What I can understand from this is how it explains voltage follower works at unity gain, since non-inverting input and inverting input (output) are almost the same. I don't understand how it connects to the previous posts though. Is the last circuit I posted an improvement or downgrade of your OS/R1 suggestion? Simulation seems to show that it at least works at unity gain.
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Hello all, I thought I'd revive this thread a bit as I am putting an amp together using this board and I have some decisions to make.
For you that are (understandably) unwilling to read this whole thread, my board is an LM3886 amp with an NE5534 preamp stage. The schematic is on the post right before this one. I won't be touching the power amp section, I am only tinkering with the NE5534 stage. Primarily because it came with a low input impedance of 2.2KOhm from the manufacturer, and as someone mentioned earlier, that is too low for a general purpose amplifier. Allow me to provide a hopefully cleaner schematic of the preamp stage:
Rz represents my source's impedance of course.
I have already changed R1 from 2.2KOhm to 33KOhm. My first question is, do I need to change anything else on the resistor network so that the preamp keeps working properly? In particular, I am suspicious of R2 which has the same value of 2.2KOhm as R1 used to have. Do I have to change it? And just out of curiosity, what purpose does R2 serve? If I understand correctly, the op amp's input has practically infinite impedance, so why have an additional external resistor there?
Looking forward for your input guys and gals.
For you that are (understandably) unwilling to read this whole thread, my board is an LM3886 amp with an NE5534 preamp stage. The schematic is on the post right before this one. I won't be touching the power amp section, I am only tinkering with the NE5534 stage. Primarily because it came with a low input impedance of 2.2KOhm from the manufacturer, and as someone mentioned earlier, that is too low for a general purpose amplifier. Allow me to provide a hopefully cleaner schematic of the preamp stage:
Rz represents my source's impedance of course.
I have already changed R1 from 2.2KOhm to 33KOhm. My first question is, do I need to change anything else on the resistor network so that the preamp keeps working properly? In particular, I am suspicious of R2 which has the same value of 2.2KOhm as R1 used to have. Do I have to change it? And just out of curiosity, what purpose does R2 serve? If I understand correctly, the op amp's input has practically infinite impedance, so why have an additional external resistor there?
Looking forward for your input guys and gals.
Hi,
Attached it is a link for the same amplifier with an schematic in EBAY. The problem it is to light to be read it. Maybe somebody can make it more darker so can be able to read it. Good luck.
https://www.ebay.com/itm/162993442604?hash=item25f32a8f2c:g:ZNYAAOSwDGhazh~T&amdata=enc%3AAQAIAAAA4FUAb%2FCUB5Kfg7cRXl6m%2FPxPGMuRAQThRWtqzCpBDT4fbWaA2uOcvovwwBu9EK12rrsrB9VYjIKScStf5Art8H5Fgy8MS7X3ocYp7BhR%2BXoUwynA896r%2FHrisevbqh8%2BaElrXlp5lCpbG%2BU1YP4kMxmzWrORfsaesoForzXcsS9BjODQvnkKOAzcGwS%2BWiiYol3d1W84mzEUB4OdXaAEOs2bEnE0r%2F%2BChePofqKieHXUKBg1%2F6DXYxy7GsiJluk22WBd3JGnzaX7MTa3AVdUppXY213GCbnkuiTH1kJ8w38j%7Ctkp%3ABk9SR_r3rcTzYg read.
Attached it is a link for the same amplifier with an schematic in EBAY. The problem it is to light to be read it. Maybe somebody can make it more darker so can be able to read it. Good luck.
https://www.ebay.com/itm/162993442604?hash=item25f32a8f2c:g:ZNYAAOSwDGhazh~T&amdata=enc%3AAQAIAAAA4FUAb%2FCUB5Kfg7cRXl6m%2FPxPGMuRAQThRWtqzCpBDT4fbWaA2uOcvovwwBu9EK12rrsrB9VYjIKScStf5Art8H5Fgy8MS7X3ocYp7BhR%2BXoUwynA896r%2FHrisevbqh8%2BaElrXlp5lCpbG%2BU1YP4kMxmzWrORfsaesoForzXcsS9BjODQvnkKOAzcGwS%2BWiiYol3d1W84mzEUB4OdXaAEOs2bEnE0r%2F%2BChePofqKieHXUKBg1%2F6DXYxy7GsiJluk22WBd3JGnzaX7MTa3AVdUppXY213GCbnkuiTH1kJ8w38j%7Ctkp%3ABk9SR_r3rcTzYg read.
