I dont have a o-scope or this would quickly diposed of, so here goes: I have noticed recently that my DIY F-5 is running quite a bit warmer than it usually does. Typically, I wonder if it is hot enough, the large M&M sinks just getting good and warm, allowing you to touch as long as one would care to leave them on, but never really hot.
Recently, it has run considerably hotter, still can be touched for over 5 seconds, at around 9 seconds I start to wonder, then I can still leave hand on, albeit beginning to get uncomfortable.
The bias is sitting fine, only a few mV of offset. I know the ambient temp is a little warmer now than it was a month or so ago, but this amp went through last summer not nearly this hot.
I havent had any other indications of instability (the amp that is
), but dont really know what they are, other than overheating and bizarre sounds.
Only the heat issue concerns. Any comments would be welcome, I suppose getting the amp to someone with an o-scope would help.
Thanks in advance for any comment, searching didnt bring up what I was looking for, although I know I have read a few threads about this here.
Russellc
Recently, it has run considerably hotter, still can be touched for over 5 seconds, at around 9 seconds I start to wonder, then I can still leave hand on, albeit beginning to get uncomfortable.
The bias is sitting fine, only a few mV of offset. I know the ambient temp is a little warmer now than it was a month or so ago, but this amp went through last summer not nearly this hot.
I havent had any other indications of instability (the amp that is
), but dont really know what they are, other than overheating and bizarre sounds.Only the heat issue concerns. Any comments would be welcome, I suppose getting the amp to someone with an o-scope would help.
Thanks in advance for any comment, searching didnt bring up what I was looking for, although I know I have read a few threads about this here.
Russellc
If you are looking for oscillation, most DMM's will not read frequencies that
high at all.
Disconnect the speakers. If it stays at the same temp, then it's probably
fine. Remember that line voltage also comes into play with this.
Thank you for your response. Does the amp stabilize when the speaker load is disconnected? How does line voltage affect this? Thanks for the info, I'm trying to understand this. ( I'm sure a scope would help
) If the amp runs considerably cooler with speakers disconnected, what would my next step be?Russellc
If the line voltage increases, so will the supply rail voltages to the amp, more voltage = more watts = more heat
The following test is crude and will not show anything below 1V.
To check if the amp is unstable, power off the amp, disconnect the speaker, connect one end of signal diode (1N4148 etc) to the speaker output, and the other end to a small polyester/polyprop cap, connect a DMM across the cap, power up the amp, connect the free end of the cap to speaker ground, If the DMM reads anything significant, you have instability. If you get a zero reading, try the test again with the diode reversed, then test again with the speaker connected.
The following test is crude and will not show anything below 1V.
To check if the amp is unstable, power off the amp, disconnect the speaker, connect one end of signal diode (1N4148 etc) to the speaker output, and the other end to a small polyester/polyprop cap, connect a DMM across the cap, power up the amp, connect the free end of the cap to speaker ground, If the DMM reads anything significant, you have instability. If you get a zero reading, try the test again with the diode reversed, then test again with the speaker connected.
Thanks for the input guys, I'm certain others have wondered about this as well.
I think my amp is fine, there was an almost 20 degree difference in ambient temp when the amp was getting hotter. Today was a little cooler and normal temps were experienced...this amp went through last summer and I just didnt remember it getting that hot. Just the nature of the beast, maybe I will build one of ZenMod's babysitter devices for this summer.
Russellc
I think my amp is fine, there was an almost 20 degree difference in ambient temp when the amp was getting hotter. Today was a little cooler and normal temps were experienced...this amp went through last summer and I just didnt remember it getting that hot. Just the nature of the beast, maybe I will build one of ZenMod's babysitter devices for this summer.
Russellc
anyway
there is a search I'm doing in last 24 hours , and just now I catch exact phrase - oscillation detector
good trick for greenhorns : http://www.diyaudio.com/forums/solid-state/112453-nap-140-clone-amp-kit-ebay-14.html#post1625589
there is a search I'm doing in last 24 hours , and just now I catch exact phrase - oscillation detector
good trick for greenhorns : http://www.diyaudio.com/forums/solid-state/112453-nap-140-clone-amp-kit-ebay-14.html#post1625589
Russellc,
This is a shot in the dark, but check for discoloration on R11 and 12.
Mine started to get lighter in color and then one went bad.
I pulled it off and the case was brittle and came apart.
Also, when the one power resistor went bad, the channel still worked. One channel was warm as usual, the bad channel wasn't getting warm at all.
What I'm trying to get at it is, maybe the value has changed due to excessive heat, causing your amp to draw more current.
Just my 2 cents. You're in good hands already.
Vince
This is a shot in the dark, but check for discoloration on R11 and 12.
Mine started to get lighter in color and then one went bad.
I pulled it off and the case was brittle and came apart.
Also, when the one power resistor went bad, the channel still worked. One channel was warm as usual, the bad channel wasn't getting warm at all.
What I'm trying to get at it is, maybe the value has changed due to excessive heat, causing your amp to draw more current.
Just my 2 cents. You're in good hands already.
Vince
They were 3w Mills. Now using 1% 3w Vishay/Dale.
I think 3w is cutting it close. They get very hot. (We don't drive mosfet close to their max ratings, why should we drive the power resistors to their limit? Know what I mean?)
I have a set of .47 ohm, TO-220, good up to 30w with heat sink, and will try them next.
A thermometer will tell you
Check wether there is temperature imbalance between the four power devices (make sure you measure at the same point or distance respective to the device)
You should not have more than a handful of degrees in difference
Check wether temperature elevation above ambient is rather constant, or not (allow enough time for the amp's temperature to stabilize)
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