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Post subject: Big Block Preamp 18v to 9v?
Posted: Fri Feb 29, 2008 8:57 pm
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Anyone know if the preamp from the Big Block P bass can be run on 9v rather than 18v?


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Posted: Mon Mar 03, 2008 2:59 pm
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It might be possible, but you need some electronics experience, soldering tools and a schematic diagram to modify the circuit board. Any device having two 9v batteries has a "bipolar" power supply configuration, that is +9v, and -9v for the amplifier chips to work. If you hook only one battery up it can only amplify one half, (top or bottom) of the output signal from your preamp, if it works at all. Modifying the board for one battery will only allow +4.5v and -4.5v power to run it and might not be enough for it to work properly.

Also, I must add, these modifications will void any warranty, assuming it's still in effect. I looked at the support page wiring diagram and the bass' preamp board DEFINITELY has the two 9v voltages clearly marked on the circuit board. I was unable to locate a schematic to see what chips are used in the preamp. Are you also aware that the jack is switched so that the batteries are connected to the board when you have a cord plugged in? I would also recommend that both batteries be replaced when you do replace them since a dying battery will cause some really funky sounds to come out of it due to the unbalanced power supply voltages applied to the board.


Hope this helps!


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Posted: Mon Mar 03, 2008 4:11 pm
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Well I've been working on guitars for 13 years now so I have all the skills... as far as warranty goes I just scored the preamp off eBay, so no worries there either.

Do you think I could wire the + to the board like in 9v standard active preamp then loop the - back from the 3rd lug of a 3 lug output jack to the preamp? So the battery + would go to the board and the battery - would go the ground (2nd lug )as in emg style circuits. The switch lug (3rd) would then lead back to the boards "-9v"

Would that cause a problem? Old Peavey preamps have a ground on the cb along with the battery + - and that's how they're wired...


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Posted: Mon Mar 03, 2008 7:21 pm
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Good to hear about your skills and the warranty issue. Most preamps have operational amplifier chips that require the +, _ voltage rails and ground, as do a lot of solid state power amps. If you know what a voltage divider is, you can, in fact, create a "false" ground using a unipolar (+V, gnd) supply, i.e., one battery. I assume that there might be a compressor chip onboard, but without a schematic for the circuit, I have no idea if the 4.5 v rails would be enough.

Your idea about routing the wiring like an old Peavey probably wouldn't work, simply because these chips do require TWO voltages AND a ground, which, by the way, is in fact the ground on your jack and plug. As I stated in my previous post, if there are two batteries it's bipolar. The old peavey might have had discrete transistors, etc. and could operate from a unipolar supply.


READ MY NEXT POST FIRST!!!!!

Now, about the divider. I would try the following if you're experimenting.

Get two 1/8 watt resistors of equal resistance, preferably 5k or more, solder one end to the + pad on the board, with the other end on the GND pad and do the same with the GND pad and - pad. Selecting the resistors is an art because too low values might increase battery drain, where too high a value might interfere with circuit operation. Two 5ks will create a .0009 A draw off the battery which is pretty low, though you might need to adjust the values a little. Then you need to wire the (-) side of the battery to the (-) pad, and the (+) to the (+) pad, preferably through an isolated switched 1/4" stereo jack like in the bass. Also do not directly connect either battery connection to the ground connection. The resistors will create the ground I discussed earlier. This might work OK, but there might be a little more to this as I haven't seen the circuit for making a bipolar chip set work with a unipolar supply in a while.

My personal opinion, without seeing a schematic, is to save all this trouble and go with the two batteries, since you'll still be losing half the voltage and most of these op-amps run b/w 5-22 vdc power supply rails.
You also might want to google for circuit diagrams to make this conversion, as my memory about this is a bit sketchy.
Good luck!


Last edited by telecaster1987 on Tue Mar 04, 2008 2:23 pm, edited 1 time in total.

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Posted: Tue Mar 04, 2008 2:20 pm
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llomens!

I did a little more research on this subject, and it would probably be a very bad idea to try this single battery conversion. Best case, the preamp will oscillate instead of amplify, worst case, damage to the amp chip could occur.

Without a schematic I cannot predict what may happen. There is indeed a way to do this but it would involve fairly extensive modifications to each amp stage. I just looked at the board drawing on the wiring diagram and there is at least 1 chip there.

If you're interested in the power supply (bipolar to unipolar) change over for op amp circuits, read this article: http://ltc.cit.cornell.edu/courses/ee47 ... supply.pdf

It'll give you an idea about how much effort is required to make this work.

I assume you either have a space limitation where two batteries might be a problem, or if you're like me, would prefer only one.

What destroys most electronics besides excessive heat, spilled liquids, and static discharge is mismatching loads or power supply voltages. Most IC chips are designed to operate within a specific range of voltages. Outside of these it'll either not work, or it'll fry.

If at all possible, go back to the two battery system, it'll will work, assuming the board is good. If the chip has a alphanumeric number on it I might be able to figure out what chip it is.

Sorry for giving you bogus info in my last post, I can't rely on my memory.

Good luck!


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Posted: Tue Mar 04, 2008 4:51 pm
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Thanks for all the info.
It seems like its not worth all the work to not have to rout out my p-bass. I guess it time to fore up the router and open up my 9v box to a double box...

Thanks again!


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Posted: Tue Mar 04, 2008 7:29 pm
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I'm so glad you read my post before I might have caused you a problem. Good luck with your project!


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