Hello!
I'm mucking around with a circuit and would love some knowledge dropped on me.
I'm wanting to make a Dual Opamp Rat but the second stage is bugging me.
Here are my questions:
– Original Rat inverts the signal – is there a reason I shouldn't keep both IC's in a non-inverting state? I'm wanting to eventually wrap a clean blend around it so I assume I should keep both IC's the same
– Will I have enough output to just run the second ic as a buffer?
– If I don't, and want to add some gain, I'm getting confused when to send something to VR vs when to send something to ground.
– I'm also a bit confused as to when to add a VR reference at the input of the IC – some circuits use it (like a HoneyBee), and others don't (like a Timmy)
Here's my schematic
(https://i.postimg.cc/rKDTrqbb/Screenshot-2023-09-12-at-1-03-21-PM.png) (https://postimg.cc/rKDTrqbb)
Hello!
I can help a bit, I've been breadboarding a Rat-related circuit and the forum members have been super helpful :)
Some answers below.
Here are my questions:
– Original Rat inverts the signal – is there a reason I shouldn't keep both IC's in a non-inverting state? I'm wanting to eventually wrap a clean blend around it so I assume I should keep both IC's the same
The original Rat doesn't invert the signal, it uses the non-inverting input to the op-amp. The inverting input also has the feedback network applied there, and the high pass RC filtering (https://www.electrosmash.com/proco-rat (https://www.electrosmash.com/proco-rat))
– Will I have enough output to just run the second ic as a buffer?
I don't think the buffer will mind much if it's being fed with a low output signal. If you want some gain from that stage (for recovery after clipping and the tone control) experiment with the value of R11
– If I don't, and want to add some gain, I'm getting confused when to send something to VR vs when to send something to ground.
Vref would be applied at R9 rather than C14
– I'm also a bit confused as to when to add a VR reference at the input of the IC – some circuits use it (like a HoneyBee), and others don't (like a Timmy)
The Timmy uses a Vref, applied to the inverting input: http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html (http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html)
Quote from: patricks on September 12, 2023, 12:47:40 AM
Hello!
I can help a bit, I've been breadboarding a Rat-related circuit and the forum members have been super helpful :)
Some answers below.
Here are my questions:
– Original Rat inverts the signal – is there a reason I shouldn't keep both IC's in a non-inverting state? I'm wanting to eventually wrap a clean blend around it so I assume I should keep both IC's the same
The original Rat doesn't invert the signal, it uses the non-inverting input to the op-amp. The inverting input also has the feedback network applied there, and the high pass RC filtering (https://www.electrosmash.com/proco-rat (https://www.electrosmash.com/proco-rat))
– Will I have enough output to just run the second ic as a buffer?
I don't think the buffer will mind much if it's being fed with a low output signal. If you want some gain from that stage (for recovery after clipping and the tone control) experiment with the value of R11
– If I don't, and want to add some gain, I'm getting confused when to send something to VR vs when to send something to ground.
Vref would be applied at R9 rather than C14
– I'm also a bit confused as to when to add a VR reference at the input of the IC – some circuits use it (like a HoneyBee), and others don't (like a Timmy)
The Timmy uses a Vref, applied to the inverting input: http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html (http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html)
Oh awesome! You've pointed out some brainfarts, so thank you!
If I could borrow your eyeballs and brain a little longer....
This was how I was going to buffer...
(https://i.postimg.cc/ZWVqSJ95/Screenshot-2023-09-12-at-5-21-01-PM.png) (https://postimg.cc/ZWVqSJ95)
and this was how I was going to add a bit of gain – I think I can send it to ground (instead of vref) because it's not on the input? Do I gain anything by sending it to ground to vs vref
(https://i.postimg.cc/gwj2c1Rq/Screenshot-2023-09-12-at-5-20-19-PM.png) (https://postimg.cc/gwj2c1Rq)
Quote from: T-Dub on September 12, 2023, 01:26:49 AM
Quote from: patricks on September 12, 2023, 12:47:40 AM
Hello!
I can help a bit, I've been breadboarding a Rat-related circuit and the forum members have been super helpful :)
Some answers below.
