Intercom

[Twiggy]

Hey guys, i'm building this intercom, but I am new to the whole audio game, so first question - will this circuit work? Second - can anyone recommend a suitable speaker from Maplin.

 

[flat5]

120 ohm speaker will be hard to find.
Use small audio transformers with 8 ohm speakers.

 

[Twiggy]

thank you very much... the circuit works then?

 

[Twiggy]

specs for such an audio transformer?

 

[flat5]

Look down the page at the second circuit.
82 ohm to 8 ohm is the one recommended.

Try anything you can find. 82 ohms or more on the primary side. If the ratio is too high you can add a resistor to lower the gain.

I can not say if the circuit works. It looks very good to me.
Quite clever.

 

[zipdogso]

I looked at this I thought of having a go at the circuit myself only one snag the speakers...

Maplin had some 64 ohm ones cat. no. YT27E but they don't appear to be in the new catalogue and are listed Out of stock so there probably wont be any more.

Rapide have some 80 ohm PCB mount speaker sound generators which might fit the bill perhaps...don't know how good they will be they are only 62p each.

As for the transformers...well not much luck there either vast majority of audio transformers are 1:1. So I have sort of given up.

Considering the low cost....next time I make an order for rapide I might add on a couple of those sound generators and see what happens since the circuit is so cheap to build....not much to lose if it's carp.

 

[tunedwolf]

Most likely get away with using a small 2-3W 100V line transformer, although size then becomes an issue. Perhaps a winding your own tx on a small RM10 core would suffice.

rgds

 

[flat5]

Google:
miniature audio transformer

 

[zipdogso]

Google:
miniature audio transformer

Yeah...and ?
most of them are 1:1

Do you really think I had'nt tried that ?

An audio transformer manufacturer said - I quote "We obviously could make this using our custom service but as you suspect it would be rather expensive. (£82 each). If you can get an old transformer form a piece of surplus equipment you could try winding it yourself. I would be happy to advise in the interest of promoting the hobby."

This is a possible but not really an idea I am interested in...

 

[flat5]

First hit was a 1000 ohm ct to 8 ohm.
Standard item years ago.
Probably good enough.

I did not spend any time trying to figure how to use the page but try here:
Audio & Signal Transformers

 

[Twiggy]

anyway to do this without a transformer, (amplifier?)

 

[Nigel Goodwin]

anyway to do this without a transformer, (amplifier?)

Use a completely different design, this one is completely dependent on high impedance speakers, or low impedance with a transformer (same thing).

But why not buy a couple of transformers from Maplin?, oder code LB14Q at £1.73 each.

 

[zipdogso]

But why not buy a couple of transformers from Maplin?, oder code LB14Q at £1.73 each.

Because neither are what the circuit demanded...since I have no doubt you know a lot more than me how about enlightening me on how I could use these...

and looking at the specs you are quoting the wrong ones anyway...I thought the primary had to be 82ohm not 12K

 

[transistor495]

Modern Intercom

The intercom circuit looks like a good one with feedback howling is eliminated by using loud speaker for double purpose. However it is a very old one and the author himself quoted that he already had many years of satisfaction from it

I don't think nowadays anybody go for a fresh assembly using an audio matching transformer which is obsolete.

This Intercom circuit seems to be a good one where the Larsen effect is reduced by a much clever design. Audio quality should be very good and it doesnt require switching over.

 

[Nigel Goodwin]

Because neither are what the circuit demanded...since I have no doubt you know a lot more than me how about enlightening me on how I could use these...

and looking at the specs you are quoting the wrong ones anyway...I thought the primary had to be 82ohm not 12K

Those transformers are perfect for the job, the primary is centre-tapped (for push-pull), use one half of it as it's a single ended amplifier.

 

[flat5]

Hi transistor495,

That is a good solution too. I'm glad you showed us that circuit.

 

[Twiggy]

Those transformers are perfect for the job, the primary is centre-tapped (for push-pull), use one half of it as it's a single ended amplifier.

very sorry but i am going to go with a couple of those maplin transformers, however i understand none of that. please explain in Newbie lingo.

 

[Nigel Goodwin]

very sorry but i am going to go with a couple of those maplin transformers, however i understand none of that. please explain in Newbie lingo.

The primary has three wires, a beginning, a middle, and an end. Use the middle and one of the other wires as the high impedance side. The speaker itself connects to the other side with only two wires.

 

[Twiggy]

Brilliant, thank you very much.

 

[transistor495]

Hi transistor495,

That is a good solution too. I'm glad you showed us that circuit.

I'm glad that we've a veteran electronics expert voice from The Netherlands