SPMSU for heater supplies

[ruffrecords]

I am building an audio mixer that uses 9 pairs of tubes and the heaters take 12V at a total of 5.4 amps dc.  I don't fancy building a single 6amp 12V linear regulator so as I see it I have a couple of choices.

1. Build nine separate 1 amp or 5 separate 2 amp regulators - a bit cumbersome but workable.

2. Use an off the shelf 12V SMPSU. I have looked at a few examples. Most seem to switch at around 25KHz and at rated current have about 200mV pp ripple on the output (content unspecified but I bet there's some 25KHz in there).


As a low cost, small size way of achieving what I want an SMPSU seems the way to go but this 25KHz power circuit worries me. OK, the PSU will be in a separate screened box some way from the mixer's sensitive circuits but still am am a little concerned.

Anyone have any experience of using SMPSUs for heater supplies in low level audio or any other comments or ideas??


Cheers

Ian

 

[Svart]

a lot of folks use AC for heaters...  Why would a little bit of ripple matter?  Just LRC filter it and you'll be fine.  It's no more noisy than junk coming from rectifiers.

 

[gemini86]

...and at 25k I would imagine that if it DID bleed into the audio at some point, 200mV is going to be hard for the human ear to hear.

A question, though- Why don't you just use two of these 3A regulators in parallel to provide 6A of regulated 12v?
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMug9GoBKXZ75%252byIzKRzQkfKJPpOqOylB7I%3d

Some easy ways to make 3 pin regulators work in parallel:
http://www.reuk.co.uk/High-Current-Voltage-Regulation.htm

 

[Svart]

actually 200mv is easy to hear depending on gain.

I had a tube box that had 20mv ripple and it was hard to use.  I reduced it down to 3mv and it's now usable.

 

[ruffrecords]

...and at 25k I would imagine that if it DID bleed into the audio at some point, 200mV is going to be hard for the human ear to hear.

And you can bet it will get rectified somewhere and appear in the audio band where it will be heard.

A question, though- Why don't you just use two of these 3A regulators in parallel to provide 6A of regulated 12v?
http://mouser.com/Search/ProductDetail.aspx?qs=sGAEpiMZZMug9GoBKXZ75%252byIzKRzQkfKJPpOqOylB7I%3d

Some easy ways to make 3 pin regulators work in parallel:
http://www.reuk.co.uk/High-Current-Voltage-Regulation.htm

Good idea. I don't need to parallel the regs though I can just split them equally between the tubes. One small point though, the parts you linked to have a theta junc to case of 2.9degrees/watt. Even with a huge 1 degree C/watt heat sink that's a total of nearly 4 degrees C/watt and worst case they'll drop 5V at 3amps that's 15W or a 60 C rise, say 90 degrees C worst case junction temp. OK that's comfortably below below Tjmax so perhaps not really an issue.  Funny thing is I looked at 3A LDO regs by Linear technology but was put off by the price - they are four times the cost of the Fairchild's.

Thanks for making me revisit them.

Cheers

Ian

 

[Svart]

yeah but I've had Fairchild parts suddenly die yet I've been using Linear's stuff for years and never had a single problem...

 

[gemini86]

And you can bet it will get rectified somewhere and appear in the audio band where it will be heard.

Not sure why you would rectify a DC voltage... Besides, a good RC filter would should clean it up?

But from were I'm sitting, a 3 pin LDO reg looks like a good solution.

 

[ruffrecords]

Not sure why you would rectify a DC voltage... Besides, a good RC filter would should clean it up?

Because it's not dc, just rectified/smoothed  ac

IAn

 

[PRR]

> 12V at a total of 5.4 amps dc ... box some way from the mixer's

That's fairly hefty power delivery.

Any PC power supply can carry it, except most need a substantial load on the +5V (and now on the +3V), and of course they are switchers and not particularly clean.

Can you not re-wire for 24V 3A? Same power, but much less fat copper.

As a brute-force hack, use 24VAC 5A PT, rectify onto 5,000uFd as 34V DC, use 3.7 ohms 50W resistor in a cage (call it "coffee warmer") onto another 5,000uFd.

You are discovering why large tube systems are a real pain, and heavy.

> use AC for heaters...

You know I advocate this a lot. We did good things before DC heat was invented. But a project this size has enough problems without fastidious heater twisting/routing fun.

> Not sure why you would rectify a DC voltage

You don't get your choice. His "DC" is contaminated with 25KC AC. Run that near diodes (grids) and it will find a way to get itself rectified.

> a 3 pin LDO reg looks like a good solution.

A linear reg which won't punk out on wall voltage dips tends to dissipate at least half the load power. That's 32W. Few 3-pin jobs are rated that much; the ones which are will need a BIG heatsink to stand it.

> regulators in parallel

You need to be sure (or force) regs to play nice together. Many common regs, whichever one is a tenth volt higher ends up with ALL the load.

As Ian says, he got 18 loads, he could slice-dice down to manageable per-reg size, though the wiring gets tangled.