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High Pitch tone / noise on F5TV2
High Pitch tone / noise on F5TV2
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Old 1st July 2013, 05:27 AM   #1
CLiu is offline CLiu
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Default High Pitch tone / noise on F5TV2

Hi All,

I have just completed my build of a F5TV2 (it has a slightly different configuration, with only 1 pair of output devices and diodes per channel with 26V rails). When I connected it to my speakers, I heard very faint high frequency tone / whining when I put my ear near the tweeter of the speakers (audible on both channels). Later on, I found that this noise varies in amplitude randomly. At times I could hear it from my listening position at about 10ft away from the speakers, and it usually quiets down to only audible when I put my ear near the tweeter after maybe 10 seconds or so, and then would come back once in a while. Also, I brought the amp from my parents place where it was build, back to my apartment, and the noises experienced were a little different. Back at my parents' place, I could even hear some clicks and pops when someone start cooking with the Microwave in the kitchen.

Could this be due to poor power supply filtering / AC noise? If I were to use some sort of power supply filter, is there any brand or model that you guys have found effective to recommend?

Thanks & best regards
Liu
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Old 1st July 2013, 10:27 AM   #2
AndrewT is offline AndrewT  Scotland
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Possibly:
Lack of RF filtering at both the input and the output of the amplifier?
Lack of attention to stability in your "as built" amplifier?
Lack of RF attenuation in other equipment around the house and your neighbours house?
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Old 1st July 2013, 11:12 AM   #3
CLiu is offline CLiu
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Thanks AdrewT, I think the answer to both the RF filtering and attenuation questions are "true". So I am exploring possible solutions for them, like line conditioners / filters for the AC part. But got some further questions:

1. Is there a way to add RF filtering at both input and output of the amplifier?
2. By stability do you mean parasitic oscillation problem mentioned in the V3 part of the turbo article? If yes, then you are right, the V2 is built exactly the same as the schematic in the article. Is there a need to add anything to solve the issue?
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Old 2nd July 2013, 10:10 AM   #4
AndrewT is offline AndrewT  Scotland
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Do RF filtering first.

A passive RC at the input.
A Thiele Network at the output.

Try to find offending equipment around the house that may be emitting RF and attenuate at source.
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Old 3rd July 2013, 04:57 AM   #5
CLiu is offline CLiu
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Thanks. I will explore the RF filtering. I assume it should be aiming to block higher frequencies from entering, so is there any frequency to start with? 22k? 100k?
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Old 3rd July 2013, 06:41 PM   #6
AndrewT is offline AndrewT  Scotland
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160kHz is about the lowest for an F-3dB to avoid cutting some treble.

Many go to 300kHz and a few go as high as 500kHz.

In my view F-3dB @ 100kHz is too low.
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Old 4th July 2013, 04:58 AM   #7
CLiu is offline CLiu
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Oh, wow, I thought 100k is already high enough....maybe not....If I do a calculation, using 470R for the resistor value, a 1800pF capacitor would give F-3dB of 188k, is the calculation correct? I hope the formula I get from Wikipedia is correct. Fc=1/(2*3.142*RC). And from your experience, is there any particular combination of values for the R and C which sounds better, or does it matter at all?

Also, a new development yesterday night (should have done that from the start, silly me). When I turn the volume of my passive pre-amp (its a Creek passive pre, not diy, so I hope I can single that out as a source of problem) all the way down, I found that the whining tone disappeared. So looks like the tone came from something before the attenuator pot. I am using a USB dac as source now, so I am going to swap to some other sources to single out the origin of the tone, whether it came from the source component or the pre-amp itself.

Thanks for the advice on the RF filtering. I learnt something anyway, although it looks like it might not be the problem after all.
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Old 4th July 2013, 05:47 AM   #8
bear is offline bear
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Good idea to put a scope on it to see if there is a parasitic developing.

Do both channels sound the same?
So, with a shorted input, no noise?

How did you do the grounding for your amp?
There may be a ground issue given that lifting the input above ground (turning the volume pot up) causes a problem.

Another thing to try is to put shorting plugs on the input of the passive pre, and turn the volume knob and see if the noise remains or not. You could also try a nominal resistive value instead of the shorted input.

Many computers produce a noise in the DAC when run off USB... I had just such a problem with a box designed for audio recording I/O... computer noise was a problem.

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Old 4th July 2013, 08:14 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by CLiu View Post
..........If I do a calculation, using 470R for the resistor value, a 1800pF capacitor would give F-3dB of 188k, is the calculation correct?.................
Yes.
Easier to source 1nF and adjust using a different resistor.
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Old 4th July 2013, 08:22 AM   #10
CLiu is offline CLiu
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Hmm....I would really love to have a look at it with a scope, but unfortunately do not have access to one.

Both channel sound similar but not exactly the same. There are actually 3 types of noise now that turning the volume down suppressed the tone completely, I can differentiate between them as follows:

1. Faint white noise like buzz only audible with ears next to the tweeters. Sounds similar in both channels.
2. High pitch tone, not sure about the frequency, but probably close to 17kHz, as it sounds similar to those mosquito repelling devices you get from diy stores. This one will be completely silenced with volume turn to 0, but audible from listening position when volume set to maximum. Sounds similar in both channels.
3. Random increase in amplitude of the high pitch tone for very short period of time, maybe 10-20 seconds, and then reduce to level of no. 2. This one appears in individual channel at different times randomly.

As for the grounding, I have 5 PCBs in total, i.e. 1xPower supply board, 2xAmp boards, 2xSpk protection boards. The Power supply, Amp boards all have ground leads that goes to the star ground, which is then connected to chassis ground via a CL-60. The speaker protection boards (purchased from ebay) does not have a common ground output, therefore I did not connect these 2 boards to the star ground, and I just assume that it will be grounded through the speaker out connections from the amp boards which has a ground wire to the star ground.

As for shorting the input, I do not have shorting plugs, and if I were to do it manually, I just have to short the signal to the ground of the input, correct?
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