Need to Convert Speaker-out to Line-out

[Alex Ethridge]

This is my first time here and my first time to post. I am not an electronic experimenter and beyond Ohm's law, I know very little about assembling electronic circuits.

I have a project I'm working on that requires me to get the sound from a 8-ohm speaker-out trerminal on an old sound movie projector to the line-in on a VCR. I have no desire to build a circuit like this myself as one mistake could end in the ruining of a perfectly good VCR. I am looking for something commercially available that has already been built and tested by experts.

Some time back, someone told me I needed an attenuator (spelling?) cable. I got one; but, I am afraid to use it as I subsequently discovered that an attenuator cable is supposedly for converting speaker-out to mic, not speaker-out to line-out.

I have searched the internet and have come up with nothing. Perhaps I don't know what search terms to use. In any case, I'm lost on this.

Can someone here help me?

 

[Nigel Goodwin]

All you probably need is a simple resistive attenuator, this is probably what's in the lead you bought. The only potential problem is how the projector might be wired.

As you already have a lead, I would plug it in and try it - if it's for a microphone input the only difference is the degree of attenuation, but the AGC in the VCR (and turning the volume up on the projector) should take care of that.

 

[Alex Ethridge]

So then, you think I won't damage anything if the attenuation is off--even if it is off quite a bit? I ask this because what liittle I know is enough to make me wary of making a mistake that will blow some part of the circuit in the VCR. I do know that the output intended for an 8-ohm speaker is quite a lot different from line-out.

It's just a cheap VCR; but, I would hate to make a silent movie player of it.

 

[Nigel Goodwin]

So then, you think I won't damage anything if the attenuation is off--even if it is off quite a bit? I ask this because what liittle I know is enough to make me wary of making a mistake that will blow some part of the circuit in the VCR. I do know that the output intended for an 8-ohm speaker is quite a lot different from line-out.

What output power is the projector?.

If it's only low power, there won't be a lot of volts out of it anyway, for example 8W into 8 ohms is only 8 volts, 1W is only 2.8V. These figures are for full volume, as you turn it down they fall rapidly, at low volumes you are probably only millivolts - you could simply use a straight lead and keep the volume low, but nicer to use an attenuator.

The attenuation can vary over a massive range without problems, if the lead is for a microphone it will most probably have too much attenuation, but will probably still be fine.

 

[Alex Ethridge]

I don't know the power output in watts. I do know it is 8 ohms. This is an old Kodak, Dual-8, sound movie projector made back in the mid- to late 1970's. It was made for home use so I would imagine it has a relatively low wattage. I have the original manual; but, it doesn't mention specifications. It just says it is the (one-quarter-inch phono plug) external speaker terminal. When I plug the phono plug into place, the projector's internal speaker goes silent.

I'll use the attenuator cable I got from Radio Shack and start with very low volume. I'll get back to you with my result.

 

[Alex Ethridge]

It worked just fine. Sound is good but not excellent; but, that isn't the fault of the connection.

That brings up another subject: This old projector's sound system offers no bass/treble control. Is there a device I can add to the connection that will give me some control over that? The sound on these old films is a little muffled and I think if I could add some treble or reduce the bass a little, it might be a little easier to understand the voices.

 

[Nigel Goodwin]

It worked just fine. Sound is good but not excellent; but, that isn't the fault of the connection.

That brings up another subject: This old projector's sound system offers no bass/treble control. Is there a device I can add to the connection that will give me some control over that? The sound on these old films is a little muffled and I think if I could add some treble or reduce the bass a little, it might be a little easier to understand the voices.

You could always build a tone control circuit, there are plenty of suitable designs on the net, but it's probably easier to use something you already have. Do you have a HiFi stero amplifier?, you could use the tone controls on that - the only problem being that the tape output sockets are usually after the tone controls - but you could always feed from the speaker outputs, just like with the projector.

A 'nicer' solution though, would be to use a small audio mixer with tone controls, this would also give you the opportunity to add extra commentry or background music to your recordings.