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

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steve3000
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Simple DIY method to get unfiltered high quality sound on your Archimedes!

Post by steve3000 » Sat Aug 26, 2017 11:27 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 most 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 1a for large-box Archimedes (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

Method 1b for older large-box Archimedes (A3xx, A4x0, A4x0/1)
Edit: 27/11/18: My original details below for A3xx, A4x0 and A4x0/1 missed out an important step. More details to follow, read this.
A310, A305, A440 (not A4x0/1 series)
Remove or disconnect at least one leg from 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 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
Last edited by steve3000 on Tue Nov 27, 2018 12:42 am, edited 2 times in total.

steve3000
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Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Post by steve3000 » Sat Aug 26, 2017 11:39 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!

Post by paulv » Sun Aug 27, 2017 12:00 am

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

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

Post by steve3000 » Sun Aug 27, 2017 12:00 am

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!

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

Post by steve3000 » Sun Aug 27, 2017 12:05 am

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

Post by pelago » Thu Nov 22, 2018 8:23 pm

Thanks for this thread. Can I please check that the circled two capacitors are the ones to remove in an A310? And the two lines are where the jumpers should go?
_20181122_195606.JPG

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1024MAK
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Re: Simple DIY method to get unfiltered high quality sound on your Archimedes!

Post by 1024MAK » Thu Nov 22, 2018 9:45 pm

pelago wrote:
Thu Nov 22, 2018 8:23 pm
Thanks for this thread. Can I please check that the circled two capacitors are the ones to remove in an A310? And the two lines are where the jumpers should go?
_20181122_195606.JPG
No, those don’t look like the capacitors the instruction is referring too. For the A5000 for example, the relevant capacitors are large electrolytic types.

Mark

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

Post by pelago » Thu Nov 22, 2018 10:49 pm

Well I thought it odd too, which is why I wanted to check. I'm pretty sure those are C22/C23 (sorry, I drew over the numbering). But the text up top says "disc capacitors" for A310. Those are cuboids.

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

Post by steve3000 » Fri Nov 23, 2018 12:42 am

pelago wrote:
Thu Nov 22, 2018 10:49 pm
Well I thought it odd too, which is why I wanted to check. I'm pretty sure those are C22/C23 (sorry, I drew over the numbering). But the text up top says "disc capacitors" for A310. Those are cuboids.
Good question, I think those are correct, apologies if I've called them disc when they aren't - I was going off the circuit diagram at the time. The capacitor types in my instructions changed with the various models because the sound circuitry is slightly different. I modified my A305, last Christmas. If you can wait, I'll dig it out at the weekend to double check what I did.

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

Post by pelago » Fri Nov 23, 2018 7:06 am

Thanks, I'm very happy to wait!

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

Post by steve3000 » Tue Nov 27, 2018 12:32 am

Well I really must apologise! The modification on my A305 is slightly different. I've managed to find my notes from the time - and in addition to removing capacitors C22 and C23 and positioning the shunts as described, one further step was needed - cutting of two PCB tracks on the other side of the PCB! Without this last step, the amplified filtered sound is still connected to the headphones socket and speaker - there is no improvement. However, the two PCB tracks were not east to find, so I had started to look for a simpler alternative, but completely forgotten to post this important point up. :oops:

From my notes, to avoid cutting PCB tracks, the following steps should work (A310/A305/A440), but it is rather more involved and I'd hoped to find a simpler method... Also I have yet to test steps 2-4 myself:

1) Disconnect a leg from each capacitor C22 and C23 (or remove both capacitors)
2) Disconnect either leg from R40, R42 (or remove both resistors)
3) Disconnect pin 1 of Q9 and pin 1 of Q11 (or remove both transistors)
4) Disconnect/snip pin 2 and pin 13 from LM324 IC13
5) Bridge pins 1-3 and 7-9 of PL2

I'll aim to try this out over the Christmas holidays, along with some photos...and also look take a photo of the two PCB tracks which can be cut to avoid steps 2-4.

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

Post by pelago » Wed Nov 28, 2018 8:12 am

Good luck, and thanks!

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