why the reluctance to use an opamp buffer on a rat?

[m_charles]

So, incorporating ideas everyone gave, here's the tweaks I came up with. thoughts?

[antonis]

Could you plz enlighten us about the purpose and particular vaue of the 560Ω resistor..??

[GGBB]

So, incorporating ideas everyone gave, here's the tweaks I came up with. thoughts?

Your 4.5V bias to the 2nd op-amp is no longer 4.5V - it varies with volume adjustment because "R?" and pin1-2 of "RV?" form a voltage divider since you removed the coupling cap "C?".

[m_charles]

Could you plz enlighten us about the purpose and particular vaue of the 560Ω resistor..??

Just thought a little resistance might smooth out some icyness that may be introduced from the opamp. Haven't tested yet, just kinda stuck it on there for now.

[m_charles]

So, incorporating ideas everyone gave, here's the tweaks I came up with. thoughts?

Your 4.5V bias to the 2nd op-amp is no longer 4.5V - it varies with volume adjustment because "R?" and pin1-2 of "RV?" form a voltage divider since you removed the coupling cap "C?".

Oh crap, that seems obvious now that you pointed it out, haha

[soggybag]

1981 Inventions DRV is a Rat built around two op-amps. It's looks pretty popular seems like they sell out.

It doesn't sound like a Rat, I don't think it was meant to. It also runs on 18v and has a second dual op-amp used as a buffer bypass.

[Rob Strand]

So, incorporating ideas everyone gave, here's the tweaks I came up with. thoughts?

The RAT has the volume control after the buffer.

This circuit has a low value 10k volume control before the buffer.

The 10k pot will affect the RAT tone control and at 10k it will even affect the clipping to some degree.

A lot more differences than just a buffer change.

Am I missing something here?

[m_charles]

I rearranged the buffer intentionally. Wanted to isolate it from the circuit in the same way as the Boss SD-1.
Also, I mentioned in one of the replies that I mistakenly put a 10k pot there. was supposed to be 100k on the schem.
I'm not going for super vintage Rat authenticity here.

[m_charles]

Or connect the "bottom" of the volume pot to 4.5V instead if ground and leave out the bias resistor and the cap between the pot and the opamp.

Any chance you could jot this down on a piece of paper and post? I'm a little confused

[Rob Strand]

I rearranged the buffer intentionally. Wanted to isolate it from the circuit in the same way as the Boss SD-1.
Also, I mentioned in one of the replies that I mistakenly put a 10k pot there. was supposed to be 100k on the schem.
I'm not going for super vintage Rat authenticity here.

To do that ideally you would *add* an opamp buffer instead of replacing the opamp buffer *and* moving the tone control.   The main issue with the latter is the significant loading effect on the tone control.

If you want to remove the JFET buffer you might be better off using a tone control like Mark Hammer's "Stupidly Wonderful Tone Control"  as that lets you load down the tone control with a volume pot without weird control interactions.

Ibanez's approach to a Boss-like format is to *add* the opamp buffer and keep JFET buffer.  (If IC1B was removed and the diodes went directly to the tone control like the Rat, which is OK,  it would be more like what you want.)

[Fancy Lime]

I just remembered that there is also this thing, when we're talking about Rat-style tone control with an opamp buffer:
http://effectslayouts.blogspot.com/2019/06/1981-inventions-drv.html?m=1

Andy

[Rob Strand]

I just remembered that there is also this thing, when we're talking about Rat-style tone control with an opamp buffer:
http://effectslayouts.blogspot.com/2019/06/1981-inventions-drv.html?m=1

Andy

That one has the volume control after the buffer, so not "Boss Style" as such.

[m_charles]

I rearranged the buffer intentionally. Wanted to isolate it from the circuit in the same way as the Boss SD-1.
Also, I mentioned in one of the replies that I mistakenly put a 10k pot there. was supposed to be 100k on the schem.
I'm not going for super vintage Rat authenticity here.

To do that ideally you would *add* an opamp buffer instead of replacing the opamp buffer *and* moving the tone control.   The main issue with the latter is the significant loading effect on the tone control.

If you want to remove the JFET buffer you might be better off using a tone control like Mark Hammer's "Stupidly Wonderful Tone Control"  as that lets you load down the tone control with a volume pot without weird control interactions.

