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Patents on Parade #1

In this first in a series of patent examinations, we want to pursue an idea on how to interface a line output, such as a tape recorder, into a guitar amp so that music can be played while a band plays along or listens to it. A simple example of how to accomplish this is with a transformer interface. Jensen Transformers makes a variety of excellent transformers that would be useful for this purpose and there is even an appication note on their site with an example:

This simple circuit can take a balanced line output and drive an input on your amplifier. It even has a volume control to make sure the levels can be adjusted to optimum listening. The image is directly out of the applications guide for the JT-11P-1 transformer.

We know that the output signal from pro audio equipment can be quite large, so it may be necessary to pad down the signal before it hits the transformer to prevent the core from saturating and causing distortion. A quick look around Jensen's site and we find an example of a resistor pad that can be adapted to our circuit in process.

This method of cutting signal level is common and has been used for many years in audio recording. If we graft it on to the input, our circuit now looks like this:

Because a transformer input is not purely resistive, there may be a peak in the frequency response that needs to be smoothed out for a better audio sound. A classic way to accomplish this is with a Zobel network... a simple resistor/capacitor combination that is placed across a transformer or voice coil of a speaker. So we go back to Jensen and we find and example like this:
The Zobel network in the above example is R1/C2 and we may have to modify the values slightly but let's go ahead and put it on our prototype circuit thusly:
Wow! Not bad. In just a few minutes using a little electronics knowledge and only taking circuit bits from one manufacturer's site, we have assembled a complete interface circuit with features such as the ground lift switch and input padding to reduce distortion along with a volume control and frequency response tailoring. This is great. Can we patent this? Surely not... it is made of common circuit building blocks that have been around for many years.

Click here to see the patent connection

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