Analog Devices Guitar Amplifier

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On the "newly introduced parts" -- I note that ADI tips their hat to guitar enthusiasts with a guitar preamp using their new low noise JFET dual IC, the ADA4625:


  • ADI Guitar Preamp.png
    ADI Guitar Preamp.png
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Joined 2011
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Opamp inverting input = GND

which means the circuit requires bipolar DC supplies like ±12V. In addition to the tube heater supply and the tube plate supply. Oh my!

And it's only available in one package, an SMD with belly solderpad. I'm sure that's the A-Number-One preference of guitar amp builders, worldwide. Oh my!


Joined 2003
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What does the 560k bring to the party? Virtually nothing.

(And why not take 470k since we already have that value in design.)

2nd-thought: when lower op-amp clips, its summing node gets trash, the 560k adds that trash to the upper "low" path. Is that even useful?
Add a couple of back to back zeners at the right arm of .022µF cap, to prevent damages to the IC at starting up /down, or cap failure.

Generally a good idea BUT in this case...

In the app notes says:
The ADA4625-1 has internal protective circuitry that allows voltages as high as 0.2 V beyond the supplies to be applied at the input of either terminal without causing damage. For higher input voltages, a series resistor is necessary to limit the input current. Determine the resistor value by (VIN − VS)/RS ≤ 20 mA where: VIN is the input voltage. VS is the voltage of either V+ or V−. RS is the series resistor.

With resistor values shown these currents will be less than 1mA with a tube B+ of +400V.

Protection diodes not required.

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Don't any of youse guys understand they did this in jest ?? -- much akin to Jim Williams using an LM317 as the error amplifier for a 75TH vacuum tube pass element.

Mbe there is a break-out movement within ADI to resurrect some of the LLTC mirthfulness.

I was talking about the other stuff, Jim called me years ago after LT copied some of my datasheet applications including the parts that did nothing and asked if they were there just to catch that. Of course I said yes, what else could I do? I could probably find out where that app came from, it is obviously tongue in cheek.
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IF anything, the architecture is backwards.

Use the very high quality, very low noise Op Amp as the *front* end, and then drive the cheesy tube from it to get large voltage swing, Mojo and "tube sound".

As is, it´s using each element the worst way possible.
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I've read a lot of datasheets over the decades, and do not remember ever seeing this particular scheme. Maybe I missed it. But I think it is new, novel, and if you can define some I Claims even patentable. (Patents no longer have to be instantly "useful".)

I meant the other applications, only I would notice this is very old stuff between Bill Gross, Jim Williams and me.
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