multi-channel amplifier power supplies

[MikeA]

Hello everyone,

This is my first post and I just want to say that this is a really great, informative forum. I've taken up this hobby just recently and have been devouring as much literature on power amplifier design as I could which includes many an hour going through messages on this site. Naturally I have a host of silly questions for you all.

My first is around power supply design when dealing with multi-channel (by that I mean 3+) amplifiers units. I've read a number of conflicting opinions on whether or not seperate transformers, or bridge rectifiers or even filter caps make a difference in the quality and stability of the amp as well as the other measurable characteristics like THD, slew, and especially noise.

I understand what the benefits are (namely lower cost) but what are the risks involved in trying to build a 7 channel amp (150W RMS @ 8Ohms/channel, for home theater) with, say a 1500 VA transformer, a 35A bridge rec (comments?) and just parallel the output rails to the 7 amp boards. Is it just a matter of doing the math correctly and ensuring there is enough filter capacitance to supply adequate current or are there other aspects to take into consideration? Any comments on circuit/speaker protection and/or the design of the power supply itself are appreciated.

[millwood]

Originally posted by MikeA
I understand what the benefits are (namely lower cost) but what are the risks involved in trying to build a 7 channel amp (150W RMS @ 8Ohms/channel, for home theater) with, say a 1500 VA transformer, a 35A bridge rec (comments?) and just parallel the output rails to the 7 amp boards.

I am actually not a fan of monoblock construction for multi-channel amps. I understand that you can improve inter-channel modulation (cross-talk via power supplies), but I think the built-in "redundancy" in a big power supply, vs. several small power supplies, can be quite benefitial in a multi-channel set-up.

For example, see you use a 200w transformer per channel. in a 5 channel set-up, the "juice" in other channels power supplies cannot help out a channel that is being overloaded. If you use a 1kw transformer, you wouldn't have that problem at all.

so there are pros and cons and for me I went with a big transformer (and beefier diodes / caps) for power sharing, simplicity, and ease of construction.

[SY]

There are some interesting tricks you can do to make multichannel amps work well on a single supply^*, but I wouldn't go there to start.

The big issue is going to be crosstalk- the supply has a finite source impedance (which itself can change with large changes in load current), and the voltage will be modulated by the current draw of each stage. If the amp circuit has great supply rejection, you'll possibly be able to get away with this. But if your goal is making something that measures as close to perfect as possible, to insure freedom from motorboating, and maximizing the output from a channel when the other ones are working hard, single channel supplies are more certain to get you there.

^* One such is to pair the channels and have each member of the pair run with opposite polarity to the other. Tough with an odd number, though, and far from perfect.

[MikeA]

Given the fact that the current will be drawn against a single transformer to power all 7 boards is there a greater risk of damaging the transformer as the result of having to increase the mains fuse in the PSU if the secondaries or bridge rec short?

I figure a 150W/8Ohm load would require about 40V rails or so (with normal losses) and draw about 3A of peak current. That means I would need a mains fuse of about 21A for 7 channels is that right? I understand the chances of fully taxing all 7 channels at 1 time would be remote but I want to take into account high amplitude transients which you see in movies all the time (explosions for example) which I would like the amp to produce faithfully without sag or droop. Would this power supply take too much abuse under these circumstances considering that a 1500 VA transformer is rated for about 18.5 amps with 40-0-40 secondaries? Do I need to go 2k or is that overkill?

Thanks for your input. You are really helping a newbie out here.

[BobEllis]

Hi Mike,

Not sure I follow your calculations. 150 W RMS means ~212 W peak. V= Sqrt(P x Impedance) = 41. You seem to have used 150 for P, giving a swing of 35 volts, and added 5 for losses in the output devices (reasonable). You should shoot for 46 volt rails to get 150 W RMS.

Rail voltage is not transformer voltage. Theoretically you'll get 1.414 x Secondary voltage. Diode losses and ripple eat up some of that, so count on 1.2-1.3x secondary voltage. A 35-0-35 transformer should get you close for class AB operation. Losses will be higher if you are building a class A amp.

Current draw =SQRT(P/R)= 5.1 A peak/ 3.6 A RMS per channel. 25.2 amps for 7 channels. => 1150 Watts RMS all channels driven. Now its time for balancing costs and performance objectives. With class AB amps, you can probably get away with a 1.5KVA or 2 KVA transformer. The DIYer rule of thumb is it should be rated 2-3x expected load.

There is no such thing as overkill in DIY.

[tool49]

Humm being in Canada, you'd have to check the rating of your electricity lines in the house. Most of them are on a 15A breaker. Perhaps that a 21A fuse would not offer much protection. Anyway, for those big transients such as the one in movies, your amps will mostly rely on your filter capacitors, not your transformer. So instead of going for a bigger transformer, I'd go for more capacitance in the power supply lines.

I have built a 6 x 200W amp (Class AB) and it uses a 1.5kW toroidal transformer (the main fuse is a 12A) with about 72 000uF per rail and even at the loudest levels it never even glitches down. Given that my speakers are pretty efficient, I rarely need all that power but, with dummy loads, I've been able to put out more than 1kW using a sine wave at 10Hz, and the 12A fuse held ok.

I hope my 2¢ will help you.
Sébastien

[Nisbeth]

Originally posted by BobEllis
...There is no such thing as overkill in DIY.

Now that's true, but a professional amp designengineer I know, told me that because the energy content of music material is far lower than if you were playing sinewaves, you can get away with a far smaller PSU. In a typical consumer amplifier, the supply will be able to output around 1/8th of the current draw from the amp (output power/efficiency in %) continuously, while a professional amplifier for stage or PA-use typically has a PSU continuous output of 1/3 of the current draw. This would mean that a 7 ch. AB amp with a 60% efficiency could get by with a supply rated as little as 220 W and even for professional use the supply would still only be capable of supplying ~580W. Of course there is some performance-gains to be had by upgrading the supply, but even then the numbers are interesting.
If you wanted to play sinewaves at full output the supply for my example amp would have to supply 1750W with all channels driven (!!)


/U.

[maylar]

Originally posted by MikeA
Given the fact that the current will be drawn against a single transformer to power all 7 boards is there a greater risk of damaging the transformer as the result of having to increase the mains fuse in the PSU if the secondaries or bridge rec short?

The real risk is toasting one of the amp channels because a single fuse on the input side has to be so large. I would fuse the mains appropriately for rated transformer primary current, and also fuse the DC side to each amplifier rail.

[tool49]

I consider that putting a fuse on each rail of every amplifier channel is definitely necessary in a multi channel amp. It happened to my quite often to blow a part in one channel and seing the fuses blow on the board, leaving every other channel intact. But what if only the output of the transformer had been fused? Well probably that the spike in current generated by the blown transistor would not have been enough to blow that fuse and that the spike could have killed parts in other channels.

Fuses are cheap, mosfets aren't. I would suggest not to skimp on fusing each channel.

Sébastien

[MikeA]

Thanks for everyone's input! It's been very helpful!

This forum rocks..