Switching Regulator for Audio Amplifier

tcharges the C,

then it

Bridge yes. 1/2 the transformer feeds both rails for one channel.

50Vrms * 1.414pk * 1.04 regulation * 1.043 for 120 local voltage = ~78Vpk

That gives 39Vpk before the bridge. Right?

tcharges the C,

then it

I could has said that clearer. The bridge differential, it supplies both rails directly.

On a sunny day (Sun, 14 Sep 2008 12:00:25 -0700 (PDT)) it happened Fred wrote in :

____________________ + (27 . sqrt(2) - .7) = 37.48V | k |k D1 D2 | | --------------------| | | 27V | | T1 | | |------ GND 0V | | T2 | | | 27V | | -----------------------------------| |k |k D3 D4 | | -------------------- - 37.48 V

So, 2 windings of 27V?

Are you trying to obfuscate? :)

27 volts would be too high, you omitted the transformer no load rise and adjustment for local mains voltage.

I'm thinkin'

_____ ___ +( 50 * sqrt2 * 1.04 regulation * 1.043 local mains )/2 | k |k - 0.7 = aprox +38V D1 D3 | |

-------------| | | | | | | | 0 | t1 50Vrms | | | | | | | | | | |

-------------------| \\\\\\ |k |k D2 D4 | | --------- aprox -38V

On a sunny day (Sun, 14 Sep 2008 13:30:08 -0700 (PDT)) it happened Fred wrote in :

No, this is a real circuit. I am also assumimng you will run both left and right channel from the same supply, needing 2x value of caps and current.

Ok, use a lower voltage then, but the issue was about the type of rectifier configuration.

I get the strange feeling you have ground floating in your diagram, this is not true now is it? that 50V has a middle tap I hope?

There are actually a standard range of torioid transformers for amplifiers in the shops, I dunno if 50V (or 2 x 25 what it should be) is a standard value.

supply,

configuration.

not true now is it?

the shops,

My bad, They would have to be center tapped windings. The transformer could have 2x50 center tapped windings, but I think that is not standard. One winding would power the left channel, the other winding the right. I have been looking and it appear standard values are 2x45 and 2x50 in the 500VA-600VA range. In that case I have to get a custom one wound. And if I did that I could order a 2x48ct

500VA, which would give me a larger voltage safety margin.

supply,

configuration.

not true now is it?

the shops,

The data sheet for that TDA7294 has an interesting booster arraignment. Did you try that? I think it would work for the device I used too. I've seen bridged and bridged-parallel arraignments for the chip I used, but nothing like those booster darlingtons. I wonder if any cross-over distorion shows up.

Fred

That is an interesting way to transfer some of the heat out of the amplifier into the darlingtons. But I would have to do a lot of analysis and testing to make sure it didn't degrade the specs.

For your switcher supply, have you considered a pair of switcher outputs (separately powered) that sit on top of (in series with) the capacitor filtered supplies that just produce the needed voltage at no load and maximum line voltage. This way, you could allow considerable ripple at that filter and just add the small additional voltage needed to hold the total constant, without having to switch the whole power. So they boost the low and ripply capacitor filtered ordinary DC bus to achieve a regulated total but with only a small fraction of the total power converted.

--
Regards,

John Popelish

On a sunny day (Sun, 14 Sep 2008 17:19:28 -0700 (PDT)) it happened Fred wrote in :

Yes

I see no reason why not use the same supply for left and right? After all supply rejection is good.

Usually, toroids for audio power come with 2 windings, like 2 x 27 V for example.

Holy golly, oh me oh my, ooops, 500VA, for 50 W out!

If indeed the chip is your problem, and you already killed a few, then why not use a decent chip from some other manufacturer that is less critical, or go discrete. A am not saying National makes bad chips, I am saying others make sometimes better ones.

Transformers are very expensive, heavy. Can you get a TDA7294 over there? I run it with 2 x 33V transformer, single phase rectification. The advantage of single phase is less diodes, but also as the + is at maximum, then the - is at minimum, keeping the total more constant. Once killed one when I accidently shorted the mounting tab against ground, for the rest it works at full power now for more then 8 years. If I drive it full, then I start smelling speaker voice coil, that chip is 100% stable. Yes it is +- 50V no signal, and they say +- 40V with signal (whatever that weird spec may mean), so I run it at 46 volt, meet halfway ;-) and it works fine. (That voltage will drop a bit at full signal, so maybe it is in spec anyways :-) ). It has overload protection, needs no RC network to be stable, it is a good chip. That chip is less then 10 Euro here, or say about 15 $. Then you should be able to use your existing transformer perhaps.

Anyways, just my view and experience. Make sure you have a decent fuse too.

Going discrete is not difficult too, what I used to do before there were these chips. And may be just as expensive, or even cheaper.

On a sunny day (Sun, 14 Sep 2008 19:04:44 -0700 (PDT)) it happened Fred wrote in :

No, I would go discrete for higher power.

37.48V

critical,

100% stable.

weird spec may mean),

:-) ).

chip.

chips.

The people I discussed this with seemed to think a separate transformer for each channel was best. Failing that, at least using a separate winding for each channel was recommended. The idea of using the same winding for both channels seemed to be appalling to most.

