RIAA preamp problems, so frustrated! - diyAudio
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Old 16th February 2008, 04:04 AM   #1
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Join Date: Dec 2006
Default RIAA preamp problems, so frustrated!

Hey guys,
I could really use some help here. My dad has a b&o turntable from the 70's. I recently found a whole b&o system from the late 80's on ebay. The 4500 system has no riaa preamp built in however. The matching 4500 turntable (which I don't have) instead had the preamp. And likewise the 70's turntable doesn't have a preamp, as the receivers back then had them. So in order to connect my dad's old turntable to the 4500 system I needed an riaa preamp. As I was in the diy mood, rather than just buying a preamp I bought the piaa circuit kit. The other reason I wanted to do this was so that on the enclosure, I could use the 7-pin DIN plugs b&o used and avoid DIN-RCA adapters.

So having finished the enclosure, and getting pretty excited, I plug the preamp box in, and as soon as the turntable needle drops I'm greeted by the most awfully horrendous garble of cover-your-ears noise. Even with the receiver/amp set to the lowest volume the speakers were blaring noise at max power. I tried remelting some of my solder joints, tested for shorts, and then soldered an 1/8" plug cord onto the board so I could try it on my computer just to see what would happen. And this way I only had to worry about damaging a cheap computer sound card rather than the 4500 system and speakers.

Well of course the turntable and preamp sound perfect going through my computer. I even tried jiggling wires and the preamp board in an attempt to replicate what happened before. I had thought maybe there was a signal to power short which had caused the intense noise. Well after a couple weeks of listening to the turntable and preamp on my computer, I venture downstairs again today to try plugging them into the 4500 system, wondering if my resoldering and messing with stuff had cleared the problem. This time I plug in some cheapy yamaha speakers in just so the b&o speakers wouldn't get ruined. No luck, I get the exact same blaring of noise as before.

I'm pretty much afraid to keep trying things, as I feel like I'm going to ruin the receiver. Btw, I realize the noise can damage the speakers, but can the receiver also be damaged by trying to drive that noise? As I said it sounds how I would imagine it would sound if the signal was shorted to the +/- 12v rails. I realize newer receivers would have an overload protection, but I don't know whether ones from 20 years ago would.

So do you guys have any ideas as to what could cause this? I had thought about the possibility that I may have miswired the connection from the preamp board to the pins on the DIN socket, but amongst the noise I can still hear brief moments of the actual music. Furthermore, I figured miswiring the DIN socket pins would probably give me either no sound or switched channels.

And I know you guys are wondering why b&o. I know b&o is ridiculously overpriced, but I love the style of some of their older systems, and I'm willing to sacrifice some sound quality for something that looks so gorgeous. I mean would you guys really turn this down for $500 (minus turntable, but including speakers, which I did have to refoam)

Thanks so much for any help or suggestions!

Austin



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Old 16th February 2008, 04:16 AM   #2
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Join Date: Mar 2007
Location: Canandaigua, NY USA
I don't know anything about the B&O stuff, but it sure sounds like the receiver has a huge gain. Are you absolutely sure it doesn't have an RIAA preamp? Did you try connecting the turntable direct? Or maybe they did the EQ in the table, but still had most of the gain in the receiver. You could try putting a temporary level control on the output of your preamp and see if the receiver operates normally with a much smaller signal. The fact that the preamp works fine with the computer suggests weird receiver inputs.
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Old 16th February 2008, 05:08 AM   #3
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Join Date: Dec 2006
Oh my god Conrad I can't believe it. I just tried plugging the turntable directly in and the receiver does have a preamp built in! And you know what, both my dad and brother, who know little about audio compared to me had said I should try plugging the turntable directly in to see what happens. And I just brushed off the suggestion telling them I know the receiver didn't come with a preamp.

The official b&o specs say there's no preamp, and I've only ever heard people say there's no preamp. When I was looking over the factory repair manual last week, they made reference to a part number for a preamp board, but I never imagined the previous owner actually had that installed. I feel like such an idiot now for having spent hours making the preamp, with a nice aluminum enclosure, and even a little white power led on the front. Ugh, but the important thing is it works now and sounds great.

Hmm, I suppose I should try plugging it in with my preamp into one of the tape inputs, to see if I can hear a difference between the b&o preamp and mine.

Thanks again!
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