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| Just how bad is it? Audio mixer with headphone amp. |
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| paulca:
Well the prototype board worked out okay. In so much that it was laid out okay and soldering went well. I only did one channel. On testing it however audio plays through it fine, but it's attenuated, the gain pot does nothing and the opamp pulls 30mA and gets hot after a while. Sometimes is a-miss somewhere. I've had 2 beers, so I'm not starting to scope it tonight. Tomorrow maybe, starting at that gain pot to see whats wrong there. |
| paulca:
Got it. The outer feedback line is coming off the - input, not the output. That will impress it. I'll fix it in the morning. |
| paulca:
So progress! At last. Having fixed the fact that pins 6 and 7 where completely backwards on my perfboard, it now works perfectly. Still gets a bit warm and still pulls too much current, but at least it's only about 13mA this time for a single channel. I also made progress with the design, (below). I'm still a bit baffled with impedance. For example the LineInputStages all have 100R's on the outputs, which is then followed by a 22K, so that makes it 22.1K and will alter the summing amp gain a little, but more importantly when I split the L/R_MIX to the AUX buffer amps do I need separate resistors on each leg of the split, or is it fine the way I have it? I haven't done the headphone amp yet and somehow I have to invert the signal in it, due to using another of the Baxandall blocks for the master volume, which inverts phase. I might change this to make a slightly different master volume block based on the same idea. A few design issues remain. 1. The active volume controls do not reduce to zero, I believe due to there being a path through the active gain pot to the output, so the original signal leaks through. I could be wrong. This is not a worry on the inputs, but is an issue on the master volume. I would like "OFF" or "Infinite". Expensive pots or a proper fader will provide this, but probably ££££. For the headphone amp, Small Signal Audio Design book actually recommends... wait for it... an array of 14!!! NE5532s in parallel to drive headphones. 14! I don't think I need that many, it sounds fine with 2 driving 34Ohm headphones. I might go to 3, 4 or maybe look at a better op amp for the output. |
| Audioguru:
Why do you have so many coupling capacitors? Most of your opamps have a gain of only 1 so their output will have the tiny maximum DC output offset of 0.004V without a coupling capacitor and resistor load to ground. Who cares about an offset of only 0.004V max? The offset voltage might even be nothing. Why do your coupling capacitors have the huge value of 100uF? Feeding 22k ohms then they pass earthquake frequencies down to 0.07Hz! That is one cycle each 14 seconds. Why not use 1uF then the audio -3dB cutoff frequency is 7Hz then will be flat down to the 20Hz lowest frequency that we can hear. The formula to calculate a coupling capacitor value is very important and is simple. Can you hear a difference between true phase and reversed phase? Does every radio in your house produce true phase? Do you want to add many other mixers to this mixer? An audiophool would use 14 NE5532 opamps in parallel to drive headphones through solid gold Monster cables. An audiophile uses only one fairly high power NE5532 opamp through ordinary copper cables to drive 600 ohm Sennheiser headphones. |
| paulca:
I'm borrowing building blocks from that book! Now, granted the book is basically his experience in designing real pro-audio 16/24/32 channel mixing consoles, but... a 100uF cap and 22K block is about 10p, I might as well. Removing them would then require I understand much more possible have to redo the maths of the various parts. On the coupling caps, there is a reason for the larger uF caps after the summing mixer. I only scan read it but basically the summing portion has a LF roll off, so the coupling caps should be kept large to prevent further LF damage. On the gain. The Baxandall stages have a gain of around -70db to +17db. The mix sum amp, I believe has a gain of 2. The Aux buffer is driving un-known external inputs, so I figured decoupling it would be wise. On the 5532 headphone amp, re-reading it, he shows THD graphs for a single 5532 driving 5Vrms into 200Ohms and how adding a second parallel 5532 drops that THD to almost nothing. 4 can drive 100Ohms with virtually no distortion. 12 are used for 25Ohms. I doubt I would notice the distortion talked about with 4, driving a 34Ohm load. So I'll maybe aim there. On phase.. you are right. Unless I ever intend to mix the output with another, phase will not matter. |
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