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Car Audio Amplifier power supply

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EDY19

Mon Jul 10 2006, 04:40AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Im looking to build my own car amplifier fairly soon, and I am looking into making a power supply for it for a ~300W amplifier. Car batteries are ~13.8V, so for this power level I am looking at about 25A in the primary, so im looking at mosfet devices in the 40A range. However, I am wondering what voltage rating is neccessary on these semiconductors for 13.8V input. Will peaks go above 50V? My main question is what type of switched power supply should I build, and what voltage and topology is neccessary for these power levels in a 4 ohm speaker? Does the supply need to be a dual supply with a + and - output with ground as well? I guess I am looking for some good websites on switching power supplies. Lastly, should the switched supply for the amplifier be regulated or isn't this really neccessary?

rev

Mon Jul 10 2006, 05:11AM

Registered Member #112 Joined: Fri Feb 10 2006, 01:19AM
Location:
Posts: 48

i hopr your not trying to build a suply to be cheep. your can easily get a 1000w amp for ~$50 already made with heat sinks that your gona need already built in.

if for just the simple fun in building one then ok.

i had some info about this that i have lost and forgoten. i'll see what i can do to retrieve this knowlege for i think i would like to make one also.

good luck to us both

...

Mon Jul 10 2006, 06:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well... Peaks shouldn't get over 50v regardless of topology... If you want to run some type of a mazzilli circuit (well it is dead simple, efficient, and pretty cheap) then I would get the next size over 50v.

I would recommend you use a push pull tl494 topology driving a ferrite core from a atx power supply...

As to your questions about the type of supply required, that depends on the type of amp you are building... If it is a sub amp then you will probably go class D, which requires only a single supply. But if you are looking for a full range amp then you will probably want a class ab, which I believe requires a dual supply.

Somewhere in the circuit you will need the supply to be regulated... You can either do it with the power supply yourself or put out a higher voltage than you will need and regulate it down, but somewhere it has to be regulated.

I would also have to second rev that it will be cheaper to just buy one...

Dr. Slack

Thu Jul 13 2006, 07:31AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Will the voltage ever go above 50V?

It depends what converter topolgy you use.

The simplest is to use a step-up transformer with a centre-tapped primary. Centre tap goes to +12v, each off device goes up to 24v by transformer action when the other one is on. Once you have a transformer, you can wind the secondary and rectivfy it how you like for single ended, or split rail or whatever. At 300W, you might use an iron-core trasnformer, or take the frequencuy up and use a smaller ferrite.

Check the input voltage specification of your amplifier carefully. You may find that you can get away without any regulators, as the input voltage to your transformer is reasonably well controlled. The on-charge voltage for a car battery should be around 14v, but may be a little higher with an enthusiatic alternator. Your would not want to contibnue to use the battery once the discharge voltage had dropped below 11v, there's little charge left and you are into shortening the life of the battery seriously if you hammer it beyond that. So that's an input voltage range of 11v to 15v. The PSU you build will have some droop as the load goes up, so you should reckon on an unregulated convertor perhaps giving you up to 2:1 output voltage varaition between on-charge battery off load, and discharged battery on load. If your amplifier will take this variation without blowing up at the top end and not misbehaving at the bottom end (though with reduced power output), then you don't need regulators. You might even put a tap-change on the transformer for different step-up ratio depending on whether the battery was being charged (14v) or not (12v).

A minor difference is a full bridge drive of a sinlge wound primary. Each device will now only see rail volts, there are two in series, but the transformer has lower losses. Not usually the choice at this voltage level.

An completely different topology is a boost convertor. Here you have regulation built-in to its action, and the device sees the full output voltage on it. The main difference between a transformer and a boost convertor is that in the former, the core does not store energy, whereas in the latter, the energy is stored and released every cycle, so for 300W you will want to design your power inductor and its operating frequency carefully.

Steve Conner

Thu Jul 13 2006, 02:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

300w into a 4 ohm load would normally be done with +/-50V DC dual supplies. As far as I know, the power supply for these things is just a ZVS-like circuit (except voltage fed, with no DC link choke, and driven by a TL494 etc rather than self-oscillating) driving a toroidal ferrite transformer. The output of this is rectified using much the same circuit they use for +/-12V in ATX power supplies. I think many of them don't bother with regulation.

Car amps often have pretty optimistic power ratings too. >_< A genuine 400 watts RMS probably needs about six TO-3 or TO-247 power transistors. I'm working on a design of about 400-500w just now, it uses eight TO-3 trannies with fan cooling.

Bored Chemist

Thu Jul 13 2006, 05:41PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I'm suprised nobody has come up with the solution used by many commercial manufacturers.
Get a 10W amp and label it as "300W PMPO"

robert

Thu Jul 13 2006, 08:26PM

Registered Member #188 Joined: Thu Feb 16 2006, 05:18PM
Location:
Posts: 67

If these inexpensive 2kW car amps would deliver 2kW it would be good...
That is because i happen to have a old 3-Phase MIG Welder transformer around (cont. Power about 10kVA) and our photovoltaic system could be rearranged to 12v at "good" current, including good batteries.
So using 3 "2Kw" Car amps and feeding pase shifted 50Hz sinewaves into them (simple task involving 3 2206, a counter and a 150Hz source) and using the output to drive the transformer would give 400v at 5kVa.At good efficienty because the amp heatsinks look like they could perhaps dissipate 200w each.
Theoretically.
Practically a "2kW" Amp doesnt even supply 200W RMS without overheating, and that only for limited times.

Steve Conner

Fri Jul 14 2006, 09:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Meh Robert. Can't you use your elite electronics sk1llz to build a three-phase bridge that drives the transformer directly off whatever DC voltage your PV system puts out just now? Some slow IGBT bricks would do fine. MOSFETs are more efficient than IGBTs at low voltages, but no matter how you do it, it'd be more efficient than using linear amps driven by sinewaves. Note: the cheap car amps are NOT Class-D, they're conventional linear amps with a switched-mode step-up power supply. Hence they could never be more than 66% efficient while producing sine waves, and would suck at handling reactive loads.

I've built an inverter like this before, and I'd be happy to help if you got stuck, since it's in a good cause

Bored Chemist: That's cool, I can rebadge my design as 25,000W PMPO

TheMerovingian

Fri Jul 14 2006, 11:55AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

i have built a push-pull +/- 20V converter for amplifiers for a power of 100W (140W at 70% eff). The voltages/powers can be freely changed changing the turn ratio, core cross section (NOT GAPPED OR THE MOSFETS WILL BLOW) or frequency. Use shkottkies to rectify. Use a filter choke (yellow inductor from PC power supplies) to damp engine spikes.
I got 85% efficiency at 100W and more than 75% at 120W. It feeds a 60W class ab amplifier with 500W pmpo.

heres the link:

EDY19

Sat Jul 15 2006, 04:55AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

If I do build this, I think i am going to reduce the power significantly from the first post. I don't want a "bumping" bass system in my car, I just want to remove some of the lows from the 6x9 speakers as well as the 5.25 speakers in order to get a little more highs out of them, and leave the bass to the other amp (maybe home made

) So I think output power of ~100W would be plenty, especially if i can find a efficient speaker (~8 or 10 inch probably). Any changes suggested to the power supply? Im still thinking ferrite core (flyback core) with a center tapped primary and secondary, push-pull, center tap on secondary grounded with a supply voltage of + or - ~30?? Volts into 4 ohms.

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