Using Quickie to drive a prewar loudspeaker

[Unclemanly]

Hello everybody. Here goes, first post.

I've got a 1920s loudspeaker for which I'd like to build a small battery powered amplifier. Early speakers were very sensitive, high impedance and often designed to run directly off an amplifier's output valve, so the Quickie seems a real option. This is what I've got:-
 
http://www.radiomuseum.org/r/philips_2007.html [nofollow]

The speaker's impedance is switchable between 500, 1500 and 2000 ohms. Its maximum power is 0.3W.

I currently drive it from the headphone socket of a modern hifi amp but wouldn't it look sensational with a Quickie standing next to it? I've already tried it with a cmoy but I get distortion before the volume gets anywhere near high enough.

So my question is, is the output from a Quickie sufficient to drive the speaker and would I need to modify the amp to do so? Since the Quickie is a stereo amp and I've only got one speaker, can I connect the two channels of the amp in series to double its output? If so, is it sufficient to simply take the ouput from one channel and feed it into the input of the other, or is there more to it than that?


Questions, questions....


Rob

[Grainger49]

Interesting!  I think I saw something like that in Huntington W VA.

The specs say no power needed.  I don't know why but maybe you can take it on face value. 

The Quickie is not an amplifier.  It is a preamplifier.  Therefore I doubt it would be even advisable, much less effective, to put both channels in series.  But you could parallel the inputs and outputs.  It doesn't produce any power just an increased voltage swing compared to the input.

To test the "theory" that the speaker requires no power wire your source into it.

[Unclemanly]

Interesting!  I think I saw something like that in Huntington W VA.

The specs say no power needed.  I don't know why but maybe you can take it on face value. 

Thanks for replying. In this case "no power" is a red herring. It simply means that its a permanent magnet speaker, just like any modern speaker. Other early speakers used electromagnets and needed a supply to the magnets as well as the audio signal.

Rob

[Paul Joppa]

Into a 2K load, a stock (unmodified) Quickie will make a hair over 1mW of power - that would be about 1.5 volts RMS.

You can get 3 or 4mW with a matching transformer and re-optimized operating point, and maybe 10mW if you go all out with paralled tubes in pentode mode. That's only another 10dB, calls for a new design and transformer, and loses the triode damping factor. I don't think that would get you enough loudness.

If you look up the data sheet for the 3S4 tube, you can get as much as 270mW in pentode mode with a 90v supply. That's probably your best bet; you could double the 9-v batteries for 72v and seek a 5K:500 ohms output transformer that will take 7mA of DC, operating the speaker at 500 ohms. That transformer may be hard to find! There's a circuit for a portable radio in the back of the RCA tube manual (mine is RC-19) and I am just describing the last stage of that.

[Unclemanly]

Well thanks for that Paul. Realisticly it looks as though I will need to go down another route for a small amp to drive the speaker. Probably something solid state. Ho hum.

Rob

[Paul Joppa]

300mW into 2000 ohms is 25 volts RMS, which is 70 volts peak-to-peak. Unless you use a step-up transformer, nothing can do that without at least 70 volts of power supply, probably at least 100 volts in practice. (At 500 ohms, half the voltage and twice the current.) Just for perspective...

[Unclemanly]

300mW into 2000 ohms is 25 volts RMS, which is 70 volts peak-to-peak. Unless you use a step-up transformer, nothing can do that without at least 70 volts of power supply, probably at least 100 volts in practice. (At 500 ohms, half the voltage and twice the current.) Just for perspective...

No future in persuing this route then?......

http://beavishifi.com/projects/LittleLebowskiUrbanAchiever/ [nofollow]

Rob