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test amps without speaker load?

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test amps without speaker load?

Postby hooty2 » Mon Oct 28, 2019 12:24 pm

Hi,
this question is in part for me and part for a friend, but mostly for my peace of mind!

Friend: wants to test his older Camco power amps that have not been used for a few years, and he asked if i can make my l-acoustics MTD boxes available to do this. I don't know if he wants to buy speakers & use them or sell them...
But i'm cautious!
Is there a first step or simple way to test that the amps work without attaching speakers?

Me: i contacted Camco and got the tech sheets for the amps.
In everyday terms, when asking about the 'power' of the amps (?), should i use the:
1 kHz Sinus @ 220 V 500 W or the Peak 730 W rating (this is into 8ohms)?

The amps are:
Dl-1000C-P, Dl-1500C-P, Dl-3000P

Your advice would be appreciated.
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Re: test amps without speaker load?

Postby ef37a » Mon Oct 28, 2019 1:01 pm

"Be afraid, be very afraid".

Several catastrophic scenarios are possible. An unstable amp that will fry speakers, inadequate loads and then itself. A serious "offset" fault that will do same (well, the amp's already buggered in that case) Other possibilities come to mind.

On the load side you want something indestructable. You do NOT want NO load. A 1kW fire element or in extremis a kettle, the actual resistance does not matter for the first test.

Across the load you need a DC meter and check the offset is less than +or- 50mV, preff' less. A scope is invaluable to show instability but an old Avo type MC meter will respond up to 30kHz or so (not accurately of course) . The input needs to be shorted for this initial safety test.

Once you are sure the amp is not going to destroy anything you can connect a speaker. I would use a gash guitar 12" or multiples then feed some signal in and check for decent repro.

Dave.
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Re: test amps without speaker load?

Postby hooty2 » Mon Oct 28, 2019 1:10 pm

great stuff...thank you:)
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Re: test amps without speaker load?

Postby James Perrett » Mon Oct 28, 2019 3:03 pm

If they've not been used for a very long time (say 10 years or more) I'd think about using some kind of variable voltage supply (a variac) or some kind of current limiting supply when first switching on. This will hopefully prevent any dried out capacitors from exploding. I'd do this with no load connected as most transistor amps are perfectly happy working with no load connected.

I'd agree with Dave that you shouldn't go connecting any valuable speakers to the amps until you've checked them with a dummy load or some gash speakers.
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Re: test amps without speaker load?

Postby ef37a » Mon Oct 28, 2019 4:00 pm

James Perrett wrote:If they've not been used for a very long time (say 10 years or more) I'd think about using some kind of variable voltage supply (a variac) or some kind of current limiting supply when first switching on. This will hopefully prevent any dried out capacitors from exploding. I'd do this with no load connected as most transistor amps are perfectly happy working with no load connected.

I'd agree with Dave that you shouldn't go connecting any valuable speakers to the amps until you've checked them with a dummy load or some gash speakers.

Sorry James but my experience is different. Running a transistor amp O/C can result in instability which fries the Zobel resistor closely followed by all the transistors!

As for running the amp up on a Variac, usually fine but some designs, mostly MOSFETs, will not start correctly unless the rails come up at the right voltages and this can cause failure.

What you CAN do is bang the mains on for one second then off then check for residual voltages, possibly popped fuses.

But, no matter how careful you are *t'appen!

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Re: test amps without speaker load?

Postby hooty2 » Mon Oct 28, 2019 4:42 pm

hmmm!...all above my pay grade and no locals are replying to my requests to check this out properly.
however, as i need to be in Dusseldorf soon, i'm waiting to hear back from Camco in Wenden, a short hop on the autobahn.
The discussion here is important to hear, so many thanks
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Re: test amps without speaker load?

Postby ef37a » Mon Oct 28, 2019 6:33 pm

hooty2 wrote:hmmm!...all above my pay grade and no locals are replying to my requests to check this out properly.
however, as i need to be in Dusseldorf soon, i'm waiting to hear back from Camco in Wenden, a short hop on the autobahn.
The discussion here is important to hear, so many thanks

Sorry if this is getting complicated for you.
Bottom line: You need a load on the amp IMO that will not burn out if the amp goes ape**** in some fashion. An old stye incandescent 100W lamp would do at a pinch since the cold resistance is much lower than the 230V burning R. The lamp will probably glow if the amp does go instable. (we used a 90V neon on 100V PA amps)
Then you need something to measure the offset voltage, that is the residual volts that in theory should be zero but never quite is.

You don't want to feed in any signals until you are sure the OP stage is behaving itself. If you have a specific point we have raised that boggles, please tell us.

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Re: test amps without speaker load?

Postby hooty2 » Mon Oct 28, 2019 6:51 pm

i appreciate that...thank you.
I'm going to read through with a colleague and i surely will come back to clarify etc.
on and up (but not just yet):)
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Re: test amps without speaker load?

Postby cyrano.mac » Mon Oct 28, 2019 7:22 pm

DIY solution:

Insert a 12V, 5W car lightbulb in series with the speakers. If something is wrong with the amp, it will light up, or even blow. Should protect against most mishaps.

If it doesn't light up, increase volume slowly.
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Re: test amps without speaker load?

