col Posted May 12, 2010 Report Share Posted May 12, 2010 Just a quick 'heads up' for folks messing around with LM386 based sustainer projects (eg. Fetzer Ruby). The Zobel network in the LM386 datasheet uses a 47n cap and a 10ohm resistor. These values are necessary because the circuit it is intended for full range audio - songs etc. The downside of this is that it doesn't do enough to offset the problem of inductive reactance rising at higher frequencies in a sustainer system. Explanation: Assuming a driver with inductance of around 1.2mH (the basic design as developed by Pete), even with the datasheet zobel network, the impedance at 2.5kHz is about 16ohms, at 5k it's more like 40 ohms. Look at the graphs in the datasheet, you can see that the maximum power available before the distortion (THD) gets high gets much lower as the impedance rises... with an 8ohm load, you hit 10% THD at about 0.75W, whereas with a 16Ohm load, 10% THD comes at about 0.45W. This becomes a problem as soon as there is clipping in the signal chain - clipping generates high frequencies in the signal. With a standard zobel network, these will cause nasty distortion a plenty from the LM386. Solution: Change the values of the zobel components. We don't need to drive anything like the full audio range, we only need from 82Hz to say 1.5Khz. So if we change the values of the resistor and cap in the zobel to 8ohm and 10u, we don't lose anything we need, and as the signal rises to the point where the inductive reactance of the driver causes the impedance to rise to distortion levels, the zobel provides an alternative route for the current So that the LM386 just sees a nice friendly 8ohm impedance. The result is that if you drive the circuit hard, instead of a really nasty fizzy distortion, you get a more musical rumbly crunch type of thing (at least I do), so if yo are having trouble with fizz, this might help a bit. remember, Zobel values: cap 10u, resistor 8ohm. cheers Col Quote Link to comment Share on other sites More sharing options...
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.