Here are my questions:
– Original Rat inverts the signal – is there a reason I shouldn't keep both IC's in a non-inverting state? I'm wanting to eventually wrap a clean blend around it so I assume I should keep both IC's the same
The original Rat doesn't invert the signal, it uses the non-inverting input to the op-amp. The inverting input also has the feedback network applied there, and the high pass RC filtering (https://www.electrosmash.com/proco-rat (https://www.electrosmash.com/proco-rat))
– Will I have enough output to just run the second ic as a buffer?
I don't think the buffer will mind much if it's being fed with a low output signal. If you want some gain from that stage (for recovery after clipping and the tone control) experiment with the value of R11
– If I don't, and want to add some gain, I'm getting confused when to send something to VR vs when to send something to ground.
Vref would be applied at R9 rather than C14
– I'm also a bit confused as to when to add a VR reference at the input of the IC – some circuits use it (like a HoneyBee), and others don't (like a Timmy)
The Timmy uses a Vref, applied to the inverting input: http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html (http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html)
Oh awesome! You've pointed out some brainfarts, so thank you!
If I could borrow your eyeballs and brain a little longer....
This was how I was going to buffer...
(https://i.postimg.cc/ZWVqSJ95/Screenshot-2023-09-12-at-5-21-01-PM.png) (https://postimg.cc/ZWVqSJ95)
and this was how I was going to add a bit of gain – I think I can send it to ground (instead of vref) because it's not on the input? Do I gain anything by sending it to ground to vs vref
(https://i.postimg.cc/gwj2c1Rq/Screenshot-2023-09-12-at-5-20-19-PM.png) (https://postimg.cc/gwj2c1Rq)
You're welcome! I've been helped out so much on the forum with many brain farts, I'm glad I can pay it forward :)
The buffer option looks good to me (but I'll defer to larger brains if I've missed anything).
The gain option looks good, too. My understanding is that you want to reference that feedback network to GND rather than Vref to keep it as "clean" as possible. Have a read through this thread (https://www.diystompboxes.com/smfforum/index.php?topic=130621.msg1267065#msg1267065) where Antonis helped me puzzle through the very same thing, also with a Rat schematic I'm modifying (I'm up to the stage of having prototype PCBs in hand but haven't built it up yet. So close!)
R8 and R10 are unnecessary. There's no harm in R8 except that it robs about 10% of your output. Without R10, POT1 (volume) can be smaller e.g. 10k (R10 and POT1 in parallel form a single resistor). You probably don't need to add gain to the output buffer, but if you do, R11 and R12 should be bigger like 100k and should go to ground through C15 which should be ~220n. If the plan is to tame high end with C13, it does very little because gain is only 2. But making it significantly larger e.g. 10-100n will reduce most other frequencies as well (not dramatically because gain is low). As you raise the gain of this stage, C13 will have a bigger effect and will have less effect on lower frequencies.
Quote from: GGBB on September 12, 2023, 04:55:51 AM
There's no harm in R8 except that it robs about 10% of your output.
It could be of somewhat lower value (down to 100R say) for oscillation prevention in case of shorted (or connected to a vely low impedance) outpout..
Quote from: T-Dub on September 12, 2023, 01:26:49 AM
This was how I was going to buffer...
(https://i.postimg.cc/ZWVqSJ95/Screenshot-2023-09-12-at-5-21-01-PM.png) (https://postimg.cc/ZWVqSJ95)
This looks fine to me.
Quote
and this was how I was going to add a bit of gain – I think I can send it to ground (instead of vref) because it's not on the input? Do I gain anything by sending it to ground to vs vref
(https://i.postimg.cc/gwj2c1Rq/Screenshot-2023-09-12-at-5-20-19-PM.png) (https://postimg.cc/gwj2c1Rq)
No, you have to send it to Vref, but it's nothing to do with it "not being on the input". You have to think about the DC levels. Ignore the audio for a moment and just think about where the voltages sit.
The buffer is fine because its +ve input is fixed at Vref by R9/1M. The audio input and output are capacitively coupled, so no DC from them can interfere. So the op-amp sits happily at a mid-point voltage waiting for audio to come along.
