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PostPosted: May 8th, 2021, 10:38 am 
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Paul McGowen(PS Audio)
said this is his daily blog

"Careful on the input
One of the ways we designers make good sounding amplifiers is to lightly limit the input frequency while at the same time extending its high-frequency response.

That’s something that might seem counterintuitive but it works.

For example, at the input of a power amplifier, I like to form a light low pass filter of around 30kHz but within the amplifier’s circuitry, extend its bandwidth to as high as is practical—hopefully somewhere close to 100kHz.

This combination of limiting what the amp has to deal with while making sure what does come in is easily handled makes for a wonderfully open and easy presentation of music.

I like to think of it as a car with more power than it needs, and then a light foot on the accelerator pedal.

Easy in so the amp never breaks a sweat."


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PostPosted: May 8th, 2021, 10:51 am 
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What sources does he think are putting anything out above 30kHz anyway? What's a 30kHz filter do if there's nothing up there anyway? Any changes he's hearing from the 30kHz filter are more likely due to phase shift below 20kHz as a result of the filter.

Roscoe

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PostPosted: May 8th, 2021, 11:17 am 
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Roscoe Primrose wrote:
What sources does he think are putting anything out above 30kHz anyway? What's a 30kHz filter do if there's nothing up there anyway? Any changes he's hearing from the 30kHz filter are more likely due to phase shift below 20kHz as a result of the filter.

Roscoe

Actually RFI/EMI "saddles up" and rides along with the audio, LP playback can have ringing out past 30-kHz., and digital playback ultrasonic artifacts also can cause issues such as "fogging" and limiting inner detail.

The "old school" designers played that game that Paul talks about such as Syd Smith (Radiocraftsman/Marantz) along with the Heathkit guys that did the same thing. I am sure there were others.

I've noted where others close down the feedback loop to provide a low-pass function.

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PostPosted: May 8th, 2021, 12:37 pm 
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Instead of using the 30 KHz point why not use an RF filter since most of what would get in would be in the RF range. I would be hesitant to use a 30 KHz filter because of possible phase shifts in the audible range.

Tom


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PostPosted: May 8th, 2021, 1:09 pm 
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tomp wrote:
Instead of using the 30 KHz point why not use an RF filter since most of what would get in would be in the RF range. I would be hesitant to use a 30 KHz filter because of possible phase shifts in the audible range.

Tom

Seriously!!!! Those front-end filters are simple low-pass filters that shunt the noise and aid stability. I've attached an example.


Attachments:
Input stage W5M.pdf [33.73 KiB]
Downloaded 183 times

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