philips: FM De-emphasis R and C

FM De-emphasis R and C

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I have noted that many Philips radios from the 50's already seem to be designed for 75 us de-emphasis although they have been sold in the continental european market. So i suggest that you check the frequency response before making any changes. Apparently, some european broadcasting companies did not decide whether to use 50 or 75 us until the early sixties.

Anyway, the difference between 50 and 75 us de-emphasis is not very large, it corresponds to just a small change in the setting of the treble control.

Note that this radio has a feature to limit the high frequency response when receiving weak FM stations (just like most modern car radios), so its frequency response will vary with the signal strength.

The rightmost diode in the EABC80 is forward biased by the cathode voltage of one of the output valves via resistor R62. But it is also connected to the negative ratio detector voltage, which increases with signal strength, via resistor R44. As far as the resulting voltage is positive, the diode will cause shunting of high AF frequencies to ground via capacitor C83.

So, the de-emphasis circuit consists (at least) of three capacitors - C80, C81 and C83 , and of three resistors R40, R41 and R42, in series with the ratio detector's source impedance which cannot be easily readout from component values, it has to be measured in the real circuit if one is interested in its exact value.

It should also be observed that many FM receivers from the mid-50's have a relatively narrow IF passband which also can influence the AF frequency response.

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Torbjorn,

Thank you for the information.  Looks like I will have to establish a frequency response test procedure in the shop.  Even if I do not change any values my interest has been peaked and needs satiated.

I thought there would be more than just two simple parts to the RC network.  But there was hoping.

Thanks again,

Paul

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If you like, you can find here the Values of 50 and 75 μsec Deemphasis.

Used in Europe and USA.

knoll

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knoll,

This will save me some researching time.  Thanks for the information!

Paul

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I am glad to see this discussion opened up again, because most of us in North America DO hear a difference when listening to radios from Europe, and ones intended for the American market.

Lately, I have been restoring a Grundig  4085, and have added capacitance to the 220 pF capacitor in thge ratio detector, with an extra 120 pF.  Interestingly enough, the bass thump, was even worse than when I started!

I'm thinking, that I might have to do some modifications with the negative feedback.

How would one go about, making this radio compatible with todays broadcasting equipment in North America? 

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Omar,

Please be careful with the Audio negative feed back circuit.  It should have been engineered to be flat or what the acoustic engineers wanted from the radio, speaker, tubes, transformer and cabinet combination.

I have read too many unsubstantiated (untested) comments and conclusions on the audiophile chat boards.

Try inputting a Pink or White noise using a Real time Analyzer into the PU or Tape and check the response.  OR sweep (by hand) the audio range with an Audio Generator to see the actual audio response, of the acoustic system, before modifying the feed back circuit.

Of course this all assumes you replaced all the degraded capacitors and resistors that are suspect.

I hope that I am not speaking out of turn.

Paul.

Additoinal Information.  

This freeware software link, will display a spectrogram of frequency vs amplitude.  You can use this to analyse the Audio section of a radio.  There are other Spectrogram freeware programs available.  I intend to use it for De-emphasis once I figure how to transmit a test signal through the FM tuner.

You should be able to attach your computer's sound card to output of the amp and send in an audio signal.  (Use an attenuator/voltage divider network. See text of the freeware.)  Use an audio generator that you sweep or try some white noise.  Also monitor the input signal with one of the stereo chanels of this program.  That will give you a reference point.

If the system is FLAT the output signal will follow the input signal.  But I doubt if that will work too well.  Because the  volume control compensation networks.  So be aware of where the volume and tone controls are set. 

I intend to sample the audio of the FM receiver at the Tape output.  It should be flat at that point, unaffected by any tonal engineering.  That way should be able to see the de-emphasis.

You will have to option the software for maximum sampling of 44khz.  Select the graph you want to use.  Optioning happends when you start the program from the FILE tab SCAN INPUT.  See the attached picture file for my options.

Put a capacitor in series with the hot lead of both sound card input channels.  Us from about .01 to 0.01 cap to keep any B+ from frying  your computer.  Watch you do not chop off the bass with too small a capacitance.

It is a start.

Let me know if  you did this.

Paul Pinyot.
 

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This absolutely makes sense! 

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Hi Paul,

I would not change C81, but I would add, in parallel, around 520 pF and see what happens.  The rest sounds like you would have to find out about the tone "coloration".

I don't think that this radio was set up for North America!

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