Wayne's 1570 summing amp

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JR.
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Re: Wayne's 1570 summing amp

Post by JR. »

It may be a GBW or loop gain issue, not slew rate per se.

The bode plot of opamps is usually in terms of "voltage" gain so what we really need to see is a plot of input voltage to output current. If the output of that opamp has an input voltage term riding on top of it (since servo output voltage is cap voltage plus the - input pin voltage), that could affect frequency response. The following passive pole should reduce that.

This should be simple enough to confirm or deny, just look at the output of that servo opamp with 20kHz input, and at the 1570 inputs.

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mediatechnology
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Re: Wayne's 1570 summing amp

Post by mediatechnology »

It may be a GBW or loop gain issue, not slew rate per se.
I've been wondering if it was a current slew rate limitation (mA/us) - something that's never specified. Or it could just be GBW. Easy enough to find out.
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mediatechnology
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Re: Wayne's 1570 summing amp

Post by mediatechnology »

Lookie here for some OP07 servo tests: viewtopic.php?f=6&t=419&p=4999#p4999
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Re: Wayne's 1570 summing amp

Post by juniorhifikit »

I wanted to add some functionality to this stereo bus circuit, and allow the user to access the passive summing node before the 1570 amplifier. There are a lot of folks who like to experiment using different mic pre's to get their makeup gain. I tried this with my Hardy M1's, which have a Jensen transformer on their input at an input impedance of 150 Ohms, and only succeeded in making the 1570 circuit go into wild oscillation. The interaction with R1 & R3 on the servo is what I suspect. Any thoughts?

Several other things come to mind... I wanted to be able to cascade two units together before the summing amp. Currently Rin is at 2k2, which at 16 channels gives a resistance of 137.5 Ohms. Two os these in parallel would be 68.75 Ohms. Is this too low for the 1570 and the real world?

Also, what would be the best way to protect the circuitry from someone accidentally injecting 48v of phantom power into it?
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mediatechnology
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Re: Wayne's 1570 summing amp

Post by mediatechnology »

There are a lot of folks who like to experiment using different mic pre's to get their makeup gain. I tried this with my Hardy M1's, which have a Jensen transformer on their input at an input impedance of 150 Ohms, and only succeeded in making the 1570 circuit go into wild oscillation.
If I understand correctly, you were having the 1570 output directly drive a 150 Ohm (differential) transformer load? Or was the output of the M1 driving the 1570?
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Re: Wayne's 1570 summing amp

Post by juniorhifikit »

The 1570 output drives a 1240, which in turn feeds a 1646 line driver. I was attempting to pick-off the summing node where all of the 2k2 resistors are joined which is feeding the 1570's input, and feed it into the M1's 150 Ohm input in parallel. By not disconnecting the summing node from the 1570 input, and simply connecting in parallel to the M1 input, it caused the 1570 & servo to oscillate. I guess I was wondering if there is a way to make this connection without having to disconnect it from the 1570 input. I'm not too concerned with the sonic outcome of this regarding the 1570, as using a mic pre for the gain negates the need for the 1570 circuit. I just don't want it to oscillate, drive the meters into the red, and generally wreak havoc.

A simple switch would probably solve the problem.

Also worried about what will happen when someone accidentally sends +48v of phantom into the circuit.

Like this:
Image
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mediatechnology
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Re: Wayne's 1570 summing amp

Post by mediatechnology »

OK, so the 150 Ohm input transformer bridges the summing node.

Well it is a summing node. Placing a transformer there will result in the 1570 losing feedback. You could, as you suggest, use a switch. But I wouldn't. You don't want any stray capacitance there.

I'm also pretty sure that you'd have some difficulty protecting the summing node from phantom faults.
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JR.
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Re: Wayne's 1570 summing amp

Post by JR. »

Bzzt... not gonna work...

Passive sum fed to a mic preamp, is a voltage feeding the gain stage.

The active virtual earth sum bus, is held to a low impedance by NF.

At best your mic preamp would just amplify the error voltage on the virtual earth, at worst you will cause instability in the virtual earth sum amp, which i think you have experienced.

You absolutely can not have both at the same time... they are not compatible. You can alternately switch the bus to one or the other, while this will likely cause clicks due to servo.

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Re: Wayne's 1570 summing amp

Post by juniorhifikit »

JR. wrote: At best your mic preamp would just amplify the error voltage on the virtual earth, at worst you will cause instability in the virtual earth sum amp, which i think you have experienced.
That pretty clearly sums it up (no pun intended). One or the other it is.
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