the sleeve lug on the other jack to minimise earth loops.
  6. gingerbeer


    Jan 9, 2005
  7. ex-tension


    Jun 11, 2009
    The preamp module should already have a reverse polarity protection inside. I mean it says Spector on it.

    The schematic in the link shows a standart (Boss) center negative wiring configuration. So, it's OK. Also there is information about the type of DC plug.
  8. RobbieK


    Jun 14, 2003
    Oops, you are absolutely right. My apologies.
  9. What does switching the input & output of the pot accomplish? This is how I have the same pre wired in my P-bass and it seems to be fine, with a nice smooth roll to it. Keep in mind that you're looking at the top of the pot in this pic.

    Thanks for the info on the light. I had forgotten about putting an indicator on it.

    I plan on doing a dual volume setup. 1 on the input for a Gain & then a Master volume on the way to the Output jack. That should help me balance everything out. The non-trim tonepump pre-amp can be quite hot, so I want as much control over it as possible.

    I don't believe any of the Euro series Spectors have active/passive switches. I can ask them about the internals & see if it has a bleed resistor.

    So, some insulating rubber between the jack and the casing should accomplish the isolating, right? Then I just run all the wires back to a central point that's insulated & then to the input jack ground?

    I do plan on running it with a 9V Boss adapter.

    Would I not want to move the battery to the Input jack? That would make more sense, wouldn't it? I don't want the thing turning on when the output's connected but the input's not, right?
  10. It alters the taper and sometimes causes a more noticeable loss of treble when rolling down. Usually, the only time you see the wiper terminal wired as an input is when you don't get a choice, because there are multiple volume pots, and rolling one down would mute everything. That is not the case here, so there is no reason to accept the compromise.
  11. evermelon


    Apr 20, 2010
    Phoenix, AZ
    IME you will lose a lot of top end and it will sound quiet and muddy if you install a pot after the TP. I wanted to tame it as well, so I tried a trimmer pot, and that was the result.
  12. ex-tension


    Jun 11, 2009
    You're right and it's wired as you think (input jack switching on the battery) in the schematic.
  13. jimmybc91


    Oct 8, 2013
    Los Angeles
    Sub'd! :D
  14. RobbieK


    Jun 14, 2003
    I didn't mean any disrespect to Stuart and his products. I just have never noticed much in the way of polarity protection on the un-potted preamps I've come across over the years. Who would change their battery with their bass still plugged in? And if you put the battery up to the clip the wrong way it's only for a moment, right? In a pedal, it's probably the same story, but the diode is to give the circuit a fighting chance if some one puts an ac voltage on the dc plug. For a few cents, I think it's not a bad idea.

    Sorry, I should have clarified that. You should go with a 10K or 25K pot for a post pre volume. A 250K or 500K here will cause those probslems.

    All true, and also, it will increase the input Z to it's max. Which is a good thing, especially if your bass is passive. Remember, when this is installed in a bass, theres only one pot between the coils and the input of the pre. With a passive bass, there'll now be 2 to 4 pots plus a guitar cable.

    In audio electronics, you have to know your load. Having a pot before a Hi Z is normally a good thing to minimise the tonal change through the sweep of the pot, but in a bass, the load a (250K) pot has on the coils has more of an effect than the load a cable and amp has on a (post pre, 25K) volume pot. In the real world, a bass vol is rarely run below 80% and will almost never see an Z of less than 50K, plus most onboard pres are made with opamps, many of which will happily drive a 1K or less Z. So having it (unbuffered) on the output is perfectly fine.

    Having said this, many basses have the vol on the input of the pre for two reasons. Firstly, if it has an active/passive switch, you need a 250-500k pot for the passive setting to sound OK, and if this is on the output of a pre, it will sound bad (in the active setting). The other reason is that most pickups are voiced to sound best when directly driving a pot or two. I know this is a big generalisation, and I don't want to spark a big OT discussion about pickup winding, but IME, most pickups can sound a bit sterile if directly driving a 1Meg+ Z. Of course if you've got two pickups, you blend pot will usually load the coils up just nicely.
  15. Sacha Lévesque

    Sacha Lévesque Supporting Member

    Jun 3, 2011
    Montreal, QC
    Endorsing Artist: Spector Basses, EMG Pickups and Schroeder Cabinets
    Sub'd! Please post pictures of the process...would love to do the same with my TP
  16. ex-tension


    Jun 11, 2009
    Actually, my experience is opposite. None of the onboard preamps and the schematics I've seen are without a diode. Yeah, you're right, the probability of changing the battery while the bass is plugged is very low. But still it's possible.
    Anyway, I wasn't worried about a few cents but a few volts :) Most probably there is a diode in the module. And if you put one more the voltage drop will be around 1.4V. I wouldn't be happy with losing 1.4V of 9V. But it's up to OP. If he wants to be on the safe side...
    Of course it's possible to ask Mr. Spector about it, too.
  17. Ah, I'll have to do this on my bass, too. I currently have it wired with a 250K pot the way the wiring above shows, but it seems to have a nice linear sweep to the volume without degrading the sound.

    Ok. So, where in the circuit would the diode go & what value do I need?

    The trim version of the 1 p-up Tonepump has a 100K volume on the output & no volume on the input. I was planning to put a 100K for the master volume, not a 250 or 500K. Would this be alright, or should I get a 25K or 10K? Should I be using a 250K pot for the Gain (input vol), or should I use a 25 or 10 for that, too?

    I really wish most of this didn't go right over my head. :( I'm very new to figuring this aspect of audio out.

    I do know that there's no opamp in the Tonepump circuit.

    Ah, ok. So, put a 250K put for the input volume & use a 25K or 10K for the output.
  18. Sacha Lévesque

    Sacha Lévesque Supporting Member

    Jun 3, 2011
    Montreal, QC
    Endorsing Artist: Spector Basses, EMG Pickups and Schroeder Cabinets
    Reviving this thread. Does anyone have a schematic of this? Id like to put my preamp in a casing (with bass, treble, volume, blend, on/off switch) like the one attached. Thanks

    Attached Files:

  19. andertone

    andertone Supporting Member

    Mar 22, 2000
    Southwest USA

    I have an older style (rectangular bar) Aguilar OBP-1 preamp with all potentiometers that is just
    sitting around. Anybody interested? Would make a great stomp box item