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| Just how bad is it? Audio mixer with headphone amp. |
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| paulca:
Thanks Ian. So a little more gain on the input amps. I was searching around and getting annoyed with the usual "All about circuits" type random audio circuits and found this: https://carvinaudio.com/pages/mixer-schematics And a single mono input channel. http://carvinimages.com/schematics/DX%20Series%20Channel.pdf Seems to be the actual schematic for a single input channel on a mixing desk. It's a bit hard to calculate the gain as they are doing funky stuff with the XLR input and the Line, though the Line passes through a voltage divider and the XLR doesn't R9/R10. A lot of it is beyond me, but I can identify stages and components.... mostly because they are labelled LOL. Another interesting thing is that the variable gains on the filters at least seem to break a supposed rule of not using feedback as gain. I remember in High school the first opamp circuit I seen had a pot on the feedback loop. It has always seemed a bit odd to me that people either boost then attenuate or vice versa, instead of just altering the gain of the amp directly. I'm sure there are reasons, but this channel circuit uses variably amp gain. I also count a whole 4 dual op amps in the input stage for mono. To replicate that for a stereo channel that would be 8 dual op amps and they haven't even mixed it down yet. Granted most of the opamps seem to be to do with active gain filters, LOW, HIGH and a sweep-able MID (not sure f it's fully parametric with Q or just sweeped). I don't need anything this complicated. It would be nice to have Low,Mid,High but maybe next time. I just thought I'd add it to see what real mixing schematics look like. |
| dmills:
The auxout buffer should have 47 ohm or so resistors between the opamp output and the socket to reduce the chance of oscillation with a capacitive load. 100k is a bit on the large side for the 5532 (Which is a bipolar opamp with relatively low noise impedance), 10k or even 2k2 would be more like it, also lower value resistors are generally quieter due to lower intrinsic thermal noise. If I was doing this, I would dump the input amps and do the gain in the summing amplifiers at the moment you have a per input gain to the output of Av = 2/3 so you actually attenuate. Make the summing stage inverting and give it some gain, make the pots maybe 5 or 10k (To give yourself space to have the summing input resistors be large enough not to disturb the taper too much) and invert a second time at the output buffers. Where is the decoupling? State variable filters (Which are what that mixer input strip uses) consume about 4 opamps per band, so swept mids are expensive on parts count. Regards, Dan. |
| BrianHG:
Don't go too crazy on the gain. You'll begin to add hiss. Also, having gain on you mixer stage means the primary input opamps wont have to go all the way to your rails to provide the same 2vrms output since their output is divided by 3 in the mixing. Unless you go to Audioguru's negative scheme at the mix stage (when done right, it's used quite often in mixers), stick with my recommendation of having a gain of 2, or 4 if you want more at the input amps and the same gain again at your mixing middle amps (unless you choose 3x deliberately to compensate exactly for the mix) & don't forget to shrink those overly large sized 100k feedback & GND gain resistors. Also remember, the higher you make the gain on your op-amp, the more lousy the high frequency response. Another good reason to divide the gain between the input amps and you mid mix amp. |
| paulca:
--- Quote from: dmills on February 12, 2018, 10:54:44 am ---If I was doing this, I would dump the input amps and do the gain in the summing amplifiers at the moment you have a per input gain to the output of Av = 2/3 so you actually attenuate. --- End quote --- Av=2/3... how? A 1:2 divider on the feedback side should equal a gain of 2. Anyway, as stated, while it may be more complicated than it needs to be, I feel I want normalised inputs. I have played the gain structure game before and hated it. To turn one channel up you have to turn the output gain up and then turn down the other channels. It's all very well when you connect 2 or 3 channels of equal or nearly equal output together, but when you get one channel that is 1/10th the power of the others it gets really irritating pulling all the other channels down to that level and then amplifying all channels along with all channels noise. As a random example, I was watching a YouTube video on OpAmps and the presenter was quite interesting, but he was also mumbling and the level on the video was barely audible. I had YouTube at max, PCM at max, master at max and the headphone amp at max and if I had decided to play music at the same time I would have had to wind the music input right back and there still wouldn't have been enough gain on the output amp, if there was it would mostly be amplifying the noise. Granted this is a for a desktop headphone amp to combine all my sources and outputs into one box, so, no I probably don't 'need' input pre-amps, but I have my mind set. |
| BrianHG:
--- Quote from: paulca on February 12, 2018, 11:20:28 am --- --- Quote from: dmills on February 12, 2018, 10:54:44 am ---If I was doing this, I would dump the input amps and do the gain in the summing amplifiers at the moment you have a per input gain to the output of Av = 2/3 so you actually attenuate. --- End quote --- Av=2/3... how? A 1:2 divider on the feedback side should equal a gain of 2. Anyway, as stated, while it may be more complicated than it needs to be, I feel I want normalised inputs. I have played the gain structure game before and hated it. To turn one channel up you have to turn the output gain up and then turn down the other channels. --- End quote --- dmills may be speaking as after you make my recommended changes a few posts up here: https://www.eevblog.com/forum/beginners/just-how-bad-is-it-audio-mixer-with-headphone-amp/msg1424158/#msg1424158 Which solves this problem, as well as a bunch of others... Same goes for all my subsequent posts, I expect you have updated you schematic to my recommendation. |
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