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Old 21st July 2008, 07:40 PM   #1
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Default Low Volume Cutoff Problem

Here's the deal: A little over a year ago I came here and got some advice for an iPod project I was building. Sadly I never made it back to share photos. I still need to take some good ones, because it's just gorgeous.

Even more unfortunately, I fried the amp a little less than a year ago. I used the T-Amp 5066. The box includes twin 4.5 Ah 12V SLA batteries (dual to balance the weight as it's designed to be portable). I set up a knife switch to isolate the amplifier from the charger, which was plugged in the back via color-coded banana plugs. The amp and the charger had to take turns accessing the batteries. (The batteries provided weeks of enjoyment many hours a day between charges. It was fantastic.)

Tragically I bridged the contacts on the knife switch by bumping it with something (I don't even remember what now) and the amp went up in smoke. I was in the middle of a move and a job change so the whole project went into storage.

Fast forward to the (almost) present...

Recently I purchased an odd reading chair. It was designed as a gaming/music chair (the "iRocker") with a built-in amp and speakers. But the speakers sucked so I pulled the amp out hoping to use it for my acrylic box.

It sounds great. A bit more power than it had with the T-Amp (certainly attributable to the 15v power supply; obviously I could have done the same with the T-Amp). But there's a really obnoxious flaw: the volume completely cuts out when the input is below a certain volume.

Any spoken word track, or any song that starts with (or at any point reaches) a low volume is completely unenjoyable. It just cuts in and out. Moderate to high volume sounds fantastic.

I'm starting a new business and I have no extra cash for this project, so I'm hoping this can be fixed easily. I have no idea if it can, but I figure the friendly experts here can point me in the right direction. My brother is an electronics geek (his idea of a fun Saturday afternoon is going through the latest Digikey catalog), but he doesn't know audio design. I love audio, but I can't read a schematic much less design anything functional (but I make pretty damn good looking enclosures and loudspeakers that sound at least as good as anything you can get at BestBuy).

I've posted close-up photos of the amp, preamp, chair, and enclosure:

http://www.sunbeamphotography.com/Amp/

The main amp is a TDA7377. The preamp is a 4558C. Neither seem to have this problem in other implementations I've read about, and both seem to be quite capable, so I'm hoping there's something I can replace or just remove and this will magically work. I realize how arbitrary and pathetic this sounds...

I realize the presentation in the photos is ugly, but I'm not going to modify the box to attractively hold this new amp until I know it works properly.

Thanks or thanks anyway!


- Jon
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Old 21st July 2008, 08:35 PM   #2
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By the way, I didn't mean for the BestBuy comment to impress anyone. I'm just saying that my designs are more attractive and just as functional as off-the-shelf systems, though nothing compared to the remarkable designs I've seen from audiophile friends and the members of this site.


- Jon
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Old 22nd July 2008, 07:46 PM   #3
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Check the voltage at pin 7 during start-up.

Does it switch in the same rhythm as the signal? Then check the components that are connected to pin 7 and replace the broken ones.

If it doesn't, check the voltage at pin 10 and compare your readings to the diagnostic information chapter in the datasheet.
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Old 22nd July 2008, 07:53 PM   #4
lineup is offline lineup  Sweden
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you have any resistor in series with THE WIPER
This is the output leg of a volume pot.

Try with one 1k0 - 2k2 kohm resistor.

The input of op-amps / amplifiers so not always like it
when the potentiometer comes close to GROUND Level.

One such resistor from the wiper can sometimes BE NEEDED.
It will not change much in performance, usually.
The circuit will work the same.
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