Is it possible to run the Esp Guitar Preamp from batteries? - diyAudio
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Old 6th February 2007, 11:12 AM   #1
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Default Is it possible to run the Esp Guitar Preamp from batteries?

Hi,

I am looking to build a battery power guiatar amp and was wondering if i could run it off the 2x sla in the system for the power amp. The issue i see is if i use centre point between the two sla batteries this is actually at 12v. Ok it has +12v point above it and a -12v point below it but since the actual 'centre point is at 12v instead of gnd the will be issue plugging this thing into anything.

Is there anyway around this with redesigning the circuit?
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Old 6th February 2007, 12:09 PM   #2
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What you propose is fine you just haven't got your head round it. The centre point is your 0V, below it is -12V and above it is +12V.

Remember voltage is just relative so if you take the centre point as ground 0V then that is the baseline where everything is referred to. What you seem to be thinking is that the bottom battery is 0V simply because it's at the bottom. This is not the case. For example you could connect your system ground to the top terminal and then you would have a -12V and -24V supply.
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Old 6th February 2007, 12:16 PM   #3
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Yeah but the way the system is used with a guitar the ground needs to be common to all the amps in the system. It this setup it would be because the power amp would use the batteies in series so its ground point would be at the bottom. Also it the guy plugged in a fx unit which ran of a 9v battery they use, that would expect the 0v to be that where as it would actually be at 12v so the system would then see -3v to the plus of the fx unit battery instead of 9v.

Maybe i am talking crap but surely you need a common ground in a system?
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Old 6th February 2007, 12:45 PM   #4
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If the pre-amp is the only thing connected to the batteries then ground can be whatever you make it. If the batteries are shared by something else already (seems the power amp maybe) then ground will already be nominated and you have to follow that.

If the grounding is such that it's a single ended supply then the pre-amp can be converted to work with that but it would take an amount of work and I doubt the PCB will be amenable to that.
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Old 6th February 2007, 01:09 PM   #5
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yeah the batteries have to be shared else the whole unit will get too heavy/ complicated.

I think i have worked out what to do. It seem to me every inside the input and output cap has to be changed so that they run off a +24v GNd and vref mid point instead of ground. This would mean that the input ground is connected straight to GND, R3 goes to vref, the tone control 'ground to vref and the clipping diodes ground to vref. However the bc549 setup does confuse me.

I agree that the ESP pcb would be any use cos the input rail connects to gnd where as it would need to connect to vref.

This might all seem like total gibberish. I am try my best to work through it in eagle but i have very little experience with this sort of thing.

phil
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Old 6th February 2007, 01:33 PM   #6
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I think you have it right, I'm assuming Vref you mean the midpoint.

Just to make sure: the op-amp ground references need changing to suit single supply. You can't just make ground the lower rail as it mucks up the reference point.
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Old 6th February 2007, 02:28 PM   #7
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I think so, i would like to post up a schematic but i think it might fall foul of ESP copyright so i won't.
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Old 6th February 2007, 09:00 PM   #8
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You are talking past each other...
with bipolar supplies, inputs must be referenced to ground...
If V- is ground, then the input resistor must be referenced
to 1/2 V+
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Old 6th February 2007, 10:55 PM   #9
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Hi,

This is what i have done i think i have created a 12v supply using 100k resistors between the gnd and 25v lines to create a 12v supply. I have the used this as a reference volatge for the input resistor.
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Old 6th February 2007, 11:11 PM   #10
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Does this look any good?
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