Inverters and audio

[peter purpose]

Gentlemen,
A friend has a small mixer (500mA max) that he would like to power from a 12V DC battery pack and I've come to the conclusion that his best bet might be one of these 12V dc / 230V ac inverters.

I understand that some output a square wave of some sort, while others output a pure sine wave.

Is the square wave variety (much cheaper) a no no for audio?

Thanks for any and all guidance.

peter

 

[NewYorkDave]

I've never seen an inverter that puts out a real sine wave. Some put out what's euphemistically called a "modified sine wave." I had to power an amplifier off the latter type once and it was noisy as hell; a square-wave type would have been even worse.

In other words, I don't expect good performance with either type of inverter.

 

[peter purpose]

Many thanks Dave..... I thought it too good to be true.

 

[mikep]

[quote author="peter purpose"]Many thanks Dave..... I thought it too good to be true.[/quote]

how about a 60Hz sinewave oscillator feeding a large car stereo (12V) class-D subwoofer amp? These things can be had quite cheap these days. If you use a stereo amp, feed the inputs in parallel and hook up the outputs in series, it *could* work. you can pull this off with big mains powered amps. But I notice alot of the more powerful car stuff is rated into 2 ohms. makes me wonder how much voltage a "1000 watt" car stereo amp can swing. Any ideas?

mike p

 

[bcarso]

[quote author="mikep"][quote author="peter purpose"]Many thanks Dave..... I thought it too good to be true.[/quote]

how about a 60Hz sinewave oscillator feeding a large car stereo (12V) class-D subwoofer amp? These things can be had quite cheap these days. If you use a stereo amp, feed the inputs in parallel and hook up the outputs in series, it *could* work. you can pull this off with big mains powered amps. But I notice alot of the more powerful car stuff is rated into 2 ohms. makes me wonder how much voltage a "1000 watt" car stereo amp can swing. Any ideas?

mike p[/quote]

By the time you get to where the power into a given load corresponds to a voltage like even just 115VAC (let alone 230), it's no longer an inexpensive amp! Crown does a multikilowatt one now but I haven't noted what the load Z is for that power. And it ain't cheap!

And you cannot hook up the outputs in series, not at least with any amps I know of (for that matter you cannot do it within a single chassis mains-powered amp either---although you can take single-ended stereo amps and get a single bridge-mode amp out of it, usually). Although the stepup switching power supply in principle could provide transformer isolation, in practice things are referred to the automotive system ground. So it would at best require extensive modification; and once done, you will have to feed each amp with a floating oscillator signal. At least those could be easily provided using low frequency transformers.

Probably a better strategy: filter the output of the quasi-sinusoidal inverter. These have a three-level output with the non-zero pulse width selected to null out third harmonic IIRC. But by themselves as reported above they are indeed horribly noisy. I got one to run a boom box in my car for a long trip when my cassette player had died, and with casettes it was totally unusable. The CD player portion of the boom box was o.k., but then the problem was the car had to be nearly dead still for the read optics to achieve a lock. So I could only change CDs at rest stops!

 

[Svart]

how about a 60Hz sinewave oscillator feeding a large car stereo (12V) class-D subwoofer amp? These things can be had quite cheap these days. If you use a stereo amp, feed the inputs in parallel and hook up the outputs in series, it *could* work. you can pull this off with big mains powered amps. But I notice alot of the more powerful car stuff is rated into 2 ohms. makes me wonder how much voltage a "1000 watt" car stereo amp can swing. Any ideas?

Yo mike, we already discussed that.

http://www.groupdiy.com/index.php?topic=14178&highlight=inverter

I'm sure it can be done and some tests I have done seem to agree with that but I ran out of free time to work on it for now.

And there ARE *true* sine inverters, they just cost a whole lot. Well, true sine isn't quite the word for it, more like the DC steps are so small you and your transformers won't care.

"modified sine" wave inverters are nothing more than alternating polarity square waves, Bcarso points out that "quasi-sine" are a little better to start with but with anything that has a PSU trafo you are headed for trouble without a lot of work. If the PSU to be fed is a switcher, then you should be ok.

However, this doesn't take into account any noise that gets into the system from EMI/RFI etc.

 

[peter purpose]

Thank you gentlemen,
I think I'll keep an eye out for a second hand pure sine inverter to experiment with. Failing that, it'll be dc/dc converters and bollocks to the phantom.

