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Old 5th June 2011, 07:45 PM   #1
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Default Regulated supply for power amp

I'm building a power supply for a power amplifier, basically the toroidal transformer feeds a bridge, then on to a 0R7 power resistor and then to a 100,000uf reservoir capacitor (i.e. RC circuit) then on to an LT1083 regulator dropping the output down to 15volts followed by another 100,000uf feeding a load that pulls 2.5 amps.

My reasoning for this approach is that I have a few excess volts to burn off from the transformer, so having an RC filter will drop some volts and help the bridge rectifier by not allowing such a huge surge current at switch on. The regulator can then take a few volts off (and kill any remaining ripple that might be present) and the last capacitor will "slug" the output of the regulator to make it is as inaudible as possible. Component cost is not a consideration as all the parts needed are already in the parts bin waiting to be used, but is there any good reason not to go this route?

The LT1083 is an LDO regulator - but how many volts should be across it for best performance?

I was going to have fuses on the transformer secondary windings - the fuse boxes for blade fuses used in cars seem to be a neat solution, but i don't know what characteristics automotive blade fuses have - can anybody advise?
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Old 5th June 2011, 07:57 PM   #2
tomchr is offline tomchr  United States
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My suggestion: Let the regulator do its job. I.e. remove the cap on the output - or better yet, move it to be in parallel with the reservoir cap. Implement a soft start circuit so you con't fry the diode bridge, caps or - over time - the transformer.

If the regulator is separate from the amplifier portion of the circuit, I would suggest having a small-ish cap at the power entry to the amp module. Small-ish being 10~100 uF in parallel with a good X7R dielectric 100 nF ~ 1 uF ceramic cap.

About fuses. I don't see any reason for fuses on the secondary side. Just put one on the primary rated at about 1.3*([Trafo VA rating]/[Mains Voltage]). So for a 500 VA trafo on 230 V, roughly 1.3*500/230 = 2.8 A. A 3.15 A would probably be fine. Slow blow.

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Old 6th June 2011, 08:33 AM   #3
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I agree, you should not need fuses on the transformer secondary since you should certainly have one on the AC line right after it comes into the chassis.

Because you are regulating this, there is no need for 100K uF, it will just cause more turn on surge current issues without any real benefit. How about 10,000uF before the regulator and 10,000uF after it? However, you should be able to use much less than 10,000uF after it, even 1,000uF is theoretically plenty unless your transformer isn't capable of 2.5A current at the target input voltage before the regulator.

I agree with tomchr that if your regulator board is separate from the amp board you should have more capacitance on the amp board for power rails, but I would assume you already do as that is typical for the (modular) amp board designs almost everyone is dealing with.

Why cut it so close with an LT1083 rated for 3A? They aren't expensive, I'd at least go with an LT1084 or 1085 instead. To cut thermal losses I'd want as little voltage drop across it as possible, the performance should not change much if you stay above the spec'd nominal 1.0V forward drop mentioned in the datasheet (though it is guaranteed at a higher 1.5V drop at max current so I use that figure below), and you don't want to go much higher than that if you wish to avoid using a more expensive and larger heatsink since even 3V drop across it, * 2.5A output to load, already puts you at 7.5W which starts to take up a fair amount of real estate for the 'sink.

If your design allows for it, and you aren't going to be going above the max voltage the amp can tolerate, what I would do is tweak the voltage to keep the drop across the regulator as low as possible. In other words, with amp operating at full output powering a power resistor as a load instead of speakers, adjust the LM108(x) regulator so the output voltage is not much more than 1.5V less than the input voltage to the regulator.

The characteristics of the fuse you pick don't seem very important, automotive type would work although I would think them harder to neatly use compared to an inline fuse holder with a pigtail if it is on the AC mains wire coming into the amp, or one soldered to the PCB that uses barrel fuses, though I imaging somebody makes blade fuse holders that solder to a PCB too, otherwise the automakers would have a hard time using them too.

