DIY Mains supply for Car Subwoofer Amp

[Pikefish]

The last few weeks I've been tinkering away on arguably my most explosive project to date, a power supply to run my Car Subwoofer Amp in the house. It seemed like it was a good idea since I haven't got a car, but I've got a car amp more powerful than my house one. It's a little silly in that the amp will step up the voltage once it gets inside, but since it's Class D the heat wastage is about 50W max, better than belching out 200W for less usable power.

 

I picked up the amp (Directed Audio 250d) off tardme for $100. It's rated at 250W into a 2ohm at 14.4V supply. Damping factor of 50. The book says it's happy with anything up to 16V, so I've built a 15.8V 25A power supply. It's also got a regulated +12V line required to turn the amp on, and it can run case fans if they're needed. I'll attach pictures and a schematic, but first here's the running specs, with an 8ohm speaker load (2 x 4ohm venom 12" subs, sealed with Qts of 0.7):

 

Voltage unloaded: 15.8V

Voltage with above average use: 15.5V

Minimum voltage with severe thrashing (prodigy): 14.5V

Ripple voltage then: 0.3V, padded by transformer internal resistance I'm guessing, about 35ohm.

Current then: 25A, I have an inline meter and it wiggles pleasingly.

 

After playing an entire album at an uncomfortable (for my walls) level (after gradually working up and monitoring temperatures):

 

Temperature of warmest part of the amp: 28

Temperature of the diode bridge: 23

Ambient temperature: 16

Temperature of all the capacitors: ~Ambient (whew!)

Transformer: About 20.

 

Here's the guts of it.

 

So that's what it does, here's how it does it. I've used a rather large capacitor bank, so added a soft starter, mainly to test my ability to make them, and protect my slightly underpowered transformer.

Here's the schematic.

When the power switch is thrown, the capacitor bank is charged through a 3.3ohm resistor (it could have been smaller), and a resistor in series with the first relay divides the voltage so that it sees the 10V it needs to switch when the bank is at 15.5v. When it does, one of the gangs in it is used to turn on the second relay (to increase current handling and reliability), and the 12V regulator which turns on the amp. There are then 5 gangs rated at 10A at 30V each, and I beefed up their internal lead wires with speaker terminal-cone braid, and drilled cooling holes in the cases so they should be reliable!

 

Construction:

The chassis is mainly aluminium, with checkerplate top and bottom ($5 at a scrap metal dealers), the sides came from two fisher & paykel gentle annie power supplies (as were the two relays and some of the caps), and the steel mesh end panels are the door from a microwave. Ample passive ventilation and heatsinking I think.

The transformer is cooled by being mounted with thermally conductve (A lot moreso than the rubber rings jaycar gave me! I did some science with my two thumbs and a boiled jug.) silicon/rubber? sheets & sleeves, come to think of it I got most of them from the same F&P power supplies aswell. There's an aluminium heastink wrapped around the outside of the toroid, with the same heat conductors, and it felt just as warm as the transformer after running. Success!

After the photo was taken I also added a draining resistor on the 12V supply, and after the first power-up I moved the relay control resistor to the SMOOTHED side of the 3.3ohm resistor.

The diode bridge and regulator have some leftover Arctic Silver 5 thermal paste, and on the outside of the chassis behind the diode bridge is an old cast aluminum CPU cooler with the same paste.

When you turn the power switch off, the bank is drained through a 220ohm resistor, but the 12V does not switch the amp off until it gets undervoltaged. I have to ensure it drains the 12V cap before the main caps get that low to prevent reverse-voltaging the regultor, and I think I've done that, but if it blows up I've got 4 more of them to get it right.

 

Cost:

Including everything I've paid for that went into the PSU (even the solder) I get $90. The majority of this was the transformer, at ~$65, and I may have to shell out for the 300VA one if this one blows up. It's got a thermal fuse at least. If you didn't have all the wires and caps laying around it'd probably cost at least double that.

 

Now I'm going to go screw the lid on so I can crank it up again, and take a photo of it finished.

 

Recommendations/criticisms welcomed!

 

PS: I 're-formed' the cap bank at about 1mA up to 18v before plugging it in, took about 3 hours, and not even one of them exploded! It was draining 0.3mA when it reached that supply voltage, through the caps, and/or the diodes.

Attached files

[nixon76]

Interesting. When I was a yoof I and thought using car audio subs/amps for a HT sub was a good idea I went on a hunt for suitable power supplies. I was told the current draw was the problem. Nothing off the shelf for student (loan ) money was available. I even asked at the car audio shops what they used for in-store demos. They said the had megabuck transformers for the job and even they couldn't handle the load when the wick was turned up. Although to be fair they had to handle banks of amps/speakers.

 

So seeing a 'home' job do the biz is great to see. Nice one.

[Pikefish]

Yeah current ability is the big one working with such low voltages, I'd previously tried to cascade down a transformer with a heavy gauge one I wound myself, only to learn the internal resistance of the first transformer was impractically high. Not alot of transformers around that can pull the kind of currents these amplifiers need. They need too much copper, and aren't in demand enough.

For anything more powerful I'd probably abandon the idea, or try to hack in ahead of the voltage booster inside the amp, but wouldn't try that with a Class D amp.

[Owen Y]

Hi Pikefish,

That's terrifyingly good DIY work, well done

 

I know little about SS/semiconductor stuff but...

