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Wiring from stereo amplifier to active speakers

Postby Hol » Wed May 15, 2019 8:12 pm

Hi everyone,

Hope somebody could give me some guidance on this one:

Basically, I've bought a new TV, which only has an optical output. I've got an old active sub and two KRK Rokit 5's.

So, what I've done is take two XLR cables (one for left and right) - and a couple of phono/RCA leads. Strip the plugs off the phono leads, wire one end into a banana plug, plait the other ends into one wire - then strip an XLR cable, and wire that into the lead I've just created.

So 2x banana plug => 2 leads => 1x XLR => Sub => KRK's.

My plan is to plug this into the speaker outputs, into the sub, and then hopefully as long as I keep the volume down on the amp itself (input via the TOSLINK digital in) then I can't see why this wouldn't work fine. I did something similar a number of years ago with an old Marantz amp, ran it from the speakers and wired it into a large jack plug into a guitar amp (also powered, obviously) which seemed to work quite well.

Any thoughts on this? I've not actually plugged it in yet, waiting for the amp to arrive: another option if this isn't a good idea is I can just wire speaker out into phono in on the sub.

I saw some random post on the internet someone saying doing anything like this would cause everything to explode - but based on previous experience (as above) I can't really see why it would!

Hope someone can help - sorry for the long post! :)

Hol
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Re: Wiring from stereo amplifier to active speakers

Postby blinddrew » Wed May 15, 2019 8:34 pm

So you're taking the optical out from the tv to the sub, then taking the output from the sub to your active monitors?
Is the output from the sub a line level output or a speaker output?
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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Wed May 15, 2019 8:54 pm

Sorry if I wasn't clear :)

Optical out goes from TV into Yamaha amplifier optical in: output goes from amplifier's speaker outputs via banana plug to sub's XLR input, sub's XLR outs go to KRK's XLR inputs if that helps?
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Re: Wiring from stereo amplifier to active speakers

Postby Hugh Robjohns » Wed May 15, 2019 8:58 pm

There's a very good chance you'll destroy the speakers.

You might get away with it if you keep the level very low, but it will only need a slip of the finger or a glitch somewhere to send lots of Volts into something designed for only a couple of Volts, and then the magic smoke escapes...

There's also the issue that working this way will result in a seriously degraded signal-to-noise ratio,

Rather than buying a power amp you don't need and bodging the connections, you'd be much better off buying a straightforward stereo D-A converter to handle the conversion from the optical S/PDIF signal to analogue line level, and then feed a proper line level signal to your KRK in a sensible way.

Alternatively, if the Yamaha amp has a preamp line output or tape recording output, you could use one of those that to feed the KRKs. A tape record feed will be at a fixed level, whereas a preamp out will be controlled by the amp's volume control.

H
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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Wed May 15, 2019 9:11 pm

Thanks! - the thing is it would need a remote control for the volume, as when the TV runs optical out it disables the volume control, hence this workaround...I can attenuate the sub's input as well as the KRK's having a separate volume control which can lower the signal. Would that reduce the risk of providing too many volts sufficiently for it to not be dangerous?
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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Wed May 15, 2019 10:18 pm

Thinking about it, I can just as easily run the headphone output to the sub's input, which is exactly the way I had it set before from the old TV that had a headphone socket! And it worked like a charm for years :)
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Re: Wiring from stereo amplifier to active speakers

Postby James Perrett » Wed May 15, 2019 11:09 pm

Headphone output into the hifi is exactly how I have the TV set up here. It isn't a great idea to feed a speaker output into a line input although I've done it in the past with low power speaker outputs. If you really need to involve a stereo amplifier you need to find one with preamp outputs.
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Re: Wiring from stereo amplifier to active speakers

Postby ef37a » Thu May 16, 2019 7:03 am

"Optical out goes from TV into Yamaha amplifier optical in: output goes from amplifier's speaker outputs via banana plug to sub's XLR input, sub's XLR outs go to KRK's XLR inputs if that helps?"

So the sub has speaker level outputs on XLR? That is most unusual and asking for trouble (I know XLR 3 was used on some very early band PA amps but the practice died out pretty quickly).

