10 July 2012, 03:42 | #1 |
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Audio output caps (again)
Hello, I know this has been beaten to death a bit but I'm shortly to be recapping 3 or 4 amigas. Last time I did a couple I just used direct replacement SMT electrolytics not realising there was a a problem with the bipolar output.
Now I want to use more suitable parts but I want to avoid bulky film caps or leaded components. Anyone think of a reason why these new fangled MLCCs would not be a good choice - http://uk.farnell.com/tdk/c4532x7r1e...f/dp/1843167RL Apart from the fact they are out of stock, (and the stocked equivalants are ~5 times the price for some reason), these look perfect to me. Should last as good as forever and be easier to solder in behind the ribbon on A600s. Am I right to think that an 1812 SMD package should be a good fit for the pads that are there? |
11 July 2012, 13:58 | #2 |
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Ok, no objections. I'll give it a try then. Seemingly these large value ceramics are designed to replace electrolytics and tantalums so I think they will be good for this. I've ordered some of these -
http://uk.rs-online.com/web/p/products/741-6865/ They are a bit pricey but I'm too impatient to wait for the cheaper ones to comes back in stock and they will probably outlast me. They are a bit bigger too, a 2220 package, kinda wide for the pads, maybe they will fit nicely on edge. |
11 July 2012, 15:06 | #3 | |
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I would'nt say these are ment to replace electrolytics. Which problem are you referring to on the audio, cause Im yet to see it / hear it. Audio sounds perfect on every amiga with smd electrolytics. |
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11 July 2012, 15:28 | #4 |
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I've read in a bunch of threads here about cap replacement where the choice of polarised output coupling caps is thought to have been penny pinching by commodore and decreases the lifespan of those electrolytics due to bipolar audio output of the audio circuits opamp. I don't think it's a problem with sound quality but reliability.
I confess I haven't measured the signal at the relevant places or attempted to analyse the output circuit to see where it idles and if the signal is actually bipolar, just hearsay, but it seems reasonable since the opamp has a bipolar supply. The data sheets for these MLCCs, (multi layer ceramic capacitors), does say they are intended as alternatives to aluminium and tantalum electrolytics so I don't think it can hurt. |
11 July 2012, 15:57 | #5 | |
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Fair enough, But I would watch how you word it as it sounds your saying ceramics can replace normal polarised electrolytics. |
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11 July 2012, 22:38 | #6 | |
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Anyway, I won't be testing my theories in other positions in the amigas, on price if nothing else. Just the audio outs seemed like a reasonable plan. I'll let you know how it turns out. |
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11 July 2012, 23:03 | #7 |
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11 July 2012, 23:58 | #8 |
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Hi,
I would not replace all the electrolytics with ceramics, you could make the system unstable. Ceramic capacitors have very low Equivalent Series Resistance (ESR), typically between 0.01 and 0.2 ohms, electrolytics and tantalums have ESRs ranging from 0.05 ohms to 10-15. Too low an ESR can cause higher inrush currents and I have seen it cause issues with power converters. Unless you understand the operation of the circuit, always replace like for like technologies. The bipolar audio circuit output stage is something I need to look at. With electrolytic capacitors, you can drive them 0.6 to 0.7V (a diode drop) negative, safely, indefinitely. I have seen this on DC clamped video circuits that have been in service for a number of years. The parts you selected will work perfectly. Ian |
01 August 2012, 22:53 | #9 |
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Well, they fit and they work and I now know they are not necessary.
As you can see from the pic the ones I used are pretty chunky but still easier to solder in that position than the electrolytics. The 1812 package would be a nicer fit and easier still to solder plus a bit cheaper. Like these - http://uk.rs-online.com/web/p/cerami...layer/7416815/ I will use these again just cause they are easy to work with and probably will never need replaced, but I no longer think that electros are a problem at the audio outputs. I stuck a multimeter and scope on the caps and there is in fact a DC bias on the opamp outputs. I measured about 2.2V. This place mentions the A4000 output should have DC bias of 2.5V at the opamp- http://amiga.serveftp.net/audio_repair.html Using the audiotest utility from that same link, I measured max audio signal at about 3V peak to peak. So there is no problem using a polarised capacitor in this position. Except the fact they are a bit of bitch to desolder and resolder jammed in behind the keyboard connector. EDIT: looking again at that A4000 audio page, it's audio output is quite different. It has an additional opamp stage. Apparently this has a +1V DC bias and a signal upto 8V p-p. I would say this is a problem for its polarised output caps. Not so on the A600 and I think A1200 also. Last edited by jimbob; 02 August 2012 at 00:19. |
02 August 2012, 00:58 | #10 |
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Hi,
Nice soldering job on the Ceramic capacitors, very neat. A while ago I measured the audio output of the A600, there is no DC bias, that is why there is an AC coupling capacitor. A capture is attached. That was the maximum waveform amplitude I could get. The Marker with '2' shows the ground level of the signal, thus it swings +/- 0.85V. Have been trying to work out, sensibly if electrolytics can get damaged in an AC coupling config, they should not. The voltage drop across them is a few mV at best, it depend on the impedance and current flows. Technical details aside, glad you've fixed your Amiga's audio. What games you going to play to test it? Ian |
02 August 2012, 01:50 | #11 | |
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Yes, DC is not present on the audio outputs. The DC bias is present before the 22uF caps in the circuit. It is of course removed by the cap. This means that the signal never swings the cap voltage below 0V. I'm pretty sure I didn't make a mistake measuring this. On the scope I could see the 1Khz from the the test program. If I switched from AC to DC coupling on the scope input the waveform shifts up ~2.2V. So the caps never have voltage across them in reverse to their polarity. The only thing I can think of to cause a problem would be exceeding the ripple current rating of the caps by connecting to an amp input with low enough impedance. I don't know how likely this is. |
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