[raduray]

I recently picked up a second hand LaRocco PPA in a Hammond case powered by a 24V Elpac Walwart. There's no power switch and I'd like to add one to the back, next to the power connector. I assume I connect it in-line with the power from the power connector to the board? Any recommendations for the switch?
Radu

 

[NeilR]

You can just put the switch between the DC jack and the board, as you are thinking. If you can mount the switch next to the jack, it is a simple mod and keeps the wiring neat.

The right switch probably depends on the layout (plus your preference in style). Plus your casing skills. A toggle or round rocker is easiest to install (just one round hole with a standard drill). A square rocker would need a sqaure hole, which is much tougher to do right.

I think some people have had trouble with the PPA and M3, etc., where a huge amount of bank capacitance is used. You might want to check on the total capacitance of the row at the edge of the board, plus the 6 caps spotted around on the other side. If there is too much capacitance, it can stress the power supply when you turn the amp on. When the PSU and amp turn on at the same time (as it is now configured), the turn on inrush is more gentle. Some people have blown out power supplies because of this. Just something to consider; I'm not familiar with the Elpac. Maybe someone else can comment on that.

 

[tangent]

Quote:

Originally Posted by NeilR I think some people have had trouble with the PPA and M3, etc., where a huge amount of bank capacitance is used.

That shouldn't be an issue if the power switch is at the PPA's power inlet. The problem comes when you build your own power supply and put the power switch on the power supply, so that both pieces go down together. In that situation, the rail cap bank discharges into the power supply's input. If there's a power switch between them, the amp circuit discharges the rail cap bank.

 

[NeilR]

Tangent,

I was thinking about situations posted here or in headwize where people have blown regulators when they fire up an amp after firing up the power supply. I have never had the problem myself; I was just thinking about what I have seen posted when I researched my M3. My recollection is that it is the current inrush into the cap banks, and it was in situations where huge amounts of capacitance was installed- I think 5000uF or more. I believe this was documented in the M3 build discussion thread, and it was a turn-on problem, not a turn-off problem. AMB recommended NOT putting a switch on the amp to help resolve that. I assume a PPA could do the same since it has room for lots of fairly big caps. The turn-off problem is something else that I was not addressing.

 

[MisterX]

I kinda like this ugly pushbutton switch:



Installed example---->

Quote:

I assume I connect it in-line with the power from the power connector to the board?

or you can wire the switch to the solder pads on the board.

 

[tangent]

Quote:

Originally Posted by NeilR I believe this was documented in the M3 build discussion thread, and it was a turn-on problem, not a turn-off problem. AMB recommended NOT putting a switch on the amp to help resolve that.

Sorry, but I don't buy it.

The LM317 has current limiting, so there should be no way you can kill it by having too high a load. The worst I've seen in that sort of situation is that the regulator just sort of "sulks", refusing to regulate with its output only a few volts. Reduce the load, or let the regulator come up first, and the regulator comes up nicely. (This tends to happen with LDOs.)

Contrariwise, the need to protect the output from the load capacitance on shutdown is documented in the datasheet. It's the reason for those protection diodes in the STEPS and TREAD. But if the load is so high that it kills the diodes, it will kill the regulator the next time you power cycle the system.

 

[NeilR]

I guess you can argue that out with AMB. I'm just a messenger passing on info. I'm not arguing any particular point of view on my own behalf.

http://headwize.com/ubb/showpost.php...=44978&fpage=4

 

[amb]

Quote:

Originally Posted by tangent Sorry, but I don't buy it. ...

Well tangent, I didn't make this up. This came to light when an M³ builder kept blowing his LM317 on his STEPS at power-up (and his M³ was populated with the usual ~3000µF total of capacitance). His power switch was between the STEPS output and the M³ pcb. So, the LM317 was already powered up at full voltage when the switch was closed.

I asked him to do an experiment, with the power switch kept in the "on" position, unplug the AC cord and let the amp turn off completely. Then plug the cord back in. This is equivalent to having the power switch inline with the AC mains. The result of this experiment was that that the LM317 was fine and everything powered up ok.

The only explanation I could offer is that the fully-discharged capacitor bank on the M³ looks like a dead short for a small moment after the switch is turned on. Thus the transient charge current is very high. Since the LM317 is already at full voltage, despite the current limiting the it still croaked.

Contrast that to the experimental scenario. In this case the LM317 is powered up after the switch is closed, and comes up more slowly due to the transformer impedance and charge-up time of the rectifier caps. Also, the "ripple reduction" cap from the LM317's adj pin to ground also provides a soft-start function. The result is that the regulator's output voltage rises more gradually, and the transient charge current is dramatically reduced.

Since the builder still preferred to have a switch between the STEPS and the amp, I advised him to use a LM338T regulator which are rated at 5A instead of 1.5A, and that resolved the problem completely.

The National LM138/338 datasheet has an interesting paragraph that's relevant:

"A unique feature of the LM138 family is time-dependent current limiting. The current limit circuitry allows peak currents of up to 12A to be drawn from the regulator for short periods of time. This allows the LM138 to be used with heavy transient loads and speeds start-up under full-load conditions. Under sustained loading conditions, the current limit decreases to a safe value protecting the regulator. Also included on the chip are thermal overload protection and safe area protection for the power transistor."

The National LM117/317 datasheet mentions current limit protection, thermal overload protection and SOA protection, but has no similar verbage as the above. I don't think it is quite as advanced as the one implemented in the LM338. I doubt that the current-limiting is capable of protecting the regulator if the output is suddently subjected to a short circuit.

