Dropping speaker levels to line levels

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tom hardwick
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Dropping speaker levels to line levels

Post by tom hardwick »

Hi Team,

I'm after some sort of converter box into which I can feed my amplifier's speaker cables, and have the box convert this down to line level to feed my sub woofer. Any units out there you know of?
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John Roberts
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Re: Dropping speaker levels to line levels

Post by John Roberts »

Hi Tom, this can be a bit tricky!

I assume you are talking about a 'domestic' stereo Hi-Fi amplifier's speaker outputs as opposed to a mono guitar or bass amp? There are a myriad of converter boxes available on the like of Amazon and eBay which claim to do that. There are a number of issues to watch out for though.

The main issue is the coupling of the 'earth/common/0V/negative' connections (call it what you will) of two amplifier outputs - some class of amplifiers do not take kindly to having their 0V rails connected together and this can lead to earth loops at best or catastrophic failure at worst. Many budget converter boxes use electronic components which utilise a common 0V rail, but if the amplifier that's being connected to it is not designed using the same configuration, it could spell disaster for the amp. If both left and right output channels are required, then a safer bet might be to use a converter with transformer based coupling, completely isolating both amplifier channels and line level outputs.

The good news is that almost all sub bass frequencies do not have any significant stereo separation, and therefore taking a feed from either the left or right channel should provide sufficient sub bass content. Using just one channel of a generic speaker-to-line converter box avoids the possibility of amplifier damage and should work well. Some generic converters also include a small preset control to allow the line level output to be varied according to how loud the amplifier feeding it is set.

If you still require a stereo sub bass feed into your (assumed) mono sub bass unit, a dedicated 'Y' type cable could be used to 'sum' the left and right line level outputs (i.e. a two phono to one phono lead) although this might require a small resistor, say 1K, connected to each of the outputs to avoid shorting them together. This depends on the circuitry of the line level output, and I have done exactly this in my own studio but did not need any additional resistors in the cable as they were already built-in to the left and right audio output feed of the mixing desk I am using.

One other thing to bear in mind is that when using a generic converter, a full-range frequency spectrum will appear at the line level output, and therefore care needs to be taken if the sub bass amplifier itself does not have any built-in filtering to remove unwanted high frequencies. Some sub bass units have adjustable low pass filters set to 200Hz, 100Hz and 50Hz etc, so they can be better matched to the other speakers in the system. It all depends how 'audiophile' you want to be!

Hope this helps :)
tom hardwick
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Re: Dropping speaker levels to line levels

Post by tom hardwick »

Thank you so much for your very comprehensive reply, John. Of course I'm disappointed to know that it's not simply a £5.99 eBay tin box I need, but it might help if I give you more details.

I'm old fashioned enough to want to play my collection of CDs, MiniDiscs and cassettes, and I have a neat little Sony MD373 that does all of this and more. The supplied speakers are 'ok', but the addition of a bass box (sub), improves things enormously. I'm no audiophile as you can tell.

My AEGO-M sub with its built in power amp and tiny little satellites brings the music alive, but this AE unit needs to be fed line levels. That's ok, I can feed it from the tape in sockets, but the volume control is now on the sub, and the sub doesn't have a remote control facility.

Ergo - if I could feed it speaker levels from the Sony mini stereo I could use the Sony's remote, effectively varying the line level that the AE receives. Make sense? And does that clarify things and make you suggest alternatives?
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John Roberts
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Re: Dropping speaker levels to line levels

Post by John Roberts »

Hi Tom, your methodology of connection makes perfect sense, and in reality connecting the Sony MD373 to extra speakers via its own speaker outputs is the only way to achieve the control you are after. My cautions above were intended for certain - possibly higher powered - amplifiers, whose output stages are designed in such a way that the 'negative' speaker connection is not connected to 'ground.' Some amplifiers are configured in what is called 'bridge mode' that in effect uses two amplifiers per speaker; one amplifier handling the positive half of the cycle and the other amplifier handling the negative, and each of these amplifier outputs are connected to their respective speaker terminals. Connecting the negative left and right speaker outputs together usually results in an early November 5th display!

