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Looper's Delight
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On Tue, 1 Sep 1998 Edward_Chang@mail.amsinc.com wrote: > A while back somebody had asked a question about getting signal from >their > guitar rack w/o an amp and you had given an answer of making a line out >for > the guitar amp. You said - > > "Get a little project box. It needs a 1/4 jack going in and rwo 1/4 >jacks > going out. The input jack should be connected to the regular speaker >jack > on your amp. For the two output jacks, wire them in parallel. > Put a 1000 ohm 5 watt resistor in series with one of the jacks. This is > the line out. > You should be able to plug it directly into mixers, etc... Make sure the > speaker is plugged into the other jack! It is bad for tube amps to not > receive a proper load.....It requires almost NO electronics knowledge!" > > What I'm trying to do these days is very similar, except that my amp is >not > a guitar amp but ANY amp (electronic toys, Casio PK-1, portable >tuntable). > I have a turntable which is from the 50's and doesn't have a line out. >So > a friend of mine soldered the wires leading to the built in speaker to an > output jack. When I plugged it in to a guitar amp. a capacitor exploded > inside the record player. Then I remembered your post about resisters >and > loads. Should I try it again with a resistor in series? Should the > resistor be in the ground or the hot wire? What's the formula to >determine > ohms and wattage for any kind of electroinic sound source (powering a >small > speaker) to get it to a line out level? I assume this old turntable has a tube amplifier? The problem may just be its age... old capacitors often fail due to aging. In fact, any tube amplifier dating back to the 1960s or earlier should be re-capped by a professional for safety reasons. Cap failures can take out other components - especially expensive vintage output transformers. A cap job won't change your amp's sound much, but a new output transformer can ruin the sonics of a beautiful vintage amp (oh, and as long as you're upgrading vintage equipment, add a three-prong power cord and a ground lift switch (for safety), and have all the tubes tested (failed tubes can also kill other parts)). Another possibility is that your friend's wiring job, a bad cable, or something else shorted the output transformer - another recipe for disaster. Also, did you keep the original speaker connected to the output, or was it disconnected? It is VERY important that tube amp outputs see the correct impedance loads, or else you risk damage. In the model i suggested, the line out is effectively in parallel with the original speaker. By Ohm's Law, a minimum 1000 ohm load in parallel with an 8 ohm load makes a nearly 8 ohm load (close enough). Voltage across the 1k load will be equal to the voltage across the 8 ohm speaker, but power will be greatly reduced. Here's where computing power output comes in... watts = volts^2/resistance. So if your amp puts out ten volts, the power across an 8 ohm speaker is 10^2/8 = 12.5 watts. If you have a 1k line out in parallel, its power is 10^2/1000 = .1 watt. See, most of the actual power is going through the speaker, even though the same voltage goes across the line out. I believe Kim suggested putting another resistor across the line out as well. This is good advice. Moreover, the output of our little line out box is *awfully* high for a line out, and should be attenuated somewhat (especially if you're plugging it into a guitar amp!) Here's an alternative suggestion - get a 1k ohm pot from Radio Shack. Wire terminals 1 and 3 across your speaker terminals. Connect pin 2 to the tip of your line out jack, and pin 3 to the ring. This gives you a line out with a 1k impedance (low enough to match most equipment) and a volume control as well! > Again I would be greatly indebted to you for any enlightenment... Hope this helps. :} -dave Practice beautiful randomness and act kind of senseless. <dstagner@icarus.net>