Ibanez's approach to a Boss-like format is to *add* the opamp buffer and keep JFET buffer.  (If IC1B was removed and the diodes went directly to the tone control like the Rat, which is OK,  it would be more like what you want.)

I didn't move the tone control..?

[Rob Strand]

I rearranged the buffer intentionally. Wanted to isolate it from the circuit in the same way as the Boss SD-1.
Also, I mentioned in one of the replies that I mistakenly put a 10k pot there. was supposed to be 100k on the schem.
I'm not going for super vintage Rat authenticity here.

I didn't move the tone control..?

Without a schematic I have no idea what you are doing.

[antonis]

Am I missing something here?

Without a schematic I have no idea what you are doing.

[antonis]

Could you plz enlighten us about the purpose and particular vaue of the 560Ω resistor..??

Just thought a little resistance might smooth out some icyness that may be introduced from the opamp.

So we can consider it "warmth" resistor..

P.S.
It has nothing to do with icyness smoothing..
Most designers put it there almost automatically, as a prevention for op-amp oscillation (in the mean of it prevents the start of oscillation and not attenuates it..)
Op-amp output has a (small but finite) resistance which forms a LPF with load capacitance..
This might cause an additional phase shift for the loop gain that can bring the op-amp (under closed-loop conditions) to its stability limits..
By adding that resistor, you prevent the loop gain (for very high frequencies) to approach zero - instead the loop gain reaches a finite value 'cause "new" LPF is formed by op-amp output resistance in series with that resistor (and load capacitance) but the resistor is outside the feedback loop..
(some kind of phase-enhancement to avoid instability..)

As for is value, it should be chosen to produce a zero app at 25% - 50% of the cross-over frequency (for unity gain feedback identical to the op-amps transit frequency)

And last but not least, it serves as a current limiting resistor in cases of very heavy loading..
(short to GND,charging an empty capacitor, etc..)

P.S.
Just as something to chew on (and earn much more by trying to study anything relative to my unintelligible interpretation..)

[Fancy Lime]

I just remembered that there is also this thing, when we're talking about Rat-style tone control with an opamp buffer:
http://effectslayouts.blogspot.com/2019/06/1981-inventions-drv.html?m=1

Andy

That one has the volume control after the buffer, so not "Boss Style" as such.

True. I just meant it as an example for replacing the JFET buffer with an opamp buffer. I am still not quite clear how the OP intends to implement the tone control, with an extra buffer between tone and volume or without. Full (tentative) schematic would be helpful.

Andy

[Rob Strand]

Am I missing something here?

Without a schematic I have no idea what you are doing.

I just remembered that there is also this thing, when we're talking about Rat-style tone control with an opamp buffer:
http://effectslayouts.blogspot.com/2019/06/1981-inventions-drv.html?m=1

Andy

That one has the volume control after the buffer, so not "Boss Style" as such.

True. I just meant it as an example for replacing the JFET buffer with an opamp buffer. I am still not quite clear how the OP intends to implement the tone control, with an extra buffer between tone and volume or without. Full (tentative) schematic would be helpful.

Andy

I might have sounded a bit harsh but the way I see it as we are at post #36 and don't even know the structure of the circuit!
(Hard for any of us to give advice.)

[m_charles]

So sorry for taking so long to show the schem. All my "real", software is on my work computer, and haven't been back there because we're working remotely for a couple more days. I don't do electronics for work, but stuck the software (kicad) on that computer long ago so I could mess with it afterhours. big monitors, etc.
to improvise, I grabbed a rat schem and did the best I could to draw on the screenshot.
But this is pretty much it. Just a rat and I'll add some of the mods that are easily found online. more gain, bit more bass, might fiddle with the tone section values, add a switch for diode clipping. nothing ground breaking.
I flipped the resistor and cap after the buffer compared to what I had before. This seemed more common to do?
Anyways, here it is:


forgot to mark it, but very last resistor would be 100k.
Antonis, was also going to change that 1M after reading what you said about the 5532 opamp. Maybe make 470k? Or am I understanding totally wrong?

[StephenGiles]

I tried many different opamps  - singles/duals/quads (thanks TI!!!) in a breadboarded Rat. At low volume slight differences could sometimes be heard, but with volume cranked using test recordings, they all sounded the same!!