@tauro0221 yes that's the one. The manufacturer schematic I uploaded a few posts earlier. But I would like to focus on the preamp and its schematic I uploaded right after that. Thanks for your input.
Friendly bump. I have replaced R1 from 2.2KOhm to 33KOhm. Do I need to replace anything else? Also, what does R2 do?
R2 is often included with the idea that it helps stability. It often does, depending on layout.
Note that you normally want the DC resistance that each input sees to be equal to minimize DC offset.
On the -input the DC resistance seen is 2.2k // 8.2k, at the +input it is 33k. That's quite an imbalance.
I would recommend to change R1 to 6.8k and R2 to 1k, and to change the feedback string to say 27k and 7.5k.
Rz can be 1k and then you can use a small cap // to R1 to roll of the input above say 30kHz to limit hf hash and EMI.
Jan
Note that you normally want the DC resistance that each input sees to be equal to minimize DC offset.
On the -input the DC resistance seen is 2.2k // 8.2k, at the +input it is 33k. That's quite an imbalance.
I would recommend to change R1 to 6.8k and R2 to 1k, and to change the feedback string to say 27k and 7.5k.
Rz can be 1k and then you can use a small cap // to R1 to roll of the input above say 30kHz to limit hf hash and EMI.
Jan
Thanks for your response @jan.didden. Isn't the - lead seeing just 2.2k? Why is 8.2k entering the picture?
@jan.didden as opposed to the schematic input impedance of 200Ω, I need to accommodate a high source output impedance of 4-6KΩ. This was obviously my motivation to set R1 = 33KOhm.
I was wondering, in order to keep the balance you describe, why not replace the parallel resistors of the inverting input with much higher values to match the resistance seen by the non inverting input? If too high resistor values are a bad idea at that point in the circuit, I am willing to reduce the amplification ratio (I understand those two resistors are the ones which set it).
What would you recommend for a R1 of 33ΚΩ, or 20KΩ if 33KΩ is pushing the limits of this simple circuit? Amplification is roughly 12dB with the initial schematic, but I'll be fine with 6dB if necessary.
I was wondering, in order to keep the balance you describe, why not replace the parallel resistors of the inverting input with much higher values to match the resistance seen by the non inverting input? If too high resistor values are a bad idea at that point in the circuit, I am willing to reduce the amplification ratio (I understand those two resistors are the ones which set it).
What would you recommend for a R1 of 33ΚΩ, or 20KΩ if 33KΩ is pushing the limits of this simple circuit? Amplification is roughly 12dB with the initial schematic, but I'll be fine with 6dB if necessary.
@jan.didden I just played some very "dense" dance music at a volume I won't be expected to go over too often. Then disconnected the outputs and connected a TRMS multimeter on one of them. It was jumping around not being able to surpass 125mV rms. I believe a max 250mV, or even 200mV is a very safe max signal to assume. Mind you, the current source is a passive mixer with an approximate Zout of 4-5KΩ, but I'd like the preamp to be more general impedance-wise, meaning it will probably need to handle lower impedances as well (100-200Ω).
Inverting is not a problem. What follows the coupling capacitor is an LM3886 power amp. I don't think it will mind the inversion.
Inverting is not a problem. What follows the coupling capacitor is an LM3886 power amp. I don't think it will mind the inversion.
If the mixer current output is a fixed impedance like a resistor, you can use the opampin inverting mode: directly connect the mixer output to the -input and set the gain with the feedback resistor. +input to Gnd. With 5k mixer output and 12dB gain that Rf should be 20k.
But for that to work the Rout should be constant. How does that circuit look?
Jan
But for that to work the Rout should be constant. How does that circuit look?
Jan
@jan.didden good evening. I'm afraid I don't have a schematic for it. It's a passive mixer with 4 inputs and 1 mix output. Measuring its output impedance, I can tell it gets affected by if and what you have connected, and how you rotate each attenuator potentiometer. Bonus: each input has a corresponding dedicated output. If you plug something to the output, it disconnects that input from the main mix (normalling TRS jacks at play). Then what you have is an independent passive attenuator with an absolutely fixed 4KΩ output impedance.