The stage with gain has the same input, so that's good and sets a DC level. The output will therefore go to that same level. However, the foot of the feedback network is at ground, so there's a DC potential (of about 1/4 of the supply) being fed back to the -ve input by the divider R11/R12. That puts the -ve input at a different potential to the +ve input, and op-amps hate that! They *always* try to have their inputs at the same level. If you take the bottom of the divider to Vref, there's no DC potential across it and the problem goes away.
However, there is another better solution which you already have on your schematic, which is to add a cap at the bottom of the feedback divider (like C6 or C7 on the first stage). No DC can cross the cap, so the divider becomes AC-only and will provide feedback for audio but not affect the DC level. Another way of stating this same thing is to say that the cap forms a highpass filter and the gain rolls off to zero at DC.
Quote from: ElectricDruid on September 12, 2023, 10:05:39 AM
However, the foot of the feedback network is at ground, so there's a DC potential (of about 1/4 of the supply) being fed back to the -ve input by the divider R11/R12. That puts the -ve input at a different potential to the +ve input, and op-amps hate that!
You could just tell that there is also DC gain (1+R11/R13), Tom.. :icon_wink:Sorry, didn't read the following.. :icon_redface:
Quote from: ElectricDruid on September 12, 2023, 10:05:39 AM
Another way of stating this same thing is to say that the cap forms a highpass filter and the gain rolls off to zero at DC.
Quote from: patricks on September 12, 2023, 12:47:40 AM
The Timmy uses a Vref, applied to the inverting input: http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html (http://revolutiondeux.blogspot.com/2008/05/wanna-build-timmy-overdrive.html)
Not really... :icon_wink:
2
nd op-amp is directly DC biased at Vref by 1
st stage output (no DC blocking cap..)
3k3 resistor Vref connection is another cap saver common minor(*) design flaw..
(*) In an ideal world, ideal op-amps shouldn't mind at all but in a real world op-amps suffer both from input bias currents (especially bi-polar ones) and input offset voltages..
Ah, cool, thank you :)
For the OP this is another great point for reading up on, RG had a good article at Geofex about op amp biasing and making sure differential voltages at the inputs don't get too wide
Dylan159 (https://www.freestompboxes.org/download/file.php?id=39126&mode=view)and Kanengomibako (https://drugscore.blog.fc2.com/blog-entry-262.html)did dual opamp RAT.
Wow, honestly I am chuffed.
I now have my underlying confusion cleared up around vref vs ground, and how the cap blocks the dc so therefore can go to ground.
Even got two examples with the two different implementations.
Thank you all so much
Quote from: 287m on September 12, 2023, 09:09:28 PM
Dylan159 (https://www.freestompboxes.org/download/file.php?id=39126&mode=view)did dual opamp RAT.
A decent design with IN-OUT in phase and no Vref connections puzzlement.. :icon_wink:
(with a minor objection about R6/R7 values..)
Quote from: antonis on September 12, 2023, 08:33:29 AM
Quote from: GGBB on September 12, 2023, 04:55:51 AM
There's no harm in R8 except that it robs about 10% of your output.
It could be of somewhat lower value (down to 100R say) for oscillation prevention in case of shorted (or connected to a vely low impedance) outpout..
Or preventing oscillation beacause of cable capacitance... It only aplies when volume pot is maxed though. When it's not the upper half of the vol pot will do the same.
If you want to prevent the oscillation with minimal "signal robbing", you can put the resistor between the pot wiper and output. 10% is under 1dB though, so it should not make much difference.
Quote from: Elektrojänis on September 14, 2023, 03:24:06 AM
If you want to prevent the oscillation with minimal "signal robbing", you can put the resistor between the pot wiper and output.
In such a case, when Vol pot is turned all the way down (dead silent output) op-amp has to fight with DC blocking cap..
(it's not good to have a severely oscillating amp even if you don't hear anything, is it..??) :icon_wink:
For unknow or dynamic load, it should be better to place that resistor inside the NFB loop..
(with just feedback cap one leg rewiring..)
