Little Things: Using a Zobel Network to Reduce Harshness with Electrostatic Speakers

If you have some electrostatic speakers, and they sound harsh, it is probably due to your power amplifier having trouble with the resonance of the transformer in the speaker. This transformer raises the amplifier voltage to a higher voltage that goes to the stators. This problem can be alleviated by the use of a Zobel network.

A Zobel network consists of a capacitor, resistor, and sometimes, an inductor.

Some of my reference speakers are electrostatic, and I did notice that the sound had a harsh character to it. So, with the help and advice of some electrical engineers, we came up with a Zobel network, which in the case of a Pass Labs power amplifier (Xs-300) and MartinLogan CLX speakers in one of my reference listening rooms, was a 10 ohm resistor and a 0.22 µF capacitor connected in parallel with the spade lugs across the amplifier binding posts.

Here is a close-up, showing the values of the resistor (10 ohms) and capacitor (0.22 µF).

Shown below is a diagram of the problem. The amplifier, without the Zobel, sees capacitance C1 and inductance L1 from the transformer, and capacitance C2 from the electrostatic panel, which causes a resonance at a very high frequency, say 80 kHz. This causes deterioration in the audio band that make the sound harsh. By placing the Zobel network (R2 and C3) in between the amplifier and electrostatic speaker, this resonance is reduced. Resistance, from the electostatic panel stators, would be to the right of C1 and L1, as the stators are connected to the secondary winding of the transformer. C1 and L1 represent the primary winding.

Sometimes an inductor is used, but running a simulation, using an inductor, showed no benefit, so it was decided that just the resistor and capacitor would be best. It turned out to work very well. All the harshness that had been present without the Zobel network disappeared, and the sound was satin-smooth, as it should be.

Shown below is the Impedance/Phase plot of the MartinLogan CLX. Notice that, above 10 kHz, the impedance is below 2 ohms, and near 20 kHz, it is less than 1 ohm.

I cannot generate an actual measurement of the impedance beyond 20 kHz, so here is a simulation plot, showing the 80 kHz transformer resonance with the Zobel (blue line) and without the Zobel (red line) in the circuit. (Click on the plot to see the full-sized version.)

So, if you have the electrostatic speaker harshness issue, a simple Zobel network could be the solution.

Please note that most amplifiers already have some form of Zobel network in place to prevent oscillation, but it might not be sufficient to reduce the harshness caused by an electrostatic speaker. So, you have to be very careful in designing a Zobel for your particular amplifier/speaker combination. The one that I put in for the Pass Labs Xs 300 power amplifiers and MartinLogan CLX electrostatic speakers is specific for that combination. If the amplifier uses no global negative feedback (from the output to the input), this would require a Zobel network that would be different than for an amplifier that uses negative feedback. My advice is to Google Zobel Networks in Power Amplifiers, and plow through the articles you will find. Some of them have formulas that you can use to calculate the values of the parts in the network. It may be necessary for you to add an inductor as well. If you put in a Zobel network with components (resistor, capacitor, inductor) that have the wrong values, your amplifier or speaker could be damaged. This article is really just to show you that there is a path to getting great performance from your electrostatic speakers if you are having a problem with harsh sound. Neither I nor Secrets can take any responsibility for what happens if you put in a Zobel network for whatever purpose you have in mind. All I can say is good luck, and I hope you solve your problem, if you have one. The safest approach, and the one that will get the best results, is to contact the company that made the amplifier and tell them what speakers you are using. Any reputable manufacturer will give the best advice.

John E. Johnson, Jr.

Editor

I wish to acknowledge the assistance of Nelson Pass in the preparation of this article.