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Switching amp hash filter

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Frank Koenig:
The power amps in my life these days, both large (Powersoft K) and small (Parts Express hobby boards), are switch-mode. They all exhibit a few hundred mV to several V of switching hash on their outputs. This is of no consequence in normal operation but makes it pretty much impossible to see any meaningful low-level output on a scope.

With nothing better to do yesterday I cobbled together a simple second-order Butterworth low-pass and put it in a little box with banana/binding-post input and BNC output. It uses an ancient, 88 mH, air-core, basket-weave inductor, a 440 pF capacitor and a 10 kOhm load resistor which yields a cutoff frequency of ~27kHz.

At 20 kHz it's .6 dB down and introduces -72 deg of phase shift (lag?  ;) ) so cannot be used for critical frequency response measurements but it cleans up the hash and I hope will be useful for looking at low-frequency noise, DC offset, low-level distortion and other problems. We'll see.

--Frank

Guillermo Sanchez:

--- Quote from: Frank Koenig on November 11, 2021, 11:42:18 AM ---The power amps in my life these days, both large (Powersoft K) and small (Parts Express hobby boards), are switch-mode. They all exhibit a few hundred mV to several V of switching hash on their outputs. This is of no consequence in normal operation but makes it pretty much impossible to see any meaningful low-level output on a scope.

With nothing better to do yesterday I cobbled together a simple second-order Butterworth low-pass and put it in a little box with banana/binding-post input and BNC output. It uses an ancient, 88 mH, air-core, basket-weave inductor, a 440 pF capacitor and a 10 kOhm load resistor which yields a cutoff frequency of ~27kHz.

At 20 kHz it's .6 dB down and introduces -72 deg of phase shift (lag?  ;) ) so cannot be used for critical frequency response measurements but it cleans up the hash and I hope will be useful for looking at low-frequency noise, DC offset, low-level distortion and other problems. We'll see.

--Frank

--- End quote ---

Hello Frank, any news on this? Did it worked as you intended?

Frank Koenig:

--- Quote from: Guillermo Sanchez on April 26, 2022, 10:08:12 AM ---Hello Frank, any news on this? Did it worked as you intended?
--- End quote ---

I haven't used it in anger yet but in initial testing on one of the little Parts Express amps it worked fine. If and when I get one of the big pro amps on the bench for some reason I'll take some scope screen shots and put them up here. --Frank

John Roberts {JR}:
I don't know what you are using for distortion measurements but if it has a distortion product output, feed that into a spectrum analyzer.

Back in the 70s I feed the product output from my cheap heathkit distortion analyzer into an old spectrum analyzer (that I paid hundreds of dollars for used). The spectrum analyzer only had a 50 dB dynamic range but the combination allowed me to measure the distortion introduced by the Heathkit audio path. By running the 0dB level through the Heathkit -10dB added 20dB net resolution to my test bench measurement floor. 

JR   

Frank Koenig:

--- Quote from: John Roberts {JR} on April 26, 2022, 12:35:04 PM ---I don't know what you are using for distortion measurements but if it has a distortion product output, feed that into a spectrum analyzer.

Back in the 70s I feed the product output from my cheap heathkit distortion analyzer into an old spectrum analyzer (that I paid hundreds of dollars for used). The spectrum analyzer only had a 50 dB dynamic range but the combination allowed me to measure the distortion introduced by the Heathkit audio path. By running the 0dB level through the Heathkit -10dB added 20dB net resolution to my test bench measurement floor. 

JR   

--- End quote ---


I'm using an HP 8903B distortion analyzer that a friend gave me when he got an Audio Precision (lucky dog). The HP indeed has a distortion product output on the back panel which I've fed into a Tektronix DPO 3034 digital scope and viewed in both time and frequency. (The scope includes an FFT-based spectrum analyzer function.) The thing that didn't make sense to me was that the fundamental wasn't as attenuated as I thought it should be. When I do a loop-back test on the analyzer (connecting its internal sine generator directly to the analyzer input) it reads within spec -- .03% or so, as I recall.

I got distracted and moved on. I need to go back and figure out why the spectrum analyzer display and the THD reading failed to agree. Maybe the distortion product output is after a pre-filter but before the synchronous detection, or whatever the analyzer usus, to make the super-notch?

One interesting thing about the analyzer is that it works with any roughly sine-wave input -- you don't have to use the internal generator. So it's useful for measuring distortion of whatever sine-source you've got or in cases where you don't have analog access to the system input, such as a Bluetooth receiver. It's a pretty cool box and it behooves me to understand better how it actually works internally.

--Frank

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