Purpose of RC Circuit on Amp Outputs?

[shredhead]

Hi everyone,

I am restoring an old Phase Linear 400 amp of mine from 1974 and I noticed that on the L&R speaker outs, bridging the positive and negative terminals is a 5 Ohm resistor in series with a 100nF film capacitor. Then I started noticing the same thing in some designs that some of you here have posted.

The only thing I've found about this online says that it is to isolate the amplifier from the different possible loads that might be used. I don't really get it. Wouldn't this change the impedance that the amp sees from the speakers that are hooked up to it? Would it alter the frequency response of the amp?

 

[maton00]

It´s called boucherot cell, its function is to damp high frecuency oscilations that may occur in the absence of a load, also is used to stabilize or flatten the impedance from the inductive component of the load or loudspeaker (so then the amplifier could see a constant load) and to prevent further oscilations

 

[Ian Finch]

If you search the topics "Zobel Network" and "Boucherot cell" (same deal) at the top of the forum page, you'll find an overwhelming amount of posts, threads etc. on them already because few people coming from other areas of electronics encounter them other than as "snubbers" which is another general term. Believe it when you see them on virtually every linear audio power amplifier - at least the ones that haven't burnt to a crisp by oscillating themselves to oblivion.

This is detailed in all the standard texts referred to here like Bob Cordell's Designing Audio Power Amplifiers and Douglas Self's Audio Power Amplifier Design Handbook. And here: http://sound.westhost.com/amp_design.htm#op-stability
Wikipedia also gives you the full technical rundown: Zobel network - Wikipedia, the free encyclopedia

 

[AndrewT]

The amplifier Output Zobel (the R+C in series from Output to Power Ground) does NOT help isolate the amplifier from reactive/difficult loading.

The Output Zobel is an extra load on the output that changes the high frequency (HF) stability of the amplifier.
Because it is ONLY effective at HF, the inductance of the R+C route from output devices back to Power Ground MUST be kept VERY LOW.

Do not fit this away from the output devices, not at the edge of the PCB and definitely not at the speaker terminals.

 

[DF96]

The amplifier Output Zobel (the R+C in series from Output to Power Ground) does NOT help isolate the amplifier from reactive/difficult loading.

Yes it does, in the sense that it provides a fairly low resistive HF load in parallel with whatever external load is present. Of course, it works best in the presence of an output inductor too. These networks make almost no difference to audio frequency response.

 

[AndrewT]

The Zobel does not include any output inductor.

The Zobel does not need any load.

The Zobel provides the ONLY HF load and must be of low inductance to achieve this.

The Zobel is an extra load on the output that changes the high frequency (HF) stability of the amplifier.

 

[AndrewT]

If one wants/needs additional isolation from reactive loadings, then one uses either form of the Thielle Output Network.