03-29-15: Kijanki
My class D amp switches at 500kHz. This square wave is filtered by the Zobel network at about 60kHz. Harmonics are most likely filtered out and main problem is about 1% residue of 500kHz switching frequency present on the speaker cable. makes sense Kijanki - 500KHz is 3.05 octaves away from 60KHz. Zobel network/filter is most likely a 2nd order filter which is 12dB/octave implying 36.6dB of attenuation of the 500KHz. That works out to 1.48% of residual 500KHz on the speaker binding posts. |
Thanks for the info Bombaywalla and Kijanki.
I tried the test again using the AM radio. I couldn't even find an AM station that was tunable. There was too much buzzing on the whole range. I'm thinking this is caused by RF leakage from my 3 class D amps, all located within a 5ftx 2ft area.
I plugged the radio into an outlet in my adjacent dining room (less than 20ft away) and the buzzing (hash?) went away and I was able to tune in a station in the low AM band.
I then moved the radio as close to my amp rack as the cord would allow (about 10ft away) and there was no interference; the station continued to be clearly audible without hash/buzzing. I was thinking the interference would increase as I moved the radio closer to my amp rack (from about 20 feet to about 10 feet way) but it did not.
These somewhat mixed results have me a bit confounded. I think further testing is called for, perhaps with a better AM radio, before conclusions are drawn. My current thought is that at least one of my amps is producing RF interference but I don't know for certain at this point.
I'm going to get a better AM radio, plug it in in my dining room using a 20ft extension chord, tune in a station on the lower AM band and retest by walking toward my amp rack in the adjoining living room. I'll post again with results.
If anyone has a better idea, please let me know.
Thanks,
Tim
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Hi Tim,
If you are going to get another AM radio, consider purchasing a portable one that is battery powered. Also, the test will be most sensitive if you tune to a frequency at which no station is present. If a station is present the radio's AGC (automatic gain control) circuit will reduce its sensitivity, especially if the station's signal is strong.
Best regards,
-- Al
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Hi Tim, I recommend that you repeat test walking around with the radio as follows....
Amp 1 on, 2 and 3 off;
Amp 1 off, 2 on, 3 off;
Amp 1 and 2 off, 3 on;
Amp 1 and 2 on, 3 off;
Amp 1 on, 2 off, 3 on;
All three amps on.
G. |
Guido and Al,
Both good ideas; I'll get a battery powered portable radio (I've been needing to buy a new hair dryer, anyways) and set the dial to a low frequency on the AM band that lacks a strong station signal. Next I'll test with the various amps on/off according to the permutations Guido listed.
This amateur RF interference technician will report back on results soon.
Thanks,
Tim
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Tim, if you use a battery powered radio, and you find that one of the amps seems to leak, try sliding the radio along/around the amp case, and then along the power cord, the speaker wires, and the input ICs.... There is no telling from where the leak might originate... It might even come from a totally different device... Make no assumptions, as reasonable as they might seem.
G. |
03-31-15: Noble100
Thanks for the info Bombaywalla and Kijanki.
I tried the test again using the AM radio. I couldn't even find an AM station that was tunable. There was too much buzzing on the whole range. I'm thinking this is caused by RF leakage from my 3 class D amps, all located within a 5ftx 2ft area.
I plugged the radio into an outlet in my adjacent dining room (less than 20ft away) and the buzzing (hash?) went away and I was able to tune in a station in the low AM band. why do you need to get a new AM radio? Why can't you repeat the test (where you cannot even tune into any AM station due to the supposed RF interference from one of your class-D amps) with the existing AM radio & simply switch off one class-D amp at a time until you can tune into a AM station using Almarg's suggestion for the AM freq? isn't this equally valid (as Guidocorona's suggestion of switching on the class-D amps one/2 at a time)? thanks. |
Bombaywalla,
You're right, I could do as you suggested but I don't want to be tied to a cord and handheld portable radios aren't expensive.
Guido,
Okay, I'll keep an open mind and see if I can pinpoint the leakage point if RF interference is detected.
Rookie Midwest RF Interference Technician,
Tim |
My feeling is that a portable battery powered AM radio would be preferable. Not only for the obvious convenience factor, but because it may make it possible to more accurately locate the source of any RFI, and to more accurately assess its relative amplitude as a function of distance. With an AC powered radio there may be ambiguity as to whether what is being picked up is being picked up by the built-in antenna, or by the long power cord acting as an antenna, or that is perhaps being routed into the radio via the power wiring itself.
Just my opinion. Best regards,
-- Al
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All,
For something that has absolutely nothing to do with the sound of your system, RF interference/leakage testing is sure a pain in the butt; mainly since I'm handicapped and walk with a cane.
