Amplifier Benchtesting?

[Miss-HiFi]

Hello everyone! I would like to know WHAT "benchtesting" involves? What ACTUAL tests are performed on an amplifier when it is "bench" tested?

The reason I ask this IS because I recently had to take my receiver (well, actually my brothers) in to be serviced. I took it to a local audio shop as opposed to shipping the 55lb hunk of metal back to the manufacturer. The sound from the Right Speaker becomes garbled after playing at a moderate level for 10-15 mins. At first, I was told it COULD be a problem with my speakers but the same thing happens to the LEFT speaker if I reverse the wiring to the receiver. So, after ruling out the possibility it could be a problem with my speakers, I took it in to be serviced.

Well, after 14 days with my receiver, I was charged $50 and told NOTHING was wrong with my receiver. Now I KNOW I'm not as audio savy as most of you around here but I DO know my way around a "basic" stereo thanks to my older brother who has always been an audio hobbyist! I asked the technician what he "did" exactly to test the amp and he was very evasive! I have a feeling that they just probably hooked it up to some small bookshelf speakers for a minute or so and heard sound from BOTH speakers and concluded that I must have made a mistake in my initial diagnosis because afterall, I AM a female, and most of us gals DON'T know all that much about Audio/electronics etc.

So, I got it home, connected the speakers up AGAIN and guess what? Garbled sound out of the right side! I've been told that IF the technician DID basically connect a pair of smaller speakers to the receiver, the problem may not have presented itself because my Tower Speakers require more "power" from my receiver AND thus make it work harder than it would have to in order to power a small speaker. I Guess that makes sense! Oh, the same person ALSO mentioned that it "SOUNDED" like the receiver had a "bad" output. (That is definitely beyond my basic audio knowledge).

I'm afraid to take it back to the previous shop because frankly, I don't think they have the knowledge/training to work on my receiver (which is a B&K AVR 307 by the way). And, so, I don't quite know where to go from here BUT it would help if I at least KNEW what type of tests are SUPPOSED to be performed on an amplifier to test its Output and overall performance/condition????


Thanks for all your help; AGAIN;


Jacinta~

 

[T2T]

Chances are, they did hook it up to scopes to monitor the output of the amplifier. That's part of the test. Such a test could also indicate the true output wattage of the receiver (e.g. 115 watts) vs. the 100 or so advertised watts - I'm not sure what your B&K has for output power, so I was just guessing.

In regards to the "outputs" being bad, it very well could be some of the chip components (output transistors) are the issue. I experienced this problem with one of my Parasound power amps and had to purchase a dozen or so of them for the 2 of 3 channels that were bad. However, in my situation, 1 of 3 channels was working and the other 2 were completely dead. I never experienced a degrading of sound - or, one that would have led me believe that the channels were on their way out, such as you're experiencing.

Now, the question comes to mind, what do you consider the receiver to be worth? Such repairs can be costly. In my case, it was $300 and took over 2 months of time to complete due to parts deliveries, etc. If your receiver has pre-outs, you might consider buying an external power amp and running that for the 2 front channels and call it a day. There's used B&K amps that have 105 watts per channel for as low as $169 on Audiogon. Certainly, that is less cost than a more involved repair on the receiver might cost.

Good luck to you!

Edit: I looked up the specs via the PDF on B&K web site. It does appear you receiver has pre-outs allowing for the connection of external amplifiers. You receiver produces 150 watts at 1kHz. Probably, the closest match in an amplifier from B&K would be the ST-2140 or the Ref 125.2. Both of these are a bit more - along the lines of $250 to the low $300 mark. Yet, I see this model of receiver selling used in the $450 or so range. I guess it's kind of a toss-up at this point as to what you'd want to do.

 

[adwilk]

If your receiver has pre-outs, you might consider buying an external power amp and running that for the 2 front channels and call it a day. There's used B&K amps that have 105 watts per channel for as low as $169 on Audiogon. Certainly, that is less cost than a more involved repair on the receiver might cost.

