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Safely testing a pre/power amp

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Posted on February 9, 2016 at 00:39:05
graciouslee
Audiophile

Posts: 244
Joined: September 29, 2003
 Hi.
I have inherited a system from a late relative, comprising a Quad 99 CD/pre/power set up with a pair of speakers. I got it home and plugged the speakers onto my system and discovered that there is no sound from the tweeters. I've double checked that the HF/LF links are in place on the back of the speakers (they were) and have also tried removing them and connecting my speaker cables straight to the 'HF' sockets, but still no sound from the tweeters.

Knowing this, and also knowing that problems with amps can sometimes lead to tweeters/speakers blowing, I am reluctant to just plug in the pre/power amp to my own system in case there is a problem with the power amp output which could damage my speakers. I obviously can't ask to find out what caused the tweeters to fail (and I will be investigating the tweeters further at a later stage), so want to check the output of the amp for safety before hooking them up. I have a multimeter and was wondering if I can measure anything on the power amp to see if it is safe to connect to my speakers?

*edited to add that I turned the power amp on, not connected to a pre or speakers, and left it to idle for about 30 minutes, then checked each speaker output for DC voltage: left output measured 32.4mV and right output measured 24.8mV. That certainly doesn't seem like enough to fry a pair of tweeters, so maybe something else happened to them?

 

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RE: Safely testing a pre/power amp, posted on February 9, 2016 at 06:47:33
John Elison
Audiophile

Posts: 23900
Location: Central Kentucky
Joined: December 20, 2000
Contributor
  Since: January 29, 2004
 The DC offset should probably be less than 10-mV, but what you measured is not enough to do any speaker damage.

It's possible there is nothing wrong with the power amp. You can easily blow tweeters by simply playing music too loud. Perhaps that is what happened to the speakers.

Did you look for AC voltage at the output of the amplifier to see if there is a hum problem or something oscillating? You probably should short the inputs for that measurement.

If both AC and DC voltage at the output look okay, you can connect the speakers and try playing some music to see if its working properly. Be advised, it's possible there could be a high-frequency oscillation at the output that your mulitmeter will not show. Most multimeters have limited frequency response that begins falling off after 1-kHz or even less. The AC function on many multimeters is designed to measure line frequency only.

I don't think there is much else you can do with your limited test equipment. Therefore, if you do not want to risk connecting the amplifier to the speakers, you should probably take the amplifier to a qualified technician to have it checked out.

Good luck,
John Elison

 

RE: Safely testing a pre/power amp, posted on February 9, 2016 at 11:53:27
Awe-d-o-file
Dealer

Posts: 21037
Location: 50 miles west of DC
Joined: January 10, 2004
 Take that meter and measure the resistance of the tweeter and woofer connections. If the tweeter is dead it will measure infinity/open. What are the speakers anyway?

I sold that Quad gear and it was pretty good. Is it a 909 power amp? Johns post covered a lot of good points. If you actually played a source with the volume up you'd measure that as AC. As John said with nothing playing if DC and AC measure very low you'll probably be fine.

ET


ET

"If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936

 

DC does not blow tweeters, posted on February 9, 2016 at 18:31:40
madisonears
Audiophile

Posts: 1587
Location: midwest
Joined: September 6, 2006
 Any crossover cap will block all DC, assuming the cap is intact, and almost every tweeter has a cap in series. Distorted AC output from an amp playing beyond its linear output is usually what blows tweeters. Or they could have just died of old age or suffered mechanical damage. Or perhaps the xover cap has gone short circuit and passed DC or low frequency AC to the tweeters and they are blown. Or perhaps the cap has gone open circuit and passes nothing; the tweeters might be fine. You need to do a little more investigating.

DC will usually not wreck woofers until it gets above one volt and plays for a long time. It's not a good thing, but it probably won't wreck anything.

Try measuring at the amp outputs while you cycle power off and on again. Then with the amp already powered, with signal at low volume, just touch the speaker cable leads to the speaker terminals, NOT securely attached to them. Use the inherited speakers, not your own good ones. If you don't hear anything bad, attach the speaker cables and play the amp for a while, turning up the volume in small increments. If you don't hear severe distortion or notice excessive heat, it's probably fine.

Peace,
Tom E

berate is 8 and benign is 9

 

DC offset should always tried to be set to....., posted on February 9, 2016 at 19:27:40
Cougar
Audiophile

Posts: 4574
Location: SoCal
Joined: June 25, 2001
 0 mVdc unless manufacturer says different like in the case of my Edge M8 it's suppose to be set from 0 mVdc to -10 mVdc. DC offset jumps around as AC line voltage varies. So try your best to get as close to 0 mVdc

Sometimes adjusting will affect the bias setting so you MAY have to adjusted as well just depends on how far out the DC offset is.

Hope this helps.

 

RE: DC offset should always tried to be set to....., posted on February 10, 2016 at 07:00:29
graciouslee
Audiophile

Posts: 244
Joined: September 29, 2003
 Thank you all for the replies and advice.

I tried the CD player separately before seeing the replies and found that it worked fine into my own system (via RCA cables), which is good. I have now checked the speaker outputs for AC and got a 0.000 reading on both left and right outputs.

So, tentatively, I connected it all up to my own speakers, now with the Quads connected using their own Ampbus cabling, and turned on (with my fingers crossed). A small switch on thump through the speakers as the power switch on the rear of power amp was turned on, plus a small click/pop through the right speaker when I powered it all out of standby. Loading in a CD and pressing play, then gently turning up the volume and I have music! No signs of audible distortion that I can hear, nor any buzzes or hums through the speakers with my ear next to them and no music playing, so all looks good so far. Its a 99 pre, 99 CD-P2 CD player and 99 stereo power (not the 909) amp, by the way. The speakers are the 77-11L, and I have tried to remove the tweeters but initial finding is that they appear to be bonded (as well as screwed) to the cabinets in some way, same goes for the mid/bass drivers too. If I can find something long, thin and strong enough then I may be able to push the tweeters out through one of the ports on the rear of the cabs, though not sure what to use for this yet as there is a metal mesh of some sort in the back of the port which blocks access a bit.

Anyway, thank you again for the tips. I guess the power amp may need a service given the switch on thump and then the click/pop when it came out of standby, though I can't detect anything which may damage my own speakers as yet so at least it is mostly fully working. Sounds great, too.

 

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