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Author Topic: Grounding and RFI  (Read 8697 times)

Frank Koenig

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Re: Grounding and RFI
« Reply #20 on: February 19, 2014, 11:58:27 am »

TIM is an overly esoteric explanation for a simple and fairly well understood mechanism by many

This is exactly right (of course!). The whole TIM brouhaha was discovered (invented?) in the late '70s when folks noticed that some amplifiers -- not just power amps -- had significant IM when at least one of the input signals was near the upper end of the pass-band.

AES even came up with a standard test involving the sum of a sine wave and a square wave and a certain accounting of the resulting intermodulation products. I set this up in a lab at Stanford where one of the profs took pity on me and let me use a very nice (analog) HP spectrum analyzer to test a bunch of amps, both large and small. I recall that the 5534 op amps, which were the hot new thing, did very well.

The Crown DC300A was considered by many to be the villain at the time (I did not get to test one) and I believe Yamaha came up with their "Natural Sound" line to differentiate themselves in the marketplace.

--Frank
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John Roberts {JR}

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Re: Grounding and RFI
« Reply #21 on: February 19, 2014, 12:26:11 pm »

This is exactly right (of course!). The whole TIM brouhaha was discovered (invented?) in the late '70s when folks noticed that some amplifiers -- not just power amps -- had significant IM when at least one of the input signals was near the upper end of the pass-band.

AES even came up with a standard test involving the sum of a sine wave and a square wave and a certain accounting of the resulting intermodulation products. I set this up in a lab at Stanford where one of the profs took pity on me and let me use a very nice (analog) HP spectrum analyzer to test a bunch of amps, both large and small. I recall that the 5534 op amps, which were the hot new thing, did very well.

The Crown DC300A was considered by many to be the villain at the time (I did not get to test one) and I believe Yamaha came up with their "Natural Sound" line to differentiate themselves in the marketplace.

--Frank
[esoteric veer]
For some reason audio has always attracted a lot of quasi science..... IIRC Matti Otolla was the primary TIM pimp (there were a couple other superfluous new distortions floated around that time). There was a funny comment in an interview with an old graybeard tube designer (Baxandall?) who was involved with the HF radar signal processing design during WWII remarking how this (speed related distortion) was well known to those skilled in the art back in the '40s.

The bottom line is that early solid state power transistors were as slow as dirt (2n3055 anyone?), so the slewing speed of early SS audio amplifiers were marginal. The more power the more voltage swing and more apparent the speed issue.  Marshal Leach an EE prof at Ga Tech published a short but seminal AES paper about how to bandwidth or rise-rime limit an active NF amplifier such that it could not be slew limited for any valid (unclipped) input voltage. Too fast signals are just harmlessly rolled off.  As the semiconductor technology evolved the devices became faster, and that created another group of snake oil inclined marketing types to over sell slew rate (not unlike over selling damping factor and other poorly understood specs). Perhaps ironic, a well designed amplifier can not be slew rate limited, and some amplifier makers would have to defeat that part of the circuitry to measure a big slew rate number for the sales manager to publish on the data sheet. The customer is always right and they wanted faster slew rates even though they did not have a clue about how much they actually needed.   :o

Enough is good, more than enough, is just more, not necessarily better.

There was a sub genre of self-appointed audiophile experts ginning up these faux design problems. like their livelihood depended on it, because it did.  8)

JR   [/esoteric veer]
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Stephen Swaffer

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Re: Grounding and RFI
« Reply #22 on: February 19, 2014, 12:36:56 pm »

JR, I'm not really a teacher... just a student trying to figure it all out. Lot's of fun, isn't it?  8)

Can't speak for Mike-but I know I was a lot smarter 25 years ago....seems the more  learn the less I know.
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Steve Swaffer

Mike Sokol

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Re: Grounding and RFI
« Reply #23 on: February 19, 2014, 12:55:27 pm »

The bottom line is that early solid state power transistors were as slow as dirt (2n3055 anyone?), so the slewing speed of early SS audio amplifiers were marginal.

But back to my original point. Under the right set of circumstances (poorly designed amps, cables just the right length, etc...) it's possible to have the speaker cables act like an antenna feeding RFI into the amplifier's circuitry with is then detected (rectified) and output as noise, or even DJ's talking. Of course, this type of interference also happens on the front end with poorly shielded/bonded snakes and XLR cables with mic preamps that aren't playing nicely.

RFI is a cruel master....
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Stephen Swaffer

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Re: Grounding and RFI
« Reply #24 on: February 25, 2014, 10:08:53 pm »

I have experienced the most difficulty with RFI on phantom powered condenser mics. Is this typical?  What causes this?
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John Roberts {JR}

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Re: Grounding and RFI
« Reply #25 on: February 25, 2014, 10:23:25 pm »

I have experienced the most difficulty with RFI on phantom powered condenser mics. Is this typical?  What causes this?

I do not like to make sweeping generalities, but there are many different phantom powered mics, with sundry different circuitry inside the mic. It's not hard to imagine something as simple as an emitter follower rectifying RF if it is large enough in the cable (>500mV). 

Passive microphones do not have any active semiconductor junctions so will pretty much ignore RF. So will well designed mics.  8)

JR
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Steve M Smith

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Re: Grounding and RFI
« Reply #26 on: February 26, 2014, 02:48:23 am »

and output as noise, or even DJ's talking

Is there a difference?!!!


Steve.
« Last Edit: February 26, 2014, 06:35:30 am by Mike Sokol »
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Re: Grounding and RFI
« Reply #26 on: February 26, 2014, 02:48:23 am »


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