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hot resistors on output stage at idle, ok ? mosfet

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wayward91 View Drop Down
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    Posted: 07 April 2017 at 10:59pm
Hello , I have recently been messing around with a (new to me) lds pa500l power amplifier designed as part of a magnetic shaker rig. I have shorted various of its sensing circuits designed to monitor the safe operating conditions for the test rig as I intend on seeing how the thing sounds on a subwoofer. I today got it to the step of playing audio but I noticed that the large ceramic resistors got pretty hot when the thing has just been sat at idle/ very quiet music output for a short period of time. The resistors at the back seem hotter than those at the front. The amp has dc offset protection and that isn't triggered but I was just wondering If the heat is something to be concerned about/ may be a bias issue. The resistors pictured burn your finger within a second or so of touching them. The output stage seems kinda self enclosed so cant quite make out if it has variable bias . Any input would be welcome .
The amp is designed to operate at 500w 20hz to 14khz and the shakers it is supposed to connect to seem to be between 4 and 8 ohms . I shall have to give it more testing tomorrow.
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Cheers All Ed

Edited by wayward91 - 07 April 2017 at 11:44pm
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M4trix View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote M4trix Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2017 at 2:35am
Well, I chuckled a bit when I read that you're using a shaking rig for listening the music. LOL
Jokes aside. I can't find a single document/schematics about LDS equipment. Ouch
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simonp1100 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2017 at 10:06am
The resistors seem a bit high in value (680 ohms), are they the original parts fitted ????, if they are used as emitter / current sensing resistors, i am sure these need to be a lot lower as used in many other amplifier designs (around 0.22, 0.33 ohms etc).

I could be wrong, but i would check if you can, this would explain why they are getting hot.

SmileWink
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BJtheDJ View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BJtheDJ Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2017 at 10:32am
Originally posted by M4trix M4trix wrote:

Well, I chuckled a bit when I read that you're using a shaking rig for listening the music. LOL
<snip>

IIRC from my hazy days back in the 60s, weren't Crown DC300 amps originally designed for shaker table duties - hence their ability to drive right down to DC ??
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madboffin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madboffin Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2017 at 5:27pm
Given that they are mounted where that fan can cool them, and on pairs of ceramic standoffs, I'd say they are expected to run at high temperature. Probably OK when the lid is fitted and there is a good blast of air across them.

--------

And yes the original DC300 was used for shakers, I think there was a link somewhere to select AC or DC input coupling.


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wayward91 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote wayward91 Quote  Post ReplyReply Direct Link To This Post Posted: 08 April 2017 at 7:20pm
Thanks for all the the input . I am just going to see how it gets on. I usually test stuff on a cheap speaker first but got it straight onto my ohm moon sub (after 5 min on a little speaker) and sounds pretty awesome. Not finished putting the case back together yet, the input isn't grounded , I need to figure out a way to get the ground back onto it that is a job for another day. I am surprised at how low the current is compared to the voltage , the front panel has a live read out of either current or voltage, had it up to 20v and the current seems to stay below 2a (8 ohm load) , there was me thinking the 19a trip limit may be an issue lol . The max output voltage is stated at 40V rms for a 6.4 ohm load 18A rms and 54A max random . Not the most practical amps but It was worth the effort , I fancied seeing if it would do it , had no idea if it would even work before I connected it, thought it should but didn't know.


Edited by wayward91 - 08 April 2017 at 7:43pm
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