A1200 sound issues

[Castellen]

@rkauer:
Potentially good advice, but after reading the old "AC coupling needs bipolar capacitors" myth for the umpteenth time, I feel an explanation is long overdue.

The function of these capacitors in the circuit is a DC block/AC coupling, to prevent the op-amp's DC bias from being affected by any load connected while allowing an AC signal to pass freely.

The average voltage on these capacitors will of course be DC, the value determined by the bias of the op-amp.  The capacitors will have a very low reactance at audio frequencies.  Xc = 1/(2 x pi x 1kHz x 22µF) = 7.2 Ohms, just to throw some numbers in.  So in each AC cycle, the capacitors's plates are seen as a near short circuit, as opposed to charging to each alternate AC cycle.  Therefore any instantaneous charge measured on the capacitor's plates remains nearly identical to the DC value.  It's what's known as a long time constant, as the large capacitance takes a very long time to charge, many times longer than the period of the AC waveform.
Simply put, it physically can't charge/discharge fast enough to gain the polarity of each alternating AC cycle.  Think about it; if it did charge fast enough, obviously meaning there is a large capacitive reactance (i.e. a huge resistance at that frequency), then you would simply have very little audio output.

If you had a much higher reactance (so in this circuit, a much lower capacitance), forming a very short time constant where the capacitor would fully charge and discharge to each peak of the AC signal, only then do you need to be concerned about capacitor polarisation.

A standard polarised electrolytic capacitor in this circuit is perfectly adequate.  It's more to do with the long term stability of the electrolyte which causes leaking and other premature failure.


Sorry for the kindergarten-level lesson in analogue electronics, it's just that I cringe every time anyone pipes up with this same old housewife's tale without any background knowledge on the subject :-P  It's basic electronics people, not rocket science!


Back to the problem at hand....
I've fully thrashed this subject to death on various A4000 audio threads here.  You'll find my online article on Amiga audio fault finding and repairs here.  Nearly all of it applies to the A1200 as well.
Feel free to Email me for any more detail.

[Castellen]

Errr... it's not too tricky to understand once you appreciate that a larger capacitance takes longer to charge.  It works a *little* bit like a battery, and like a battery it takes time to charge and discharge.

You have this thing called a resistor-capacitor time constant (the maths goes: Time (seconds) = Capacitance (Farads) x the series resistance (Ohms).

Basically the audio frequency is changing (i.e. swapping polarity) at a rate much faster than the capacitor can charge at, therefore it cannot possibly adopt much charge from the alternating + and - AC cycles.

If you like maths (I don't!) then to bounce a few numbers about....
The series resistance is 1k Ohm (inside the A1200) + the impedance of what you plug into the RCA socket.  10k Ohm would be typical.

So series resistance is 1 + 10 = 11k Ohm.

Capacitance = 22µF = 0.000022 Farads

Capacitor charge time (to one time constant, or 63.2%) is 11000 x 0.000022 = 242mSecs

The frequency of a 0.242 second periodic waveform is = 1/0.242 = 4.1Hz

So if you had a 10k load connected, and you were somehow generating large amplitude frequencies below 4Hz, then the capacitor would begin to get noticably charged and discharged with the alternating current.  Then you'd probably start wanting to look at bipolar capacitors.

I forget the specs on the Amiga's D to A converter, but I doubt it goes too much below 10Hz.

There'll be a heap more detail on the internet about RC time constants.  A quick Google search found this article which explains it quite well.  There's many badly written posts on Wikipedia on the subject too.  Should probably avoid those!

Time for another beer before I keep ranting on all night :pint: