Signs your Firstwatt DIY is unstable?

[Russellc]

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

[Bill Fuss]

Hi Russel,

I remember reading about using a small cap on the inputs to kill any RF signals from getting into the amp. The F5 has an outragously wide bandwidth and will happily amplify RF, causing overheating.

Best, Bill

[flg]

Do You have a DMM? Put it on AC and look for a signal level at the output FETs Gate or Drain... If it's only DC your good the reading will be 0...

[Russellc]

Originally Posted by flg

Do You have a DMM? Put it on AC and look for a signal level at the output FETs Gate or Drain... If it's only DC your good the reading will be 0...

I do have a DMM. will try that tonight.

Russellc

[Nelson Pass]

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.

[Russellc]

Originally Posted by Nelson Pass

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

[Itsmee]

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.

[Russellc]

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

[Zen Mod]

in that case - we must discuss royalty issue ;

waste one beer for me , and we are even

[Zen Mod]

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 : NAP-140 Clone Amp Kit on Ebay