The Phase Linear 400 Guinea Pig

[premiumplus]

Just finished the test tunes, I'm tellin ya guys. The Pl platorm is something special now. Joe, Don, Jer, and the rest of us crazy mo-fo's whio build these critters, they are something to be very proud of !!


Congratulations folks!! I thank each and every one of you for your contributions to my listening pleasure!!!!!!!!!!!!!!

I'm looking forward to getting my WOPL 700B's to fully complementary status...they sure sound great as is, but after I get the new filter capacitors and the full-comp backplane boards installed they are gonna be legendary.

 

[laatsch55]

Lee, what do you mean by cycling the power cord? I want to make sure I avoid doing that by accident

The 400's don't have power switches, I thought I had tripped the DCP and to reset you have to unplug it. I have tripped the DCP's before on MAX output tests, they see a hard clip as DC and trip. Instead of checking the rail fuses, I cycled the power. There was no destruction of parts till then.

 

[Northwinds]

Gotcha, thankfully Jer installs power switches on mine. I guess he plans for contigencies with me

 

[laatsch55]

The Pigs Max test was 288-287 per into with both loaed and driven. It is now back to pushing the K's to almost insane listening levels. It really takes a 700 to get insane with em.

 

[Gepetto]

Yea, it wouldn't have blown them had I not immediately cycled the power cord. I believe that's when Q7R/L went. The 47uf/100volt cap also vented. I had things pretty warm Joe. Yes on the 5 amp fuse.

Hi Lee can you check the condition of D4 and D5 in the amp that you blew Q7 on please.

 

[laatsch55]

Certainly Joe. I'll do that right now.

 

[laatsch55]

That cap should not have vented if those are intact?

 

[laatsch55]

Forward .614, rev, infinity, on all 4.

 

[Gepetto]

Forward .614, rev, infinity, on all 4.

Thanks Lee

 

[laatsch55]

You're welcome Sir Joe. Anything else?

 

[Gepetto]

You're welcome Sir Joe. Anything else?

Since D4 and D5 are intact as well as Q5, we will have to work on reducing the peak current through Q7, based on the fact that you are wiping out Q7 and then the in line fuse. Since you are driving this to the limit, try changing out R35 to 22 ohms instead of 1 ohm. That will reduce both the peak Q7 base and collector current value.

Right now you are blowing the 5A main fuse and then the gain on the lower half collapses and poor Q7 has to try to keep up with driving the load without predrivers or output drivers. It is no match for that Lee.

 

[laatsch55]

OK, why is it the negative half that takes the hit?

 

[laatsch55]

Should that be a 1/2 watt Joe?

 

[Gepetto]

OK, why is it the negative half that takes the hit?

Because Q5 can drive the negative side into a very hard clip because of the low impedance path into the base of Q7 from the collector of Q5. A 22 ohm drive.

On the topside when you go into clip, the base of Q10 is driven much more gently by the 3.9K and 22 ohm resistor series combination. When you are near the main fuse limit, the bottom half will always tends to go first for this reason, then Q7 fails and takes out the pico fuse.

 

[Gepetto]

Should that be a 1/2 watt Joe?

No a 1/4 watt Lee, want it to act as a soft fuse in this application. Really trying to save Q7 in these severe overdrive situations where you have blown the negative fuse.

 

[laatsch55]

OK, want me to push real hard after the R35 swap to see what happens?

 

[Gepetto]

OK, want me to push real hard after the R35 swap to see what happens?

Do I have any choice Lee

 

[laatsch55]

Well yeah, right now. You're calling down changes and I'm makin em. It seems the only way to see if that was a cure is to duplicate the push.

R35 in. got a little rough with Q7R last time and ripped the base trace, thank goodness that doesn't see much. Managed to get a small sliver back in the hole and a good solid solder job.

 

[Gepetto]

Do I have any choice Lee

You will notice I scaled back the pico fuses on the 700 backplane kit to 500mA to work on saving Q7 when the main fuses give out. That change can apply to the 400 backplane kit as well. The normal currents involved are about 70mA peak so even 500mA is serious overkill.

 

[laatsch55]

I see. If the 22 ohms look like the cure should we run that in the 700 board too?