I have an audio amp based on the LM3886. In order to get 50W out of it I need to use a transformer who voltage rises too high for the LM3886 to tolerate during no or light load times.
I have modeled a linear regulator for the amp, but is is very wasteful, it dissipates over a hundred watts a channel. I had to parallel four regulators on each rail in order to keep their individual dissipations manageable.
I have started considering a switched mode power supply, and have been looking at chips such as the LM5116. Some time ago, I heard using SMPSs in audio amplifers in problematic because of the difficulty in filtering out the switching noise from the output.
The specs on this amp aren't particularly great. You could probably buy an off the shelf amp for less than what you are trying to build. My recollection is Randy Sloan's amp design book takes a dim view of regulated supplies for power amplifiers.
(1) The higher the frequency of the switcher (within reason) the easier it is to keep it out of the audio. Remember to make sure that the servo loop of the design is very well damped. You don't want it ringing up if a certain frequency is in the audio material
(2) You may not need the switcher and may be able to do better than the linear.
(2A) Place a large MOSFET between the rectifier and the storage capacitors. Sense the voltage on the storage capacitors and if it is going too high turn off the MOSFET for the next cycle. If you are careful to turn on and off the MOSFET when there won't be any current in it you won't make much high frequency noise.
(2B) Magnetic amplifiers can be kind of cool. A saturable reactor in series with the transformer could knock the voltage down by 25% under the light load case.
It'd certainly be manly to have an amplifier that needs it's own foundation poured...
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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Oh -- and if the switching supply works at 20kHz, yes, it'd be crappy with a hi-fi amp.
But if you can put one together that works at 100kHz, then you don't have to worry too much about switching noise. Transients yes; switching noise only to the extent that it doesn't get into your front end stages and overload things.
--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
I think you are putting the effort in the wrong place. Instead of screwing with regulators, switching supplies and the like, the effort should go into the amplifier(s) and not the power supply.
If it were me, I'd design an amplifier to replace the 3886 using discrete components that had sufficient head room for any voltage I desired, unregulated. Plus and minus 20 volts, plus and minus 30 volts, plus and minus 40 volts, or more, whatever.
Remember a well designed amplifier does not need regulated power, it is in effect a high speed (regulator) on it's own, turning raw DC into controlled (regulated) audio voltage. adding additional regulation is redundant, wasteful and can be noisy. Why do it? Put the effort into the amplifier not the power supply to fix a poor amplifier.
I must acknowledge this is probably the wisest solution to using this chip: Just settle for 40 watts.
This is what the audio engineers at National said. They also said getting 50 watts out of the chip "was difficutl" without regulating the supply, and that goes against all their engineering effort to make the device have as high ripple rejection as it does.
They were aghast when I told them I put 50,000uF on each rail. And you know what...
It actually sounds better without all that reserve capacity
So basically, I guess what I want is simply a more powerful amplifier.
I have been looking at thoses 1kW rack mount PA amplifiers, and wondering how in the world do they get that power in such a small place. I know SMPSs have something to do with it. I have also seen the terms maginetic amplification and digital amplifaction thrown around in advertising for those high powered PA amplifiers made by Crown, Carver etc.
In any case, I've been wanting to get familiar with SMPSs and may be using this amplifier problem as motivation to start at the bottom a new learning curve :)
** In nearly all examples, the switching PSUs used are non-regulated - so the DC output voltage varies with the AC supply voltage and has maybe 10%
100/120 Hz ripple voltage under load.
** No such thing in audio.
You may be thinking of Carver " Magnetic Field " amplifiers, which has got to be the most misleading title ever printed on an audio power amplifier. These amps used conventional, laminated iron transformers - just rather small ones with a TRIAC pre-regulator in the AC supply.
** Another marketing department invented misnomer that refers to PWM - a purely analogue technique.
I used the LM3886 it has better than excellent rejection. It was designed to run off an unregulated supply. the problem came in trying to get 50 watts into 4 ohms. it requires slightly more then +/-24V at
5A. My 24 volt transformers would rise to +/-42 during no-load conditions. That is the absolute maximum limit of the chip. I destroyed two chips, and they have all sorts of built in protections, driving them to full output with those transformers.
By the way, Nation recommended to avoid the use of a choke in the power supply but omitted elaboration about why. Presumably, it has negative affects upon the device.
Transformers I used are old Signal units, probably 15% regulation. So if if got a larger, perhaps toroidal, transformer with 5%-6% regulation, I might be able to slip underneath the voltage limit.
