After recently doing a full re-cap of the Amiga 1000 that had been collecting dust in my back room for the last two decades, I decided on also performing the audio filter mod. A quick search led me to a number of sites, and when I came across this post I liked what I saw.
I always prefer mods that have the least amount of impact, and this one fit the bill nicely, but as I was studying the mod instructions something occurred to me. The op amp that follows the filter is configured as a simple unity gain buffer; it's not configured to provide filter gain. The filter itself is passive, so I should be able to simply bypass it with a short between the two op amps. I reasoned that the loading caused by the filter components would have minimal effect, if any, on the low impedance output of the first op amp, so I set about creating a simulation in LTSpice to test my theory. The simulation showed no difference in sound quality between the original mod and my alternate mod, so I gave it a try, and it worked just fine.
I'm posting this alternate mod as an addendum to the one described in the original post. Be aware that this mod is only to be used if you want to disable the filter permanently. If you want to make the filter switchable, then you must use the mod described in the original post.
The alternate mod is simple, and requires soldering only two wires across pins on IC U5G. The mod can be done on either the bottom or the top of the board, and is as follows:
- Connect a wire from PIN 14 to PIN 3.
- Connect a wire from PIN 8 to PIN 5.
That's all there is to it. For those of you interested in how I verified my alternate mod with LTSpice, continue reading.
When I embarked on verification of my audio filter mod the first thing I looked at, since I was curious, was how the Amiga 1000 audio filter behaved. The following figures show the schematic for one channel of the audio filter as entered in LTSpice, and the frequency response of the signal at the output of the circuit.
The filter cutoff frequency is right around 3.5KHz, and the response is fairly linear in the passband, with a drop to about -3 db at 10 Hz. The stopband response drops off sharply with a slope of about -30 db per octave. This filter produces an output with bass and midrange, but not much in the high end. The sound mimics that of coin-op arcade cabinets.
Next, I looked at the mod from the original post.
The passband response and cutoff frequency are pretty much same as with the unmodified circuit, but the drop off in the stopband is much more gradual, about -6 db per octave. The sound is definitely much brighter with more high end than with the filter in the circuit.
And finally, I compared my alternate mod to the original mod.
The simulation showed that the frequency response of the circuit with the alternate mod is identical to that of the circuit with the original mod.