My thoughts exactly when I was reading the first post. Thats a BEAST of a receiver, but unfortunately I don't think you're the first to experience that issue, at least thats what a google search turned up. I would look into adding an external amplifier, but at a rated 150wpc from the receiver, I'd at least look into possibly upgrading or at least equaling the existing output. You'll have tons of options as far as that goes. The other option would be to sell the receiver 'as it' and you could probably still afford some nice gear.

 

[TLS Guy]

Hello everyone! I would like to know WHAT "benchtesting" involves? What ACTUAL tests are performed on an amplifier when it is "bench" tested?

The reason I ask this IS because I recently had to take my receiver (well, actually my brothers) in to be serviced. I took it to a local audio shop as opposed to shipping the 55lb hunk of metal back to the manufacturer. The sound from the Right Speaker becomes garbled after playing at a moderate level for 10-15 mins. At first, I was told it COULD be a problem with my speakers but the same thing happens to the LEFT speaker if I reverse the wiring to the receiver. So, after ruling out the possibility it could be a problem with my speakers, I took it in to be serviced.

Well, after 14 days with my receiver, I was charged $50 and told NOTHING was wrong with my receiver. Now I KNOW I'm not as audio savy as most of you around here but I DO know my way around a "basic" stereo thanks to my older brother who has always been an audio hobbyist! I asked the technician what he "did" exactly to test the amp and he was very evasive! I have a feeling that they just probably hooked it up to some small bookshelf speakers for a minute or so and heard sound from BOTH speakers and concluded that I must have made a mistake in my initial diagnosis because afterall, I AM a female, and most of us gals DON'T know all that much about Audio/electronics etc.

So, I got it home, connected the speakers up AGAIN and guess what? Garbled sound out of the right side! I've been told that IF the technician DID basically connect a pair of smaller speakers to the receiver, the problem may not have presented itself because my Tower Speakers require more "power" from my receiver AND thus make it work harder than it would have to in order to power a small speaker. I Guess that makes sense! Oh, the same person ALSO mentioned that it "SOUNDED" like the receiver had a "bad" output. (That is definitely beyond my basic audio knowledge).

I'm afraid to take it back to the previous shop because frankly, I don't think they have the knowledge/training to work on my receiver (which is a B&K AVR 307 by the way). And, so, I don't quite know where to go from here BUT it would help if I at least KNEW what type of tests are SUPPOSED to be performed on an amplifier to test its Output and overall performance/condition????


Thanks for all your help; AGAIN;


Jacinta~

The tech could be totally incompetent, or it could be he tested it with a power resistor, which is accepted usual practice. The problem is a resistor is not a speaker, and the complex load of your speakers is causing the output stage to malfunction.

When testing though, you should really push the amp hard into a hefty resistor, before passing the amp.

You need a better service center, or need to send it to the manufacturer.

 

[KEW]

Just to be sure, when you swapped your speaker wires (to test whether the problem was with the speaker or with the receiver), did you swap them at the speaker or at the receiver. If you swapped at the speaker, it is possible that something is wrong in the wire between the receiver and speaker.
It would be a very long shot for wires to go bad, but over the years, I've had a puppy who liked to chew on cords (it took a jolt of 120V to break him of that) and a cat that liked to attack any cords on the floor.

If you are uncomfortable with the shop and assuming they are the only game in town, it sounds like repair isn't a good option.

 

[highfigh]

Hello everyone! I would like to know WHAT "benchtesting" involves? What ACTUAL tests are performed on an amplifier when it is "bench" tested?

Well, after 14 days with my receiver, I was charged $50 and told NOTHING was wrong with my receiver. Now I KNOW I'm not as audio savy as most of you around here but I DO know my way around a "basic" stereo thanks to my older brother who has always been an audio hobbyist! I asked the technician what he "did" exactly to test the amp and he was very evasive! I have a feeling that they just probably hooked it up to some small bookshelf speakers for a minute or so and heard sound from BOTH speakers and concluded that I must have made a mistake in my initial diagnosis because afterall, I AM a female, and most of us gals DON'T know all that much about Audio/electronics etc.

