TELEPHONE QUESTION - Ringing a Western Electric 302 with power amp - clarification

[BCAP]

Hello, I have a production coming up very soon where I would like to ring an old Western Electric 302 phone using a power amp.


I would first like to reference this historical thread here, as I am looking for some clarification, and earnest thanks to those who contributed to that thread:

Anyone using Amp to ring phone?

I've seen several threads on using an amplifier to help generate a ring signal to old style rotary phones. I've looked at the circuit diagram here: Telephone ringing circuits I have an amp, a Sony. I have a transformer, a Radio Shack model: 6v or 12v to 120v (mounted backwards). I have a...

www.controlbooth.com

I would like to use an old QSC RMX850 which is 200 watts per channel into 8 ohms, 830 bridged into 4 ohms. My experience so far trying the bridged method with no transformer, a stereo input signal, 20Hz sine with + polarity on left channel and - polarity on right has so far not been able to ring the phone. I have not measured the output voltage yet, but I believe that the voltage generated on the amp output is not enough to ring the phone.

So hence, my questions.

Referencing from the original thread here:

Telephone ringing circuits

Telephone ringing theory and circuits.

www.epanorama.net

Can anyone recommend a make / model of transformer? It seems to me that a Hammond 266PA12 should get me enough voltage out to ring that bell if I wire the secondary to the output of the power amp with 10 ohm resistor in series and the primary to the phone, with a 1000 ohm resistor.

266PA12

Hammond Manufacturing is a leading manufacturer of industrial enclosures, electronic enclosures, racks & rack cabinets, transformers, outlet strips and climate control products.

www.hammfg.com

That said... that is a pretty expensive transformer at $70. The good thing is it can handle a lot of power.

I wonder if I could hack it with a standard US doorbell transformer I can find at Home Depot for $14.95 120V to 16V. I am not so sure those can handle as much power as the Hammond, but it might work.

Someone on the original thread recommended an 100V distribution transformer (presumably for 100v/70v speakers) - again would like to ask for make / model recommendation here if possible. My inclination so far from searching is that those are also pricey.

Next question - my 302 is pretty beat up. I don't mind if I have to disconnect the bell it will never be used as an actual phone. Is there any reason why I shouldn't just connect the output of the primary right to the bell leads red and black, if I would like to bypass the operation of the hook switch and all other features of the phone. Seems like it should work, but...


Thanks in advance. And yes, a recording of a phone ring is an option. This is a radio play, so I would prefer to do a live ring if I can.

 

[Crisp image]

All I know is here in Australia the old phone system before we converted to NBN was a line voltage of 50vdc and when the phone rang the voltage was boosted to 90vac. There was a capacitor involved somewhere but I cant remember where.

Here is a link to some information. This might be useful. How a POTS rings

Regards
Geoff

 

[Jay Ashworth]

90vac, 20Hz, for a standard non-partyline ringer.

There are boxes that generate ring current to spec, like this:

TELEPHONE RING GENERATOR for Stage or Screen Productions, Props, Tele Ringer $ q | eBay

This is the standard for ringing phones. generator using standard modular connectors. or any application where you need to have an accurate sounding ring.

www.ebay.com

or you could build one, if you're that sort of nerd:

Telephone Ring Generator

Schematic and brief description of a standalone generator of telephone ringer signals.

siber-sonic.com

 

[microstar]

A couple of other resources:
pretty much the community theatre standard unit http://www.tele-q.com

and a DIY component

Black Magic Telephone Ringing Generator Square Wave Model 5 Vin | eBay

Find many great new & used options and get the best deals for Black Magic Telephone Ringing Generator Square Wave Model 5 Vin at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[FMEng]

An 830 Watt amp only produces 57 Volts, and you need 90V, so that's why it won't ring. A phone ringer is a 7k ohm load, at about 1 Watt, so the current is negligible. Try almost any transformer, wired as a step up. The only trick is going to be find one that has enough efficiency at 20 Hz.

 

[DrewE]

The resistors in your proposed setup are quite unnecessary; the impedance seen by the amplifier is the impedance of the phone ringer, as scaled by the transformer, and plenty high enough for the amplifier to be happy. (It's not harmed or troubled by running open-circuit, after all.)

 

[BCAP]

Thank you very much for all the replies. My intent long term is to purchase a phone ringing unit such as the ones microstar and Jay Ashworth mentioned. In the short term, my show starts in only a few days and between holidays I'm betting the post office is overwhelmed so I think I am going to try the amplifier with a doorbell transformer in reverse as step up and see if I can get that to work. I don't even have time to have the resistors shipped so DrewE thank you for mentioning resistors should not be critical. I believe a doorbell transformer will not be very good at 20Hz, but it's worth a try I think in the spirit of getting something working in the time I have.

 

[Amiers]

Blutooth speaker hidden under the actual phone to give it that real ring effect without having to actually wire anything up.

 

[BCAP]

Thank you Amiers. I'm aware of all of the alternate options, bluetooth speakers, iPhones, playing a sound through the PA system, etc. and I appreciate the suggestion and will keep that in mind as a last ditch option. However, my intent here is to ring the actual phone. It's a radio drama.

Just an update as well, I switched to a more powerful amp, QSC RMX1450 bi-amped with a full on input signal can produce 9.7VAC on output, with a doorbell transformer and no resistors, up to 64VAC and that's not only probably not enough to ring but it's stressing the amp pretty bad I can hear it, but... it was a cheap experiment haha.

 

[Amiers]

Thanks Amiers. I'm aware of all of the alternate options, bluetooth speakers, iPhones, playing a sound through the PA system, etc. The point here is to ring the actual phone. It's a radio drama.

