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Old 1st December 2014, 09:41 PM   #1
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Default Newbie Question: Troubleshooting Simple Line Mixer

I built the mixer in the attached schematic (which I pieced together from a couple of circuits I found online).

The mixer works, but the output is slightly distorted and there is no bottom end.

What am I doing wrong?
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File Type: pdf LB-02.pdf (15.9 KB, 30 views)
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Old 2nd December 2014, 01:17 AM   #2
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You're using a single 9 volt battery, as shown? Then in essence you're operating the TL072 at +/- 4.5 volts, which is too low. The data sheet calls for at least 6 volts.

It might (or might not) fix everything to use two 9 volt batteries in series, total of 18 volts, which is, in essence, +/- 9 volts.

While we're at it, I'm suspicious of your output circuit. I think C7 might better be something like 220uF, and I wonder whether R9 is needed at all.

Adding to the pile, batteries are not the perfect power source they at first glance might appear. They're actually a turgid chemical factory manufacturing electricity. For this reason C6 might well be increased to be a 10uF electrolytic--although i wouldn't call this critical. I personally use electrolytics rated at 50 volts, 105 degrees centigrade.
.

Last edited by bentsnake; 2nd December 2014 at 01:21 AM.
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Old 2nd December 2014, 04:28 AM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by rasmus b View Post
The mixer works, but the output is slightly distorted and there is no bottom end.
With the input 0.1uF caps, the 10k pots, and the 10k summing resistors, you have a LF corner between 159 Hz and 318 Hz,
depending on the position of the pot. I'd change the input caps to 1uF, for a 15.9 Hz to 31.8 Hz rolloff instead.
There is also DC on the pots, which will cause noise. The best way to stop this is to use a bipolar power supply of +/- 9V,
two series batteries with the center connection grounded. This would also fix the too low power supply voltage problem.

Last edited by rayma; 2nd December 2014 at 04:35 AM.
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Old 2nd December 2014, 07:06 AM   #4
JMFahey is offline JMFahey  Argentina
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Agree with bass loss because of too small input caps, rayma Math is spot on, and scratching noise on the pots, to which I add that current through pots and mixing resistors is destroying your bias, voltage on IC output (pin 6) approaches or meets +9V so you are losing the upper half of anything you amplify.

Talk about distortion !!!

You need to add another 10uF cap between the mixing resistors and Op Amp -input, pin 2 ;+ sign pointing towards the Op Amp.

After that, it will work reasonably well even with a single 9V battery.
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Old 2nd December 2014, 10:28 AM   #5
AndrewT is offline AndrewT  Scotland
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It's the input capacitors that are limiting the LF end.
The output capacitor loaded with a 10k receiver will extend to F-3dB @ 1.8Hz

Your 0.1uF loaded with 10k||10k has an F-3dB @ 318Hz when gain is set to max and improves to ~160Hz when gain is set to inaudible.

If you need low bass then even 1uF is too small.
Try at least 4u7F
A pair of 22uF electro back to back will give the equivalent of 11uF and take the F-3dB down to 3Hz
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regards Andrew T.

Last edited by AndrewT; 2nd December 2014 at 10:31 AM.
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Old 2nd December 2014, 11:08 AM   #6
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C7 is reversed........pin 1 of the IC will be at 4.5V +ve

Change C7 to 100uF and move it to just ahead of RV6 connection 3 (+ve end towards pin 1 IC ) . This will stop DC unnecessarilly flowing through RV6 .
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Old 3rd December 2014, 12:05 AM   #7
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Good stuff... my mixer works as a charm now!

Here's what I did:

I replaced the input caps with 1uF which brought back the low end. I actually started out with 1uF caps but switched to 0.1uF for some reason that I don't remember... perhaps I just screwed up :-)

I tried 4.7uF caps as well but I couldn't hear or see any difference (using a software spectrum analyzer and my PC sound card). Also, for my purposes (making music) rolling of frequencies below ~32 Hz is probably a good thing, so I'll stick with that.

I also decided to use a bipolar supply as suggested. For practical reasons I need to power the mixer from a 9V DC wall wart, so I used an LT1054 to create the negative rail.

While the bipolar supply solved the problem with DC on the input pots, I also realized that a "virtual earth mixer" with a single rail supply might not make a lot of sense. As I understand it, the whole idea with this design is to create 0V at the point in the circuit where the signals are mixed in order to eliminate cross talk between channels (as well as DC on the input pots).

I also decided to run the signal through the other side of the TL072 to eliminate the phase shift. I used 10k resistors which I'm pretty sure is safe but I wonder if a lower value might be better.

On the output, I removed the 100k resistor to ground (which I see no reason to keep with a bipolar supply) and put the capacitor before the pot to eliminate any DC offset as suggested.

Updated schematic attached.
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File Type: pdf LB-02.pdf (19.6 KB, 12 views)
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Old 3rd December 2014, 09:10 AM   #8
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Now you have gone to dual supply the polarity at pin 7 IC could be either +ve or -ve so you will need to measure the voltage across C7 to make sure it is now the correct way round .

VEE is -ve with respect to ground so you have C9 reversed
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Old 3rd December 2014, 07:42 PM   #9
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Yes, C9 was obviously reversed in the schematic attached to my last post.

C7 is more tricky though. I measure +5.8 mV on pin 7 of the TL072 so I guess this one is in the right way.
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