Burn in of new amps

[Brian Donaldson]

I've read a lot over the year about burning in an amp.(Quiter litereally with Pass products). I just fired up my second over biased Aleph 2. I'm still waiting for my latest Ebay purchase, a sig generator, and the family is already in bed, so I'm using the calibration square wave on my scope for a test tone as I ramped up my latest baby.
When I first turned the variac past 30 volts, the output did its usual up and down, then the square wave appeared. I checked all the critical stats. (you guys all know how nerve racking this can be when all of you love, blood, sweat and tears first begins to pay off as you pray for all of the magic smoke to remain inside the fets) Everything looked good so I put my dvm on a source resistor to watch the bias as I ramped it up to full power.

[Brian Donaldson]

Pardon the extra post, but If I messed something up in the posting, I don't want to loose the whole thing.


I was alsowarching the scope and noticed a little overshoot. on the output. I installed pots for R 19 and R 21 so I could tweak the values before I chose a final one.

I played with the pots and set it for 5A bias and checked a few other things. (ps ripple, offset, etc.) But the overshoot was still there. I installed my temp probe and left it to burn while I cracked open a few beers to quietly celebrate with myself.

I checked back on my darling creation and noticed the overshoot didnt seem as bad. I checked the probes and they were dead on (the trimmer cap), so I had another beer and read some of your posts.

Now that it's been running for about 3 hours, the overshoot is completely gone.

[Brian Donaldson]

To make a long story short, There is definatly something that has to burn in in a new peice of audio gear. I didn't think I heard it, but dismissed it because I had a few beers, I Know I saw it and It dissapeared without me doing anything.

Has anyone else seen this happen ?

[AudioFreak]

Electrolytic capacitors certainly have to burn in (form) before they perform at their optimum. Some other components probably also change a little in the 1st few hours of use.

[chris ma]

Originally posted by AudioFreak
Electrolytic capacitors certainly have to burn in (form) before they perform at their optimum. Some other components probably also change a little in the 1st few hours of use.

When it was new the bias was set and 0V DC offset obtained, after 30 hours or so, I had to reset both of them due to components burn in change I guess, after six months passed I have to reset them once again, but now they are rock steady in almost a year passed.

Regards,
Chris

[Brian Donaldson]

On my first amp, I connected the diff pair with a peice of aluminum L, I still need to make one for this amp. Just to see how hot they were, I touched one and the offset shot from 35 mv to 80 mv. I touched the other one and the offset went from 80 mv to -6 mv. I danced the offset back and forth 3 or 4 times untill the fets were both skin temp. Just goes to show how temp sensitive they really are. I will certainly make that peice of L tomorrow.

[jarek]

Now that it's been running for about 3 hours, the overshoot is completely gone.

It is probably due to change of characteristics of transistors with temperature. Turn the amp off, let him cool and repeat measurements.