 

[ppl]

Note that LaRocco PPA had an onboard regulator on the negative supply rail the PCB positions that are intended to connect the power switch and battery board are the same holes used so put your switch inline between the PCB and power jack see attached

 

[tangent]

Quote:

Originally Posted by amb Well tangent, I didn't make this up.

Not sayin' you did. I just didn't see how it can happen when I wrote that.

Quote:

The only explanation I could offer is that the fully-discharged capacitor bank on the M³ looks like a dead short for a small moment after the switch is turned on.

Yes, but a dead short alone isn't sufficient to explain it. The current and overtemp limiting should save you if you simply short the regulator's output. What it may be is some weakness in the protection scheme during power-up.

Do you agree that the STEPS could also be killed in this configuration, as I described above? In several of these instances where STEPSen have died, we've found dead protection diodes. I guess the moral of the story is that large rail cap banks are just bad news all around, because you can kill the STEPS no matter where the power switch is.

For the future...I dunno. Larger protection diodes? Soft start features in the unregulated stage of the STEPS?

Quote:

I advised him to use a LM338T regulator which are rated at 5A instead of 1.5A, and that resolved the problem completely.

That's reasonable. Now, if you change to a high current regulator and put the switch on the PSU's input, then I think you'd have a bigger problem than before. High current regulators have lower output impedance, so once the protection diodes are killed, the regulator's even more likely to die the next time you power cycle the system.

Quote:

The National LM117/317 datasheet mentions current limit protection, thermal overload protection and SOA protection, but has no similar verbage as the above. I don't think it is quite as advanced as the one implemented in the LM338.

Thanks for the info.

 

[amb]

Quote:

Originally Posted by tangent Yes, but a dead short alone isn't sufficient to explain it. The current and overtemp limiting should save you if you simply short the regulator's output. What it may be is some weakness in the protection scheme during power-up.

I think the LM317's current limit function isn't good enough for a dead short, which is what caused it to blow. The scenario where the regulator was blowing, it was already up at full voltage, so it isn't only a weakness in the regulator's "power-up" phase.

Quote:

In several of these instances where STEPSen have died, we've found dead protection diodes. I guess the moral of the story is that large rail cap banks are just bad news all around, because you can kill the STEPS no matter where the power switch is.

Agree that too much capacitance is bad news. This is why I recommend no more than ~3000µF on the M³.

In the specific example I cited above, though, the protection diodes were all fine.

Quote:

Now, if you change to a high current regulator and put the switch on the PSU's input, then I think you'd have a bigger problem than before. High current regulators have lower output impedance, so once the protection diodes are killed, the regulator's even more likely to die the next time you power cycle the system.

I built two STEPSen for my M³ and Millett, both of these have Amveco 70062 transformers and are fairly "stock" except for the LM338 regulators, and the power switch is at the AC mains side. I've encountered no problems at all, and the 1N4001 protection diodes are all fine too. These two STEPSen have probably gone through hundreds of power up/down cycles and they have never given even a hint of trouble.

My M³ has 3000µF of capacitance across its rails and my Millett has much less. The unregulated side of the STEPS has 4000µF. Given that the M³ draws close to 300mA and the Millett around 200mA, the post-regulator capacitor(s) should drain down fairly quickly after power-off, and there shouldn't be any possibility that the protection diode would actually become forward-biased (which is what would have to happen in order to kill the diode in this application).

 

[tangent]

Quote:

Originally Posted by amb Given that the M³ draws close to 300mA and the Millett around 200mA, the post-regulator capacitor(s) should drain down fairly quickly

That could well be significant. I'm talking mostly about PPA configurations, which generally draw less than 100mA.

 

[NeilR]

Quote:

Originally Posted by tangent That shouldn't be an issue if the power switch is at the PPA's power inlet. The problem comes when you build your own power supply and put the power switch on the power supply, so that both pieces go down together. In that situation, the rail cap bank discharges into the power supply's input. If there's a power switch between them, the amp circuit discharges the rail cap bank.

Tangent,

Doesn't the PPA's D1 protect the PSU if the amp is not switched off first?

I'm curious here because I am in the middle of stuffing my PPA board and I have a little more capacitance in C4+/- than I probably should (330uF because of an ordering snafu) and I'm trying to decide if this is worth placing a special order for something smaller. My C1's will be 7 or 8x 330uF. I'll be using a battery board and using your standard wiring setup for that.

Regards,
Neil

 

[tangent]

Quote:

Originally Posted by NeilR Doesn't the PPA's D1 protect the PSU if the amp is not switched off first?

Not if it dies in the performance of its duty.

A 1N4002 can soak up 1 A continuously, or up to 30 A for a very short amount of time. But ya wanna take a guess at how much current can flow from a big rail cap bank, especially in one of these heaphone amps, where we've taken some pains to ensure that the copper connecting to the rail caps is thick and wide?

 

[NeilR]

Quote:

Originally Posted by tangent Not if it dies in the performance of its duty. A 1N4002 can soak up 1 A continuously, or up to 30 A for a very short amount of time. But ya wanna take a guess at how much current can flow from a big rail cap bank, especially in one of these heaphone amps, where we've taken some pains to ensure that the copper connecting to the rail caps is thick and wide?

I thought the current rating ability was relevent only for startup- forward current. When the PSU is powered down, there is no current flow as long as the reverse voltage (100V here) is not exceeded. Where am I confused?

Have you ever actually had a PPA's D1 fail from too much bank capacitance?