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Designing and building a mixer circuit.
Designing and building a mixer circuit.
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Old 19th April 2019, 10:59 AM   #21
Mooly is offline Mooly  United Kingdom
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Designing and building a mixer circuit.
There are probably quite a lot of those kind of things on line... and a good result
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Old 21st April 2019, 04:10 PM   #22
edbarx is offline edbarx  Malta
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After I downloaded a few equilizer schematics, I came up with the attached circuit which is still in the beginning stages of optimisation. The emulated inductors' quality factor is too high. This resulted in a frequency response with narrow and very deep valleys which is consistent with theory. With much lower quality factors I got a maximum attentuation difference of 12dB. 'Difference' refers to the dB values at the maximum and minimum setting of a frequency channel. The attached schematic should have the following channel frequencies:

32Hz, 64Hz, 128Hz, 256Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz, 16kHz.

I am after a bigger attenuation, but it seems, this requires the use of a high resistance in the output of the input buffer amplifier stage. High resistances on signal paths do not bode well for a good signal to noise ratio. Therefore, I am trying to avoid this.
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File Type: asc corrected_equaliser.asc (13.3 KB, 4 views)
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Old 21st April 2019, 05:45 PM   #23
Mooly is offline Mooly  United Kingdom
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Designing and building a mixer circuit.
Thats a very very nice simulation and a curious choice of opamp...

I tend to find that pure noise isn't particularly objectionable in practice, now obviously that doesn't mean it should be hissing at twenty paces but on the other hand are you chasing specs that might have little audible impact apart from being a bigger or smaller number on paper.
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Old 21st April 2019, 06:55 PM   #24
edbarx is offline edbarx  Malta
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I chose that opamp to facilitate simulations until I get a circuit that simulates well. By further reducing the Q factor the frequency resonse has been flattened further but it still looks undulant.

I have to further tweak the resonant frequencies to get make them closer to what I want.

The new circuit is attached.
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File Type: asc corrected_equaliser.asc (11.1 KB, 1 views)
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Old 21st April 2019, 07:42 PM   #25
Mooly is offline Mooly  United Kingdom
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Designing and building a mixer circuit.
This one looks a lot better but it is still far from flat at around -/+4db.

Have you looked at any commercial equalizers witha view to simulating one of those for comparison?
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Old 22nd April 2019, 07:33 AM   #26
edbarx is offline edbarx  Malta
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The attached file is the latest version which should have a flatter frequency response. The response's shape is like a 'flat' hill top, although there is still room for improvement.

Logic tells me that the middle channels have a lower pass impedance because both side channels contribute to them, whereas this is not true for extremity channels. This means, adding extra resistors to counter this effect should result in a flatter response.
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File Type: asc corrected_equaliser.asc (12.4 KB, 2 views)
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Old 22nd April 2019, 08:34 AM   #27
Mooly is offline Mooly  United Kingdom
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Designing and building a mixer circuit.
That looks a lot better

Just a thought, Doug Self did a brilliant (imo) tone control many years back with variable turnover frequencies and he reckoned it was of more use than a simple graphic equaliser type design.

This was a simulation I did of it a long time ago, and I also built the real thing. Its great.

The sim has two versions on the same page, one a low impedance version (lol, noise ) but it really is extremely quiet in standard form.

You'll figure it out if you run it. The audio output is at the first opamp and as it stands its inverting of phase.
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File Type: jpg Annotation 2019-04-22 093119.jpg (49.6 KB, 26 views)
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File Type: asc Tone Controls Low Impedance Version 2.asc (15.8 KB, 3 views)
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Old 22nd April 2019, 10:02 AM   #28
edbarx is offline edbarx  Malta
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While research reading about gyrators, I encountered the concept of a gyrator that implements a variable inductance. Following that, I read that these can make variable frequency resonators (filters) that are more useful than graphic equalizers.

Since I am also interested in electronics, I would like to first understand how the circuits you posted work. I am assuming they use the previously mentioned concept of a variable resonator. The fact that the posted circuit is by Douglas Self, makes building it much more motivating.

This will greatly reduce the mixer's complexity.

Finally, I presume, there is a ready designed PCB for this. There are two circuits: which one would you suggest?
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Old 22nd April 2019, 11:46 AM   #29
Mooly is offline Mooly  United Kingdom
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Designing and building a mixer circuit.
Not exactly, it was part of a complete preamp design (for which boards were/are available). I hand made a PCB many years ago now but I'm not certain I have the artwork as I used Diptrace with an old Vista PC. I'm not going to promise I can come up with anything there, it may be lost in the mists of time.

Anyone built/listened to D Selfs precision preamp.

The low noise version simply had the reistors scaled down and the caps scaled up. Marginally lower noise in theory but a tougher load for the opamps. I'd go with the original I think.
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