So, I got it home, connected the speakers up AGAIN and guess what? Garbled sound out of the right side! I've been told that IF the technician DID basically connect a pair of smaller speakers to the receiver, the problem may not have presented itself because my Tower Speakers require more "power" from my receiver AND thus make it work harder than it would have to in order to power a small speaker. I Guess that makes sense! Oh, the same person ALSO mentioned that it "SOUNDED" like the receiver had a "bad" output. (That is definitely beyond my basic audio knowledge).

I'm afraid to take it back to the previous shop because frankly, I don't think they have the knowledge/training to work on my receiver (which is a B&K AVR 307 by the way). And, so, I don't quite know where to go from here BUT it would help if I at least KNEW what type of tests are SUPPOSED to be performed on an amplifier to test its Output and overall performance/condition????


Thanks for all your help; AGAIN;


Jacinta~

Any shop that services equipment should have a service warranty. If they said it "sounded like it has a bad output", they should have repaired it. Ask for the manager and tell them what was said and by whom. They're supposed to describe any work done in writing and if they didn't, tell them that you want them to actually try to replicate the problem. If it didn't do anything wrong in a week, they should have asked you to bring the speakers in so they could find out if they were the cause.

Bench time isn't, and shouldn't, be free. They should have explained how they charge, though. If that's their minimum charge, they should have told you and your state has specific requirements for this kind of thing.

A basic bench test would include connecting the output to speakers and listening to it over a period of time, which is what they apparently did. A more advanced test would include connecting it to test loads, which are fixed resistors that will be attached to an oscilloscope and they can stack these to create different loads. They would then pass various sine wave signals through it so they could see if there's any major distortion. If they're a well-set up shop, they'll be able to do an advanced bench test and that would include using a distortion analyzer, transistor tester and testing for current draw. With the low cost now, there's absolutely no reason they shouldn't. I recently saw a USB-based function generator/oscilloscope/distortion analyzer for $100, so there's no reason not to. Having someone who knows how to use this equipment is another story. Whether they checked the actual power output is up to their capabilities and equipment but for $50, I doubt they did this because their shop rate is probably higher than what they charged you.

If this receiver is under warranty, it should have been taken to a B&K authorized shop.

 

[mtrycrafts]

....

Well, after 14 days with my receiver, I was charged $50 and told NOTHING was wrong with my receiver. Now I KNOW I'm not as audio savy as most of you around here but I DO know my way around a "basic" stereo thanks to my older brother who has always been an audio hobbyist! I asked the technician what he "did" exactly to test the amp and he was very evasive! I have a feeling that they just probably hooked it up to some small bookshelf speakers for a minute or so and heard sound from BOTH speakers and concluded that I must have made a mistake in my initial diagnosis because afterall, I AM a female, and most of us gals DON'T know all that much about Audio/electronics etc.

Oh, the same person ALSO mentioned that it "SOUNDED" like the receiver had a "bad" output. (That is definitely beyond my basic audio knowledge).



Jacinta~

What speakers are you using and what volume levels do you have this issue?
Just because you are driving a tower speaker doesn't mean it needs more power than a small bookshelf speaker.

 

[Seth=L]

I'm sorry for not reading every response on this thread before responding (as this may have been covered already).

Miss-HiFi,

Have you tried connecting a difference source or swapping the left and right output from the source to see if the source is causing the problem? It's rare that a source would fail like that, but not impossible.

Edit: I used to have a B&K Reference 20 preamp/processor that had intermittent audio output problems. It's possible that when they where testing it the problem was not replicating during the time of testing.

Also, when you experienced the problem was the source connected by digital or analog connections? The B&K platform is set up much like a computer system and relies on firmware to run correctly. If the firmware were to become corrupted, maybe the battery backup died and caused corruption. With the B&K components it's hard telling what it's doing.