I suspect that if you try to use the configuration you suggest with only 0R7 resistor, your turn on surge with 10,000uF capacitance after the regulator is too great, that you will blow the regulator eventually if not immediately. I suggest putting a NTC thermistor in series to limit that turn on surge current more, or using a much smaller value capacitor after the regulator IC, OR using the regulator IC to drive a pass transistor rated for higher current. I'd still use an NTC thermistor though, since your amp board is going to have more power rail capacitance too I assume.
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Old 6th June 2011, 11:19 AM   #4
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Default Regulated power amp supply

Hi Guys,

Thanks for your response.

The transformer does indeed have a fused primary. Fusing the secondaries was a belt and braces precaution because of the high currents the transformer is capable of supplying (it is 1kva). The bridge rectifier is 40amp item mounted to the chassis, so should handle a bit of surging. The automotive fuses would be on the secondary side of the transformer only (an integrated IEC socket/switch/fuseholder will take care of the primary side) - the fuseholder units I have in mind would hold multiple fuses and protect each individual secondary winding rather than an inline fuseholder which would be impossible to mount to the chassis without fiddly clips, etc).

The LT1083 is rated to 7.5 amps according to linear's data sheet (you're thinking of the LT1085 rated at 3amps) - It will be mounted on a 1 degree/Watt heatsink, so pulling only 2.5A through it will be fine (i'm estimating the heatsink will be at no more than 20 degrees above room temperature with two devices mounted to it, probably a lot less).

I already have the 100,000uf capacitors and they are very good quality caps - using anything else will necessitate buying more stuff (and leaving the big caps in the parts bin waiting for some other project that i'll struggle to get around to doing). There are smaller caps sprinkled around the power amp PCB but not much. The purpose of the big cap on the output was to swamp any audible effects of the regulator more than anything else. I expect the regulator to current limit output current to about 10 amps for a few seconds when charging the output caps.

The NTC seems like a good idea though....

Last edited by Jason Hubbard; 6th June 2011 at 11:36 AM.
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Old 6th June 2011, 07:50 PM   #5
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Haha, indeed I got the numbers on the LT regs backwards thinking as number increases so does current capability. Though it does seem the LT1083 will current limit, I'm still feeling that it's a bit of a mismatch for a 1KVA transformer and 200,000uF total rail capacitance... it may work, but kind of like putting a tiny engine in a dump truck and having it work because you picked the right transmission.
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Old 6th June 2011, 09:03 PM   #6
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i em not all that sure it would worth the efforth and trouble at all.
but yet again, i could be so terribly wrong.
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Old 6th June 2011, 09:10 PM   #7
agdr is offline agdr  United States
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Suggestion: use the two big caps up front in a RCRC filter, then use something around the 150uF minimum the datasheet shows for loop stability on the output. You can probably afford additional voltage drop in another series resistor since you are using an LDO. The datasheet shows just 1.2V minimum dropout at 2.5A. A second RC section will give you more smoothing going into the regulator, which may help more than a load of capacitance after it. After all, the regulator regulates!

If you can't tolerate the additional (0.7R)(2.5A)= 1.75V drop, then I would second what tomchr said, put those big caps in parallel on the input RC.
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Last edited by agdr; 6th June 2011 at 09:27 PM.
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Old 6th June 2011, 09:46 PM   #8
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I would use 2 off 10,000uf with fused secondary.
No soft start, I have lost count of the threads on DIYAUDIO where soft starts have gone faulty. Less means less to go wrong.
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Old 7th June 2011, 09:29 PM   #9
hesener is offline hesener  Germany
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Best not to rely on the current limit of an integrated regulator, by putting large caps after it, that will be a big thermal excursion at every turn-on, potentially reducing the lifetime of that regulator.

my recommendation would be to put the big caps before the regulator and a small (foil) cap at the output.

just my two cents....
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Old 7th June 2011, 09:41 PM   #10
CBS240 is offline CBS240  United States
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Those regulator ICs can be fragile when it comes to big caps on thier outputs without current limitation. If the transformer is a toroid, could you not fairly easily re-wind the secondary(s) to the proper voltage? Seems a waste to just waste heat un-necessarily.
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