What cutoff value do you guys select when using passive RC input low pass filters? I've heard you need to select it much higher than 20khz for some reason, what's the lowest you can go without affecting the audio range.
This is for a phone headphone output going into a chip amp via a 3.5mm jack.
Thanks.
This is for a phone headphone output going into a chip amp via a 3.5mm jack.
Thanks.
That depends upon the load impedance, inpit impedance and the chosen frequency.
This will give you the nformation you require; RC Low-pass Filter Design Tool
The human ear cannot hear anything above 18kHZ or 14kHZ when age advances.
Anything higher than those frequencies, is a waste of power.
This will give you the nformation you require; RC Low-pass Filter Design Tool
The human ear cannot hear anything above 18kHZ or 14kHZ when age advances.
Anything higher than those frequencies, is a waste of power.
But wouldn't the impedances of the source and amp not matter since the RC values can be adjusted to get the desired corner frequency?
I'm just looking for some ballpark RC cut-off frequencies really, something that'll be good enough.
My current corner frequency is currently 160khz, is that ok?
I'm just looking for some ballpark RC cut-off frequencies really, something that'll be good enough.
My current corner frequency is currently 160khz, is that ok?
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An RF filter is installed to prevent the amplifier from overloading with interference. Industrial installations work in the range specified by you - lamps, SMPS, inverters, etc.
RC filter is far from brick wall filter. It provides some attenuation at much lower frequencies than its corner frequency. It also introduces some phase shift. 200kHz is perhaps sufficiently high to ensure negligible impact in audio band and it goes even higher in many commercial amplifiers. However, 50-60kHz might be acceptable to, depending on individual taste. Some experimenting is recommendable.
Given corner frequency can be achieved with various combinations of R and C. Higher R and lower C vs lower R and higher C. Output impedance of the preamp adds to R. Therefore higher R/lower C combination depends less on preamp impedance. However, high R might increase noise with bipolar input transistors (JFETs are much less affected). So R should not be too high. Perhaps 1k – 2k2 for bipolars and up to 10k with JFETs.
Given corner frequency can be achieved with various combinations of R and C. Higher R and lower C vs lower R and higher C. Output impedance of the preamp adds to R. Therefore higher R/lower C combination depends less on preamp impedance. However, high R might increase noise with bipolar input transistors (JFETs are much less affected). So R should not be too high. Perhaps 1k – 2k2 for bipolars and up to 10k with JFETs.
Should be OK. Audible difference between 100kHz vs 400kHz is very subtle (if any) and which one is better is subjective decision. When everything settles you can experiment little bit and decide if you prefer some other value. 160kHz is good starting point.
I would be interested hear what RC values you use for input?
For the present I have used 2k2 and 220pF.
This gives like 300kHz.
Maybe this is too high?
For the present I have used 2k2 and 220pF.
This gives like 300kHz.
Maybe this is too high?
I have now a power amplifier which have upper bandwidth freq 900kHz.
What input filter should I use?
Input potentiometer is 10k and the R is 2.2k
What would be a good C to use?
What input filter should I use?
Input potentiometer is 10k and the R is 2.2k
What would be a good C to use?
What is the output impedance of the preamp? Is it variable or constant? If it is a passive preamp the output impedance can vary substantially over the volume pot wiper sweep.
Is the 10k pot at the very front end of your power amp? That pot is effectively another passive preamp seen by the 2k2 R.
What is the input impedance of the power amp seen by the shunt C in question, looking into the input stage? Is it much greater than the sum of your preamp output impedance, a 2.5kohm (1/4 of the 10k pot), and 2.2kohm? If it's much greater, like 5-10 times greater, it can be neglected. Otherwise take it as a resistance in parallel with the above mentioned sum of impedance in calculating the total R value.
Is the 10k pot at the very front end of your power amp? That pot is effectively another passive preamp seen by the 2k2 R.
What is the input impedance of the power amp seen by the shunt C in question, looking into the input stage? Is it much greater than the sum of your preamp output impedance, a 2.5kohm (1/4 of the 10k pot), and 2.2kohm? If it's much greater, like 5-10 times greater, it can be neglected. Otherwise take it as a resistance in parallel with the above mentioned sum of impedance in calculating the total R value.
use a 330 pf cap for c8.
also, put a 100 pf cap between the base and collector of U1
That should get you around 200Khz
The bandwidth of the amp may have little play here. The amp's slew rate is a lot more in context. The input low pass filter is for minimizing the chances of overly fast signal slew-overdriving the amp. Generally anywhere 150-500KHz for the corner frequency is ok, lower being more robust in protection.
I settled for 150pF cap.
This will give 250kHz to 400kHz depending on the potentiometer setting.
This will give 250kHz to 400kHz depending on the potentiometer setting.
Note it is wise to stop RF at the door, i.e. have a capacitor on the input connector itself, before any of this low frequency stuff. RC filters on the input have to be a compromise as the R introduces more noise - however make R too small and C too large and you risk making it hard to drive by being a capacitive load, so its best to keep R at or above 100 ohms. The input RF cap has to be small for the same reason, but even 100pF will do a lot to combat WiFi, phones and other microwave interference that's now commonplace.
An RCRC filter is also a possibility (well, with the RF cap on the input connector its an CRCRC filter), which gives better ultimate roll-off.
An RCRC filter is also a possibility (well, with the RF cap on the input connector its an CRCRC filter), which gives better ultimate roll-off.