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Old 1st November 2010, 08:09 PM   #6481
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Quote:
Originally Posted by Joshua_G View Post
Of course, tough where I live now, in the country side, RFI carried by mains supply may be much worse than RF radiation inside my house.

Is there any way around it? Is there any better solution?
There are some things that help:
- smallest possible loop area of each circuit stage, to reduce the electromagnetic 'moment' (leverage)
- 'stopper' resistors VERY close to transistor or valve input pins;
- simpler circuits arranged to avoid accidental demodulation;
- circuit architecture with input AND output referred to GND e.g. folded (shunt) cascode - has much higher immunity to EM-coupled noise - together with Circuit surrounded with copper ground.

I find that circuits built like this have excellent rejection of coupled noise, all the way from transformer fields to DECT & mobile telephone handsets held right up to the circuit.
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Old 2nd November 2010, 04:06 AM   #6482
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Been finishing a system, but I have to put my $.02 in.

Shunt regulators have advantages over series regulators in particular when driving swinging inductor loads. Sometimes the load can source current rather than just sink it.

I prefer a resistor fed from a voltage regulator rather than a current source to feed a shunt. Both the series regulator and the current source at high frequencies pass more noise due to what may be modeled as "capacitance." With a resistor driving the shunt element, the "capacitance" of the shunt now loads the resistor and provides passive attenuation assuming the active feedback is approaching unity or less. There also are issues of two active devices in series with slight or even not so slight nonlinear behavior interacting.

It is nice to use circuit theory to model much of what we do, but when you are hitting the limits, expect nonsense.

I do find it interesting how much attention has been paid to shunt regulators since June 2007th. Which must be their re-birth date. (Note this is an ego exercise.)

High frequency noise can be addressed by treating the equipment enclosure as a Faraday cage. Small openings, no straight line from the openings to internal wiring, bypassing almost everything to the case and understanding that at high frequencies electromagnetic fields like to "stay tight". (Sorry I will not elaborate on that, there is good material elsewhere and if I remember where I will post the links.)

I also like to use simple fixed voltage at the base of a transistor and the output from the emitter for a regulator. This can be modeled as a gyrator and has all sorts of complex behavior. Properly tuned this may provide "musical" benefits.

One of the secrets to audio power amplifier design is the "voicing" of the power supply. Having the right internal impedance for the VA output will affect low frequency sound, clipping effects and whole lots of other stuff. The idea of having a massive transformer feeding steriod grown capacitors is nice, but even those are limited by power line sags. Adding small RF (EMI to the hip) filters changes the power supply voicing. Please note that the stiffer the transformer the smaller the phase angle for capacitor charging and the greater the line sag effects.

Now for an aside, I just tried to install an AM transmitter in a new arena. The local AM station is 50KW at 15.7 miles from the venue and I have to use an external antenna and distribution system to get any reception inside the building! The idea was to rebroadcast the event coverage with a small transmitter inside the building.

Legal power limits are .1 watt and a small antenna. Using this, the range to my "classic 6 transistor" radio was less than 25 feet. Try number 2 was 212 feet of cable attached to a class D transmitter with a final input power of 30 watts. I had to use fan cooling as even the output inductors were getting really hot. Final range 90 feet!

It seems each light is on a dimmer, the LED display boards (Hundreds of them), the LCD TV's, the cell phone repeaters, the internal data networks (telephone, TV, sound, building control, security all are ethernet based) all emit legal limits of RF, but when combined are stronger than the local broadcasters!

Most folks understand the sine(x) + 1/3sine(3x) etc. At 120 hertz rate the 1/833sine ain't much by each contributor but the entire choir is really loud. Maybe you do not have as much at home, but it is the wave of the bluetooth mobile future!

ES
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Old 2nd November 2010, 12:48 PM   #6483
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Quote:
Originally Posted by MRupp View Post
the aforementioned gear
A pic is worth 1K words, total of 10, including the 6 on the PS board.

(the slick 069 CD player has them as well, also 6 of the BNX on the PS board. Same setup in the 808-MK5, 070, etc.)
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Old 2nd November 2010, 01:07 PM   #6484
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Quote:
Originally Posted by simon7000 View Post
One of the secrets to audio power amplifier design is the "voicing" of the power supply. Having the right internal impedance for the VA output will affect low frequency sound, clipping effects and whole lots of other stuff. The idea of having a massive transformer feeding steriod grown capacitors is nice, but even those are limited by power line sags. Adding small RF (EMI to the hip) filters changes the power supply voicing.
Indeed, unless the chokes on those small RF filters are made of really thick wires, about 1mm or more. As far as I can see, RF chokes made of really thick wires shouldn't impact the mains power sag. Unless there is something about such RF filters and chokes that I didn't account for.
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Old 2nd November 2010, 01:53 PM   #6485
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any significant power sag on RF coils would probably burn them up quickly or not?
I would like to see a practical example where appropiate RF filters change clipping effects etc.
Regards
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Old 2nd November 2010, 02:29 PM   #6486
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Default And now...for something totally different...

This is a bit off the current topic, and I don't know if this is common knowledge here, but I had not seen it yet discussed. I recently came across this link to the Bell System Technical Journal archives 1922-1983:

Bell System Technical Journal

BSTJ is sort of a holy grail for some of us old audio/radio engineering farts. Much original research into just about everything having to do with audio is contained in these pages. I looked around for an index, but no luck. It has not stopped me from spending way too much time perusing the links. Oh - the speed and availability are pretty limited, but a couple of tries usually gets the DL.

Back to the discussion of interference, a subject near and dear to my heart as a radio engineer.

Howard Hoyt
CE - WXYC-FM 89.3
Chapel Hill, NC
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Old 2nd November 2010, 03:27 PM   #6487
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Quote:
Originally Posted by Juergen Knoop View Post
any significant power sag on RF coils would probably burn them up quickly or not?
I would like to see a practical example where appropiate RF filters change clipping effects etc.
Regards
No, the duty cycle is very low and the average power stays the same. It is a simple test just about anyone can do it.
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Old 2nd November 2010, 03:46 PM   #6488
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Quote:
Originally Posted by simon7000 View Post
No, the duty cycle is very low and the average power stays the same. It is a simple test just about anyone can do it.
Ed were you talking about filters on the mains side or after the reservoir caps?

jan didden
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Old 2nd November 2010, 04:01 PM   #6489
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Quote:
Originally Posted by 1audio View Post
Not to knock this study in emi noise reduction off track (I have opinions but this may be more interesting) here is a really novel amp measurement from the upcoming AES:
P7-5 New Techniques for Evaluating Audio Amplifiers via Measuring for Induced Wow and Flutter and Differential Phase Distortions—Ron Quan, Ron Quan Designs - Cupertino, CA, USA
Tough question would be; "Since all the information about the signal is also contained in the Fourier domain what could not be done with a sufficiently sampled signal and DSP ?"

At least there are no claims of in-harmonic signals.
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Last edited by scott wurcer; 2nd November 2010 at 04:09 PM.
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Old 2nd November 2010, 04:04 PM   #6490
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Quote:
Originally Posted by janneman View Post
Ed were you talking about filters on the mains side or after the reservoir caps?

jan didden
Filter on the mains.
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