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Old 28th August 2004, 09:39 AM   #1
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Location: London
Default Help: LCAudio module putting out 42v DC

I have a couple of these LCAudio Zappulse modules. The modules are wired up with the LCaudio power boards

However, with no input signal the output across the speaker connections is 42V!

I have checked all connections and also checked for shorts, but can't see anything obvious. The power supply is a 42Va torroid, which makes about 60v through the power supply module.

Any ideas what I might have done wrong? Have I wrecked something in the output stage somehow?

Thanks for any ideas
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Old 28th August 2004, 10:10 AM   #2
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Aha, hmm. Looking closer at the specs on this amp they suggest not testing without a speaker load. Hmm.

However, I am pretty scared about wiring a speaker up now.... Will have to dig around and look for a low ohm resistor I think
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Old 28th August 2004, 10:11 AM   #3
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Hi Ed

First of all you should check + and - supplies to be 60V like you would expect. If both + and - supplies are right, you can then check to see if there are any difference in the input GND voltage, compared to the power GND (that one on the speaker terninal).

If you find 0 V difference, then your module is wrecked, and you can get a new one on warranty. (From L C Audio).
If there is a voltage difference, something has wrecked the 2.2 Ohms power resistor found near the input terminals of the ZAPpulse module.

If you post your findings, i will try and help you further.

All the best from

Lars

PS you can normally test the amplifier for DC measurements without a speaker load, it only gets problematic if you try to sweep high frequency response and HF coupling without a load. Then it can damage the module. All ZAPpulse modules are factory tested without load on the output, and checked to have DC below 25 mV.
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Old 28th August 2004, 11:49 AM   #4
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Oh darn. Power supply is fine +/- 60V ish. However, there is 0v between speaker output ground and line input ground...

With the speaker connected there is 0v across the speaker, and without a speaker load there is 40v across the speaker terminals.

The line input is unconnected, but has a 2 foot length of cat 5 cable (just something that was to hand) soldered on with an input socket. Everything is checked for shorts.

From what you have said this suggests that the module is blown. And since you have clearly checked that it was fine before leaving the factory, this strongly suggests that it was something that I did which caused this. But before I swap in another module, how do I work out what I did wrong...? I mean these things arent exactly hard to wire up are they (3 wires), and it sounds like the input and outputs can be shorted with no ill effects even if that were something I had done (which I can't see any evidence that I did of course.)

It's extremely kind of you to offer to replace these under warranty, but I'm worried about blowing another one up now! Any thoughts? I can send a picture of my lashup - it's just on a wooden table right now, loose, and I swept everything carefully first to make sure there were no loose wire ends anywhere... Powersupply was actually tested yesterday before wiring up to the module (everything was fine).

I will have a think for an hour before doing anything else and double check everything. If you don't suggest not to, then I will carefully try another module with only the power wired this time, and leave the inputs and outputs floating - from what you described this will be fine and allows me to check everything for DC first?

Thanks Lars - I wasn't expecting a note from you on the weekend! Very impressive!
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Old 28th August 2004, 12:08 PM   #5
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Hi Ed

Yes please send me a photo of your amp, maybe i can see something.

The 2 ft unscreened input wires, don't happen to be snaking around the speaker wires?

All the best

Lars
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Old 28th August 2004, 12:51 PM   #6
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by Lars Clausen
Hi Ed

Yes please send me a photo of your amp, maybe i can see something.

The 2 ft unscreened input wires, don't happen to be snaking around the speaker wires?

All the best

Lars

Hi Ed,

I would be carefull with the 2ft unscreened wires. These wires will probably pick-up some signal and if you have bad luck you may get oscillations etc. I would advise to terminate those wires with an RCA plug that has the input grounded to short, I did that during testing and then everything stays clean when no input signal is connected. I smoked out a resistor of the zobel network (the 2.2Ohm resistor between the two big blackgate caps when I touched the open input with my finger, so before I shorted the input). I guess my body pickup enough signal to make the whole thing oscillate at a very high frequency smoking out that resistor. When the zappulse is connected to a preamp it will also see a very low resistance between inpuit and GND, so shorting GND and input should be OK.

Best regards

Gertjan
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Old 28th August 2004, 01:18 PM   #7
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I have uploaded two pictures (large) here:

http://wildgooses.com/downloads/amp1.jpg
http://wildgooses.com/downloads/amp2.jpg

Depth of field is a little low though.

Nothing obviously smoked at any point. There is no smell of burning anywhere, and nothing obviously blackened. The only suspicious thing is those big square Dale resistors. The one on the +60v side is bulging a little...? (Is that normal?)

The input wires are not deliberately wound around the speaker wires, but yes they are hanging off the table together and so they are in the vacinity of each other.

Gertjan: What you say about these things being so easy to force into oscillation is a bit worrying. The intended application is to take the input from a PC, and of course these things sometimes need to be removed to replace stuff inside - unplugging and replugging the audio is not a totally infrequent event - and since this usually involves a blind stretch behind something, touching the signal wire is a possibility. However, the final design will have small blocking caps on the signal input - perhaps this will be enough to prevent such problems in practice?

I will setup one of my other modules to try. Initially I will leave inputs and outputs floating then and test from there. This should be fine yes?

