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Old 6th January 2013, 05:21 AM   #1
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Join Date: Nov 2004
Default Adding line level outputs to an amp

I'd like to add line-level/aux RCA outputs to an amplifier that lacks them. Is there a relatively easy way to determine where to tap the printed circuit board? Can I locate these connections using a DMM while the amp is operating? Would a photo of the boards be necessary for advice?
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Old 6th January 2013, 05:47 AM   #2
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You'd have to explain why you'd avoid a simple pair of Y cables. If you want to pluck the signal after some signal processing that occurs in the amp then you'll definitely need the schematic for the amp.
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Old 6th January 2013, 06:19 AM   #3
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You mean extra outputs from a preamp? Or pass-through line level outputs on a power amp? Not sure what you really want.

Are you sure what you're doing? If not, DO NOT put anything into the amp while it's operating, including a DMM or any other probes or your fingers.

Get a pair of Y adapters. Save money, time, possibly your life.

Tom E
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Old 6th January 2013, 05:13 PM   #4
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It may sound like a frivolous project especially at the level of discussion here. In fact it's a little embarrassing to explain...

....but I have a fifteen year old, Japan made, Sony tabletop Bose Wave clone. I recently bought it in its box never used for five dollars at a garage sale. It has an internal 3 inch rear "subwoofer" and two 3 inch forward speakers. It has separate bass and treble control. Built like a brick. It's a beautiful little unit, dark gray fabric grilles and chassis, and gloss black panel under the controls.

Its sound quality is pretty astonishing for a little tabletop AM/FM CD player. It occupies a kitchen counter top... the room where I actually spend a great deal of time and WAF prohibits the installation of anything more conventionally "audio" in the kitchen. No integrated amps, separate small speakers, multiple components, etc. But it would be easy to hide a sub under a cabinet.

I've got lots of odds/ends laying about. Half a dozen sub plate amps and a dozen or so drivers of different sizes. I thought I might hide a subwoofer somewhere near this player as it sits in a corner of the kitchen where I spend a lot of time.

Should I just open the chassis and tap into the main speaker connections and connect to the speaker level inputs on the plate amp even though there would be no speakers connected to those outputs? Would that be just as well as connecting via line level inputs to the plate amp?

Last edited by peace brainerd; 6th January 2013 at 05:19 PM.
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Old 6th January 2013, 05:55 PM   #5
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Starting to make more sense now. Music in the kitchen is a lot of fun.

So you want to run an outboard sub from the existing "sub" drive of the Sony? That sounds easy enough, especially if your plate amp has speaker level inputs. Those usually use a couple resisitors to drop the main power amp output voltage so it can be accepted by the plate amp's inputs.

A couple things to keep in mind. There might be some signal splitting going on inside the Sony to direct a summed low frequency output to the single "sub" speaker. If that's the case, you won't get a full range signal going to your outboard sub. That might still be okay, possibly even better.

Also, those little radios can have a lot of hum and low frequency buzz that you can't hear through the tiny built-in "sub", but might become apparent when using a proper outboard sub with decent response. I would experiment a little before going all out to mod the Sony.

You can take the signal in a couple places from the Sony. If you tap the circuit at line level, you'll need RCA output jacks and IC's to feed the line level inputs of the outboard sub. If you take the actual powered "sub" output of the Sony, you'll need speaker wire to make the link to the speaker level inputs of the outboard. Either way will work: the line level might be harder to figure out where to tap and be more subject to noise; the speaker level might suffer losses from a long run of wire, but the tapping point is pretty obvious.

Could be fun little project, but don't expect high fidelity or decent imaging. I expect the sound will wander back and forth from radio to sub because you'll need to use a pretty high crossover point. With a three inch mains speaker, I wouldn't use anything bigger than eight inch for the sub. Six might be even better. You'll still get plenty of additional thump, if that's all you want.

Tom E
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Old 7th January 2013, 12:45 AM   #6
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There are no jacks at all on this thing. Going speaker level I was just going to connect to the front stereo speaker pair and run the wires about four feet down under the cupboard to feed the plate amp.

A smaller sub driver is an excellent suggestion too. Thanks. I was going to use an old car audio jbl 10 inch that I have but I'll use a little six inch instead.

"Thump" is not really what I'm after. I've never been that technically precise with balancing subs and mains. I generally set a sub level much much lower than the average listener in order to just imperceptibly fill the lower end for richness. I've always been amazed what a tiny little level of sub addition it takes to widen and fill a soundstage, even such a relatively poor one to begin with as this.

Last edited by peace brainerd; 7th January 2013 at 12:50 AM.
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