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ETI Vocoder constructional project reproduced with kind permission from Wimborne Publishing Ltd. http://www.epemag.com
The use of different op amp types (bipolar, JFET input) in the vocoder project has caught me out 🙂
ETI Vocoder constructional project reproduced with kind permission from Wimborne Publishing Ltd. http://www.epemag.com
The use of different op amp types (bipolar, JFET input) in the vocoder project has caught me out 🙂
When was the circuit designed and built? If I was doing this back in the late 70s, when I was tinkering with stuff, my supplier of choice would have been Radio Sack, of necessity (limited market), and they didn't stock very many of the better opamps like the TL080/CA3140 variety, but you could usually get a bunch of the 1458s. So, most likely, he built the filters and then put the better ones in the most critical (to his ear most likely) stages. Id go with all higher performance amps, and I'd not bother with the 1458 at all, and I'd get rid of resistors.
I would be interested in seeing the original article which this is based on, but haven't located a copy online so far.
If you don't have it, you really should get yourself a copy of Don Lancaster's Active Filter Cookbook.
Couldn't you simply put channels 9 and 13 on one board, 10 and 11 (and 12) on another? Surely that's just a wiring fudge from the control panel.
I could be wrong but if I remember correctly, the phase poles can be pulled a bit by the current on the input of the op-amp (IE the phase angle changes). Those phase angles are critical for a Sallen and Key implementation of what looks like a Chebyshev filter. The stop band attenuation seems far too hard to be Bessel and even those can be a bit finniky. How ticklish they are is dependant on frequency and the exact topology of the filter. Specifically individual component values as well as the phase angles of the poles and the frequency. That's why I still use Bessel single pole filters on noise sensitive and phase sensitive kit as you can control both . Sallen and Key is a damn good compromise but it's still a compromise.
I suspect he has used the JFET op amps as the frequencies that they cover as the key area for the vocal range
I need you to use electrolytic capacitors!! Please 🙂
Just another thought, since there are 4 channels that specify the jfet op amps, why not put them on a pair of boards and build the other channels with bipolar op amps separately. Surely the channels do not have to be in strict order on the array of boards.
It could be something as simple as those were the only op amps he had available when he built the circuit.
Why dont take channel 9 and 2, 3+4, 5+6, 7+8, 13+14 with bipolar opamps. Channel 1+10, 11+12 with TL082?
I am pretty confident it will work just fine if you stick TL082 in all of them. The R6 difference is because of impedance balancing on the bipolar opamps, which others have explained also. Jfet opamps dont care about the impedance because, as you say, they have so high input impedance
My guess: The resistor to ground is to minimise offset current problems in what was presumably the cheaper bipolar op amp. Where that is not feasible a (more expensive) op amp (with lower input currents) has to be used. As others have said, I'd use a tl072 throughout.
When using op amps in this type of circuit, the prudent designer will keep the dc resistance at both inputs the same to minimize output offsets and temperature drifts due to input bias currents, regardless of the type of amplifier used. R5 and R6 should have the same value in every stage. The following stages in this design are all dc coupled, so any offsets will be amplified by the following stages and added to their own and may become problematic.
Bipolar OPAMPs have leakage so DC impedance on both inputs must be the same to avoid amplifying DC leakage.
JFET OPAMPS have no leakage (<1nA) so no compensation for leakage is required.
I suggest you use JFET OPAMPS for everything and ground negative input. I hate bipolar!