test problem

[KevO]

Doc,

Thanks again for the help yesterday. Oscillation has stopped. Can't say why. Just glad it did.

Only one anomaly now. I power up the source, then the beepre, wait 2 minutes and turn on the usually well behaved amp.

I get 2 Bzzt's from the mid/tweets and then 2 full excursion pops of the woofers.

That is followed by beautiful music. Detailed. palpable... but I digress.

My other tube and SS preamps do not have this issue on this system.

I will test beepree with another amp for comparison.

Oh, amp is ATI 6012 and between amp and preamp is Linkwitz active analog crossover. I always leave the x-over powered on.

Any ideas? Oscillation lite?

Thanks!

Kevin

 

[Doc B]

Following our discussion yesterday, it sounds like the safety ground and signal ground may not have good contact with each other.

 

[KevO]

Doc,

Thanks for the reminder.

Kevin

 

[KevO]

Doc,

I reflowed the ground tabs and tightened both up more. Same issue, both channels and effects all frequencies. Woofers shake the cabinet. Afraid I may blow a tweeter!

I can disconnect speakers from amp, power up and reconnect. Bypasses the audible issues that way, but certainly no solution.

I hate giving up, so will review wiring.

Sound had been too bright and bloated in mids. Last night the preamp 'gelled'. About 25 hours of play. I had no idea it would sound this *&%$!! fantastic. We both sat and listened to album after album until sleep won out. We are delighted with the music. Palpable, natural and nuanced.

Thanks for a stunning design
-Kevin

 

[Paul Joppa]

If everything else is on and stabilized, and the power amp gives a jump, one would suspect the power amp first. But you said it is well behaved with other preamps. The only thing that comes to mind is a possible DC bias on the output of the filter that is not there with other preamps. That's possible if the BeePre has some leakage around the output capacitor AND the filter has no input or output coupling caps. Can you post a schematic of the active filter?

 

[KevO]

Thanks Paul,

I will look for the schematic. As you can see from my posted measurements I did detect a small amount of DC on the outputs.

I am really curious why the noise stops. I can warm up the preamp and as soon as the amp goes on, 2-3 seconds of noise and then all is well.

Maybe I need a much longer warm up for the beepre. Would a mute switch help?

Since this is Linkwitz proprietary, I can't post it. I'll PM you tomorrow and we can figure this out. I am also checking the Linkwitz forum.

-Kevin

 

[Paul Joppa]

I downloaded the ATI 6012 manual, and I see that its input is AC-coupled. Therefor a DC offset voltage (if present) won't have an effect on the power amp. The time constant of the BeePre output 10uF/475K is 5 seconds, and it will settle to less than 1mV in 60 seconds.

My conclusion is there must be a power line transient when the big amp switches on. That seems on the face of it unlikely, since the ATI supposedly has a soft-start to prevent just that. Still, you might put an AC voltmeter across the power line and watch is as the power amp is switched on.

How rapidly does the speaker oscillate? i.e. what frequency is the short impulse? I know, that's hard to tell - but you can probably see if it's closer to one per second or 10.

 

[KevO]

Paul,

The ATI is very well behaved. I don't suspect the power line. Could be an interaction with the Orion active crossover.

I think the best clue is the noise lasts 2-3 seconds, then is over.

The woofers are maxing out and I am afraid of blowing a tweeter. Each driver has a 60 watt amp.

I will try again after warming up the beepre for 10 minutes.

I have sent you a PM.

Thank you,
Kevin

 

[Paul Birkeland]

I've been lurking on this thread a bit, but I'll throw in my $0.02.

I'm guessing just the woofers do this?

It is possible that the crossovers have no filter at the input, and if this is what's causing your problems, the fix is easy.

I would recommend investigating this case by simply connecting your BeePre directly to the woofer amp, then going through your power cycling.

If this ends up being the case, a C-R filter at the input with a 0.1uF cap and 249K resistor may take care of you.  You could also try lowering the 475K resistors on the BeePre to 100K (or maybe try both).

-PB

 

[KevO]

PB,

Thanks for the help.

The noise involves all the drivers, but the woofers seem to be cycling to full excursion at around 2-3 hz for 2 or 3 seconds. I am new to schematics, but I am fairly sure there is no filter at the input.

I will run your test in a few minutes.

Thanks!!
Kevin

 

[KevO]

Gentlemen,

Just tested without the crossover. 1 pop instead of 3-5. So it appears the active crossover is more sensitive.

I put the o-scope on Beepre out and watched. But I have no idea what any of it means. Time to learn more about using the oscilloscope.

I found a diagram of the c-r filter (high pass) here; http://www.learnabout-electronics.org/ac_theory/filters81.php

Not sure how to build it into the crossover since it is a circuit board, but can I put the parts on a M/F RCA jack and insert it to the end of the BeePre out cable? How much will it cut the bass? The bass sounds great right now.

Thanks!

-Kevin