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Old 23rd June 2009, 04:18 AM   #1
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Default turntable/record player and unwanted reception of broadcast sw stations

Hi
I heard often about unwanted radio stations by playback of records (radio moscow effect), especially in combination with phono preamplifier concept's without lowpass capacitors and inductive resistors arround the input gain stages. One example of radio sensitive RIAA head preamplifier is the EAR 834 (EAR834), see schematic by the URL http://www.goodsoundclub.com/Site_im...AR-834PT-1.gif
Amazing for me is, that in some cases the unwanted reception by shorting the inputs will be still present. The reason could be a parasitic inductivity between mainboard GND and metallic enclusure chassis

On the web I don't find any informations about theoretical background and the reasons for unwanted AM demodulation in the short wave (SW) aera - in most cases between 6 MHz and 7 MHz (wavelength 49m and 41m). I think, I don't know the right keywords resp. the english slang words about this topic in Google.

Perhaps one of the members know web addresses about this topic.

Best thanks for your advices

Best Regards
Andreas Kirschner
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Old 25th July 2009, 10:53 AM   #2
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nobody knows more?

about telephone I was informed, that the right keywords are the follow:
1) "RF contamination"
2) EMI ("Electromagnetic Interaction)
3) EMC (Electromagnetic Compatibility)
4) EMV (Elektromagnetische Verträglichkeit - German abbreviation)
I will check this keywords in the next time by google to find more, especially basic articles.
Quickly I have found this:
http://audiophileanswers.blogspot.com/
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Old 25th July 2009, 02:55 PM   #3
SY is offline SY  United States
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RF breakthrough is a difficult issue. I don't know what the grounding/shielding scheme is for that preamp, but it may be worth examining it closely.

As simple measures, I'd add grid-stoppers to the tubes (1k should be big enough to be effective but small enough not to compromise noise too badly), make sure the chassis is well-attached to earth ground, and put RF common-mode filters on the mains input and in the heater power supply feed.
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Old 25th July 2009, 03:34 PM   #4
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Hi,
in all cases where I have removed this undesirable shortwave reception, the cause was mostly a high proportion of parasitic inductivity between the PCB-GND and the GND of alumunium enclosure.
More rare was the cause of too high parasitic inductivity by the resistors in the input stage, especially in the line of emitter/Source/Cathode.

Amazingly for me are two things:
1) only the audio signals of radio stations (mostly SW 41m) was to be heard, but no sums, hums or other noise.
2) only in certain circuit topologies I can observe this one, especially when certain shortwave transmitter (e.g. radio moscow in Germany) in generall give strongly reception level (enhanced through larger reflections in the Ionospäre).

In the storerooms of the RIAA and AES exist with high probability technical papers about this topic and what to do is to avoid this by circuit dvelopement

P.S. By the way - "Red Light District" for a model name to amplifier is a good idea
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Old 31st July 2009, 04:57 AM   #5
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Have you actually confirmed that the signals are originating in the preamp?

There is evidence with certain amplifiers and speaker layouts of speaker wiring acting as an antenna at certain frequencies and feeding back into the power amp inputs through feedback networks from the output to the input. Lets face it - if you are getting SW reception, you have to have a pretty good antenna and speaker wiring may be it.

If you shield the preamp, do you still have the same effect? If you replace the speakers with short wires to test speakers, does the interference change?
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Old 31st July 2009, 08:57 AM   #6
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Quote:
Originally posted by amptramp
Have you actually confirmed that the signals are originating in the preamp?

There is evidence with certain amplifiers and speaker layouts of speaker wiring acting as an antenna at certain frequencies and feeding back into the power amp inputs through feedback networks from the output to the input. Lets face it - if you are getting SW reception, you have to have a pretty good antenna and speaker wiring may be it.

If you shield the preamp, do you still have the same effect? If you replace the speakers with short wires to test speakers, does the interference change?
Yes, in most cases (but only by MM RIAA amps and only by such without OP-AMPs). But your estimate are right. However, in all cases until this day, I was been able to eliminate unwanted reception.
Therefore my main question was an another:
Where are to find any informations in generall about theoretical background from this topic (please read at begin of this thread)?
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Old 14th October 2009, 12:18 PM   #7
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now I have found a few informations:
http://www.fcc.gov/Bureaus/Mass_Medi...ceHandbook.pdf
audio-nirvana.fortunecity.net/img/fish4002.pdf
http://eisenaudio.com/diy500/datashe...31appnotes.pdf
Audio power amplifier design handbook - Google Books
look from page 400 until 407
The sound reinforcement handbook - Google Books
begin at page 325
http://books.google.com/books?id=ScU...0loops&f=false
begin at page 166

Last edited by tiefbassuebertr; 14th October 2009 at 12:26 PM.
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Old 16th October 2009, 06:00 PM   #8
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Old ARRL handbooks talk about this subject in a general way, not sure that I would necessarily implement things the ways shown in some of the text.

Any good book on EMC/EMI should touch on good design and layout practices for these sorts of issues.

A couple of things I have found that help:

1. Usually it is the case that inputs and outputs are returned to a common starground which is great from an audio standpoint, but the wiring is sufficiently inductive to develop significant rf voltages between the jack shell and chassis and internal signal grounds. A very small disk ceramic from the jack shell to chassis ground right at the jack can really help.

2. Ferrite beads on the signal lead right at the jack may help, in some cases they may be audible in a negative sense so try this with several different types to see which are audibly transparent.

3. A small series resistor at the jack may help. I have found 100 - 1K carbon types to be helpful. (Reduces wiring Q etc.)

4. PCB ground planes directly connected to chassis is effective if this can be arranged to be the system star ground point - otherwise a good ceramic may be ok.

5. Decouple, filter power supply leads coming from outside the chassis.

6. Make sure sources are not able to rectify rfi signals appearing on their outputs - usually a series resistor on the output and step 1 above prevent that.

I think you have probably long since figured out all of the above..
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Old 17th October 2009, 09:57 AM   #9
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Hallo Kevin,
very good advices.
For me it is amazing the reception of unwanted radio stations by playback of records (radio moscow effect) at certain circuits without any additional hum.

Which parasitic elements are creating the unwanted RF respective SW receicer?

PCB ground planes directly connected to chassis without inductance solves the problem by my service work in all cases.
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Old 19th October 2009, 03:09 AM   #10
kevinkr is offline kevinkr  United States
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Basically the presence of rf in places and at levels that weren't intended to see them.

No really simple answer here unfortunately. Anywhere non-linearities lurk and there is sufficient rf potential difference impressed across the device for a rectifier junction to become forward biased for some part of the cycle so that detection can occur..
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