Amplifier for antique speaker?

Posted: 3/29/2015 8:33:08 PM

From: Denver, Colorado, USA

Joined: 9/6/2009

I went to the Vintage Voltage Expo today, here in the Denver area, and found this beautiful Brunswick Model A loudspeaker. It was made around the same time as the original RCA theremin. It has a big horseshoe magnet behind the cone and some sort of a transformer. I would like to use it with my theremins, but the line level signals from my theremins aren't quite strong enough to get adequate volume from the speaker. Can anyone recommend a small, monophonic amplifier that will boost a line-level signal to an appropriate speaker-level signal? Ideally, I would like one that runs on batteries and is small enough to place inside or behind the cabinet. 

Posted: 3/29/2015 8:40:39 PM

From: Colmar, France

Joined: 12/31/2007

Hi Rick,

in order to guide you towards a fitting amplifier, some more info would be needed. Today's speakers have a quasi-standardized impedance of 4 to 8 Ohms and most amplifiers are designed to deliver a matched small voltage/high current ratio. Older speakers had most times much higher impedances (up to 800 Ohms) so that they could be driven with high voltages and very small currents by a specific tube circuit.

I'll google a bit to find more information, but you could help me by looking at the speaker's internal transformer if there is a part n° or something like that.

Edit: Looks like the Brunswick Model A speaker works in the same way as the RCA 106 speaker, with separate field and voice coils. If that turns out to be true (does the speaker need a separate 110V supply?), you may forget about battery operation...

Posted: 3/29/2015 9:06:06 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Wow spring is bringing everyone out, even my tortoise.

What I would try first is just direct drive (bypass trans) the old speaker from a modern day solid-state amp. With the old speaker higher impedance no way will this damage anything.

The other way around old tube amp to modern speaker would be a problem!

I have ordered these for pitch preview, nice design good for batteries. Leave the jumper off for a 20:1 gain.

1 Watt Amplifier    A matching transformer added would be better but try without. 1 Watt is loud in single room.

* * *

10 Watt Amplifier This would be good with a larger battery. (Motorcycle?) Can the speaker handle 10 watts?

My humble opinions (-'


Posted: 3/29/2015 9:22:40 PM

From: Denver, Colorado, USA

Joined: 9/6/2009

It does work with a line out signal, but the volume is low.  The headphone output from a Moog Etherwave Plus is quite a bit louder, but not enough.  

I'll give that 1 watt amplifier a try.  Thanks!

Posted: 3/29/2015 9:23:50 PM

From: Denver, Colorado, USA

Joined: 9/6/2009

Thierry,  I couldn't find any part numbers on the inside.

Posted: 3/31/2015 5:14:22 AM

From: Denver, Colorado, USA

Joined: 9/6/2009

I removed the badly damaged, original speaker cable, then connected an audio transformer directly to the speaker.  As a test, I hooked up the transformer to an amplified computer speaker system and played some audio from my smart phone.  The speaker sounds really good and the volume should be fine for my purposes.  For authenticity, I am getting an antique-style, fabric-covered speaker cable to connect to the amplifier I've ordered for it.

It will be fun to use the speaker with my theremins and to play old-time radio shows through it.

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