(https://i.imgur.com/0wOe14W.gif)
Quote from: antonis on September 14, 2023, 05:47:02 AM
Quote from: Elektrojänis on September 14, 2023, 03:24:06 AM
If you want to prevent the oscillation with minimal "signal robbing", you can put the resistor between the pot wiper and output.
In such a case, when Vol pot is turned all the way down (dead silent output) op-amp has to fight with DC blocking cap..
(it's not good to have a severely oscillating amp even if you don't hear anything, is it..??) :icon_wink:
I probably didn't express clearly enough what I meant, but maybe a Q&D mockup pic based on OP's design will explain. This is what I meant:
(https://i.postimg.cc/PpF5r9Q9/output.gif) (https://postimg.cc/PpF5r9Q9)
I dont see how turning the volume pot down would make the opamp fight with the output coupling cap. There will be full 100kOhm of the pot between the output cap and ground.
Quote
For unknow or dynamic load, it should be better to place that resistor inside the NFB loop..
(with just feedback cap one leg rewiring..)
(https://i.imgur.com/0wOe14W.gif)
That one probably goes beyond my understanding, but to me it seems like placing the resistor inside the feedback loop partially cancels what I wanted to do. I wanted to separate the cable capacitance connected to the output of the pedal from the output of the opamp and the feedback arrangement. The cable capacitance (you could think of it as C
L in that image) could actually stop the feedback for high frequencies as it will form a low pass filter with R
X. C
F should mitigate that if it's big enough though.
Anyway.. .I don't really see a need for keeping output impedance under 1kOhm when we are talking about guitar pedals. Using 100kOhm volume pot on the output will make the output impedance higher anyway if it's not turned up to max.
Quote from: Elektrojänis on September 15, 2023, 03:19:21 AM
I probably didn't express clearly enough what I meant, but maybe a Q&D mockup pic based on OP's design will explain. This is what I meant:
(https://i.postimg.cc/PpF5r9Q9/output.gif) (https://postimg.cc/PpF5r9Q9)
My bad, then.. :icon_redface:
(mistakenly imagined configuration..)
OK gang, think I've got my head around it - does this clean blend make sense?
(https://i.postimg.cc/qg851zVN/Screenshot-2023-09-18-at-8-06-16-PM.png) (https://postimg.cc/qg851zVN)
...and one more question – will a simple low pass on the clean work/be enough?
(https://i.postimg.cc/47yM3TWp/Screenshot-2023-09-18-at-8-19-08-PM.png) (https://postimg.cc/47yM3TWp)
It makes sense but only for Vol pot FCW setting.. :icon_wink:
For Vol & Blend pots setting at 50% say, you don't have Dry-Wet "equal" mixing..
Better move Vol pot in place of R14 (make it 100k or so), make Blend pot 10k(*) and add 1k or so series resistors on both IC1.A & IC2.B outputs..
(*) For more effective mixing, retain Blend pot 100k value..
Quote from: T-Dub on September 18, 2023, 04:20:17 AM
...and one more question – will a simple low pass on the clean work/be enough?
(https://i.postimg.cc/47yM3TWp/Screenshot-2023-09-18-at-8-19-08-PM.png) (https://postimg.cc/47yM3TWp)
1. Why you think you need a 30kHz LPF on a buffer output..??
2. IC2.B LPF corner frequency is 1.6MHz..!!
3. Is there any particular reason for C16 to be wired to Vref instead of GND..??
Hopefully this is cleaned up a bit :icon_redface:
(https://i.postimg.cc/zLrfct3K/Screenshot-2023-09-18-at-10-34-56-PM.png) (https://postimg.cc/zLrfct3K)
Really appreciate you taking the time here.
I *think* that the clean blend is what you suggested now – 1k off each output, R15 and R14. I'm guessing I should get rid of c12 so the mixing IC doesn't need a bias?
R15/C16 were mean to be a low pass at ~200Hz on the clean so it would slope off the high frequencies if I was using this for bass...fixed the dodgy RC there. Was hoping to put the C16 on a switch to engage for chopping the top off, or even putting a clean tone pot to blend between caps there.
Also changed c13 to be an LPF at ~8K to smooth the top end of the drive out...that's what I'm hoping anyway!
