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Old 28th December 2011, 08:23 PM   #1001
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Originally Posted by NebuK View Post
I'm just putting together my shopping-carts on distrelec
Be fast and effective with Distrelec, because they do not seem to save shopping-carts, as other vendors do, so you can add things and come back later with more.
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Old 28th December 2011, 08:59 PM   #1002
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Or these if you find a "single" DIP style opamp you want to try. http://www.ebay.ca/itm/180749610268?...84.m1438.l2649
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Old 28th December 2011, 09:22 PM   #1003
Reality first!
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Exactly
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Old 29th December 2011, 01:25 AM   #1004
erin is offline erin  Australia
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Semiconductor Opamp adapters from Parts Connexion if you are ordering from them anyway...
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Old 29th December 2011, 01:19 PM   #1005
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Hum problem.

I'm having some issues with my DAC passing mains hum to it's output. I'm feeding it to a Pre-amp board I bought from Ebay (to buffer, and provide a Volume control), and then into an ICEPower 125ASX2 module. With the DAC disconnected from the Pre-Amp, I have silence, even at full volume (ok, the slightest bit of hiss if I listen really close to the tweeter). But with the DAC connected, I get mains hum - not terrible - but definitely audible with no music playing at listening volumes.

I'm feeding the DAC with the recommended R-Core transformer, and have kept signal paths short, screened, and paid attention to layout. If I ground one output channel of the op amp on the DAC board, the hum reduces marginally, and if I ground the other, it stops almost completely. I wondered if I had a faulty regulator smoothing capacitor - I had replaced the originals with 100uf Electrolytics, per the BOM. (and the regs were also replaced per the BOM). I tried putting in some different 220uF capacitors, but still to no avail. I've checked component values in the LPF (modified per the BOM) and all ok, and checked and resoldered connections.

Unfortunately I don't have an oscilloscope which is hampering my diagnostics. I've tried running a ground wire to the AC centre tap to bring the circuit to chassis earth and am running out of ideas. I still suspect either a problem with the board (why would grounding one signal channel completely kill the hum, vs the other one?). I've tried changing the Op-AMP too in case it was faulty.

I'm about to try putting 10uF caps directly at the Op Amp supply rails, but I'm not sure what else to try! Anyone else experiencing hum from this board? Any other ideas?

Last edited by Simes123; 29th December 2011 at 01:30 PM.
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Old 29th December 2011, 03:30 PM   #1006
F1 FAN is offline F1 FAN  Canada
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Does it only happen with the digital cable connected?If so it's likely a ground loop.I had a similar issue when connected to the sp/dif output of my PC.
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Old 29th December 2011, 04:04 PM   #1007
erin is offline erin  Australia
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Originally Posted by Simes123 View Post
Hum problem....
A photo of your build might help.

Last edited by erin; 29th December 2011 at 04:07 PM.
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Old 29th December 2011, 04:10 PM   #1008
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No - with the digital cable connected it is the same. If I remove the digital cable, I get white noise superimposed on the same hum. I've tried adding the 10uf caps to the Op Amp supply rails, and that made no difference. I have just reduced the gain of the following pre-amp buffer from 2, to 1.2 as I realised that I was achieving almost full volume too early on the volume control - that obviously improves matters in that the hum appears at a higher apparent volume setting, but the problem is still there. When I prototyped it outside the case, I'm fairly sure I didn't have the hum (but not 100% - I might have dismissed it or not noticed it - it's fairly low level, albeit noticeable). So it *could* be a ground loop. I think I'll try moving everything out of the case and recreate my prototype scenario to see if anything changes.
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Old 29th December 2011, 05:46 PM   #1009
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OK, making good progress; it was an inadequate ground from the centre tap of the R-core supply to the DAC and the chassis. That's got it to around 97% of perfect.

I probably still have too much gain in the pre-amp buffer - what I would consider full (clean) volume is somewhere just over the mid-point of the volume knob. I've reduced gain of that stage to around 1.2 (56k/47k) but I'm wondering whether I should reduce it to Unity gain (will I risk stability issues?). To reduce it further, I'd have to reduce the input to the pre-amp buffer, and I'd rather avoid that.
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Old 29th December 2011, 06:59 PM   #1010
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OK, making further progress. Ignoring the gain issue for now. I detect, with preamp volume at Zero, a very low level of hum at the speakers. Have tweaked the layout of cables a little more and am trying to reduce that hum next. Still getting a slight hum from the preamp stage at full volume in addition, but will tackle that in a minute. They might be the same source. My expectations might need adjusting before long!
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