When to break / filter power

This is something I've wondered for a while. Are there any rules of thumb for when to break up and put filtering between amplifier stages? For example, in the EZ1073 I see that there are different series resistors off of the 24V rail -

12R + 2x 470uF to ground to the output line amp stage
120R + 470uF to ground on the EQ power rail
270R + 470uF to ground on the mic pre power rail

I've also seen this kind of interstage break up / filtering to current drive stages or between gain stages. When do you need it, and how do you size those series resistors?

As for sizing the resistors, you need to consider the DC bias drawn by the stage and the resulting voltage drop through the resistor. The power supply voltage must be made larger to accommodate for this voltage drop, and/or the resistor needs to be made small enough to result in sufficient supply voltage after the isolator.

An opposing requirement is that it's generally good to make the 3dB frequency of the RC filter below the audio band, probably several Hz if you can use large enough capacitors. The numbers you show above are good general ranges - 470µF with 100-150Ω will be outside of the audio band, and thus will filter transient currents out of the supply rails (and move them to the ground point of the shunt capacitor).

As for why you'd do this, there are a number of reasons. One is to move transient currents out of the power supply and into  the local circuit ground, which will tend to make the signal current loops in the circuit smaller, and thus less troublesome to other circuits. It also allows you to essentially ignore the power supply impedance, and to some extent, power supply noise as well, since the RC isolators will filter out some power supply noise, and the shunt capacitor impedance alone will determine the effective supply impedance over the audio band.

Another reason why this could be useful is if each amplifier or amplifier stage has very low (or no) power supply rejection. This is the case with a common cathode tube amplifier stage - plate voltage variations can go straight into the output of each stage with little attenuation, making some filtering and isolation worthwhile. So, filtering schemes that start at the output stage, and provide a chain of RC filters that work their way to the input stage, will make good use of the voltage drops and provide the greatest filtering and isolation at the first stage where there is little feedback to counteract power supply variations.

With op amp circuits, which have extremely high power supply rejection, you may not want to do this at all, and instead rely on a regulator to provide a low impedance supply, which will inherently have isolation between stages because of its low impedance. The transient currents will then be kept out of the ground system, and will stay in the power supply rails, and this can be good or bad depending on the overall design.

I hope this helps!

dogears said:

This is something I've wondered for a while. Are there any rules of thumb for when to break up and put filtering between amplifier stages? For example, in the EZ1073 I see that there are different series resistors off of the 24V rail -

12R + 2x 470uF to ground to the output line amp stage
120R + 470uF to ground on the EQ power rail
270R + 470uF to ground on the mic pre power rail

I've also seen this kind of interstage break up / filtering to current drive stages or between gain stages. When do you need it, and how do you size those series resistors?

Click to expand...

There are probably multiple rules of thumb...

The "need" for it is related to the PSRR (power supply rejection ratio) of the electronic path.

The holistic approach to circuit design involves following the current (kind of like following the money in politics).  The current comes from the rails, through the driver to the load, "and" returns to the power supply... You don't want the audio signal corrupted by IxR voltage drops anywhere around the loop.

As usual it depends, but try to look at the whole picture.

JR

> Are there any rules of thumb for when to break up and put filtering between amplifier stages?

A chum and I were yakking. I now believe the convention of decoupling (in tube amps) may be just superstition, an old-spouse tale. I have a hard time showing motorboating when violating convention. At least for audio bass, when eating wall-power and needing to tame buzz. While three stages can be unstable, two stages is degenerative, and 3 4 or 5 stages it seems the couple degeneration exceeds the whole-amp regneration and it is stable but a bass roll-off.

> 12R + 2x 470uF to ground to the output line amp stage
> 120R + 470uF to ground on the EQ power rail
> 270R + 470uF to ground on the mic pre power rail

One is high level and high current. One is low-level and low current.

At high current you want "low" voltage drop in the resistors. I suspect 12r is <1% drop, teeny, however the filtering need is also small.

At low-level you want low buzz, but at low current, so try 270r. This may be 5% drop, not very significant at low level.

12r+470u is 30Hz, so little effect on subsonics and not much on 100/120Hz. But the far end of the 12r is probably a <1r regulator which will suck much crap.

270r+470u is 1.3Hz, so will cover a few octaves below the audio band, and give 37dB buzz filtering.

There's no absolute reason they used 470uFd everywhere. However since signal level and current may scale in the same direction, it makes sense that the cap won't change much. In that case it makes commercial sense to buy a BIG box of all the same, get the bulk price, reduce worker confusion.

Just to add to what PRR said, at the input end you have low current and low signal so you can use a bigger R to get more decoupling. The reasons you want more is because the signal is smaller and it is going to get amplified by later stages.

Cheers

Ian

Awesome insight guys, makes a lot of sense. Thanks.