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Preamp and amp power supply
Preamp and amp power supply
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Old 28th December 2017, 04:34 PM   #1
Yosefu is offline Yosefu
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Join Date: May 2013
Default Preamp and amp power supply

Greetings, earthlings;
I'm making an instrument that has a battery powered amp inside with a preamp. The preamp is a variation of the Supreaux Deux found in the RunOffGroove website; the amp is a TDA2003.

When I try to power them both from the same batteries (two 9v batteries in series), no sound comes off it, or a very weak and distorted sound. Works just fine using different batteries for each. I guess it has to do with current draw, but this is no problem when I try to power two TDA2003s from the same batteries, which puzzles me a bit.

Must I use two separate power supplies? The problem with this is, for the final design I intended to use 12 rechargeable AA batteries, but the price is around 50. Doubling that would really be painful, and heavy as well. 9v batteries aren't much cheaper and die off quickly. Also, I'd like to be able to recharge the thing with a single cable connected to a 18.5v supply. I could use a stereo cable, sure, but then the problem would be overcharging one of the two battery packs.

What if I used a higher voltage, say 24v?

Have you faced similar problems? How did/would you solve it?

Thanks!
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Old 28th December 2017, 05:07 PM   #2
radtech is offline radtech  United States
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Have you measured the battery voltage when in use? The preamp should only draw a couple of milliamps, but it is voltage sensitive... the bias points will change if the supply voltage changes.
The TDA2003 will draw something like 40mA with no signal, and up to several amps with a signal, which could be drawing down the battery voltage. It will probably be less voltage sensitive, so while it may work with the battery voltage varying with current draw, it could be changing the voltage enough to affect the preamp operation, this would be why they work okay on separate batteries but not on the same one.

You should probably run the preamp off of some kind of regulator or converter to keep it's voltage constant. They recommend no lower than 15V for it, 18V is better.

12 rechargeable AA batteries is only going to get you 14.4V (12 * 1.2V per cell). You would need some arrangement that would give you between 8 and 18 V (from data sheet) for the TDA2003, and a regulated 18V supply for the preamp, possibly using a charge pump.

Last edited by radtech; 28th December 2017 at 05:14 PM.
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Old 28th December 2017, 05:08 PM   #3
voltwide is online now voltwide  Ireland
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Greetings, alien
You know that TDA2003 consumes 44~50mA before any sound is produced? 9V-blocks and TDA2003 are not compatible.
You should monitor the supply voltage during your test, I presume it drops down to several volts.
If you are looking for a simple, low-power amp, have a look at LM386.
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Old 28th December 2017, 05:14 PM   #4
voltwide is online now voltwide  Ireland
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Quote:
Originally Posted by radtech View Post

You should probably run the preamp off of some kind of regulator or converter to keep it's voltage constant. They recommend no lower than 15V for it, 18V is better.

12 rechargeable AA batteries is only going to get you 14.4V (12 * 1.2V per cell). You could try a couple of 12V gel cells in series, run the power amp off the 24V and an 18V LDO regulator for the preamp.
I run both pre-amp and power amp with one 12V SLA for my mobile guitar amp. If you want to increase power, use 2xLM386 in full bridge configuration, or some full-bridge class-d-amp based on TPA3116/TPA3118.
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Old 28th December 2017, 06:18 PM   #5
radtech is offline radtech  United States
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Note that I edited my earlier post when I realized the TDA2003 is only rated to 18V supply voltage.
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Old 28th December 2017, 06:59 PM   #6
Yosefu is offline Yosefu
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Join Date: May 2013
Thank you guys!

So, if I understood correctly, using rechargeable AA batteries in series to reach >18volts, then using a couple of regulators* for preamp and power amp... should it work? Would a higher voltage be better?

As for the Lm386, i never managed to make it sound loud enough, not as loud as the TDA2003.

Thanks for helping!

* what are those, chips? which one would you recommend?
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Old 29th December 2017, 02:56 PM   #7
radtech is offline radtech  United States
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With NiMH or NiCad batteries the voltage shouldn't vary as much as with an alkaline or lead acid, so you might be able to get away without any regulation with those.

Otherwise, one option would be using a 9V LDO regulator such as an LF90C (http://www.mouser.com/ds/2/389/lfxx-974166.pdf) to power a MAX1044 circuit like you'll find here: Coda Effects - JHS Superbolt clone / ROG Supreaux

The TDA2003 could run right off the batteries, you'd just use the regulator for the preamp.

You're also going to have to think about charging circuits. Do a bit of research on that, you don't want to just throw a power supply across a NiMH/NiCad battery.
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Old 31st December 2017, 10:27 AM   #8
AndrewT is offline AndrewT  Scotland
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For battery powered power amplifier/s you really have to look at very high efficiency switching amplifiers and very high efficiency speaker/s.
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regards Andrew T.
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Old 9th January 2018, 04:02 PM   #9
Yosefu is offline Yosefu
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Thank you all for your answers! Sorry I couldn't read you before, first week of the year has been madness.

Quote:
Originally Posted by radtech
With NiMH or NiCad batteries the voltage shouldn't vary as much as with an alkaline or lead acid, so you might be able to get away without any regulation with those.

Otherwise, one option would be using a 9V LDO regulator such as an LF90C (http://www.mouser.com/ds/2/389/lfxx-974166.pdf) to power a MAX1044 circuit like you'll find here: Coda Effects - JHS Superbolt clone / ROG Supreaux
I think I'm gonna go with a 24v battery made of AA NiCad or NiMH cells for different reasons. Could it be enough with a couple of diodes (to prevent the TDA2003 sucking current, but also to lower a bit the voltage)?
If not, do you know of any dc-dc voltage regulator that would operate in that range 24v->18v?

Quote:
Originally Posted by radtech
You're also going to have to think about charging circuits. Do a bit of research on that, you don't want to just throw a power supply across a NiMH/NiCad battery.
Oh, I thought you could charge rechargeable batteries simply by pushing a slightly larger voltage across them as long as you are careful not to leave it for too long. But I'll research on the subject.

Quote:
Originally Posted by AndrewT
For battery powered power amplifier/s you really have to look at very high efficiency switching amplifiers and very high efficiency speaker/s.
I don't like much class D amps despite their efficiency, but if you know of any other class AB that sounds warm and nice and consumes less than the TDA2003, I'd like to try it.

Edit: as for voltage regulators, I just found the L7818, which outputs 18v. Does it need any extra circuitry, or do I just solder the pins to Vin, Vout and Ground?

Last edited by Yosefu; 9th January 2018 at 04:08 PM.
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