This is slowly becoming less alchemy and science so I highly appreciate your much more trained mind on this one.
Looking much better. 8)
You've got bias for the final blend buffer IC1B coming from the input buffer IC1A, even with C12 in place. There's a direct DC path from the output of that chip (which is biased at Vref) through R15 and the Blend pot to the +ve input of IC1B.
R15/C16 will do what you want, but they're also going to hit the volume, especially for higher notes. That might make using the blend tricky, since lower notes will see less cut than higher ones (-6dB/octave). You'll have to try it and see for that. The science can give you the numbers, but the numbers don't tell you whether it sounds how you want it!
Quote from: ElectricDruid on September 18, 2023, 07:10:58 AM
Looking much better. 8)
Indeed..!! :icon_wink:
BUT... :icon_frown:
IC2.B
might be permanently hitting the positive supply rail.. :icon_wink:
@T-Dub: I let you find the reason and heal it..
(hint: You have to roll-off DC gain to unity..) :icon_wink:
Quote from: antonis on September 18, 2023, 08:09:51 AM
BUT... :icon_frown:
IC2.B might be permanently hitting the positive supply rail.. :icon_wink:
Ooh, yeah, he's right. I missed that.
Quote from: T-Dub on September 18, 2023, 06:43:36 AM
Also changed c13 to be an LPF at ~8K to smooth the top end of the drive out...that's what I'm hoping anyway!
As I mentioned earlier, this is not very effective - because the gain of that stage is only 2 so C13 can't reduce gain of high frequencies very much.
If you want to do this more effectively, add the 18n cap after R14 to ground.
Quote from: antonis on September 18, 2023, 08:09:51 AM
Quote from: ElectricDruid on September 18, 2023, 07:10:58 AM
Looking much better. 8)
Indeed..!! :icon_wink:
BUT... :icon_frown:
IC2.B might be permanently hitting the positive supply rail.. :icon_wink:
@T-Dub: I let you find the reason and heal it..
(hint: You have to roll-off DC gain to unity..) :icon_wink:
Mentioned earlier:
Quote from: GGBB on September 12, 2023, 04:55:51 AM
R8 and R10 are unnecessary. There's no harm in R8 except that it robs about 10% of your output. Without R10, POT1 (volume) can be smaller e.g. 10k (R10 and POT1 in parallel form a single resistor). You probably don't need to add gain to the output buffer, but if you do, R11 and R12 should be bigger like 100k and should go to ground through C15 which should be ~220n. If the plan is to tame high end with C13, it does very little because gain is only 2. But making it significantly larger e.g. 10-100n will reduce most other frequencies as well (not dramatically because gain is low). As you raise the gain of this stage, C13 will have a bigger effect and will have less effect on lower frequencies.
As an alternative to adding C15 (from the original schematic version not the latest) - connect R12 to VR instead of ground.
Quote from: GGBB on September 18, 2023, 12:17:30 PM
Mentioned earlier:
But not utilized.. :icon_wink:
P.S.
Let's be forgiving with OP 'cause, IMHO, he tries to implement discrete building blocks without understanding the way they work.. :icon_wink:
Having a hard time quoting here but yes, patience is greatly appreciated and I am doing my best to understand the building blocks properly
Quote from: T-Dub on September 21, 2023, 01:07:09 AM
I am doing my best to understand the building blocks properly
IMHO, that's an essential requirement for
intentionally making them working
improperly.. :icon_wink:
(e.g. for making an asymmetrically clipping amp you have to know when and how it clips..)
Guilty as charged with using building blocks, without quite knowing what's going on yet :icon_redface: :icon_lol: I'm learning heaps here though
(https://i.postimg.cc/tsq7hg38/Screenshot-2023-09-21-at-5-23-23-PM.png) (https://postimg.cc/tsq7hg38)
Ok, so when it was mentioned to roll off DC gain to unity, I *think* that means to add a capacitor going to ground (c13 here), which I think is the same bit of info I accidentally skipped over that GGBB mentioned (I've implemented his suggestion there). Am I right that instead of placing the capacitor to ground I could have put R12 to Vref?
