25 June 2024, 10:49 | #61 |
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I'm going to remove the op amp and regulator board anyway, I was just interested to see if there would be any difference but I don't think Commodore skimped at all really. I'll see if I can lower the volume and if so will record from the beginning to get an idea of noise.
BTW Stedy, do you have an A600 shielding with those boards you received? I plan to sell my A600 to make back some money but mine seems to have gone AWOL. |
25 June 2024, 14:52 | #62 |
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Capture volume is set at 100% so the clipping must be down to Commodore? unless there's something I'm doing wrong.
I've remove the bodge board and replaced the op amp back to TL084. https://ufile.io/f/b9ga0 https://postimg.cc/5HrKPzF2 I'm not really sure why they drift out of alignment over time? the time between powering on which is the spike at the very beginning and the intro sound is the same but then it drifts out after... |
25 June 2024, 22:49 | #63 | |
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That's why lot of audio DAC's use or passive I/V network or fast OPAMP's. To get "14" bit you need address this. |
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26 June 2024, 09:46 | #64 |
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It might be worth someone who knows what they're doing looking into reducing the gain some more? I have A1200 2B spec and it was almost constantly in the red zone slamming 0dB and probably beyond, I think Audacity recommend input levels to be between -3 and -6dB.
The problem is if I change the feedback resistor value it might change what's needed as the feedback capacitor as from what I've read it is a stabiliser and it's beyond me to measure what's best, or maybe I can just tweak the voltage divider near to the RCA's? Also, I'm going to guess the alignment issue was down to slight difference in floppy drive reading time. |
26 June 2024, 23:05 | #65 |
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Perhaps you can adjust input mixer level on your capture board?
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26 June 2024, 23:58 | #66 | |
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Mine's packed for storage so can't check it but they should be 390R-510R, not 2K as shown on schematics at AmigaWiki. This will give a true line level output. To truly answer the output levels from Oaula, I'd need the spec of the chip, it's one of the few custom chips I don't have the spec for. I assumed 8mA, a common value for the I/V converter and it worked in simulations which matched reality. I don't think I have A600 metalwork, like I said, all my Amiga gear is packed away for a few months, whilst building a new workshop @pandy71, do you have any idea how filter and op-amps work? You're spouting a lot of rubbish. A filter at the I/V converter, will attenuate higher frequencies above 27KHz, the slew rate will easily handle the required changes in output over a 20KHz range, any talk of 3.58MHz is irrelevant, the circuit works at ~20 KHz due to the filter. I've spent the last 18 years specialising in precision analogue and power electronics, kinda know what I'm doing (mostly). |
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27 June 2024, 00:06 | #67 | |
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Good for you Steady - don't get me wrong but I/V converter in Paula is active not passive - PWM volume control is not 20kHz sinewave for sure. If you do passive I/V and low pass before OPAMP then i agree with all you wrote. |
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27 June 2024, 13:12 | #68 |
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I just used line in on the sound card so I would assume it doesn't amplify the input signal?
360 ohms. I built the whole sound stage the same as the A1200 2B schematics and also checked with motherboard photos to make sure that's what Commodore actually used. Last edited by Mick; 27 June 2024 at 13:19. |
27 June 2024, 17:09 | #69 |
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Amplify - probably not, but attenuate probably yes so you can reduce level by 50% (-6dB from 0dBFS) or more to prevent clipping if Amiga level is so high (usually supported maximum input level in consumer devices is around 2.504 Vrms)
Last edited by pandy71; 27 June 2024 at 22:05. |
27 June 2024, 17:36 | #70 |
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It's too late run tests again now, well it's not but I'm just too demoralised now. I just thought that if Amiga levels are still too high (even after all of the messing around Commodore themselves did) then it might be better to bring them down to modern day recommended output levels, assuming that's what Audacity's recommendation is. If the Amiga output is clipping in Audacity then wouldn't it be reasonable to assume it may also be clipping when connected to other devices?
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27 June 2024, 22:11 | #71 |
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It looks like clipped - but perhaps everything is fine (source dependent) - best test will be some sinewave with level for example -6.0206dBFS so easily maximum output level can be calculated (basic multimeter can be used to measure level if your sinewave frequency will be like 50 or 60Hz - depends on country).
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13 July 2024, 01:54 | #72 |
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I've thought about the filter in the Amigas many times, back in the day and over the last 20 years. In the early days when filtering was too muffled, there was the enthusiast movement for the very few, to delete the filter internally, and filter externally.
But this is not correct. Not because it removes the filter (as best you can), but because it lends itself to psychoacoustics. You'd be surprised how many think their horrible "hifi systems" and "Marshall amps" sound "just right". Everyone will end up with a different experience, and (more importantly) different from the author's, whose ears were the monitors. This way lies madness - I should specify that: Everyone will end up with different conclusions = not Science. If consumers at large had as good ears as studio engineers, I would agree that deleting the filter is correct in an instant! But we know this couldn't be further from the truth. There are currently 3 distinct filter levels: Amiga with LED filter on, A500 (or corresponding) with LED filter off, and A1200 (or corresponding) with LED filter off. Some correct way would be the same filter mod (and LED filter off, obviously) for all Amigas: 7.5kHz with a gentle knee to 13kHz. But this would then instantly transform all sounds and compositions published so far in the ears of everyone. The ears will adapt with time however, and this could be the standard for the future. A third correct way is the aforementioned monitor ears of the authors. Change nothing. Sound or music is made for the original hardware, and you play it with the same settings, and you hear the sound or music exactly as intended. Yes, even with the LED on. (Excluded from all this are emulators and PC tracker clones, until they select and specify a target platform filter that they emulate and measure for accuracy.) |
13 July 2024, 21:29 | #73 |
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Situation is even more complicated - you can have programmable filter (like switched capacitor filters) - in case of Amiga you could simulate "variable" LPF using software PWM/PDM modulation to OFF/ON LED filter to create some crude programmable filter.
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18 July 2024, 10:04 | #74 | |
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The output is then fed through a 47k resistor which (if my calculations are correct) lowers the mid-point voltage to ~3V. I'm not sure why they did this, perhaps the idea was to get some positive voltage across the coupling capacitor in the next stage. |
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18 July 2024, 18:18 | #75 | ||
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What do R760/R770 do? Last edited by Mick; 18 July 2024 at 18:35. |
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23 July 2024, 21:58 | #76 |
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So I just noticed that the Amiga 3000 doesn't even have C430/C440 (equivalents to C321/C331) populated at all, not so important after all then?
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