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Old 23rd Sep 2022, 8:22 am   #1
llama
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Default Stylophone dud speaker

A recent family conversation had "Stylophone" mentioned so, knowing I had at least one of them, I popped a PP3 in it. No sound.

I plugged an amplifier into the line out socket and everything worked as it should, but no sound from the speaker. It was open-circuit. It's clearly marked 75 Ohms - I guess that means super-thin voice-coil wire so more vulnerable.

I downloaded the circuit from the Internet, having determined that I'd got the PUJT version. The audio output stage is a single transistor driving both the line-out socket and the 75 Ohm speaker.

I don't think I own a 75 Ohm speaker but found an identical footprint 8 Ohm speaker that'd come out of an old PC. I suppose I could have re-designed the output stage to directly drive this speaker but lack the skills to do so - at least in a sensible time-frame. So I thought of using perhaps a dual or quad op-amp all paralleled-up to drive it, taking advantage of the class B output stages. In the end I thought a single 741 might do it so quickly knocked-up a voltage-follower and fed the speaker via a 100 Ohm resistor to limit output current. This worked but was too quiet. So I gradually reduced the series resistor and ended up with just a wire.

A bit of tidying-up and the boxed up Stylophone now works to my satisfaction and possible use by the grandchildren.
Graham
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Old 24th Sep 2022, 11:41 am   #2
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Default Re: Stylophone dud speaker

This is a good solution using parts that were to hand.

Another possible solution could have been a small output transformer from an old pocket transistor radio. Although not intended for the standing current of a single transistor output stage, it would probably work OK.
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Old 24th Sep 2022, 2:49 pm   #3
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Default Re: Stylophone dud speaker

An LM386 could also be used. These can work with a very low component count if the application isn't very demanding.
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Old 8th Oct 2022, 4:23 pm   #4
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Default Re: Stylophone dud speaker

Are 64 Ohm speakers still available? If so then I wonder if one would be a suitable replacement.
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Old 9th Oct 2022, 8:57 am   #5
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Default Re: Stylophone dud speaker

Yes, CPC have this one :- https://cpc.farnell.com/cqr-componen...ohm/dp/LS01466
and I can confirm that it works well - however, when I was looking for a replacement for my Stylophone, I found out that it's too big unfortunately..

Andy
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Old 9th Oct 2022, 7:28 pm   #6
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Default Re: Stylophone dud speaker

What is the actual speaker diameter required? Small medium impedance speakers are often used in the handsets of door entry phones.
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Old 9th Oct 2022, 10:48 pm   #7
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Default Re: Stylophone dud speaker

I think it's 57mm/2.25".
At least, that's the size of the one in my original Stylophone.

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Old 10th Oct 2022, 10:48 am   #8
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Default Re: Stylophone dud speaker

Yes,it's 2.25 inches diameter, overall. No fixing flange, just held in by a plastic ring. The PC speaker on the other hand was held in place by a plastic bracket fitted onto the speaker magnet.
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Old 10th Oct 2022, 11:05 am   #9
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Default Re: Stylophone dud speaker

I've just looked at one of the speakers in the handset earpiece of one of our door entry phones. They are 50 Ohm, so not too far out impedance wise but unfortunately only 40mm diameter, rather too small in this case.
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Old 10th Oct 2022, 4:50 pm   #10
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Default Re: Stylophone dud speaker

Try some old telephones. Some of those have 50-75 ohm speakers for the loudspeaker function, the business, multi line units particularly.
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Old 11th Oct 2022, 1:00 pm   #11
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Default Re: Stylophone dud speaker

Found one in a Norstar Tele 7100. 50 ohms and about 55mm diameter.
Photos available!
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Old 27th Oct 2022, 5:14 pm   #12
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Default Re: Stylophone dud speaker

Thanks for the interest in the topic guys. Not that I'm about to take it apart, but with this new wealth of information I'd now be inclined to try a 4T earpiece out of a 706/746 phone and see if it would be near enough replacement for the 75 Ohm speaker. A bit harder to mount but probably no worse than my little 741 mod.
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