However, the good news is that like most low to medium powered units, the MD373 uses a more traditional amplifier setup of an IC-based power output stage. Because effective cooling is an issue, most IC amplifiers do not pass high power and therefore usually work in push-pull as opposed to bridge mode. This means that a common 'ground' is very likely. I checked the schematic for the MD373 and indeed it is the case:

Image

If you're interested, the negative speaker outputs (marked SP-L GND and SP-R GND at top of the far left of the diagram) can be traced back via the thick black lines to a common point towards the right of the diagram, and on to CT POWER which is the Centre Tap of the power supply. This is the 0V rail or 'ground' that will be common throughout the MD373.

Having established that any inadvertent coming together of the negative speaker outputs should not cause any issues, most of the speaker to line level converters should work OK. One found on Amazon that seemed to get good reviews, is 'passive' and therefore does not require any additional powering up, has individual left and right trim controls and has been used in exactly the same scenario as you are intending, is on the following link:

https://www.amazon.co.uk/Speaker-Conver ... B004UA753G

Hope this helps, Tom :)

By the way: no liability accepted for an early fireworks display :lol: :wink:
tom hardwick
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Re: Dropping speaker levels to line levels

Post by tom hardwick »

It certainly does help John, and many thanks for all your care and attention to detail.

An associated question. If I do feed the powered sub (with its own satellites) then of course I don't want speakers connected to the Sony MD373 amp outputs. So is it safe to just disconnect the Sony's speakers and leave the power amps open circuit as it were?
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John Roberts
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Re: Dropping speaker levels to line levels

Post by John Roberts »

Hi Tom, disconnecting the speakers should be OK and I can't foresee any problems. Most IC amplifier stages have protection circuits built in to safeguard against short circuits or low impedances, and an open-circuit output shouldn't be a problem.

However - if you have no intention of using the MD373 speakers, have you thought of simply running a cable from the headphone socket of the MD373, which will be post volume? A 3.5mm stereo jack to 2 x phono will work just fine to connect the MD373 to your sub amp system, and might be something you already have in your toolbox. Using the headphone output will usually disconnect the speakers as a matter of course anyway.

I use the same procedure for connecting a multitude of items to others, and because the input impedance of the sub amp system is greater than the usual impedance of headphones, there will be no loss of gain or additional loading on the headphone output.

Just a thought :D
tom hardwick
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Re: Dropping speaker levels to line levels

Post by tom hardwick »

And a good thought on that John; thank you for all your help.
Now it looks as if I would only ever need the Amazon spkr>line level box if I wanted remote control of the head unit. Guess I'm sorted.
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John Roberts
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Re: Dropping speaker levels to line levels

Post by John Roberts »

No problem, Tom - glad to help where I can.
tom hardwick wrote: Sun Jan 09, 2022 12:47 pm Now it looks as if I would only ever need the Amazon spkr>line level box if I wanted remote control of the head unit. Guess I'm sorted.
Not quite sure what you mean by this, though. I've checked the schematic again and as I expected the headphone output is taken directly from the amplifier output stage, via a couple of resistors to drop the high speaker level down to a more manageable signal for headphones. Therefore, everything that is happening at the speaker outputs is also happening at the headphone output, including all your remote control functions of volume and tone etc.

In effect, the headphone output is almost doing the same job as any generic speaker to line level adaptor box, because, apart from a slight mismatch with impedance and signal level, a 'line level' is what will appear at the headphone output. The only reason to use a speaker to line level adaptor box is if you wanted to use the speakers on the MD373 as well as your sub and satellite speakers, because using the headphone output will disconnect the MD373 speakers. Because this is what you are after anyway, I would say just plug into the headphone output and all should be well :D


As an aside - I used to use minidiscs a lot in my studio, especially as media carriers for live performances. At one time I also used a Yamaha 8-track multitrack recorder that used datadiscs (the double capacity minidiscs). Minidiscs really were fabulous, I thought - the ability to add and delete songs, rename them, re-order them and cut/paste/split and edit them etc was something no other media could do at the time. The players (and recorders) were small and I never had any issues in the years I used them. Possibly the most advanced hardware-based media carrier ever? I still have all mine!

John R
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