Thanks for all your help
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Old 28th August 2004, 01:34 PM   #8
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Hi Ed

That's some pretty nice photos (What camera are you using?). I would love to see there the input wires end up, since you have connected both the + and - input to the cable.
I have seen some examples where people have used the - input of the module for GND. That goes wrong!

Apart from that everything looks just fine!

Best regards

Lars
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Old 28th August 2004, 01:40 PM   #9
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by ewildgoose
I have uploaded two pictures (large) here:

http://wildgooses.com/downloads/amp1.jpg
http://wildgooses.com/downloads/amp2.jpg

Depth of field is a little low though.

Nothing obviously smoked at any point. There is no smell of burning anywhere, and nothing obviously blackened. The only suspicious thing is those big square Dale resistors. The one on the +60v side is bulging a little...? (Is that normal?)

The input wires are not deliberately wound around the speaker wires, but yes they are hanging off the table together and so they are in the vacinity of each other.

Gertjan: What you say about these things being so easy to force into oscillation is a bit worrying. The intended application is to take the input from a PC, and of course these things sometimes need to be removed to replace stuff inside - unplugging and replugging the audio is not a totally infrequent event - and since this usually involves a blind stretch behind something, touching the signal wire is a possibility. However, the final design will have small blocking caps on the signal input - perhaps this will be enough to prevent such problems in practice?

I will setup one of my other modules to try. Initially I will leave inputs and outputs floating then and test from there. This should be fine yes?

Thanks for all your help

Hi Ed,

I`m of course not sure that I had oscillations, however, the fact that the zobel resistor smoked and burned out in a few seconds strongly suggests that there was high power output at a very high frequency.

I would be extremely careful with hot-swapping the input of the modules. I would turn of the amp before connecting/disconnecting to anything and definitely a PC. In fact I think you should do that with any amp, not only the ZAPpulse.

When I made a test setup of the ZAPpulse modules and connected them to a pre-amp and then turned them on, everything was fine and everything worked. Just be carefull when nothing is connected to the inputs.

Best regards

Gertjan
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Old 29th August 2004, 12:52 AM   #10
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Hi Lars,

Thanks for the note. The camera is a Canon 10D Digital SLR with L series glass (in this case a 17-40L). That picture was taken on "low" resolution, and although it's quoted as a 6mp camera, the quality is streets ahead of a "consumer" digicam. The difference is quite apparent if you zoom into the individual pixels on the canon where they are still detailed vs zooming in on one of my (many) other digicams where you just get pixel junk. All in all I love it!

Actually I bought it for a trip to Africa when I had a chance to take some weeks off work and go travelling. Pictures here:

http://www.wildgooses.com/gallery/sabbatical


Anyway, back to business: I installed one of my remaining three modules, no inputs or outputs, and bingo - straight away I see a blue LED that I have never seen before! (Actually it looks rather cool, and I remember now that I have seen other people comment on it!). I'm not sure if this gives any clues as to what I have done to my other module mind..?

I then wired up the input to a tiny length of cable with an RCA jack, and used a normal interconnect. Same speaker cable as before. Bingo - sound!

...With just a mono channel, cheap speaker and cheap input source, and poor quality input connects, played for 30 seconds only it would be hard to comment on audio quality....! However, everything sounded pretty promising so far.

I'm going to put the rest of the build on hold until next week when I will hopefully beat up Henrik a bit to send me the proper connectors and some decent cable. I then need to find a reasonable case to put it in. 4 amp channels is going to need a reasonable sized box I think...

Do you have any comments about how important it is to try and keep the amplifier modules away from the AC stage...? I know that sounds like a silly question, but with 4 torroids in the case I don't really have much choice - I'm thinking about putting the torroids at the front of the case, then wedging the power supplies up against them, and the amp then shoehorns at the back of the case next to the in and out connections. The only other possibility would be to try and wedge the torroids on one side of the case, and use the width of the case to try and get some space away from the AC stage....?

The decision point on design is really going to come down to the heat output I think. With four of these amps in a case, is it likely that a standard 19" rack mount unit will have enough cooling if I simply nail all of them to the base of the steel chassis? My initial 1 min test suggests that there is actually a fair bit of heat produced - I don't feel that confident in this solution.

The heat output might therefore dictate either a case with a heatsing worked into the sides, and trying to use the "across the case" layout. Or perhaps just add some heatsinks to the modules and lay then vertically against the heatsink...?

Actually, for a while I had some ideas about building a rather nice wooden case, perhaps with a large aluminium base to act as the heatsink. I'm not sure though in my skills to route the appropriate cooling holes in the top in a way that will look neat though. (Especially if I need to veneer for example). Nice idea though.

If anyone has any thoughts on layout or heatsinking for 4 modules I would be interested to hear. I'm currently thinking about a case from here:
http://www.audiokit.it/ITAENG/Cabine.../HI-FI2000.htm
(Any other good cheap 2U cases out there?)

Oh, nearly forgot, quick question to Lars. With four modules in the case, your suggestion to reduce consumption by using the 15V line from the Predator PS seems sensible? Is the 15v supply the small hole just above each of the -ve supplies? Are there any audio benefits, positive or negative in doing this...? A 5w per module, or 20W altogether reduction in heat output and electricity bill seems like a good saving to me!

Thanks all, things are looking up! Thanks to Lars for his amazing support.

Ed W
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