Postby ef37a » Mon Oct 28, 2019 10:42 pm

cyrano.mac wrote:DIY solution:

Insert a 12V, 5W car lightbulb in series with the speakers. If something is wrong with the amp, it will light up, or even blow. Should protect against most mishaps.

If it doesn't light up, increase volume slowly.

No, if it blows, no load, instability, transistor death.

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Re: test amps without speaker load?

Postby Wonks » Mon Oct 28, 2019 11:13 pm

A bit OTT, Dave? Most amps will be perfectly fine, especially at a minimal output power. These particular German pro touring power amps, whilst a few years old now, still sell for around £1k, so are hopefully well engineered against open circuit issues as they must be well regarded to hold so much value.

I would say that 5W is rather low value bulb for testing a 1.5kW pc power amp so is very likely to blow just because of normal amp power and you then won't know if it's faulty or fine.
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Re: test amps without speaker load?

Postby James Perrett » Mon Oct 28, 2019 11:44 pm

ef37a wrote:No, if it blows, no load, instability, transistor death.

I'd be interested to know which amps you've experienced this problem with Dave. I can understand certain amps like those made by Naim or those with transformer outputs could have problems but I've never had issues with this personally and some of the amps I have here have speaker switching on the front panel which can easily leave the outputs open circuit.

I also have some B&W speakers with protection circuits that work in a similar way to the lightbulb - they go high resistance if you feed too much power to them.
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Re: test amps without speaker load?

Postby ef37a » Tue Oct 29, 2019 12:33 am

Yes Wonks and James, extreme situations I grant you but I have had an amp burn out on me sans load and when an amp comes crispy into the workshop you generally have no idea what caused the fault.

OP asked for advice re loads and testing. I gave him the benefit(?) of my experience. OTT? Perhaps but my way is not expensive nor difficult. It as easy to be that careful as cavalier.

Mention was made of "dried out electrolytics" Rarely happened in my world ( well yes, in the 60s in tellies but then the caps were crap and the stuff designed by bean counters. I don't think I have had a failed elecro cap in 20 years to date that could not be put down to other causes.) But, rare or not, something to look out for.

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Re: test amps without speaker load?

Postby James Perrett » Tue Oct 29, 2019 1:23 am

ef37a wrote:Mention was made of "dried out electrolytics" Rarely happened in my world ( well yes, in the 60s in tellies but then the caps were crap and the stuff designed by bean counters. I don't think I have had a failed elecro cap in 20 years to date that could not be put down to other causes.) But, rare or not, something to look out for.
.

I guess we all have different experiences. Thinking about it, I did have a Leak Stereo 70 that added a quiet whistle to everything and could possibly have been oscillating at inaudible frequencies - although that one also had speaker switches that could disconnect the load completely so it would have been originally designed to be stable into an open circuit.

I've also recently had a failed Rebis power supply which may be down to dodgy smoothing capacitors although I've not checked it properly yet. I'm also in the process of replacing hundreds of capacitors in a Fostex G24S. I'm not a fan of replacing capacitors willy nilly but so many of these have swollen and started to ooze that I think I need to do something drastic. So far I've replaced around 100 of the worst ones and I've got another 500 capacitors sitting here waiting to be installed.
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Re: test amps without speaker load?

Postby Folderol » Tue Oct 29, 2019 9:56 am

My 2d
I agree with Dave, in that an amp in unknown condition could well have stability issues. However, I'm not keen on using light bulbs as loads - their dramatic change in resistance with applied voltage can cause problems in itself. The simple and more accessible alternative is to use a domestic electric iron. depending on its power rating it will have a pretty stable resistance in the 25 -50 ohm range, so an effective load, but not low enough to be a problem, and you'll really have to be slamming it to get it to warm up enough for the thermostat to cut in :lol:

Also, these days moderately accurate digital meters are cheap as chips, so there really is no excuse for not having one. You will be amazed at how many uses you find once you've got it. In this situation, as Dave mentioned you need to check for any DC offset voltage, as well as some apparent signal when the input is shorted (could be hum from drying caps, or HF instability).

Incidentally, I've never had dried out caps explode. It's the other ones, the ones that have gone leaky - and might not have any visible indication at all of a problem.
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Re: test amps without speaker load?

Postby hooty2 » Fri Nov 15, 2019 12:39 pm

Just to complete...

the in-house engineer at dBaudiotechnik near me in cotswolds is having the amps into the db workshop.
this is not the first time db have offered valuable help, a wonderful thing that does involve some mild jesting at the 'walk in' with l-acoustics on his mind!
so a shout out to them:)

Also, the Camco office in Germany have been extra helpful with sending schematics and offers to do the work (i had a reason to be near them soon) and just being professional with a smile and helping hand.

nb: in relation to another post : active monitor giving shocks when disconnected
another db engineer helps out at a local repair cafe. when the electronics slot comes up, armed with schematics from Alesis, he will delve into that issue.

Basic reason for posting this: a big thanks to large players helping out small players (very small in my case)
&
a shout out to all the replies to my OP from the forum
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Re: test amps without speaker load?

Postby Hugh Robjohns » Fri Nov 15, 2019 1:06 pm

:clap: :thumbup:
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