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Old 2nd November 2004, 12:54 PM   #1
Triox is offline Triox  Australia
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Default Input capacitor

In most circuits for amplifiers there is an input capacitor.
I have seen several variations including 2.2uF tantalum, 1uF bipolar, 1 uF electrolytic.100n,4.7uF, and some such as the digi125 don,t show any input cap.

Is there a preferred or better input capacitor?

Would it be possible to replace a 1uF bipolar which is shown in a circuit diagram as the input cap, with say any 1uF cap?

If I use a standard electrolytic I understand the positive goes to the input source.?
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Old 2nd November 2004, 01:17 PM   #2
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The value of the cap is dependent on the input impedance of the circuit. As for the type, that depends on preference I guess.

It may not be a good idea to replace a non-polarised (bipolar) cap with a polarised one as it may fail. The orientation of a polarised cap depends upon the offset voltages around it and whether it will be connected to a single-rail circuit or not. In my own designs any input or output cap is used -ve as the 'free' end.
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Old 2nd November 2004, 05:07 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
Usually the input cap is either bipolar or back to back polar. In both these cases there is no polarity problem.
The longtail pair, if used as the input stage, will have a polarising voltage of about 700mV and should have a resistor tying this to the clean ground to bias the base current. The polarity of a polar input cap must be oriented to suit either the PNP or NPN used for the LTP. You can adjust the time constant of the input cap to give the bass response/roll off you require in conjunction with that bias resistor referred to earlier. Remember that large electrolytics have quite a bit of inductance and might roll of the treble hence the popularity of bypassing with a film or better. Also two back to back caps have half the effective capacitance and raise the roll off point one octave.
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Old 2nd November 2004, 05:13 PM   #4
K-amps is offline K-amps  United States
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I would experiment by replacing the Electrolytic with a nice Paper in Oil / Polyproplene or Teflon cap of same value.

I have not heard good things about tantalums.
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Old 2nd November 2004, 06:06 PM   #5
jwb is offline jwb  United States
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Electrolytics are a poor choice, as their capacitance is directly related to their working voltage. My personal rule is to never use an electrolytic in AC applications.

The Standard Advice is to use a polypropylene or metallized polypropylene capacitor. Depending on the resistor to ground this could be anywhere in the range of 1µF-10µF. Be sure to calculate the right capacitance for the desired corner frequency.

Some amps are DC-coupled but you have to apply these carefully. If I have an AC-coupled output on the source then I don't also need an AC-coupled input on the amp. But if you build a DC-coupled amp you must think before you connect it to any given source component, whereas you can safely attach an AC-coupled amp to any source.
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Old 3rd November 2004, 09:29 AM   #6
Triox is offline Triox  Australia
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Thanks for the assistance , I am building an amplifier, ETI466 "THE BRUTE" which does 300W into 4 ohm,so as I understand it, the input cap is 2.2uF tantalum 16V I would be better off to replace that with a 2uF metallized polypropylene .

I don,t know how to calculate the right capacitance for the desired corner frequency.

I can tell you that in the circuit diagram the cap goes through two 1k resistors to the base of a BC547, and also there is a 10K resistor after the 1st 1k on the input to ground.

There certainly is a lot more to solid state amplifiers than I thought.

Also as the poly cap is not polarised I guess either way inserted doesnt matter.
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Old 3rd November 2004, 12:29 PM   #7
K-amps is offline K-amps  United States
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Quote:
Originally posted by Triox
Thanks for the assistance , I am building an amplifier, ETI466 "THE BRUTE" which does 300W into 4 ohm,so as I understand it, the input cap is 2.2uF tantalum 16V I would be better off to replace that with a 2uF metallized polypropylene .

I don,t know how to calculate the right capacitance for the desired corner frequency.

I can tell you that in the circuit diagram the cap goes through two 1k resistors to the base of a BC547, and also there is a 10K resistor after the 1st 1k on the input to ground.

There certainly is a lot more to solid state amplifiers than I thought.

Also as the poly cap is not polarised I guess either way inserted doesnt matter.
You seem to be on track there. Also I would experiment with a few other types, but metalized P's are good.
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Old 3rd November 2004, 02:30 PM   #8
Mr Evil is offline Mr Evil  United Kingdom
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I prefer polyester, since polypropylene capacitors tend to be physically very large in those sizes, as well as expensive.
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Old 3rd November 2004, 04:34 PM   #9
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The formula to use is the standard 1st order formula. C=1/(2x pi) X F X R, where C is the cap value in farads, pi is 3.14159265, F is -3db frequency, and R is resistance (input Z, in this case). You certainly dont want to use a bi-polar or tantalum, if you desire the best in sound quality. Use a film type, polyester (mylar) being the entry level. Polypropylene is better, and probably the most easily found.
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Old 3rd November 2004, 05:36 PM   #10
jwb is offline jwb  United States
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Your input filter is a high-pass filter with a capacitor (size as yet undetermined) and 11K resistance to ground. The corner frequency f0 is equal to 1/(2pi*R*C), in Hz. If you wanted to pass frequencies higher than 20Hz, you could use a 1uF cap. If you used 2.2uF you'd pass everything higher than about 7Hz.

2.2uF is the practical limit of polypropylene capacitors. Anything larger than 2.2uF will tend to be physically very large.

I recommend you get a copy of Horowitz & Hill Art of Electronics, and maybe play around with this Java applet that explains high-pass filters: http://www.st-andrews.ac.uk/~www_pa/...hpass/hpf.html
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