is 60mv too high to drive a tweeter directly? - Page 3 - diyAudio
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Old 17th September 2008, 11:30 PM   #21
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well, i didnt get to test it for frequency response and noise yet, but i added a 2.2uf ceramic 1206 cap in series with the 1K resistor in the feedback circuit and just measured 1.5mv offset with no load on it..

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Old 18th September 2008, 08:21 AM   #22
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by thetube0a3
i was considering throwing a capacitor in series after the pot and a resistor to ground, so the pot wouldnt effect offset anymore.
this is the correct way to do it. But you should also add the DC blocking cap to the feedback loop.
Do not try to use mixed AC and DC coupling. i.e by omitting one DC blocking cap. Either use both or neither.
If you go the DC coupled route (neither) then i recommend a DC detect circuit at the output and a DC disconnect circuit/relay to protect the downstream equipment. Even better would be to combine this with a DC servo circuit to maintain near zero offset until it cannot correct any further.
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Old 18th September 2008, 08:26 AM   #23
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by thetube0a3
well, i didnt get to test it for frequency response and noise yet, but i added a 2.2uf ceramic 1206 cap in series with the 1K resistor in the feedback circuit and just measured 1.5mv offset with no load on it..

thanks people!
What value of capacitor do you need as a DC blocker for driving that tweeter flat down to 1500Hz?
Try replacing that 2u2F ceramic with a 100nF polypropylene film/foil.
100nF & 20k gives F-3db~=80Hz.
You could even try 47nF or 68nF as a blocking cap.
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Old 18th September 2008, 12:06 PM   #24
SY is offline SY  United States
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Andrew, this is the capacitor in series with the shunt resistor in the feedback circuit, not the input cap. So he really needs to go in the other direction, since the LF rolloff will be set by the time constant between the cap and the 1k.

For an f3 of 5 Hz, C = 1/(2pifR) = 1/(6.28)(5)(1000) = 32uF, approximately. I'd use a minimum of 68u, and 100-200u would be even better (thinking LF phase shift).
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Old 18th September 2008, 02:39 PM   #25
AndrewT is offline AndrewT  Scotland
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Whether it's the input DC blocking cap or the NFB DC blocking cap, he does not need an F-3dB of 5Hz for a tweeter amp that only requires to be flat down to 1500Hz.
About a decade below that frequency is sufficient for the input cap and half an octave lower for the NFB cap.
If the Zin = 20k then 56nF gives F-3dB ~=142Hz.
If NFB Rlower =1k0 then 1.6uF gives F-3dB ~=100Hz.
Both these capacitors are small enough to allow selection of a high quality film cap.
Using F-3dB allows the active filter and the tweeter driver to interact without the power amp response interfering with the overall phase and attenuation in the two to three octaves around the crossover frequency.
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Old 18th September 2008, 02:49 PM   #26
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i agree with andrew. the reason i used 2.2uf for both is because it is what i have on hand. i used high quality ceramics.
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Old 18th September 2008, 02:56 PM   #27
AndrewT is offline AndrewT  Scotland
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some would say there is no such thing as a high quality 1206 ceramic cap.
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Old 18th September 2008, 03:53 PM   #28
SY is offline SY  United States
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Quote:
he does not need an F-3dB of 5Hz for a tweeter amp that only requires to be flat down to 1500Hz.
Yeah, your point is well-taken. I'm too accustomed to using solid state at the bottom and tubes at the top.

If the ceramic is an NP0, it should work fine; otherwise, I'd stick something like an FKP in there.
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Old 18th September 2008, 04:36 PM   #29
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yea, i like NP0 / C0G caps my self
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Old 22nd September 2008, 03:33 AM   #30
KBK is offline KBK  Canada
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Actually, you need to be completely linear well out of the pass-band. Which means at least 10uf of decent high grade capacitance. If not, you are still using a crossover that is passive.

If you are using a capacitor there at all, it is arguable that you are not using an active crossover at all, and you have the problems of both active and passive, instead of the problems of one only.

And ceramic doesn't have anything to do with fidelity. I certainly wouldn't use them with digital circuitry either. I leave them with the companies who make them, or pull them of any circuits I own that use them, if it is an audio or video application. I can barely stand tantalum. Mylar? off the list and down in the same hole as ceramic.
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