Output of Quickie in mW?

[Dr. Toobz]

I don't know all of the Quickie's "stats" to figure out the effective output in mW at 250ohms when I'm using headphones. Just curiosity, I suppose.

Also, I find myself wondering what the output would be at 8ohms if I bought another pair of Specos and wired them as 8k/8ohm autoformers? (The current ones have the 8ohm taps clipped off). I tried a strange experiment this evening and hooked one of my Klipsch Icon bookshelf speakers up to the headphone jack, wired off the 500ohm tap. Obviously, there was a huge impedance mismatch and a resulting crappy sound, but I wanted to see if they'd even make a sound to begin with. They did, and are loud enough that on 8ohm taps they might be passable for near-field computer speakers, 1-2 feet from my ears.  Any thoughts?

 

[xcortes]

Answer from PJ on the old forum:

The Quickie is designed as a preamp; we just tacked the Speco transformer onto the output to drive headphones. Power should be 10 or 15mW - less than the 50mW normally specified for headphones, but close enough for many high-sensitivity phones. Understand that my proof-of-concept prototype was turned into a product before I had a chance to do much in the way of measurement, so all specs are - ummm - speculative at this point!

Decent performance will be obtained with 150 to 600 ohms using the "10W" tap on the transformer, or 3 to 12 ohms on the "8 ohm" tap. There are no intermediate taps, though I hope eventually to have a better headphone transformer made which will be usable with this thing as well. Note that 10mW is 20dB down from one watt, so you'd need 122dB/m/w sensitive speakers to get true hifi loudness. It might drive 114dB horns to listenable levels though.

Each tube has its own D cell for filament power. The tubes were designed for carbon/zinc dry cells, so they will probably operate acceptably with 1.2v from NiCd or NiMH cells. Haven't tried it though. IMHO this thing provides a great platform for experimenting with power supply designs, even including solar power!

You can get more output by increasing the plate voltage, up to the 67.5v maximum specified for the screen. This will increase current consumption as well as increasing the voltage enough that you'll want to keep tiny fingers away. I'd suggest a bunch of AA cells (up to 45 of them) since they are particularly cheap in quantity. However, I don't think the Speco OPT can take more than a couple mA of DC current - you'd need a plate choke and parafeed output. Might be an application for the surplussed iron from a SEX upgrade though.

 

[Dr. Toobz]

For some reason, I'm a little confused as to how one would wire these for 8ohms, since the 8ohm taps are wired to the black (common) wires of the primaries, and the black wire on the secondary side is tied to ground. I'm using the red 500ohm taps right now, and cut off the 4ohm taps from the secondaries. So, how would this work?

 

[Paul Joppa]

For 8 ohms, just wire the speaker across the 8 ohm wires. Really. It's that simple.

By wiring the bottom of the primary winding to the top of the secondary, the transformer is converted to an autoformer - byt the 8 ohm winding is still an 8 ohm winding.

I guess I'd better try to sketch up the various output transformer connections that are possible, since there is so much experimenting going on these days.

Incidentally, I'm thrilled - the whole point of the Quickie was to get something out there that was cheap enough to encourage playing around and trying things. That seems to be happening in spades!

 

[Dr. Toobz]

Duh, why didn't I think of that? My brain still must have trouble trying to "picture" the autoformer arrangement after always using transformers in the usual fashion.

The Quickie has been great fun, and I suspect that many of us will keep tweaking and playing around with it as time goes on! I suspect this will become the "new" classic Bottlehead product, similar to the Foreplay of yore.

 

[Len]

For 8 ohms, just wire the speaker across the 8 ohm wires. Really. It's that simple.

By wiring the bottom of the primary winding to the top of the secondary, the transformer is converted to an autoformer - byt the 8 ohm winding is still an 8 ohm winding.

I guess I'd better try to sketch up the various output transformer connections that are possible, since there is so much experimenting going on these days.

Incidentally, I'm thrilled - the whole point of the Quickie was to get something out there that was cheap enough to encourage playing around and trying things. That seems to be happening in spades!

But why not use it as a regular transformer, using the 8 ohm tap on the secondary? Wouldn't taking DC off the output be desirable?

 

[Dr. Toobz]

Correct me if I'm wrong, but I think there is a sonic advantage to using these el-cheapo transformers as autoformers (by connecting the primary and secondary and keeping DC out with the 2.2uF coupling caps). Serial feed would put 36VDC on the primary of the transformers, whose cores would probably saturate quite easily, thus affecting the audio quality. I've read that one of the benefits of parafeed is it lets you get away with smaller irons since there is no DC to worry about. Good quality transformers for serial feed would probably be more expensive than the $4 I paid for each of the Specos!

 

[Paul Joppa]

The 8 ohm tap is fine if you have 8 ohm headphones. Or probably even up to 30 ohms if they are sensitive enough.

Toobs is right, there are advantages to parafeed. But FWIW, I tested a Speco and it can - just barely - take the 2mA of plate current in the Quickie, so it's reasonable to experiment with series feed. In that case, or with choke-loaded parafeed if you have a tiny choke, I'd raise the cathode resistor to 2K ohms. This will reduce the plate current to about 1.2mA, raise the bias slightly so full power takes a bit more drive, and re-optimize for an 8K load. The result is more output power - still not much, but more than before.