Although it could be viable, I believe the inverting preamp would be too much of a fuss with my board. Also, I suspect it would limit the kind of sources I can use and perhaps be a bit noisier.
So, assuming I stick to my non inverting schematic: I suspect that with this simple layout I can't get high input impedance in an acceptable manner. I'm guessing that 10KΩ should probably be an upper limit, so let's fix R1 to that value. Would 1KΩ be reasonable for R2? Also, how close should R3 || Rf be to R1 + R2 (very approximately) in order to avoid the DC component imbalance you described earlier? Lastly, to avoid noise, what upper limit would you put on Rf given this layout?
I have a feeling that really soon I will be buying my first breadboard for experimentation and measurements with NE5532/NE5534, trim pots, and my DAC/ADC.
Although it could be viable, I believe the inverting preamp would be too much of a fuss with my board. Also, I suspect it would limit the kind of sources I can use and perhaps be a bit noisier.
So, assuming I stick to my non inverting schematic: I suspect that with this simple layout I can't get high input impedance in an acceptable manner. I'm guessing that 10KΩ should probably be an upper limit, so let's fix R1 to that value. Would 1KΩ be reasonable for R2? Also, how close should R3 || Rf be to R1 + R2 (very approximately) in order to avoid the DC component imbalance you described earlier? Lastly, to avoid noise, what upper limit would you put on Rf given this layout?
I have a feeling that really soon I will be buying my first breadboard for experimentation and measurements with NE5532/NE5534, trim pots, and my DAC/ADC.
As long as you use an output coupling capacitor the DC unbalance will not be a problem, except possibly at switch-on or switch-off.
For resistor values, I would personally not go higher than 10k, with probably max 20k for the feedback resistor.
It is not a hard limit, and if your signal level is relatively high you can go higher.
It's easy to check: listen to the noise and experiment to see if a) the noise is objectionable and b) if you can lower it by using smaller resistors.
I can give you the equation for resistor noise (somebody will probably do that after this post) but that doesn't really tell you what it does in your circuit.
Bottom line: there's no substitute for just going ahead and play and blow things up ;-)
Jan
For resistor values, I would personally not go higher than 10k, with probably max 20k for the feedback resistor.
It is not a hard limit, and if your signal level is relatively high you can go higher.
It's easy to check: listen to the noise and experiment to see if a) the noise is objectionable and b) if you can lower it by using smaller resistors.
I can give you the equation for resistor noise (somebody will probably do that after this post) but that doesn't really tell you what it does in your circuit.
Bottom line: there's no substitute for just going ahead and play and blow things up ;-)
Jan
Hello, I'm back with a vengeance. Simple question: What benefits does one reap by doing the more complex wiring that does the filtering, instead of the simple voltage follower version where you just connect the op amp output to one of the inputs?
Hello all, it's been a while. That project is still unfinished and I think it's a shame, so I thought about revisiting it.
If you 're with me, let's take this as a starting point:
I have chosen Rf = 20k as the feedback capacitor and Rg = 6.8k as the grounding capacitor to set the ratio. I believe 20k is on the high side for such a circuit, I have heard recommendations of not over 10k for the feedback resistor. But I decided I wanted something like a 12dB voltage gain, and I am planning to use relatively high value resistors on the non-inverting side of the op amp too. So, for the time being let's take Rf = 20k and Rg = 6.8 as a given.
Now, the part I would like some help with, the input impedance resistor Rz (and the auxiliary resistor at the non-inverting input, Raux). I would like to have adequate impedance because my source will be relatively high output impedance. So I would like to go for at least Rz = 10k, ideally Rz = 20k. I am reading that using higher value resistors can have detrimental effects on noise and DC offset of the opamp.
What I did was recreate the circuit off the amp (minus the filtering capacitor and the 56k output resistance) to check things in practice.
Concerning the first issue of increased noise, I did a frequency sweep of the circuit being inside a grounded metallic enclosure (old tin box of cocoa). I mainly wanted to see the difference of Rz = 10k vs Rz = 20k.