Failing the separate winding for each channel, in order to avoid having to buy two transformers or have a custom one wound, I suppose I could use two sets of filter caps at the value we determined and add some additional diodes to prevent a load on one channel from drawing down the caps on the other.

Aside from that, I have acquired a design for a discrete MOSFET based power amp with good specs in the 150W to 200w range. Although, less then vastly, it is significantly more elaborate then this.

I believe it would be uh,,, prudent of me before delving into that, to at least get this current relatively simple product working to my satisfaction. :)

Fred

On a sunny day (Mon, 15 Sep 2008 09:36:58 -0700 (PDT)) it happened Fred wrote in :

Are those the same people who recommend oxygenated copper for the speaker leads?

You can do that with the single phase circuit as in my amp.

Supply rejection of that chip is ?dB? If you know the max voltage change a sudden load variation in one channel can cause, (say from silence to full power 1000Hz), then apply that supply rejection, and that is what you get as transient in the other output. As the channels likely carry related audio material, maybe you will not be able to hear it.

There are simple and complicated amps, build something simple, but especially something you understand, run a simulation in LTspice, play with it, LTspice is free, and easy to use, that saves buying transistors, and shows you in a few seconds what would otherwise takes hours of measuring and experimenting.

leads?

rejection,

I've been using LTSpice for years now.

Its awesome! :)

Fred

What's the state of the art in terms of Class D (or other PWM architecture) audio amps in this power range?

Perhaps you could give up the idea of trying to clean up the supply for a linear amp and really save yourself some power.

--
Paul Hovnanian	paul@hovnanian.com
-----------------------------------------------------------------------
Have gnu, will travel.

I think this problem is solved. I just have to verify that by buying a $130 transformer and a couple mnore filter caps.

I actually have a chip set for a class D amplifier. Like the discrete design it significantly more elaborate. Looking to the future though...

Fred

Well, that would be weird. for 130$ (89 Euro) you buy a 2 x 100W amp here with the 200VA transformer in it, in 19 inch rack, with XLR connectors, temp controlled fan, volume controls, etc. AND filter caps :-) Yes it is discrete. PA3000

Yes and one transformer for both channels.

In these times, with markets crashing, I'd think trice about spending dollars. It does not make a lot of sense to build something if you can buy it cheaper with the same or better quality.

So, you want me to be a consumer rather than a producer?

Its actually a very scary thing that the bulk of our manufacturing has all been outsourced to east asia, if you are economically aware.

I have learned so much about our global economic situation, with the world bank want to rule in totalitarian fashion a single world government, that it is depressing me to death.

Everywhere there are nay sayers. Don't do anything creative, Don't do anything on your own initiave. Just do what you are told. Work in service industry.

The service industry is basically all thats left in this county now. I heard them talking ten years abo, about the economy of this country moving toward becoming service industry oriented. Now I realize what they meant.

What ever happens, in the end there will be the list of the thing I created. Things I chose to do when I was able to, instead of shivering with fear of the unknown. I'm truly beginning to think in the end that will be my only solace.

Anyway, enough of that unpleasantness.

I have an amp to build, and its gonna rock!

On a sunny day (Mon, 15 Sep 2008 15:06:07 -0700 (PDT)) it happened Fred wrote in :

Well, lemme put it this way, you can produce all you want, but what if you cannot sell it? That makes little economic sense.

There may be an educational aspect, then I would _still_ buy the Chinese amp, as it has all parts and more for less money. And play with those parts - now that is 'green'.

But there are many things that you can design that you cannot buy cheaper. My last home project was a digital SWR power meter with LCD display. Very cool :-)

Does not scare me at all :-) It allows us to have many cool things, from DVD players to scooters, for a low price, and the cheaper the more you can do with your money.

mm

Well, do what you must do, do your thing. Creativity comes in many ways, I did maintenance of complex systems for many years, it can be as fulfilling as designing something. Fulfilment is an inner experience.

Designing for a job can be extremely demanding. 'Service' can be extremely demanding. Both can be fun too. It is up to you.

Creativity includes not being stupid. 'do what you are told to to?' I dunno, what makes authority is you listening to it.

Well, I dunno where you are, with that gmail address, but yes, service industry always will be important.

See, if you want to design, you have to be better then the rest, cheaper too perhaps, else it will never be a product that can be sold. There is always the competition, so using a ridiculous big transformer sort of makes your design void from that perspective.

I looked at the PA3000 before. It omits mention of whether that is watts music power or watts RMS. I noticed the ST chip quotes figures large letter the specify music power. The output of that chip is close to the NS chip when you look at the RMS figures. The only improvement I see is that you can get

60-70 watts RMS out of it into 8 and 16 ohm as well as 4 ohm loads.

The price is good. The bet the manufactures buy lots of 10,000 transformers at a time etc. I've looked into temperature controlled fans. It's easy to add on. My units will lack attenuators, but will have a mute switch. That's all it needs to fulfill it purpose.

What is it they say about ignoring the things beyond your control, doing the things within your control, and having the ability to tell the difference...

Fred