The device you used looks interesting, and you got 75W :)
On a sunny day (Sun, 14 Sep 2008 00:28:37 -0700 (PDT)) it happened Fred wrote in :
would be enough.
Thats transformer is really really bad. I do admit I use a 200W toroid (once I wanted to make a stereo amp), but, for a fraction of what this home build did cost me, I also bought a 2 x 180 W in 4 Ohm
19inch stereo amp ...... You cannot beat German design manufactured by a Chinese company where they work for peanuts, or maybe M&Ms. Lookup PA3000 at conrad.nl...
I work the other way around, If 42V is max, then subtract 10% for mains variation makes 37.8V DC nominal peak. This is 'top of the ripple'.
You drop 1.4V in a bridge, so 39.2V peak out of the transformer.
At 50 W into 8 Ohm you need a voltage swing of P = U^2 / R so U^2 = 50 * 8 = 400, that makes U = 20V eff. The peak would be 20 x sqrt(2) = 28.28 V.
How much does the chip drop of this? I dunno. but say 4V (just for the example). So that makes 28.28 + 4 = 32.28.
We have 37.8 - 32.28 = 5.52V for the ripple. At 50 W the peak current into 8 Ohm is 28.8 / 8 = 2.54 A At 50 Hz mains, and a full bridge you have 10 mS between mains tops, Q = C . I = U . t So 5.52 V at 2.54 A in 10 mS makes C = (5.52 x .01) / 2.54 = .022 F or 22 mF or
22 000 uF for the filter caps.
Not counting any internal resistance of the transformer. I'd say: You cannot even do 50 W into 8 Ohm with a 'perfect' transformer with that chip.
Maybe I messed up the math....
Anyways, in audio the average loading is much less (unless you want to play continuous sine waves), and 30% power rating would be safe perhaps for the transformer for music use. Then you run into the larger internal resistance (thinner wire) of that transformer again... And somebody WILL do sinewaves for hours on end (I know somebody that did, but IIRC they told him to move his workshop after that).
I see my Chinese amp simply uses a _huge_ torroid, 250W? ...... and normal transistors, a temp controlled fan, stereo, all in a nice 19 inch rack, with separate volume controls for left and right, with clipping warning indicator LEDs, with power switch, cinch AND XLR connectors on the back.... And I payed 65 Euro (about 91 dollars) for it..... some years ago, see they are now 87.95 Euro... with more LEDs :-) Lookup 'PA3000' at
Interesting, they omitted any mention of those last two equations in the data sheet. I just looked at it again and apparently I had
17,670uF on each rail rather than what I said before. When I was listening to it loud, I thought I could hear it completely bottom out that reserve in spots.
I was using 4 ohm load.
Maybe I give this one more try with a transformer large enough to supply both stereo channels. That would be about 500VA. A toroidal that size should have a regulation of like 3%.
Also the transformers I used before were rated 115:24 @ 8A. The power here runs about 120-121. So, that was an additional 4%-5% rise.
And yes, I did want this thing to be capable of outputting 50W rms sine wave all day long... :)
37.8VDCpk nominal - Good
39.2Vpk out of the transformer - Good
50W into 4ohms = 20Vpk and 5Apk DropoutV 4 - Good
So, 37.8 - ( 20 + 4 ) = 13.8V ripple
120Hz mains = 8.33ms between peaks.
Accordingly, 13.8V * 0.00833s / 5A = ~23,000uF
National told me that all the reserve I put on the on the rails, beyond the 0.1uf and 470uf bypass caps, was unnecessary. I mentioned that I failed to understand how the chip could handle large transients without it, and they just left it at that.
In any case, I guess I'll buy a 115 to 2 x (50 @ 5A ) 4% transformer and give this another shot.
That should give me 38.4 out of the transformer with a 120 at the mains.
On a sunny day (Sun, 14 Sep 2008 09:30:39 -0700 (PDT)) it happened Fred wrote in :
That makes 27.7 V effective secondairy.
correct.
Yep.
I dunno, these numbers are from the worst case, of course the top, tha tcharges the C, is part of the 8.3 ms, so in reality you neeed a bit les C, also 4 V is really bad for a drop in a good output stage, if it is some MOS then it could be close to zero (with bootstrap C).
470 uF is way to small.
I think you mean 2 x 27 V secondary, and a bridge configuration?
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