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diblet 7th May 2010 06:35 PM

how can you convert a speaker out to line level out?
i have some older analog sources i would like to connect to my mixer (portable record players / reel to reel players) that only have speaker outputs.

when plugging these into the mixer, the signal is way to loud and causes a lot of overdrive - and when i turn the volume down enough to get rid of the overdrive i get some sort of tone that drowns out the signal.

so i figure i must need to create some sort of box that i can plug in-between the units to convert the speaker out into a line level out - how can i go about doing this?

can you make a universal box that i could use with all my units (maybe with a pot to determine how much the signal is attenuated?) - or would you recommend modifying all the units internally somehow?

thanks for your help on this

ChristianThomas 9th May 2010 02:47 PM

The gain of most amplifiers is in the region of 26dB or x20. Some are a bit higher. The simplest and perfectly acceptable way of doing this would be to make a potential divider. Either a 5k or 10k pot and a resistor 10 or 20x as large. The pot goes to ground with the voltage taken from the other side of it. The resistor connects between +ve and the pot. Put it at the mixer end and use shielded cable after it.

A 10k pot with 100k will give you nominally full output at halfway. If you need more or less gain you can move it eaither way.

bob91343 9th May 2010 05:04 PM

The problem with that solution is that it depends on the setting of the control. A better idea might be to make a dedicated amplifier with constant gain. This could be very simple and of as high quality as desired.

Another problem is that the output of many amplifiers has more noise and distortion than the signal and your line output will reflect that.

If I knew more details of your setup I could offer a solution. Basically what you'd do is take audio from before the volume control and feed it to a small amplifier (or signal conditioner if you will) to supply a leveled output. You will need one such device for each source, but it could be built into those units.

Something as simple as an emitter follower (or cathode follower if tube) might do it.

Pano 9th May 2010 09:53 PM

Here is the easy, cheap way. Go to Parts Express. Look for part #269-050
Speaker Level Adapter Costs 75 cents.

Graft a plug or spades onto the end of it and "Bob's Your Uncle." :)

Xoc1 9th May 2010 10:55 PM

I would use a DI box to acheive what you want to do.
Features to look out for would include:-
Switchable attenuation to allow inputs at speaker levels.
Ground lift to avoid any ground loop hum.
A transformer coupled output isolates the balanced signal output, allowing you to use phantom power for any mics that are connected to your mixer.
Active or passive types would be suitable in this case - Active DI boxes are more flexible, as they can also be used with high impedance sources, like an peizo guitar pickup.
Prices vary, more expensive units tend to be more robust for stage use.:)

diblet 3rd June 2010 05:26 PM

thanks for all the advice guys - i have a di box, and while it works, the suggestion from christian for a voltage divider ended up fitting the bill. (i need the di for other purposes)

i put a 10k in series with the (+) and a 100k to the ground - 10:1 works like a charm.

i only worry that i could do damage to the output transformer by not using a speaker... the jack is a closed circuit and subsequently cuts the speaker when anything is plugged into the jack - should i re-wire the jack to be keeping the speaker on?

Th3 uN1Qu3 3rd June 2010 08:34 PM

Well if you're not using the speaker then the load is lighter so you'll actually be getting better bandwidth through that transformer. No risk of anything breaking. :)

diblet 3rd June 2010 09:26 PM

i thought i remember being told to always run an amp with the speaker load in place -- or is this only for tube amps?

bob91343 4th June 2010 01:01 AM

If you have an output transformer with no secondary load, when the tubes or transistors that drive it go into cutoff with very large signals, there can be an inductive surge that, in rare cases, will cause arcs inside the transformer. And once you have an arc, the insulation has been compromised.

A solution that works well is to use a dummy load. It doesn't have to equal the usual speaker impedance; 50 or 100 Ohms is fine.

diblet 4th June 2010 03:29 AM

great, i'll make sure to use a dummy load when the speaker is cut out of the circuit then - thanks

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