By Forr:
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Well - I had measured HF behaviour at poweramp speaker terminals and at speakerbox binding posts quite many times (always when I made a new product). For zipcord-like speaker cable and for RG-213 coax used as a speaker cable. For the RFI induced voltage, the speaker cable behaves as misterminated transmission line, as Bob has already mentioned. By placing an appropriate resistor at speaker binding posts we get a one-side terminated line. Reduction of HF content can be expected as 5 - 10 times. Some power is lost within the resistor. The effect is audible, but do not overemphasize expectations.
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Well - I had measured HF behaviour at poweramp speaker terminals and at speakerbox binding posts quite many times (always when I made a new product). For zipcord-like speaker cable and for RG-213 coax used as a speaker cable. For the RFI induced voltage, the speaker cable behaves as misterminated transmission line, as Bob has already mentioned. By placing an appropriate resistor at speaker binding posts we get a one-side terminated line. Reduction of HF content can be expected as 5 - 10 times. Some power is lost within the resistor. The effect is audible, but do not overemphasize expectations.
For less power loss, could a series RC fit the bill ?
Maybe we should think about using properly RF-terminated 50R BNC or Twinax for line-level connections too ? Especially those that come from a CD player, for instance...
Maybe we should think about using properly RF-terminated 50R BNC or Twinax for line-level connections too ? Especially those that come from a CD player, for instance...
Hi Juergenk,
---bipolar output transistors rectify rf, providing am-demodulation---
Do you have some data ?
---bipolar output transistors rectify rf, providing am-demodulation---
Do you have some data ?
oh no, just some personal experience from driving through a medium wave-antenna plant with a shut off car stereo.forr said:
The rectifying output stage seems the most likely theory to explain what I heard from the speakers.
Regards
PMA said:
Very good. The Zobel at the speaker end need not consume much audio power, since it only needs to come into play at higher frequencies, and the resistor need only be 50 ohms or so. I figure if it comes in by 100 kHz, it will probably be all that is needed. A capacitance of about 0.033 uf will do this.
Cheers,
Bob
peufeu said:
This may not be necessary, since there is usually a source output impedance from a preamp or whatever of 50 ohms or more, essentially resulting in a quasi-source-terminated situation.
Bob
I think T Holman wrote about this (BJT vs FET and RF) in the JAES in the early '80s.
I remember a 5534 problem on a mixer board I solved by replacing the 5534s.
Anyway bipolars do rectify RF
I remember a 5534 problem on a mixer board I solved by replacing the 5534s.
Anyway bipolars do rectify RF
I can confirm RFI troubles concerning 553X opamps, and 2134 or 275 improvement (though I prefer OP275 to OPA2134).
I agree with Bob that BJTs are more sensitive to HF rectification, and I prefer FETs both as output and input stages.
I agree with Bob that BJTs are more sensitive to HF rectification, and I prefer FETs both as output and input stages.
PMA said:
Hi PMA.
I think that it’s hard to generalise about which devices are necessarily better in this regard because there are so many other factors involved. You can make a JFET input stage, for example, that is just as susceptible to RF detection than a BJT input stage.
To effectively demodulate an AM signal, you do not necessarily need to rectify it. A sufficiently non-linear response to the AM signal will do the trick. A small signal stage overloaded by the RF will do. In some instances in this regard, JFET’s can be worse here due to higher bandwidth. Product detection will also demodulate AM. For instance, an AM signal limited by a high bandwidth amplifier stage can mix with a lower amplitude, non-limited version of itself in a successive stage of the amplifier, with resultant demodulated audio. At the end of the day, it is still better to stop the RF getting into the amplifier in the first place.
As a matter of fact, in an amplifier grossly infected with RF, all of these demodulation mechanisms can be contributing to the end result to a degree.
Designing a circuit that is not as susceptible to AM demodulation isn’t a very good cure, because any RF can negatively effect the performance of audio circuits in other ways.
Well that’s from my experience designing and building audio processing / modulation (AM, SSB, ect) circuits for RF transmitters anyway.
Cheers,
Glen
Glen,
comparing non-degenerated BJT input differential pair with JFET input differential pair, could you indicate how the JFET pair becomes more susceptible to RF, assuming both have similar BW?
comparing non-degenerated BJT input differential pair with JFET input differential pair, could you indicate how the JFET pair becomes more susceptible to RF, assuming both have similar BW?
PMA said:
I think it's safe to say that a non-degenrated BJT input stage would generally be worse.
Cheers,
Glen
Maybe due time for you to give a thorough lecture on ways of preventing RF to enter, young stud master.
(minus the coil)
(minus the coil)
Bob Cordell said:
50 ohms at the output of an opamp is RF-termination only if you use a high-speed opamp which has enough feedback at very high frequencies to keep its output impedance tightly controlled... this is not the case with most audio opamps.
And the impedance of audio cables is unspecified.
jacco vermeulen said:
Hmm....I'm not into giving lectures. You can either take something of value from what I said or disregard it. Thanks for the intelligent post.
Hey Glen, I think it was an attempt at humor, but the idea is sound, no pun intended, and maybe a discussion of suppression would be useful. I can basically shunt the stuff to ground with a small cap and/or block it with an inductor. I don't pay too much attention to construction techniques like completely closed boxes with feedthru caps on all connections, but maybe there's some merit there. The problem with everything I've tried is that I get measurable phase shift in the audio band because of it. Audible, I doubt, but I don't want it. Because my ultimate test is differential measurement, I make unusual demands on flat response and minimal phase shift.
Who is the real enemy? Cell phones? Computers? Light dimmers? Gasp! Hams?, Stinkin CFLs? AM and FM radio stations? Actually I've been bitten by all but hams and cell phones. Illegal CBers used to be a problem, but I haven't picked them up for quite a while.
Who is the real enemy? Cell phones? Computers? Light dimmers? Gasp! Hams?, Stinkin CFLs? AM and FM radio stations? Actually I've been bitten by all but hams and cell phones. Illegal CBers used to be a problem, but I haven't picked them up for quite a while.
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