What is normal DC Ofset on nakamichi PA-1

gto127

Member
2007-05-09 2:51 am
I have Nakamichi PA1 and think DC offset may be high. I'm getting 90mv after being warmed up on all 3 front channels & 70mv on the rears. I'm guessing this may be why I blow a tweeter about once a year. Anyone have one to take measurements from or familiar with these amps?
 
Power is what blows tweeters together with overestimated crossovers Offset of the amplifier has not much to do with it ( especially at this level ) after all in tweeter crossover circuits there is always a capacitor in between so if any DC will not pass from there ...

When DC appears at any level ( obviously far above your numbers ) the woofers will be the first to have problems since in their crossover circuit there cannot be any capcitor as a rule of thump... ( one exception i have in mind there could be more )

The ideal offset for any amplifier will be a total , nice , big and round zero at all conditions

Kind regards
Sakis
 

gto127

Member
2007-05-09 2:51 am
Thanks for replys. I just replaced the tweeter. B&W had been selling me just the diaphragm. The tech guy said It may have dried ferrofluid in old tweeter assembly which could have been causing this. I won't know if it works for about a year which is how long they have been lasting. I was able to slightly lower the DC offset on amp by bypassing main caps with a film. Makes me think I may be in need of large main caps but if it's in range I may just leave it be.
 
You may add as many film capacitors you like but these have absolutely nothing to do with offset.
You may replace as many of the main caps you like but these also have nothing to do with offset
You may actually remove totally the main capacitors ( for experiment ) and replace them with anything available from capacitance aspect as low as 100uf in the rated voltage still offset has nothing to do with it ....( static conditions )

A slight mismatching on the LTP and slight changes on the thermals ( like starting measuring with the top cover on or without ) are responsible for the small variations in the offset of that level and LTP is so sensitive that in most cases it is enough to blow your breath on it and there is going to be minor change in the offset ...

Still this behavior is expected to almost any amplifier and variations of that level are expected while these are by no means responsible to blown tweeters .

Ferrofluid does not evaporate though the chemical composition inside the driver might change if excessive temperature is produced , coil of the driver is getting toasted and leftovers of the coil is mixed with the ferrofluid ...( clearly not the case here )

Your tweeters blow from optimistic design of the xovers and the general aspect of the speaker to have a rich high mid area even though a tweeter found in the wrong power , wrong peak , wrong frequency every now and then might blow ...

Both offset and ferrofluid have nothing to do with ..Actually writing these things is between laughing or screaming

If you been reading this about film capacitors in the internet you have been probably reading the wrong fairy tale or the wrong urban legend ...
If the tech dude in the speaker shop was telling things like the above then YES you have been fooled around big time ...
If film capacitor story is your own knowledge or idea keep away from anything that has a cable on it ....for safety reasons ....
 

gto127

Member
2007-05-09 2:51 am
I don't think it's design problem on tweeter since the tweeter from other one does not blow. If it's not ferrofluid maybe problem with crossover? The film caps lowering DC was simply measurement I took after adding it. Sorry I said film-they were actually polystyrene. I will take them off to see if only coincidence.