I'm interested by the R12 100k/ C13 220nf combination in values, would that make a LPF at 7.2HZ? Also, what would be the difference between using 100k vs 1k in the R12/R11 positions?
Thanks so much!
Quote from: T-Dub on September 23, 2023, 12:51:47 AMOk, so when it was mentioned to roll off DC gain to unity, I *think* that means to add a capacitor going to ground (c13 here), which I think is the same bit of info I accidentally skipped over that GGBB mentioned (I've implemented his suggestion there). Am I right that instead of placing the capacitor to ground I could have put R12 to Vref?
Yes but only in case of exactly the same voltage on both R12 legs..
(i.e. absolutely no voltage drop across R9..)
Quote from: T-Dub on September 23, 2023, 12:51:47 AMOk, so when it was mentioned to roll off DC gain to unity, I *think* that means to add a capacitor going to ground (c13 here), which I think is the same bit of info I accidentally skipped over that GGBB mentioned (I've implemented his suggestion there).
Yes. The cap makes a lowpass filter for the feedback, and since it's negative feedback, that has a *highpass* effect on the signal, so DC gain is unity.
Another way to look at it is to think about it "from DC to daylight". At DC/0Hz, all caps are an open connection, so the amp is effectively a unity-gain buffer with the output connected back to the -ve input. At "daylight", infinity-Hz, all caps are shorts, effectively a wire, so we can ignore them and calculate as if they weren't there.
That gives you a quick sketch of what the end points of the frequency response look like. The bit in the middle is what the maths is for!
QuoteAm I right that instead of placing the capacitor to ground I could have put R12 to Vref?
From a DC bias point of view, yes, you could do that instead, since the +ve input has that level already, so there'd be no problem. However, that doesn't fix the DC gain problem. It gives you a working amplifier with gain all the way down to DC.
QuoteI'm interested by the R12 100k/ C13 220nf combination in values, would that make a LPF at 7.2HZ?
Yes, it would.
QuoteAlso, what would be the difference between using 100k vs 1k in the R12/R11 positions?
If you change R12, the cutoff frequency would go up by 100K/1K = 100, so from 7.2Hz to 720Hz. The classic Tubescreamer value. If you change R11 as well, the gain doesn't change. So you can tweak gain or the frequency response by adjusting one or the other or both.
Quote from: ElectricDruid on September 23, 2023, 09:02:37 AMQuoteI'm interested by the R12 100k/ C13 220nf combination in values, would that make a LPF at 7.2HZ?
Yes, it would.
I think Tom is sloppy enough lately.. :icon_wink:
It's in a feedback loop, so call it whatever you like. It's either a lowpass because that's what it does to the feedback, or it's a highpass beause that's what it does to the signal.
My point was that T-Dub's cutoff frequency calculation is correct, which it seems to be.
Quote from: ElectricDruid on September 24, 2023, 04:40:32 PMIt's in a feedback loop, so call it whatever you like. It's either a lowpass because that's what it does to the feedback, or it's a highpass beause that's what it does to the signal.
My point was that T-Dub's cutoff frequency calculation is correct, which it seems to be.
Really blew my mind that (of course) the lpf acts as a hpf due to it being negative feedback.
Quote from: T-Dub on September 25, 2023, 03:39:33 AMReally blew my mind that (of course) the lpf acts as a hpf due to it being negative feedback.
Same as C8/Dist pot HPF, acting as LPF there.. :icon_wink:
P.S.
It should be more convenient to memorize the most frequently existed filters around an op-amp configuration.. :icon_wink:
(https://i.imgur.com/twgo7I7.gif)
edit: C6/R5/R4 HPF might seem puzzling (same for R7/C5/Gain pot lug3-2) but simply add resistor values for equivalent R in 1/2πRC corner frequency formula..
Quote from: T-Dub on September 25, 2023, 03:39:33 AMReally blew my mind that (of course) the lpf acts as a hpf due to it being negative feedback.
Yeah, me too! The first time I actually understood what was going on there was definitely one of those "Oh wow!" moments.
Quote from: ElectricDruid on September 25, 2023, 08:42:25 AMone of those "Oh wow!" moments.
Nobody remembers the reason they call them Operational Amplifiers.