I would like to build the circuit below. However, we cannot get the Radio Shack parts shipped to us in Canada (a corporate issue after they left the country). In the meantime I have found only one other small audio transformer in town (shown below in two views). The inputs and outputs do not match the one in the diagram, but they might be usable all the same.
Can anyone tell me if this is so…and which coloured wires would go where?
(I have the potentiometers and capacitors shown in the diagram.)
The black wire on each side are the common earths ( one common earth for the primary, one for the secondary), the coloured wires are for the labelled values:
Secondary: Black-Yellow= 4 Ohm
Black - White = 8 Ohm (that would be your choice in this case)
For the primary, well, it’s a bit harder to say, as it depends on your input voltage etc, but as it’s rated at 70V, we can calculate the equivalent resistance values
W=V^2/R R=V^2/W
Black-White= 5W@70V, makes R=70*70/5=980Ohm, near enough to 1kOhm
You can check it in a dry run by connecting a multimeter to the mentioned wires, and checking the resistance it measures.
ONLY if the ohm meter operates at audio frequencies, not on DC. Not likely to have one of those at home. [:)]
He needs an impedance meter or a bridge to make this check. DC resistance is not the same as impedance.
Otis, 1K means 1000 so 128 ohms is not 1.28k ohms. The good news is that impedance will be higher than resistance as measured with the meter.
JM’s calculations indicate that this transformer should work for you. I agree. You might try each pair of windings b-w, b-y and test the results dynamically.
It’s been a few years but aren’t the inputs of these 70V transformers often 600 ohms??
Out of curiousity, what is the circuit supposed to do?
Thanks for the feedback. Perhaps I will try the transformer if you think it matches the one in the Soundtraxx circuit somewhat.
The circuit itself is what is recommended by Soundtraxx to connect their DSD DSX sound only decoder to an external amplifier rather than to some small speakers mounted under the layout table.
In an earlier post I mentioned how I had pretty good results playing the decoder sample sound files from the website through my 15" subwoofers and satellite speakers. I am experimenting with these as in the cab loco sound as the train throttles up etc.
Soundtraxx tech support says the soundfiles should be quite good even coming from a decoder designed for tiny speakers. Another fellow in the Soundtraxx Yahoo group says he got good results doing something similar. And another experienced sound man says he simply hooked up the decoder as is…as a kind of preamp without the transformer circuit.
It is an experiment. However, I don’t want to fry several $68 DSX decoders trying this, so I am trying to get the circuit as close as possible to the example given.
What you’ve got there are line transformers. They are designed for public address amplifiers which have a 70 volt output. This type of output allows you to tap off the line wherever you need a speaker, using that type of transformer. The multiple taps allow you to match various speaker impedances and set the amount of power drawn from the line which sets the maximum volume at that speaker.
It would appear that the drawing is showing a standard audio output transformer which is used in older radios etc. and in old tube equipment. Modern audio gear usually has the speaker directly coupled to the output transistors.
For what it’s worth - I’ve ordered from US Radio Shack sources and have had them ship to Canada numerous times. Has there been a change in the last couple months ???
The 33uF capacitor is to limit the base output. It eliminates the bass frequencies your speakers are unlikely to duplicate. If you want lower bass, try a smaller value.
If you run the capacitors in parallel, thier capcitance adds up. ie: 10uF + 10uF + 10uF + 3uF in parallel = 33uF. You could put smaller value in for more bass, or put more in for less bass. The advantage of taking out bass is you get less distortion, and free up more power for upper frequencies.
This appears to be a big transformer used in a PA system, and the transformer Soundtraxx is specifying is probably a small, board mounting audio transformer used with small transistorized amplifiers.
If you can get some details, you probably could match it with one of these units from Hammond.
I figure that circuit is a simple way to put 8R on the output of the decoder, and match it to a higher impedance amplifier. Which is what an audio transformer should do if you connect it in reverse.
(The point of the transformer is to make a low impedance voice coil appear to be a very high impedance load on the amplifier. Very important in the days of tubes.)
Thanks for all the helpful replies and tips on sources. Nope, the transformer I had didn’t work at all as you knowledgeable people already knew, but I tried it before you posted. With the sources listed I should be able to match the Radio Shack part.
Nope, Mark, Radio Shack in the US flatly refused to ship to Canada…or they did refuse last summer. It was a fallout from their leaving the market here completely after forcing Radio Shack Canada to cease using the name and product line. I will call again…but in the past I had to have a friend in the US reship items for me.
Anyway, going on a holiday overseas for a month. Will take this up again later.
