Low Frequency Performance Mod

for M Audio Delta Audiophile 192

or

How to go from this:

To this:

By cutting 1 trace on the circuit board, and adding 1 capacitor!

WARNING! Warranty Void zone - proceed at your own risk.

Modulation Arts assumes ABSOLUTELY NO RESPONSIBILITY. Don't do it if you're not willing to risk it.
You could destroy your card. You have been warned.

Now that we got that out of the way, and you're still here, let's continue.

The unmodified AP192 has highly mediocre low frequency performance.

The primary culprit is a High Pass Filter inside the AK5385A A/D converter.

This high pass filter (HPF in the diagram above) rolls off way too high for use with composite signals, causing huge overshoots on bass peaks. However - the data sheet tells us it's optional!

Here's the pinout of the chip:

As you can see, it's simply a matter of not pulling Pin 19 high.

The easiest way to accomplish this, is to cut one trace on the circuit board - the one leading to Pin 19.

This will improve the low frequency handling of this card by an order of magnitude.

After cutting the trace, leaving pin 19 unconnected

An order of magnitude improvement, but it still has a very slight inwards tilt. We can do better.

The remaining culprit is an undersized DC coupling capacitor.

We solve this by adding a 470uF 16-volt capacitor in parallel. Do not remove the original one - there is no advantage to removing it, and removing it may in fact affect high frequency performance.

As always with electrolytics, pay attention to the polarity.

Negative (minus) side needs to be closest to the connector edge of the card.

This modification completed, squarewave will look like this:

If all you want to use this sound card for is one channel of composite (unbalanced), then you're done already - install it into your computer and enjoy!

However, if you want stereo (unbalanced), you need to add one more capacitor:

If you want balanced input, you'll have to add two more. Looking at the front of the card, you see a group of 4 capacitors, for two of which you added two bigger capacitors. Add two more, to assist the two remaining unmodified ones.

The modifications above (cut trace, add capacitors) only affect the input.

The unmodified output of this card is also mediocre:

By adding two 470uF 16 volt capacitors here:

The negative (minus) side needs to be towards the bottom of the card, that is, towards the PCI corrector.

The output performance improves dramatically:

With these modifications, the output of the card is 100% perfect with just a hair of software tilt compensation.

However, the inputs, even with these modifications, it's not 100% perfect. DC Straight would be better. The problem will manifest itself as occasional bass overshoots (on sudden heavy bass) up to around 5%. However, it's relatively rare (depending on program material), so accurate modulation measurement is possible as long as one keeps this limitation in mind - and the DC servo in MpxTool will compensate for this without breaking a sweat.