[fishmalt]

I have been listening to my amps periodically over the last couple of months since completing them and everything has been fine.  However, while taking measurements for some acoustical treatments I plan on implementing in my room, my left side pcb F1, F2 7A5 fuses blew and R26 is clearly fried.  I was only listening to the left side (as the balance of my CAT SL-1 Ultimate pre-amp was to the left) so I could take measurements for that speaker only.  Litening to a test tone CD at about 97db at 8khz (with ear plugs in) when the 7A5 fuses blew.  The IEC fuse appears unaffected. . .

Where is the best place to start the troubleshooting proces without potentially doing more damage? 

TIA,

Mike

[AKSA]

Hi Mike,

You've just discovered that testing an amp on continuous tones really stress the rail fuses!!  They get damn hot, but in music, which is not continuous, the duty is much less and they don't overheat.


There's a very good chance all is well.  Simply replace the fuse and R26, take a deep breath and switch on.  If you want to test again, replace the fuses temporarily with 12A rating to handle the higher heat!

Cheers,

Hugh

[DSK]

Hi Mike,

Why 97db, that is very high, especially for tones ... you don't typically listen to your music at a steady 97db do you?     (I'm assuming your room is not enormous).

I'm guessing you were trying to maximise the S/N ratio for accuracy of measurements, but the accuracy of measurements performed at 80-85db should be ample for these purposes.

Cheers,
Darren.

[Tinker]

Why 97db, that is very high, especially for tones ... you don't typically listen to your music at a steady 97db do you?     (I'm assuming your room is not enormous).

I'm guessing you were trying to maximise the S/N ratio for accuracy of measurements, but the accuracy of measurements performed at 80-85db should be ample for these purposes.

Darren is right, 97dB is pretty high unless you are trying to measure an RT60 accurately, in which case 97dB is barely adequate!

I'll shut up now.

At the danger of starting a really long thread, what sort of treatments are you planning?

T.

[fishmalt]

Thanks for the quick response.  I will need to track down an R26 replacement and since tomorrow is a holiday in the US, it may not be until mid-week. 

On the topic of the high SPL, I'm using the "Rives Audio Test CD" which includes tracks that compensate for the lack of linearity in the Radio Shack SPL meter which is what I was using.  They recommend working in the 80-90db range to increase accuracy.  My reference was 1kHz at 92 db and it hit 97db at 8kHz when my fuses blew.  My room is 19' x 14' with cathedral ceilings but also opens into the rest of the house so it is a relatively large space but I certainly don't listen at those levels (very often or sober). I certainly don't claim in any stretch to be particularly knowledgeable in this area but I do know I won't be using such high SPL levels in the future. . .

I'm looking at GIK Acoustics (gikacoustics.com) for their decent looking, well tested, and affordable products.  While I'm sure my wife (who agreed to the acoustics treatments after years of resistance or should I say impedance - it came in many forms) isn't clear on how much acoustics treatment I plan to do in our living room/audio room soon to be audio room/living room, my listening experience should significantly improve. . .

Mike

[G Georgopoulos]

it is clear the amp cant handle sinewaves
i suggest you dont repeat the test
possible damage must have resulted (resistor fried)
Hugh has suggested you repeat the test using larger fuse
i dont know how is that going to help with one resistor down already
even if the o/p can take the heat (depending on time) the o/p devices
can fail!

regards

[DSK]

I thought it must have been something like that. I'd recommend setting your ref at 85dB@1khz and pausing between each tone for the same length of time that you play each tone (eg. play the tone for 15 seconds, pause the CD for 15 seconds while you write down the measurement, then play the next tone).

If you are interested in taking measurements occasionally, I'd recommend buying Acoustisoft ETF5. It is reasonably inexpensive, can use the RS SPL meter that you are using now, and saves your system from the torment of multiple tones. It sends a full range or low frequency sweep to your speakers for up to 3 seconds then instantly shows you the FR chart, low freq response, RT60, impulse response (great for identifying reflections and determining what freq ranges each reflection occurs at), waterfall plots etc etc etc. See www.acoustisoft.com or www.etfacoustic.com

[fishmalt]

I found a replacement for my "fried" R26 and put in new 7A5 fuses and all is well.  Another lesson learned the hard way. . .

Thanks again for the assistance,

Mike
Anchorage, Alaska

[AKSA]

Mike,

R26 is merely a fuse resistor, used to set up initial bias and not actually in circuit when the fuse is good.

With one rail fuse blown, the amp will still hold offset within +/-1V, which will not damage the voice coil.

R26 is a useful, olefactory indicator of circuit health, as you found!

Cheers,

Hugh

[rabbitz]

I'll second that.... that resistor has saved my butt.