what amp kit/circuit for a 28.7v-0-28.7 2A transformer? - diyAudio
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Old 14th March 2010, 06:25 PM   #1
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Default what amp kit/circuit for a 28.7v-0-28.7 2A transformer?

Hello, I have a 28.7v-0-28.7 2A toroidal transformer out of an Accurian amp, I am wondering what amp design would be best suited for this transformer, I notice most of the kit amps I see in that voltage range recommend a 3A or greater transformer.
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Old 16th March 2010, 02:33 AM   #2
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even a family of chips to look into?
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Old 16th March 2010, 02:55 AM   #3
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2 amps is not enough at that voltage range to feed a proper loudspeaker amplifier properly without serious voltage drop. Maybe ONE speaker if you are lucky.

A -+ 28.7 rail regulates to around -+ 40 volts Peak with NO load, it will sag, but using worst case here. 80 volts RMS at a sine wave is around 56 volts.
using an 8 ohm speaker indicates a peak current of 7 amps. No way that transformer is capable of that.

You could however run the windings in parallel and get 28.7 volts at 4 amps, and you could use that to drive some LM1875 chips in single supply mode. Even then, 4 A would only just barely keep 2 LM1875 chips happy driving 8 ohm loads.

Last edited by Rainwulf; 16th March 2010 at 03:00 AM.
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Old 16th March 2010, 03:25 AM   #4
sregor is offline sregor  United States
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Class D amps use less power for given output - especially at moderate levels. It's a stretch, but is as good as you can do. Also, higher impedance speakers cuts down on supply requirements, as well as high efficiency. Or use for a killer headphone amp.
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Old 16th March 2010, 09:37 PM   #5
dukep is offline dukep  United States
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Default lm3886

Now that is 28v AC right???
if so...

That would be a fine transformer for 2 each lm3886 amps. Keep to 8 ohm loads. Use a sort of separate power supplies (2 bridge rectifiers, then separate caps, etc.) That will provide separate current storage for the chips.

True, there would be a voltage drop at 4ohm max wattage loads. Usual performance of the toroid is quite good to overload currents. It would behave like a soft clipping/ current limiting.


I've actually ran that type of transformer on a lm4766 in my kit. I tried to kill it, but...It worked well with a big heatsink.
NOTE: It exceeds the voltage spec for the chip, but it sounds great when you keep it cool into 8 ohms.

recycle!
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Last edited by dukep; 16th March 2010 at 09:41 PM.
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Old 17th March 2010, 02:55 AM   #6
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dukep, you are forgetting the fact that this thing can only provide 2A. 2A is not enough for even one 3886. Maybe at low volume levels sure, but you would need phenomenally huge filter caps to maintain voltage at any decent current consumption.

Using simple DC laws, an 8 ohm load will take 1 amp at 8 volts, 2 at 16 volts. And at 28 volts, way too much.

Put it this way.
a 28 volt transformer at 2 amps is a whole 50VA.
50VA!!!

I used 1 300VA per two LM3876 chips. Ok thats into 4 ohms but still.
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Old 17th March 2010, 01:22 PM   #7
dukep is offline dukep  United States
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Default less than ideal but doable

it is a compromise... certainly the best is dual transformers, so channels can perform optimum, Altec>> Ya got 2??

1st, it's 28.7v x2 taps x2 amps x 1.2 (20% toroid) =115 to 138 VA

2nd voltage drops gradually, not like a switching supply. This provide even more VA, at the sacrifice of heat in the transformer (as I have done hammering an amp into 2ohm loads).

3rd audio is dynamic, and usually does not keep to high wattage levels for lengthy times,
>>unless you really jam>> then use and Aussie amp for 300 watt/ch! (one day I will have one, got the shell of an SAE 2500, waiting)

For the worst case currents, simple ohms law is applicable for clipping the amp, with no capacitance. Inrush current from a good cap 12kuf per rail far exceeds 2amps (--arc welder), and should level off the power consumption for audio dynamics.

Altec, What's your application goals? What speakers would you like to drive? to what levels?
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Last edited by dukep; 17th March 2010 at 01:26 PM.
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Old 17th March 2010, 01:33 PM   #8
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Rainwulf View Post
dukep, you are forgetting the fact that this thing can only provide 2A. Using simple DC laws, an 8 ohm load will take 1 amp at 8 volts, 2 at 16 volts. And at 28 volts, way too much.

Put it this way.
a 28 volt transformer at 2 amps is a whole 50VA.
50VA!!!

.
Rainwulf, it s about 113VA...(28.7 X 2 X 2A)
Also, the current will be drained alternatively
on each side, so the mean current will be half of the value
you quote for each rail..
That said, i agree that this is a strict minimum and that
200VA at least would be more adequate if the amp is to be used
at high power listening levels for prolonged periods of time.
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Old 17th March 2010, 02:10 PM   #9
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Hey, thanks for all the input guys, I only have 1 of these trafos,

I would be using it with speakers in the range of 91-93db efficiency, I'm only after a few watts, 5-10 per channel would do me fine as a maximum.

would it be ok to run a pair of lm3886 into 8ohm speakers with 15-20kuf per channel?
I would rather have quality over quantity, and by no means am I expecting to shake the house with this.
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Old 17th March 2010, 09:20 PM   #10
tomchr is offline tomchr  United States
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28.7 VAC should give something like 37 V when rectified. Usually you end up with 1.3*Vrms rather than the theoretical sqrt(2)*Vrms due to a non-zero conduction angle of the rectifiers and such. Anyway...

+/-37 V is perfect for an LM3886. At this voltage, it will deliver upwards of 90 W into 8 ohm, which is approaching the upper limit of what the transformer can deliver. As others have pointed out, your transformer is only capable of a bit over 100 VA or 100 W assuming a resistive load. That doesn't mean it'll blow up if you try to draw more power from it, just that it will get too hot if you force it to deliver more than 100 W for hours on end.

So in my opinion, get yourself a 35 A diode bridge like the KBPC35 and a couple of 10000-22000 uF caps and you're in business. Why a 35 A bridge, you ask... I use the big bridge rectifiers as they tend to survive the in-rush current better than the smaller rectifiers. I would add a bleeder (resistor) across the caps - use a 5.6 kOhm, 5 W resistor for each cap. This insures that the supply discharges to a safe level within a minute or so of turning off the amp. Remember to include a fuse in series with the primary of the transformer. You'll probably need a heftier one for this as it will see some in-rush current when you turn on the amp -- unless you limit the in-rush, of course. A 6.3 A, slow-blow is probably sufficient. Note, the fuse goes on the primary along with the on/off switch.

Follow the typical application schematic in the LM3886 data sheet to the letter and you will succeed.

Good luck!

~Tom
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