Improving Daisy (Part 3)
The third of Stephen Delft's series on building a guitar routing box.
All that remains is a diagram of the pre-amp board and details of the output transformers.
These are a special item, made by Sowter Transformers, (Contact Details). The part number is 3642, and at present, two will cost £12.36 incl post + VAT in the U. K. These transformers will remove any possibility of hum loops via the two output sockets and the switch box's own earth, and also give a small voltage gain (1.2 times) to compensate for losses in the buffer stage. They will also isolate you from any electrically doubtful equipment connected to the output jacks.
The transformers are mounted in any convenient place away from the power supply, by pairs of 6 B.A. screws. Remove all connections from both output sockets, trace two of the wires to the link switch and disconnect them there also. Re-arrange a standard three-way tag strip to give two down and one up as shown in the photograph last month, and solder to the tags of the link switch from which you just disconnected two wires. You must now identify the leads from the transformers. The secondary leads each have two inner wires. Blue and the braid connect to the sleeve tag of their output socket, and Red connects to the tip tag.
The transformer input leads have only one inner. You should still have one wire hanging around loose which comes from the sleeve tags of the input jacks and formerly connected to the sleeve tags of the output jacks, until you removed it. This wire, and the braids of both transformer input leads, connect to the turned-up tag on the three-way strip, and to nothing else. The inner wires to the transformers go to the other two tags on the three-way strip which are soldered to the link switch.
That completes the construction of DAISY. If you find that, with your preferred amp arrangement, the two amps are out of phase (see article on hum loop isolator, May, 1975) you can either reverse the loudspeaker leads of one transformer only (not the braid). If you find you need to re-arrange this frequently, you could insert a phase change switch in one transformer output lead.
The reasons for the 1k. ohm padding resistors in the output of each buffer amp are:- 1), to assist stability by isolating the buffer amp from an unsuitable (Capacitative) load, and 2), to define the worst loading conditions which can apply to each amp when they are linked.
Without the resistors, each amp would be trying to drive its signal back into the output impedance of the other amp and there would be severe distortion. The cost of this simple arrangement is that the total output remains roughly constant when both channels are linked; the signals are mixed, not added together. I think this is probably an advantage; if you want a full range booster, they are cheap enough to buy.
(If you have technical or career enquiries on this or any of my other articles, please send them to the magazine, not directly to me.)
Feature by Stephen Delft
mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.
If you value this resource, you can support this project - it really helps!