Cheers
peter

 

[SonsOfThunder]

[quote author="bcarso"]And you cannot hook up the outputs in series, not at least with any amps I know of...although you can take single-ended stereo amps and get a single bridge-mode amp out of it, usually).[/quote]Sorry to barge in here, but many class D car stereo amps (which I just so happen to know a "little bit" about...) ARE single-ended. All of the Kicker KX class D amps save the biggest one had provision to strap two amps together, built in. But you are correct that its not cheap...Nor does this project need that much current!

I have another suggestion... (related to the subject of car stereo amp technology and class D amp power supplies). You need +/- 15V and +48?. Pretty close to what is in a lot of class D amps but you need a lot less current from the +48. Build yer own little SMPS! Need a schematic?

Charlie

 

[bcarso]

[quote author="SonsOfThunder"][quote author="bcarso"]And you cannot hook up the outputs in series, not at least with any amps I know of...although you can take single-ended stereo amps and get a single bridge-mode amp out of it, usually).[/quote]Sorry to barge in here, but many class D car stereo amps (which I just so happen to know a "little bit" about...) ARE single-ended. All of the Kicker KX class D amps save the biggest one had provision to strap two amps together, built in. But you are correct that its not cheap...Nor does this project need that much current!

I have another suggestion... (related to the subject of car stereo amp technology and class D amp power supplies). You need +/- 15V and +48?. Pretty close to what is in a lot of class D amps but you need a lot less current from the +48. Build yer own little SMPS! Need a schematic?

Charlie[/quote]

I'm not sure why you think (if I am reading you correctly) that "single-ended" means that they can be put in series. They are not independent/floating. Bridge-mode from two single-ended channels, yes, with one driven in inverse polarity relative to the other, and some have provisions as you say for paralleling, where one becomes the master and the other the slave. Only if the power supplies were fully transformer-isolated could different-chassis amp outputs be stacked as if they were batteries for higher voltages.

Car stereo is, intrinsically, relatively low voltage even for the highest power. One could make a purpose-built 12V input HV class D amp that put out sinusoids, and probably that's what's in some of the "pure sine wave" inverters.

 

[SonsOfThunder]

[quote author="bcarso"]Bridge-mode from two single-ended channels, yes, with one driven in inverse polarity relative to the other, and some have provisions as you say for paralleling, where one becomes the master and the other the slave.[/quote]Exactly what I am talking about and what is done in a bunch of class D car amps. Though I have seen full-bridge amps strapped up with some modifications. We even worked up a scheme whereby you could strap 4 amps in series, though we have not tried it. Usually all you have to do is clip one resistor to get full isolation on the grounds (audio vs batt) but IIRC, its not necessary for our scheme.

One could make a purpose-built 12V input HV class D amp that put out sinusoids, and probably that's what's in some of the "pure sine wave" inverters.

Or modify an existing amp to do so. You would need to rewind the trafo for lower current/higher voltage and change the outputs for higher voltage.

But I stick with my suggestion to build a +/-15V/+48V SMPS off of 12V battery as the better, easier, more straight-forward idea. And I have done something VERY similar, multiple times.

Peace!
Charlie

 

[pstamler]

You really need a 117V sinewave? Build a clean sinewave generator and connect it to a car amp. Feed the output of the car amp into a filament transformer turned backwards -- 6.3V : 117V.

Peace,
Paul

 

[bcarso]

[quote author="SonsOfThunder"] We even worked up a scheme whereby you could strap 4 amps in series, though we have not tried it. Usually all you have to do is clip one resistor to get full isolation on the grounds (audio vs batt) but IIRC, its not necessary for our scheme.

Peace!
Charlie[/quote]

Still have to have floating drive signals to the various channels though, I would think. Those series amps are going to be at big potential differences. Transformers or fast optocouplers at the very least.

 

[peter purpose]

[quote author="SonsOfThunder"] Build yer own little SMPS! Need a schematic?

Charlie[/quote]

Yes please Charlie

 

[Guest]

Peter
I was on the distel web site at lunchtime and found this
http://www.distel.co.uk/asps/details.asp?ID=88404LL09

Simon

 

[peter purpose]

Cheers Simon, but that thing weighs 40LBs...!!!

I'm looking for something that a few ants could carry.

peter

 

[Guest]

oops - sorry!
BTW - they have some nice PPM and driver meters there - (and I am trying to convice the missus I really need one of those massive Railway digital clocks - she says NO! and mentions something about how she knows how short some things are anyway - she doesn't need to be reminded of that fact by a 2 foot clock)

 

[peter purpose]

I'll resist the obvious.. :razz:

 

[bcarso]

How does it go? Arse longa, vitals brevis?

 

[peter purpose]

:green:
No... something with a far more juvenile slant.

 

[SonsOfThunder]

I got it... :roll: