Hello there.
As part of my initiation to electronics, I would like to build my own box to select an audio input and redirect it to 2 or 3 amps.
I need to be able to manage 3 inputs, without necessarily mixing them, I just have to select one at a time.
After looking at different circuits found on the net, I came up with this circuit.
To be honest, I don't think I have a good understanding of how preamps work in a feedback loop, but this design seems to be found on many preamps.
For the input selection part, I'm thinking of using a 3-position rotary switch, with a DPST relay for each switch position (or DPDT, easier to find). On the schematic are represented two SPST relays in parallel because the software I used has no DPST.
What do you think of this design ?
There are also some questions I have:
- Regarding the capacitors that are in parallel with the 5V supply, I was inspired by this circuit, but I don't understand what they are for. Some kind of filter against parasites ?
- When the input signal is divided into 3 parallel branches for the outputs, I think that we must have a weakening of the signal since a loss of intensity. Is it relevant to pre-amplify the signal before splitting it so that it keeps its input level in the three outputs ? If yes, do I need to add a resistor in the feedback loop of at least 30kOhm to have a x3 gain ?
- I'm a bit lost in the multitude of op amps (TL072, TL084, LM324, NE5532, ...). Is there any more adapted to my use than the NE5532 ?
Thanks
As part of my initiation to electronics, I would like to build my own box to select an audio input and redirect it to 2 or 3 amps.
I need to be able to manage 3 inputs, without necessarily mixing them, I just have to select one at a time.
After looking at different circuits found on the net, I came up with this circuit.
To be honest, I don't think I have a good understanding of how preamps work in a feedback loop, but this design seems to be found on many preamps.
For the input selection part, I'm thinking of using a 3-position rotary switch, with a DPST relay for each switch position (or DPDT, easier to find). On the schematic are represented two SPST relays in parallel because the software I used has no DPST.
What do you think of this design ?
There are also some questions I have:
- Regarding the capacitors that are in parallel with the 5V supply, I was inspired by this circuit, but I don't understand what they are for. Some kind of filter against parasites ?
- When the input signal is divided into 3 parallel branches for the outputs, I think that we must have a weakening of the signal since a loss of intensity. Is it relevant to pre-amplify the signal before splitting it so that it keeps its input level in the three outputs ? If yes, do I need to add a resistor in the feedback loop of at least 30kOhm to have a x3 gain ?
- I'm a bit lost in the multitude of op amps (TL072, TL084, LM324, NE5532, ...). Is there any more adapted to my use than the NE5532 ?
Thanks
First things I notice...
1/ The 5 volt supplies are a bit on the low side. Most audio circuits like this would use at least -/+12 volts to -/+ 18 volts. The caps on the supply provide local decoupling and ensure a low impedance at all frequencies.
2/ The opamp + inputs need ground referencing via something like a 100k to 470 k resistor.
3/ The 10k's on each input only add to the noise (albeit slightly) and serve no purpose. AC coupling the inputs (the 1uF is good though)
Your circuit can only have a gain of '1' as configured (and when the + input is sorted).
4/ One opamp can easily drive the three pots meaning you don't need three opamps per channel. There would be no interaction.
NE5532 is a great device but suffers a higher than average DC offset in a configuration like this. If you AC couple throughout that isn't such an issue. Don't use the LM324, its worse than terrible for audio. The TL0 series are favourites of mine and are docile and easy to work with.
1/ The 5 volt supplies are a bit on the low side. Most audio circuits like this would use at least -/+12 volts to -/+ 18 volts. The caps on the supply provide local decoupling and ensure a low impedance at all frequencies.
2/ The opamp + inputs need ground referencing via something like a 100k to 470 k resistor.
3/ The 10k's on each input only add to the noise (albeit slightly) and serve no purpose. AC coupling the inputs (the 1uF is good though)
Your circuit can only have a gain of '1' as configured (and when the + input is sorted).
4/ One opamp can easily drive the three pots meaning you don't need three opamps per channel. There would be no interaction.
NE5532 is a great device but suffers a higher than average DC offset in a configuration like this. If you AC couple throughout that isn't such an issue. Don't use the LM324, its worse than terrible for audio. The TL0 series are favourites of mine and are docile and easy to work with.
You definitely need to add resistors from the positive op-amp inputs to ground, to set the DC voltage there. Preferably also large resistors to ground on the other side of the relay contacts.
Supply decoupling capacitors are meant to counteract the effect of wiring inductance. The wires from the power supply to the op-amps have some self inductance that may make the op-amps unstable. Capacitors right next to the op-amps can solve that.
The op-amps form a very high impedance load for the signal source, so you don't get any noticable signal loss.
LM324 is unsuitable for audio use (except for some special cases). It is slow, noisy and produces lots of cross-over distortion.
Supply decoupling capacitors are meant to counteract the effect of wiring inductance. The wires from the power supply to the op-amps have some self inductance that may make the op-amps unstable. Capacitors right next to the op-amps can solve that.
The op-amps form a very high impedance load for the signal source, so you don't get any noticable signal loss.
LM324 is unsuitable for audio use (except for some special cases). It is slow, noisy and produces lots of cross-over distortion.
Thanks for your answers.
I have modified the circuit based on your comments. Concerning the capacitor that is supposed to reduce the inductance near the op-amp, should it be placed as in the circuit below or in the feedback loop?
@Mooly, when you talk about AC coupling, do we agree that the -12V/12V control circuit power source does act as AC coupling? I just copied what I found elsewhere, but a DC power supply could work just as well? Because I'm not sure yet how to supply the circuit.
I found this module, but I don't like to buy a power supply when I have dozens of old adapters lying around....
Also I decided to add a low pass filter activated by a switch on my output 3 which is for the subwoofer amp. The cut-off frequency is set for 80Hz (I chose the value of the components according to this site).
I have modified the circuit based on your comments. Concerning the capacitor that is supposed to reduce the inductance near the op-amp, should it be placed as in the circuit below or in the feedback loop?
@Mooly, when you talk about AC coupling, do we agree that the -12V/12V control circuit power source does act as AC coupling? I just copied what I found elsewhere, but a DC power supply could work just as well? Because I'm not sure yet how to supply the circuit.
I found this module, but I don't like to buy a power supply when I have dozens of old adapters lying around....
Also I decided to add a low pass filter activated by a switch on my output 3 which is for the subwoofer amp. The cut-off frequency is set for 80Hz (I chose the value of the components according to this site).
A capacitor in series with a switch without bleed resistors on both sides will cause loud popping when you switch. You need ground reference resistors both sides of the switches. The sub filter bypass connects the outputs of two op-amps together, which is a no-no. Switch the output and not the input of the filter. You may want to mix the channels for a mono sub. An 18dB sub filter vs 12dB is recommended. An Allen-Key filter requires non equal values for unity gain, but equal RC values works for a (2x) 6dB gain. The filter requires a ground reference if the input is disconnected.
Semiconductor switches usually have some distortion but if you use them as a shunt/short instead of in series, it's not a problem.
Semiconductor switches usually have some distortion but if you use them as a shunt/short instead of in series, it's not a problem.
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AC coupling refers to the input and output coupling caps. These prevent any DC appearing at the output which is something you don't want.
If you want the 150pF caps at the input then a small series input resistance before the 150pFwould be of benefit. That is because the 150pF effectively appears in parallel with the signal source which is an unknown. The series resistor now defines the response of the input filter. You could use something like a 2k2 and increase the 150pF to say 470pF. That would give a response like this at the input: The low frequency roll off is determined by the 1uF and 100k and the high roll off by 2k2 and 470pF
Hi! Just some tips. Like Steveu says the paralleling of opamp outputs in the schematic is an error.
- The 1 µF input caps could be film caps (if necessary at all which I doubt).
- Input RC filtering today is a must like Mooly suggested.
- There are no GND reference resistors before the input caps to discharge to so plopping will be an issue. You could leave out the caps after checking your sources. Modern sources normally have 0 DC offset because of output caps or DC servos.
- Same counts for the output caps. Check if your amplifiers have input caps and you then can use DC coupling. That is 12 caps less and not a single spot with 2 caps in series!
- Why 3 volume controls? Are all 3 amps used simultaneously? Otherwise just 1 volume control before the opamps could be a more price effective choice and you could obtain a higher quality version.
- with 3 outputs and no muting relays you will experience heavy plops when powering on the preamp after the power amplifiers. System choice you'll regret.
- Possibly you'll need slight gain in your situation.
- Since it is a new design one could think of current best quality opamps in SOIC8 like OPA1642, OPA1656, LME opamps etc. There is no reason to use classic DIP8 opamps.
- it is recommended to use onboard regulators. Even old school 7812/7912 will give you less headaches than switchers. There are many nice recent regulators and some even are symmetric in just 1 IC.
- Make sure you decouple the opamps supply pins as close to the opamp as possible.
- the 100 kOhm in the relays supply lines is an error.
- The relays better have antiparallel diodes over the coils.
- The 1 µF input caps could be film caps (if necessary at all which I doubt).
- Input RC filtering today is a must like Mooly suggested.
- There are no GND reference resistors before the input caps to discharge to so plopping will be an issue. You could leave out the caps after checking your sources. Modern sources normally have 0 DC offset because of output caps or DC servos.
- Same counts for the output caps. Check if your amplifiers have input caps and you then can use DC coupling. That is 12 caps less and not a single spot with 2 caps in series!
- Why 3 volume controls? Are all 3 amps used simultaneously? Otherwise just 1 volume control before the opamps could be a more price effective choice and you could obtain a higher quality version.
- with 3 outputs and no muting relays you will experience heavy plops when powering on the preamp after the power amplifiers. System choice you'll regret.
- Possibly you'll need slight gain in your situation.
- Since it is a new design one could think of current best quality opamps in SOIC8 like OPA1642, OPA1656, LME opamps etc. There is no reason to use classic DIP8 opamps.
- it is recommended to use onboard regulators. Even old school 7812/7912 will give you less headaches than switchers. There are many nice recent regulators and some even are symmetric in just 1 IC.
- Make sure you decouple the opamps supply pins as close to the opamp as possible.
- the 100 kOhm in the relays supply lines is an error.
- The relays better have antiparallel diodes over the coils.
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Thank you for all these complements.
I lack a lot of understanding to fully understand all your remarks. I'm going to dive back into the theory and come back to you as soon as I understand better how this whole mess works.
I lack a lot of understanding to fully understand all your remarks. I'm going to dive back into the theory and come back to you as soon as I understand better how this whole mess works.
It is not a mess. We just want you to build it without errors or inconveniences straight away.
If you say what you did not understand maybe we can explain. “We” as in “the members here”.
If you say what you did not understand maybe we can explain. “We” as in “the members here”.
Here are some of the things I can't figure out.
I can't see how bypassing the low pass filter connects the two opamp outputs. And how switching the output of the opamp instead of the input would change anything. Is it because of the volume control pot?
I don't understand this: "An 18dB sub filter vs 12dB is recommended. An Allen-Key filter requires non equal values for unity gain, but equal RC values works for a (2x) 6dB gain."
It seemed to me that in this configuration, my gain is x1, so there is no dB gain either. Where does the 6 or 12dB come from?
What does "RC filtering" mean?
If I moved my DC filtering capacitors just after the output of the first opamps, then only two might be enough, but will my different outputs interfere with each other?
All three amps should be able to work at the same time, but indeed a single potentiometer at the opamp input seems much better.
Would a x10 gain be relevant ? Or is it too much? Wouldn't that correspond to a gain of 20dB?
Where would you place the onboard regulators? At the output of the power supplies? I plan to use an already designed balanced power supply, it already contains them.
I can't see how bypassing the low pass filter connects the two opamp outputs. And how switching the output of the opamp instead of the input would change anything. Is it because of the volume control pot?
I don't understand this: "An 18dB sub filter vs 12dB is recommended. An Allen-Key filter requires non equal values for unity gain, but equal RC values works for a (2x) 6dB gain."
It seemed to me that in this configuration, my gain is x1, so there is no dB gain either. Where does the 6 or 12dB come from?
What does "RC filtering" mean?
If I moved my DC filtering capacitors just after the output of the first opamps, then only two might be enough, but will my different outputs interfere with each other?
All three amps should be able to work at the same time, but indeed a single potentiometer at the opamp input seems much better.
Would a x10 gain be relevant ? Or is it too much? Wouldn't that correspond to a gain of 20dB?
Where would you place the onboard regulators? At the output of the power supplies? I plan to use an already designed balanced power supply, it already contains them.
Well, I think I answered my issue about the LPF.
"RC filtering" means the simplest form of Resistor-Capacitor LPF and have itself a -3dB attenuation. A 2 RC design gives us a 6 dB/decade attenuation. And as it is a stereo output, we get 12dB when converting to mono. Those 12dB mean that at 80Hz, we have an attenuation of -12dB ? Or does it means that we have -12dB/decade ?
In order to have a 18dB sub filter, do I need to use 3rd or 4th order filter ?
It is also now obvious, as Mooly explained, that the design of the resistor and capacitor before the first opamp make them behave as HPF and LPF.
I made some adjustments on the circuit.
I realized that a rotary switch with at least two poles could switch the different inputs and I would not need any relays. I also added a 4th input connected to a bluetooth module.
I moved the input filtering capacitors after the rotary switch and I added before some resistors connected to the ground to avoid poppings.
I set the opamps to a gain of 10, don't know if it's right.
I'm now switching the output of the filter instead of the input, not sure if that's what you actually meant.
Here is the new circuit, I added the secondary components to have all the connections represented (bluetooth module, the power supply circuit and the AC input).
"RC filtering" means the simplest form of Resistor-Capacitor LPF and have itself a -3dB attenuation. A 2 RC design gives us a 6 dB/decade attenuation. And as it is a stereo output, we get 12dB when converting to mono. Those 12dB mean that at 80Hz, we have an attenuation of -12dB ? Or does it means that we have -12dB/decade ?
In order to have a 18dB sub filter, do I need to use 3rd or 4th order filter ?
It is also now obvious, as Mooly explained, that the design of the resistor and capacitor before the first opamp make them behave as HPF and LPF.
I made some adjustments on the circuit.
I realized that a rotary switch with at least two poles could switch the different inputs and I would not need any relays. I also added a 4th input connected to a bluetooth module.
I moved the input filtering capacitors after the rotary switch and I added before some resistors connected to the ground to avoid poppings.
I set the opamps to a gain of 10, don't know if it's right.
I'm now switching the output of the filter instead of the input, not sure if that's what you actually meant.
Here is the new circuit, I added the secondary components to have all the connections represented (bluetooth module, the power supply circuit and the AC input).
One very quick comment on the above. R12 and R13 need to be reference to something. As they are, the opamp + input is floating. Easiest way is simply to connect them to the other side of C5 and C6 because the first opamp output will be at zero volts DC.
Also consider AC coupling the output of those two right hand opamps
Fwiw, I think the TL072 is a good choice (FET input and no DC offset issues).
Haven't looked at anything else in detail (out of time 🙂) but the above jumped out.
Also consider AC coupling the output of those two right hand opamps
Fwiw, I think the TL072 is a good choice (FET input and no DC offset issues).
Haven't looked at anything else in detail (out of time 🙂) but the above jumped out.
With +/-5V supply you are pushing NE5532's or TL072's past their comfort zone (or more specifically you won't have much headroom for the signal. +/-15V is pretty standard for this generation of opamps, you can also use them at +/-9V for battery applications pretty well, but +/-5V is getting problematic. More modern rail-to-rail opamps would be fine at +/-5V, for instance the OPA1642 FET opamp is rail-to-rail and high performance.
And yes watch out for isolating opamp inputs at DC - they must have a DC path for bias current or the opamp output will latchup at full supply.
And yes watch out for isolating opamp inputs at DC - they must have a DC path for bias current or the opamp output will latchup at full supply.
RC filters should be before the volume control to avoid a variable filter. Why C5/C6? And you included stop resistors but forgot them at the outputs. It would help limit current as the outputs are shorted to GND.
Interesting power supply schematic. Are you sure it works and is safe? 🙂
Interesting power supply schematic. Are you sure it works and is safe? 🙂
C5/C6 are supposed to work as DC filtering caps, in the previous schematics I was using one for each parallel output (total of 6 caps) but I figured two caps before signal division could do the job. Is that right ? The amplifier I plan to use with are very likely to have input caps but I may also use an output for some other device, so I think it's better to have them.
