Simple DIY method to get unfiltered high quality sound on your Archimedes!

Arc/RPCs, peripherals, RISCOS operating system & ARM kit eg GP2x, BeagleBoard
steve3000
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Joined: Sun Nov 25, 2012 12:43 am

Simple DIY method to get unfiltered high quality sound on your Archimedes!

Postby steve3000 » Sat Aug 26, 2017 10:27 pm

First a bit of background: I've always found it frustrating that the standard audio output from the Archimedes sounds rather muffled, even when playing very high quality samples/music. The reason for this was Acorn's choice of aggressive low-pass sound filter, which strips out the high frequencies, making samples and music in particular, sound flat and disappointing - especially when compared to certain other home computers of the time.

Thankfully Acorn included an unfiltered output on the aux. sound connector of the early Archimedes, which was fine if you had a hi-fi or amplified speakers next to your computer, providing you were happy making up a cable to a 3.5mm jack from the internal aux. sound pins. But this method was fiddly, it only worked on the big-box Archimedes, the trailing lead out of the computer never looked great and after all that effort, if you don't use you hi-fi or external speakers regularly, you'll still only hear filtered sound from the internal speaker.

So I've always wondered if there could be a better solution...and I'm pleased to say, I've found one :)

New solution: After some experimenting, I've come up with what I think are the simplest methods to bypass the sound filter completely for each model of the Archimedes range. Importantly: These methods remove the filter for both the internal speaker(s) and externally via headphones socket - with no external cables required, and can even be done with no soldering for many big-box Archies. There are two approaches described below, Method 1 is the easiest but only suitable for big-box Archimedes with the aux. sound connectors; Method 2 requires soldering of two wires to the LM324 op-amp chip, and works for all single-box Archies (A30x0's) and the A4000. I'll post pictures of my conversions below, but first here's the details of what to do for each computer.

*DISCLAIMER* - if you follow the procedures here, you do this at your own risk, please only attempt the modifications listed below if you are confident in working with computer PCBs and take precautions especially to avoid static discharge.

Method 1 for large-box Archimedes (A3x0, A4x0, A4x0/1, A540 and A5000):
This method just requires removal or disconnection of two capacitors and the fitting of two 2.54mm shunts to bridge pins on the aux. sound connector. Here's a modified snippet from the A5000 circuit diagram, showing how this method works (other computers are similar):

A5k sound filter bypass.png

No soldering is needed if you use wire cutters to disconnect the capacitors, but if you just cut one leg of the capacitors (eg. to allow reversal of this procedure), you must insulate it with tape to ensure it doesn't accidentally contact the PCB when lose.

A5000
Remove or disconnect at least one leg from electrolytic capacitors C21 and C19
Bridge pins 1-3 and 7-9 of LK3

A540
Remove or disconnect at least one leg from electrolytic capacitors C17 and C15
Bridge pins 1-3 and 7-9 of LK3

A310, A305, A440 (not A4x0/1 series)
Remove or disconnect at least one leg from disc capacitors C22 and C23
Bridge pins 1-3 and 7-9 of PL2

A440/1, A420/1, A410/1
Remove or disconnect at least one leg from disc capacitors C29 and C30
Bridge pins 1-3 and 7-9 of PL2

Method 2 for single box Archiemdes and A4000 (A3000, A3010, A3020 and A4000):
This method requires disconnection of the +ve leg of two capacitors and soldering a wire link from these legs to the LM324 op-amp, because no aux. sound connector is present on these computers. Here's a modified snippet from the A3010 circuit diagram, showing how this method works (the others are similar):

A3010 sound filter bypass.png

A3000 (fairly easy soldering)
Disconnect the +ve leg of electrolytic capacitors C65 and C70
Solder a wire from Pin1 of LM324 IC39, to +ve leg of C65
Solder a wire from Pin8 of LM324 IC39, to +ve leg of C70

A3010/A3020/A4000 (warning: quite tricky soldering!)
Disconnect +ve leg of electrolytic capacitors C60 and C68
Solder a wire from Pin7 of LM324 on Audio card, to +ve leg of C60
Solder a wire from Pin8 of LM324 on Audio card, to +ve leg of C68

Others?
Finally, if you are wondering about the A4, A7000 and RiscPC, then there is no need - these do not have the low pass filter :) (RiscPC and A7000 have a completely revised sound system, and the A4, I expect just didn't have enough space on its PCB!).

Pictures from my conversions will follow below. Please post up here if you try this out and your experiences :)

Steve

steve3000
Posts: 1705
Joined: Sun Nov 25, 2012 12:43 am

Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Postby steve3000 » Sat Aug 26, 2017 10:39 pm

First example: following "Method 1" to modify an A5000.

First open the cover and find the sound circuitry (I've highlighted this with a red rectangle).
tn_A5k_01 sound circuit.JPG

Here we see electrolytic capacitors C19 and C21 (light blue), and aux. sound connector LK3 in the centre of the picture.
tn_A5k_02 capacitors C19 C21 and LK3 up close.JPG

C19 and C21 with +ve legs removed (I used a thin solder tip and pair of tweezers to pull the legs up, but wire cutters could be used). Also see the 'jumpers' added to LK3, bridging pins 1-3 and 7-9. Remember to clean up meticulously and especially if not removing the PCB from the case, be very careful not to leave any stray solder/metal swarf on the PCB.
tn_A5k_03 C19 and C21 legs removed and LK3 jumpers added.JPG

Fold the disconnected capacitor legs out of harms way.
tn_A5k_04 C19 and C21 legs folded out of way.JPG

Add insulating tape to cover secure the legs, and over the pads on the circuit board, to avoid accidental shorting out if the capacitor ever moves.
tn_A5k_05 insulating tape added.JPG

That's it. This modification on the A5000 only took about 10 minutes in total and what an improvement it makes :D

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paulv
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Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Postby paulv » Sat Aug 26, 2017 11:00 pm

That looks really cool and simple for most machines....

I have a slightly unrelated question... Is that A5000 an Alpha with a fifth column ROM??? If so, what's in the ROM?

Paul

steve3000
Posts: 1705
Joined: Sun Nov 25, 2012 12:43 am

Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Postby steve3000 » Sat Aug 26, 2017 11:00 pm

Second example: following "Method 2" to modify an A3010.

First open the cover and find the sound circuitry, shown here.
tn_01 DSC_4431.JPG

Here we see electrolytic capacitors C60 and C68 on the right hand side and LM324 op-amp chip on the vertically mounted audio board in the centre of the picture.
tn_02 DSC_4432.JPG

C60 and C68 with +ve legs removed (again, I used a thin solder tip and pair of tweezers to pull the legs up, but wire cutters could have been used). Remember to clean up meticulously and especially if not removing the PCB from the case, be very careful not to leave any stray solder/metal swarf on the PCB.
tn_03 DSC_4435.JPG

Here I've used fine single-core wire to carefully loop behind pin8 of LM324 op-amp. This was very fiddly! I've also folded the disconnected capacitor legs upwards to provide easier access to solder, and I've added two blobs of insulation tape to the PCB pads where the capacitor legs previously connected.
tn_04 DSC_4437.JPG

Once pin8 is wired, solder this in place, then do the same for pin7 (much easier) and solder in place. Finally connect these wires to the previously removed +ve legs of capaicators C68 and C60 respectively.
tn_05 DSC_4438.JPG

The final modified PCB. :)
tn_06 new A3010.JPG

For this A3010 modification, it took me about 1 hour, the most difficult part being threading the wire connection through Pin8 of the LM324 op-amp (the hard to get to lower-pin as viewed from above). Once complete, however, the sound improvement is stunning, even through the internal speaker!

steve3000
Posts: 1705
Joined: Sun Nov 25, 2012 12:43 am

Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Postby steve3000 » Sat Aug 26, 2017 11:05 pm

paulv wrote:That looks really cool and simple for most machines....

Thanks Paul, definitely simple for the big-box machines, and really worth while :)

paulv wrote:I have a slightly unrelated question... Is that A5000 an Alpha with a fifth column ROM??? If so, what's in the ROM?

No, not an A5000 alpha (although it is moderately overclocked at 30MHz, but you really can't tell that from the photo!). The 5th column ROM is Wizzo IDEFS v3.15 (from here) - to enable me to run a 2Gb CF card HDD replacement. Another simple but effective upgrade :)



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