I’m aware. But unless the phone moves when it rings like in the movies you could achieve the same thing with a lot less headache and still make it look real. Through the PA no but a 15$ 10$ cheap chinese bathroom blutooth speaker playing a ring from a 1930-50s period spot on.

 

[BCAP]

Not to go off topic, but a long term wish of mine since I do a lot of radio shows (as the sound effects guy) is to build a ring / doorbell box with it's own power supply, self enclosed and push buttons, etc. Possibly 2 different sounding telephone bells, an alarm bell, maybe even a car horn or two in there. There are far easier ways of doing these days but I just like the thought of it. I see props like that at theaters from time to time and I think it's cool. They are no doubt a pain in the ass to carry around and maintain.

 

[Amiers]

Not to go off topic, but a long term wish of mine since I do a lot of radio shows (as the sound effects guy) is to build a ring / doorbell box with it's own power supply, self enclosed and push buttons, etc. Possibly 2 different sounding telephone bells, an alarm bell, maybe even a car horn or two in there. There are far easier ways of doing these days but I just like the thought of it. I see props like that at theaters from time to time and I think it's cool. They are no doubt a pain in the ass to carry around and maintain.

Agreed. I was one tasked many years ago with making a desk lamp period piece work wirelessly with a chain switch. So it didn’t require power and just a battery. So through trial and error and lots of shocks. I converted a disposable camera circuit to power a CFL lamp for a 2 second chain pull. I prolly invested a weeks worth of time for it just trying to figure out how to squeeze everything into the fixture. Fun times in the 2000s.

 

[DrewE]

The RMX1450 should provide a lot more than 9.6V on its output. 9.6V rms into an 8 ohm impedance would mean a power output of about 11.5W. Either your input signal isn't sufficiently strong, or whatever you're using to measure the output voltage is not capable of properly measuring the waveform or frequency you're using, or you have a high-pass (low-cut) filter selected on the amplifier. They do claim open-circuit protection for the amplifier, but I rather doubt that means shutting the channel down with no load--most common would just be having a circuit that's stable with no applied load.

(Modern power amplifiers are basically regulated voltage-controlled voltage sources. Their output voltage is more or less invariant with regard to applied loads, within certain limits--typically so long as the impedance of the connected load isn't too low. Thanks to this, the expected RMS output voltage can be computed using Ohm's law and the amplifier's power specifications, assuming the latter are at all realistic and not pure marketing fantasy.)

 

[microstar]

Not to go off topic, but a long term wish of mine since I do a lot of radio shows (as the sound effects guy) is to build a ring / doorbell box with it's own power supply, self enclosed and push buttons, etc. Possibly 2 different sounding telephone bells, an alarm bell, maybe even a car horn or two in there. There are far easier ways of doing these days but I just like the thought of it. I see props like that at theaters from time to time and I think it's cool. They are no doubt a pain in the ass to carry around and maintain.

Don't forget to include a door chime in your SFX box!

 

[BCAP]

Finally got it to ring. I removed the bell component from the phone and it works just fine connected to the doorbell transformer primary, but the amp has to be wide open in bi-amp mode. My input signal is from an iPhone - stereo sine wave file at full peak to peak, iPhone volume all the way up into the unbalanced ins of the amp.

This will work, I think for this show as a temporary solution though I acknowledge it's not the best way. I think this whole phone needs to be re-furbed anyway so I'm satisfied with this solution for now.

DrewE, I measured the output with an old Radio Shack multimeter and no load. I don't think I have any filtering engaged on the amp but I will check.

 

[microstar]

I’m aware. But unless the phone moves when it rings like in the movies you could achieve the same thing with a lot less headache and still make it look real. Through the PA no but a 15$ 10$ cheap chinese bathroom blutooth speaker playing a ring from a 1930-50s period spot on.

Yes, it is of course possible to use a recording, but by ringing the actual phone, you avoid the un-intended comic effect of the phone ringing after the person answers it. And it is always more realistic because..... it's real.

 

[FMEng]

The iPhone clearly doesn't have enough output to sufficiently drive the amp. If you are connecting the phone's TRS output to the TRS input of the amp, the amp is summing the two channels out of phase and cancelling the signal to near zero. You need a cable that breaks out the phone to unbalanced left and right.

Caution: If you correct the wiring, turn everything down before testing because you will have way more output than you did.

 

[almorton]

If you do end up building a ringer circuit, it may be worth considering adding in ringing frequencies and cadences for Europe and the UK, for added flexibility. We are UK based, but our ringer can switch between UK and US frequency and cadence, and we do use both (rarely in the same show, I might add).

 

[JD]

Standard Western Electric ring generator outputs 115 vac at 30 hz. 118 ring gen
Used to have a few around. Seen them for sale for $15. The big thing is the 30Hz. 60 will not work.
If you really want to be authentic, keep you eye out for a ks 19175 interrupter! 10vac motor and will give you the classic Bell System ring pattern. interrupter

 

[RonHebbard]

Standard Western Electric ring generator outputs 115 vac at 30 hz. 118 ring gen
Used to have a few around. Seen them for sale for $15. The big thing is the 30Hz. 60 will not work.
If you really want to be authentic, keep you eye out for a ks 19175 interrupter! 10vac motor and will give you the classic Bell System ring pattern. interrupter

I'm remembering when Bell system dial tones were 25 Hz generated by 25 Hz generators being spun by AC synchronous motors of the day.
In Canada the drive motors were 25 Hz then 60 Hz when our power grid changed from 25 to 60.

Internal PBX dial tones were 30 Hz. If you listened closely, your dial tone would drop in pitch when you dialed for an out line rather than an extension within your office.
Toodleoo!
Ron Hebbard