As I stated I would in my last post, I bought a $15 Craig portable AM/FM radio that runs on 4 AA batteries (I also bought a new hairdryer I've needed so this whole thing already has been worthwhile).
Having completed a somewhat lengthy and definitely aggravating process, I want to complete it here by documenting my findings thoroughly. What follows is probably overkill but I hope to never need to duplicate this little experiment on my system and would like it to be of benefit to others wanting to test for RF interference/leakage.
My audio rack is 5ft wide and 2ft high and is located directly below my wall mounted 5ft wide and 3ft high HDTV. The rack has a solid walnut top shelf that holds a D-Sonic mono amp on each end, with an Oppo 105 located between them. Below are 2 compartments that are accessed via 2 swing-down doors and hold my sub, center and surround amps. All system amps are class D except the class A/B sub amp.
I assigned the amps as follows:
Amp#1: D-Sonic M3-600M class D mono pair on top shelf.
Amp#2: ClassD Audio SDS-440-CS class D stereo amp located in left lower enclosed compartment.
Amp#3: Dayton Audio SA-1200 mono class A/B sub amp/controller located in left lower enclosed compartment.
Amp#4: Emerald Physics EP-100.2-SE stereo amp located in right lower enclosed compartment.
I began the testing by tuning the Craig to the lowest station on the AM band that had decent reception. The lowest range station was unfortunately mid-band at '1070 KHZ' but this setting proved to be good for testing purposes. My process was to move the radio, set at half volume and tuned to 1070KHZ, from a few feet away to a few inches away from each amp on all sides and top. I did the following tests:
Test#1: All amps off.
Result: No loss of signal, static or buzzing near any of the 4 amps. However, there was some loss of signal and buzzing between the amps as I walked from the right mono amp to the left. I was able to pinpoint the source to the small Directv satellite tv 'client server' box located just to the right of my Oppo. This box was off but the signal loss and buzzing got worse when I turned it on. The RF was emanating from the small box as well as the power cord leading to my multi-outlet surge protector power strip.
Test#2: Amp#1 (mono pair) on only with all others off.
Results: Same as above with no RF leakage detected from any amp but buzzing and loss of station when near Directv client box and power chord.
Test#3: Amp#2 on only with all others off.
Results: No RF leakage detected from any amp just from Directv box and power chord.
Test#4: Amp#3 on only with all others off.
Results: No RF leakage from any amp just from Directv box and power chord.
Test#5: Amp#4 on only with all others off.
Results: No RF leakage from any amp just Directv box and power chord.
As you can see, the results were clear that none of my amps had RF leakage but the small satellite tv client box and power chord did. I'm not going to take any action since, as Guido correctly stated, there are no adverse audible affects of this RF leakage on my system.
Tim |
Good news, Tim, and thanks for undertaking the effort. To others who may wish to perform similar tests in the future, though, I would repeat one of my earlier statements: 03-31-15: Almarg
... the test will be most sensitive if you tune to a frequency at which no station is present. If a station is present the radio's AGC (automatic gain control) circuit will reduce its sensitivity, especially if the station's signal is strong. Best regards, -- Al |
For something that has absolutely nothing to do with the sound of your system, RF interference/leakage testing is sure a pain in the butt; mainly since I'm handicapped and walk with a cane. RF itself is way above our hearing range but different frequencies can mix on any non-linear element in the system, producing new frequencies that might be audible. Al mentioned it many times. Keep your system clean from interference (shielding, keeping things apart, using different outlets etc.). Anything can be the gateway, including speaker cables. |
Hi Almarg,
I did initially try your recommended method (tuning to an area low on the AM band with no station present) but found it difficult to determine when RF interference was present. Tuning to a station with only decent reception (not an overly strong signal with very good reception) made it obvious to me when RF interference was present; the signal strength would decrease (sometimes going out completely), static would increase and a buzzing sound would begin.
Your method may work better for some than it did for me but I wanted you to know I appreciate your input.
Thanks,
Tim |
I did initially try your recommended method (tuning to an area low on the AM band with no station present) but found it difficult to determine when RF interference was present. Good! That provides further confirmation of your conclusion. If significant RF was being radiated it's pretty much inconceivable to me that you would not have perceived it under that condition, as some combination of static, noise, and buzzing, most likely at any reasonable setting of the volume control. Best regards, -- Al |
Almarg,
That's good news. And, yes, I agree that I would have noticed any combo of static, noise and buzzing. None was present even at a fairly high (1/2 of max) volume.
Thanks for your assistance and verification,
Tim |
