Help with generating negative voltage for car amplifier

[carebare47]

Hello,

My car audio is rubbish, and so I wanted to make my own amp. The problem that I have is generating the negative voltage required. I was thinking something along the lines of a 741 op-amp driving a push-pull system consisting of an IRF9630 and an IRF630. I can put a voltage divider across the +12 and 0v power supplies and take the center point of that as 0v for the circuit, then use the +12 as +6 and 0v from the battery as -6. However, this would not be enough for the FETs to give a decent output. I don't really want to have to wind my own transformer(s) to get the voltage up to 30 so that I can split it into +/- 15v. Anybody have any ideas how I can get this working, or at least point me in the right direction?

Thanks,

Tom

 

[poor mystic]

There are lots of voltage converters on ebay at nice prices!

 

[carebare47]

There are lots of voltage converters on ebay at nice prices!

Good call =) I did look, bu I guess I didn't search 'voltage converters'. There is this: http://cgi.ebay.co.uk/ICL7660SCPA-C...Supplies_ET&hash=item45fad5e50e#ht_1653wt_911 but the absolute max it can source is 40mA. I don't think there will be many in my price range that will source much more than that =(

 

[Resqueline]

How much power do you need then?

 

[carebare47]

Well I was hoping for at least a 30W amp, which at 12v is 2.5A. Even if the chip is only supplying negative voltages, it would still need to pull 1.25A.

 

[Resqueline]

This type of voltage inverters tend to be hard to find, and quite expensive if powerful.
Remember for 30W out you'll need 40W in, unless you're using a class D amplifier.

Here's one possibility but you'll have to ask them about shipping cost and if the output is galvanically isolated from the input.

Here's an example of exactly what you need, though it's more powerful than you want.

 

[carebare47]

Thank you for the links Resqueline =) But is there not a cheap(er) way that I can do it? How to cheap car radios manage it, do cars already have negative voltage lines in them? Could I tap it off the alternator? Or even just a DC-DC converter up to 30v and split that into +15 and -15.

 

[carebare47]

Also, are there amplifiers that don't require negative voltages to work properly?

 

[poor mystic]

Of course an amplifier needs to be able to swing about some sort of rail to produce an AC signal. However there is a simple trick often used in low voltage amplifiers.

You have visualised an artificial rail at nominal 6V; I suspect you envisage the output transistors driving the speakers to that rail.
In low voltage amplifiers an inverted version of the signal is often amplified alongside the erect version, and instead of driving the speakers to the artificial centre rail they are driven to the inverted output of the second amplifier.
This is called "bridging" mode.

 

[carebare47]

Thank you. So this with the +6 and -6 would give out a decent signal? Could I connect this output to the push/pull FET configuration, or would it have to be like this? And would bridged or paralleled amplifiers be best suited for this problem?

Many thanks,

Tom

 

[poor mystic]

The amplifier configuration you have drawn is bridging mode. The following remarks apply.

You can see that instead of having a maximum possible 6V peak when working to a centre rail you now have a maximum possible 12V peak.
A sinusoidal waveform has RMS power equivalent to a DC waveform about 70% the peak AC voltage.
For 6V peak this is 4.2V, hence maximum power in a single pure tone is about 4.4 Watts.
For 12V peak we obtain 4 times that power, or 17.6 Watts. So theoretically, bridging mode quadruples power.
In practise you won't get full swings to the rails, so these figures are not to be regarded too seriously.
Still you'll probably get better than 10W into your speakers without objectionable distortion and that's a lot of sound to be with in a car - you'll think its loud!

I don't know why you are asking about paralleled amplifiers - perhaps you hope that power may be further increased in this way. Sorry, no.

 

[carebare47]

I was asking that, thank you very much for this info, much appreciated. Will get started right away ^_^

 

[poor mystic]

It has just occurred to me that there is a cheeky way to use paralleled amplifiers... use separate speakers as well! In other words take your signal at pre-amp level and feed it to 2 entirely separate power amp / speaker systems. This of course doubles power.

 

[carebare47]

It has just occurred to me that there is a cheeky way to use paralleled amplifiers... use separate speakers as well! In other words take your signal at pre-amp level and feed it to 2 entirely separate power amp / speaker systems. This of course doubles power.

Would these paralleled/bridged amps be the pre-amps or the power amps? Thanks for all this ^_^

 

[poor mystic]

Hi again
I expect that the 1 pre-amp could be made to serve more than 1 power amp, so there would be no need to duplicate the pre-amp.

 

[carebare47]

Ok, thanks. I think that I will have one 741 as a pre-amp, and then a TDA2822M for the bridged op-amps, one of these setups per channel. Sounds ok? =)

 

[poor mystic]

Well, OK. I think you could do a lot better than a 741 though - that's a pretty old design and I'm certain that much better op-amps are available today. Even 25 years ago the TL071 was considered an enormous improvement on the 741 and technology has not stood still in the meanwhile.

 

[carebare47]

Ok. I do have a pair of TDA2030As that are pretty beefy, could use those. What op-amp would you use?

 

[poor mystic]

Hi again, Carebare (smile)

So you think you'd like to put 2 amplifier/speaker systems in parallel on each stereo channel to increase sound levels, and wonder how to condition the signal to suit the inputs of the TDA2030A power amps?

I looked up the chip on the manufacturer's page at http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1459.pdf and I don't think you need a separate pre-amp stage at all.

I've copied the cct diagram from the datasheet as an attachment, there are much more knowledgable people than myself who may have another opinion.