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-   -   Omitting input cap and resistor? (http://www.diyaudio.com/forums/chip-amps/227054-omitting-input-cap-resistor.html)

iggy111 4th January 2013 10:20 PM

Omitting input cap and resistor?
 
I'd like to know what you guys think about omitting input cap and/or resistor in a GC?
My GC really sounds better without the input cap, and reading the audiosector.com pdf manual I came to a conclusion that these are not crucial for safe operation. Also a pair of good sounding caps costs more than the kit itself...

Since the cap is there for DC-decoupling, when does actually the danger of DC occur - at power on/off, during the operation, or only in case of malfunction?

And (newb alert) - what does the resistor do anyway?

nigelwright7557 4th January 2013 10:46 PM

It is DC from the previous stage like a pre amp, MP3 player etc.
If the amp starts amplifying DC you could fry your speakers with DC.

iggy111 4th January 2013 11:12 PM

Yes, I understand it's from a previous stage, but do preamps or cd players exhibit DC on output normally or only if they malfunction?

nigelwright7557 4th January 2013 11:16 PM

Normally they are biased around zero volts.

The problems occur if someone has designed a pre amp that is single rail and they haven't put a coupling capacitor on the output.

nigelwright7557 4th January 2013 11:31 PM

One possible source of DC are guitar pedals which run off 9VDC.
The bias will be 4v5 which might be on the output depending on whether or not coupling capacitors are used.

Andrew Eckhardt 4th January 2013 11:45 PM

Almost all mainstream audio signal sources have around 200 ohms series output resistance added to protect against overload if nothing else, and capacitor coupling. Best thing to do is obtain a circuit drawing for your units, and see exactly what's there. The series input resistor on the power amp helps define the source impedance and on a non-inverting amp probably most important in setting up a low pass filter with a 100's picofarad range capacitor to ground. In the absence of this capacitor the second most important function might be related to noise and input bias current if there's no DC blocking cap. If you're sure you can hear the sound of an input capacitor, then you're probably okay to go fiddling around with the value of of the series resistance any way you want in the audiosector gainclone circuit pdf I've seen.

madtecchy 5th January 2013 07:27 AM

Just be sure to power up the chip amp last and power off first..

jackinnj 5th January 2013 09:39 AM

There's input bias current from the bipolar input transistors. This impresses itselfupon the input loading resistor and biases the amp with DC. It also biases the output stage of the preceeding step in the signal chain if there is no blocking cap. You can avoid the issue with a jfet buffer -

iggy111 5th January 2013 03:34 PM

So, if I got it right, the input cap could be redundant if the previous stage already has one; or it could be replaced by a buffer?
I'm using a Rotel cd-player and my pc with a E-mu 1212 soundcard to listen to music, do you think those could be dc problematic? Considering the amp is ALWAYS last to turn on and first off.

About the input resistor, if it biases the chip and is omitted, isn't there the problem of the chip working in its non-linear region? And where can I find the min and max values for it? I.e. does the chip sound better and heat more if it's biased towards a-class operation?

pacificblue 6th January 2013 07:01 PM

This is how a typical output stage in a source looks.

http://up.picr.de/13040320dp.jpg

The resistor values may be different. The transistors shunt the output to ground during power cycling for popless operation.

The electrolytic cap AC couples the output to the following device, e.g. an amp, and it forms a high-pass filter with the 100 k resistor.

Imagine you connect a DC coupled amp to that output stage.
  1. Ignore the two 100 r resistors. The amp's input resistor falls in parallel to the 100 k resistor, which means the total resistance decreases and the roll-off point rises accordingly. E.g. if you use the 20 k resistor as specified in the kit, the resulting -3 dB point will be 6 times higher than intended.
    Some sources use 1 meg instead of 100k. The resulting -3 dB point with a 20 k input resistor will be 19,6 times higher than intended.
  2. The same is true for the amp's input resistor. The actual resistance will be smaller than the input resistor value due to the 100 k resistor in the source and you will have a different DC offset than expected. Different source, different value, so you don't even have a reliable DC bias, if you calculate the resistor of a certain source in.
On the other hand if you use an AC coupled amp the two cascaded filters will add up to a steeper slope instead of a higher roll-off. And the bias will be independent of the source's output stage.

You cannot find the min and max values for the input resistor anywhere. You have to calculate them according to your application.

If you bias the chip towards class A operation it will dissipate more heat and you will have a high DC offset at the output that may force you to use a DC blocking cap at the output. That is likely to do more harm than you gain from the class A operation.


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