However, if indeed you need to feed the KRKs inputs from power amps, well, you seem a practical chap so put a 10k stereo log pot in a tin, XLR "tails" then advance slowly from zero to get a good balance with the sub.

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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Thu May 16, 2019 10:39 am

James Perrett wrote:Headphone output into the hifi is exactly how I have the TV set up here. It isn't a great idea to feed a speaker output into a line input although I've done it in the past with low power speaker outputs. If you really need to involve a stereo amplifier you need to find one with preamp outputs.

Thanks James, it's exactly how I'd set it up if LG hadn't decided that in this day and age, headphone sockets were redundant - didn't even cross my mind that there wouldn't be one on the telly!

I did a spot of digging around last night and I do at least now understand *why* it's not a good idea now - speaker level outputs are much higher voltage than line level inputs :) ahhh the learning never stops..... :)
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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Thu May 16, 2019 10:54 am

ef37a wrote:So the sub has speaker level outputs on XLR? That is most unusual and asking for trouble (I know XLR 3 was used on some very early band PA amps but the practice died out pretty quickly).

Just line level I think Dave (steep learning curve over the last 24 hours with terminology here, so bear with me!) a choice of phono or XLR - it's a standard "bedroom producer" subwoofer with a crossover, the whole thing's supposed to be powered by line level outputs from an outboard USB/Firewire sound card/DAC/mixer/whatever. All designed to be plugged into active speakers after its gone through the crossover.

The sub's been out of production for an awfully long time now, but there's still a manual floating about here: https://www.zikinf.com/manuels/tapco-sw ... -46200.pdf

ef37a wrote:However, if indeed you need to feed the KRKs inputs from power amps, well, you seem a practical chap so put a 10k stereo log pot in a tin, XLR "tails" then advance slowly from zero to get a good balance with the sub.

Dave.

Wasn't sure if you were joking or not about that so I looked it up - I might have some learning to do before I try something like that! Cheers though ;) *EDIT presumably this would only work at a fixed volume out of the amplifier though, turning the amp up could then blow the speakers anyway?
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Re: Wiring from stereo amplifier to active speakers

Postby MarkPAman » Thu May 16, 2019 11:35 am

If going from speaker outputs a pair of http://artproaudio.com/product/av-direct-audio-video-direct-box/ would make things a lot safer. Though you're then nominally at mic level and may need to turn your speakers up quite a long way....
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Re: Wiring from stereo amplifier to active speakers

Postby ef37a » Thu May 16, 2019 12:20 pm

No, you are not going to blow anything up Hol going for a pot.

I am a bit confused (old, meds) but I shall just crash on! Say you wanted to drive the KRKs from an amplifier rated at 50W into 8 Ohm speakers? That is a voltage of 20V (rms but don't fuss that atmo) Now the active speakers are unlikely to need more than TWO volts rms for full output so you could build a resistive attenuator to give a X10 loss (20dB to keep you larnin!)
The snag is, you can never be quite sure how loud you want the amplifier to be, the sub in your case so, a potentiometer is just an attenuator that allows you to "fine tune" the loss.

As I said, start with the pot at zero then run the system at the level you most often use. Probably set the monitor controls about 1/2 way then slowly turn the pot up until you get the desired sound level.

Actually, when I get a lot of "life/wife's heal t***t fixed I intend to do something very similar. I have a quite decent SEAS 4" centre speaker driven from a Panasonic surround sound DVD player but I suspect the DVD power amps are not very good, early class D jobbies. I shall therefore attenuate the level and drive the SEAS from a very clean Maplin 100W MOSFET amp I built yonks ago. (drove a 200W 15" bass cab at Towcester racecourse pavilion no sweat a few years ago) I also have an amp made with two ILP modules, will use that similarly for the L/R outriggers...Or I might just buy an AV amp!

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Re: Wiring from stereo amplifier to active speakers

Postby ef37a » Thu May 16, 2019 12:26 pm

Ha! Just looked at that (very good) user handbook.

I shall let someone else point out the error of my ways!

Too much ***t to sort out.

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Re: Wiring from stereo amplifier to active speakers

Postby Hol » Fri May 17, 2019 4:00 pm

Thanks everyone for the input and the learning points - I now have a Yamaha AS 301 plugged into my sub via the headphone output, and it's sounding fantastic! :bouncy:
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