 

The TX seems to run v cool, I had one (EI) in a valve amp some yrs ago that ran quite hot... I knew that if I could still hold my hand on it for at least 3secs, all was OK, ie. normal It was drawing ~110VA quiesecent as I recall.

 

Why not a EI core PTX? Size? Or, did this one just turn up?

The TX heatsinking is Heath Robinson, low budget genius :cool: No fans.

 

I like the paralleled miscell cap bank, with quadrupled solid core wire links, all carefully routed!

 

Kudos

 

I hoard any bits of perf metalwork, in the hope that one day it will be invaluable

[Ernie1553552694]

 

Owen Y;89882 wrote:

I hoard any bits of perf metalwork, in the hope that one day it will be invaluable

 

Look forward to the day when you post photos of your new hotrod!

[Pikefish]

Cheers Owen,

 

I haven't seen an affordable 12V EI core transformer with a VA rating over 50 anywhere. I'd tried a microwave tx with the usual HV secondary used as the mains fed primary, but perplexingly the unloaded voltage ratio appeared to change depending on which way through the transformer you were measuring. Putting a 15V test into the low turn end got a ratio of about 1:10, but into the high turns end the ratio was about 30:1, both tests done with a ~1W load on the output. I guess the efficiency is very low, high eddy losses, not to mention the resistance in the primary of 100ohm. There wasn't room enough in there if I was to re-wind the secondary with more turns anyway.

I wouldn't have minded a different type of transformer, there's no signal level stuff inside the box to get interfered with, it's just the only thing I could find that didn't cost the earth. I think back when I was first toying with the idea I found some from one of the more high-volume wholesalers around, and they cost hundreds.

I am thinking of installing a fan, there's room for a 120mm to go in there, but I haven't got any suitable ones that are slow and quiet enough. Might have to make a PWM speed controller. It'd be there mainly to blow on the amp which is mounted next to it, it gets unreasonably hot and doesn't generate the right convection. Mounting it sideways should work better. I'll do that when I get the final power cables for it, multistrand stuff.

[Owen Y]

It get's more terrific (terrifying).... a m/wave PTX wired backwards!

 

I see how you can justify such a luridly gruesome avatar

 

PS. I crudely used to run 120mm 230v pancake fans at lowered volts, to slow them down.

[nixon76]

Please excuse if this is a noddy interruption, but could you use a 120mm 'silent' computer case fan and run it off the DC output? These can be pretty cheap (~$20-40) and pretty quiet (16-35db), but they are normally setup for 10.8-13.8V. Not sure what would happen if you put 15.5V through them.

[Pikefish]

That's right, computer fans are the way to go IMHO, I've always found AC powered fans to be too thick, or have poor propeller design. I've only found that silent fans by SilenX completely live up to their name, I've got one you can't hear until it clips your ears. I've got a regulated 12V rail in there that can run one fine, but would struggle to run a non-silent one. I've got some 60mm ones that are pretty quiet, but I'll probably leave it until I go and make another order from our national silenx dealer.

I could get what I need for $30 or less, but the trouble is once I go to that site I'll come away with hundreds worth of extra fans and heatsinks and sleeves and dyes etc. Will have to go there before July though anyway.

 

Next on the workbench: HV power supply, gonna zap me a creme egg.

[Pikefish]

This project is now finished. I bought a pair of 60mm quiet fans (these aren't silent when they're this small and held fast to a plate of aluminium) and stuck them to the lid, only place I had room for them, with a sheet of Sekisui Thermal Tape, very handy stuff. Strong and sticky and thermally conductive, I always buy a few sheets when getting some fans. In this case, I lived up to my fears and bought a load of additional fans I don't need yet, but this isn't a blog so more DIY:

 

There is a switched mains output at the back of my main amp (a Kenwood KP-V5580), but it's only rated for 150W, so I rigged up a 240V relay inside an old adapter case to let me run the sub amp through it. There's also a heavy terminal block for attaching earth cables to at a star point. Now when I switch on the stereo, a sequence of 4 separate relays activate!

 

Furthermore, I made an attenuator to use the headphones out, since it has no sub out and I use it in stereo mode. It came out a little too hot and distorted in the sub amps frontend even with the gain at minimum (8V), so now it presents a 147ohm load to the HP amp (which is always on) so I made this from some scraps. Attenuation is from down a few dB, to infinity.

 

Here's the whole lot, in place, with the lid finally screwed on. Hard to catch it on camera, but it swings up to 30A short hits before any distortion is heard. I have the lowpass set to about 40Hz, above there the mains take over well enough. Judging by the huge difference in level heard when flicking the 180deg phase switch, I'd say they're operating pretty well in phase.

I've put the amp on the sub box there because the fans blow on it, this has been very effective in keeping it cool.

Attached files

[Owen Y]

..........Everyone is... speechless.

 

My hat is off

[Pikefish]

I reckon it'll blow my hat off too soon, I recently bought a 15" cerwin vega 13mm xmax "97dB@1v??" 4ohm sub off trademe for $40, when that arrives this weekend I'll put it on the end of my 18" 3m long sonotube and see what happens! These 2 subs now are in series for 8ohm, trying to keep the power in voltage rather than current for now.

 

Here's my headphone output attenuator schematic if anyone wants it:

One half, repeat with double gang if you can find one, for stereo. Surplustronics had the 100ohm pots.

Attached files