This is what I got from REW. I should note that Raux was set to 1.5k when I measured:
- For Rz = 10k:
- For Rz = 20k:
Even with the shielding, the circuit is weakly picking up the 50Hz from the power lines.
I think these clearly show that a) there is a difference in the noise floor and b) it is too small to be of concern. I wonder if you agree.
The way I think about it at the moment is I'd have no problem choosing Rz = 20k because I want as much input impedance as possible.
But the noise floor is not the only problem with opamps.
I now have a better understanding of what DC offset is, and how simple it is to measure it (I think).
What I did was disconnect the circuit input, leaving it floating, and powered it up. Then, using a multimeter in DC voltage mode I measured
the opamp's output, without the filtering capacitor.
It turns out the more I increase Rz (but also Raux) the higher the DC offset becomes. Actually lower, as it is negative. Pictures follow:
Last picture shows that Raux also plays a role in worsening the DC offset.
Should I just slap a 10uF capacitor on the output and don't think about it? Or can DC offset be somehow problematic even then?
Also, is the Raux resistor really necessary? If I lose it, how do I know everything's still working as well as with it on the circuit?
I would also love your commentary on my tin can frequency sweep results, about noise.
Thanks for your patience in reading this far, can't wait it hear your input.
If you 're with me, let's take this as a starting point:
I have chosen Rf = 20k as the feedback capacitor and Rg = 6.8k as the grounding capacitor to set the ratio. I believe 20k is on the high side for such a circuit, I have heard recommendations of not over 10k for the feedback resistor. But I decided I wanted something like a 12dB voltage gain, and I am planning to use relatively high value resistors on the non-inverting side of the op amp too. So, for the time being let's take Rf = 20k and Rg = 6.8 as a given.
Now, the part I would like some help with, the input impedance resistor Rz (and the auxiliary resistor at the non-inverting input, Raux). I would like to have adequate impedance because my source will be relatively high output impedance. So I would like to go for at least Rz = 10k, ideally Rz = 20k. I am reading that using higher value resistors can have detrimental effects on noise and DC offset of the opamp.
What I did was recreate the circuit off the amp (minus the filtering capacitor and the 56k output resistance) to check things in practice.
Concerning the first issue of increased noise, I did a frequency sweep of the circuit being inside a grounded metallic enclosure (old tin box of cocoa). I mainly wanted to see the difference of Rz = 10k vs Rz = 20k.
This is what I got from REW. I should note that Raux was set to 1.5k when I measured:
- For Rz = 10k:
- For Rz = 20k:
Even with the shielding, the circuit is weakly picking up the 50Hz from the power lines.
I think these clearly show that a) there is a difference in the noise floor and b) it is too small to be of concern. I wonder if you agree.
The way I think about it at the moment is I'd have no problem choosing Rz = 20k because I want as much input impedance as possible.
But the noise floor is not the only problem with opamps.
I now have a better understanding of what DC offset is, and how simple it is to measure it (I think).
What I did was disconnect the circuit input, leaving it floating, and powered it up. Then, using a multimeter in DC voltage mode I measured
the opamp's output, without the filtering capacitor.
It turns out the more I increase Rz (but also Raux) the higher the DC offset becomes. Actually lower, as it is negative. Pictures follow:
Last picture shows that Raux also plays a role in worsening the DC offset.
Should I just slap a 10uF capacitor on the output and don't think about it? Or can DC offset be somehow problematic even then?
Also, is the Raux resistor really necessary? If I lose it, how do I know everything's still working as well as with it on the circuit?
I would also love your commentary on my tin can frequency sweep results, about noise.
Thanks for your patience in reading this far, can't wait it hear your input.
Rz is in parallel with the source output impedance and that combination determines the equivalent noise.
Is the source capacitively coupled? If so, the Rz determines the resistance the noninverting input sees for DC. To minimize offset, you want to keep the DC impedances at each input to the same value. On the inverting input the DC impedance seen is the parallel resistance of the two feedback resistances.
Jan
Is the source capacitively coupled? If so, the Rz determines the resistance the noninverting input sees for DC. To minimize offset, you want to keep the DC impedances at each input to the same value. On the inverting input the DC impedance seen is the parallel resistance of the two feedback resistances.
Jan