On the same concern, do I need to put stop resistor for each parallel output or could I only use two before signal division ?
The power supply come already designed (more info), you just have to solder everything together and connect a transformer on the input. I just finished soldering it yesterday but haven't tested yet.
But indeed I have some concern about the safety of using a transformer and having some 230V in the box. I plan to put the transformer as close as possible of the power input pins and separated from the rest of the circuit. And only have 2 "dangerous" wires connected from the pins to the transformer.
But is there some other recommandation or things I should be careful about ?
The box will be made with plywood, transparent glass at the top and a steel plate on one side for thermic regulation of the voltage controller on the power supply module.
I have already made the input panel (see the pic). The rotary switch output is directly linked to the control volume pot, which simplify a lot cable management. But in this configuration the pot could act as a variable filter as it will be followed by caps, as jean-paul mentionned.
Maybe by adjusting C2 and C4 I could prevent an audible effect.
You can also see on the picture the planned disposition of the power supply and the transformer. The voltage regulator will be screwed to a steel plate on the right side.
On the same concern, do I need to put stop resistor for each parallel output or could I only use two before signal division ?
The power supply come already designed (more info), you just have to solder everything together and connect a transformer on the input. I just finished soldering it yesterday but haven't tested yet.
But indeed I have some concern about the safety of using a transformer and having some 230V in the box. I plan to put the transformer as close as possible of the power input pins and separated from the rest of the circuit. And only have 2 "dangerous" wires connected from the pins to the transformer.
But is there some other recommandation or things I should be careful about ?
The box will be made with plywood, transparent glass at the top and a steel plate on one side for thermic regulation of the voltage controller on the power supply module.
I have already made the input panel (see the pic). The rotary switch output is directly linked to the control volume pot, which simplify a lot cable management. But in this configuration the pot could act as a variable filter as it will be followed by caps, as jean-paul mentionned.
Maybe by adjusting C2 and C4 I could prevent an audible effect.
You can also see on the picture the planned disposition of the power supply and the transformer. The voltage regulator will be screwed to a steel plate on the right side.
Creepage. You have to remove some of the copper islands of your perfboard (easily done by overheating with a soldering iron) to ensure there is at least 6 mm (for class II) without any conductor between the 230 V connections and anything you might be able to come in contact with when the case is closed. Also check if the transformer requires a fuse and provide one if it does.
The transformer requires a 125mA fuse (datasheet). That confuses me as it is rated 15V and 11Vac. Shouldn't it be near 750mA, since it is the max expected current ?
I probably don't need such a high current but all the caps may trigger a current peak when turning on the device.
I probably don't need such a high current but all the caps may trigger a current peak when turning on the device.
Please reevaluate this and don't connect mains voltage before you do as I am sure it will be unsafe and may lead to heavy disappointment. Also normally the primary side is switched for safety reasons. It is not OK to leave the primary side energized and switch the device on by switching the secondary side as I think that is what you intend to do.
BTW it is 230V for many decades. The box should be metal (and connected to PE) not wood and glass if you want it to work safely and as desired.
Having noticed you are trying to develop skills but still are learning it maybe is good advice to build a known well performing opamp preamp with PCBs. Not meant negatively but it would more or less guarantee a working device and good performance. If you don't want that then you could copy the basic circuit with output stop resistors etc. For basic features and good performance the P88 and its PSU the P05 are recommendable. Rodd Eliott also is a pleasant guy to deal with and he supports his designs.
https://sound-au.com/project88.htm
BTW it is 230V for many decades. The box should be metal (and connected to PE) not wood and glass if you want it to work safely and as desired.
Having noticed you are trying to develop skills but still are learning it maybe is good advice to build a known well performing opamp preamp with PCBs. Not meant negatively but it would more or less guarantee a working device and good performance. If you don't want that then you could copy the basic circuit with output stop resistors etc. For basic features and good performance the P88 and its PSU the P05 are recommendable. Rodd Eliott also is a pleasant guy to deal with and he supports his designs.
https://sound-au.com/project88.htm
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