Hi all,
Given that this is my first post here I appreciate all opinions 🙂
I started designing my own 3.1 preamp because I did not find a simple, solid and expandable design anywhere. By simple I mean few parts and by solid I mean low THD and noise.
By expandable I mean usable as a stereo mixer in the future (hence the Rx04 resistors where the mixer will be split), by adding a virtual ground and an inverting opamp or adding more pots for variable filters like this. Also I plan on adding a microphone preamp but first things first.
The inputs are designed to be fed mainly from x.1 PC soundcard, and because of the jumpers someone could use full-range speakers as well.
My main concerns before actual building the prototype are the auto oscillation of the opamps and the unforeseen issues I may have overlook 🙂 Nobody is perfect.
I started looking first at simplest designs such as this and this but I did not want to compromise neither the quality or the simplicity. I think Sallen-Key filters are enough.
The +/- 15v power supply is based on LM317/LM337 split rails and it's star grounded. Decoupling capacitors are 1uF MLCC (I plan on soldering another one on the backside to get to 2uF maybe I can 'sense' something 🙂)
I will use sockets on all preamps because I plan on testing TL072, NE5532, and LM4562.
I don't think I will need Electrolytic capacitors on the actual preamp pcb given that after the regulators I have 4700uF tanks and a whole lot of pi filters everywhere using 1uF/2uF MLCCs. But I will plan on testing this anyway.
The outputs from 100ohms resistors / volume potentiometers are fed directly into a parallel 1uF||1uF capacitor network to the amplifiers (lm1875 / lm3386 but that story is for another time).
Resistors Rx05/Rx06 are used instead of a buffer as a result of esp-project-155 design notes. I used them in the subwoofer section just as for the unit look of the design, although I plan on testing the non-inverting buffer as well.
First is the twitter Left Channel preamp (Right and Center are a just a copy):
Second is the Subwoofer Channel:
Last but not least the power supply using a PI filter (I saw this somewhere and liked the idea):
Using 2uF instead of 1uF gives ripple rejection Cutoff at 7.9Khz.
As you see the jumpers are used as a bypass switch but I can always replace them with switches connected by LYCY cables.
I will also post pictures and waveforms (not sure if my UT81B handheld oscilloscope is up to the task) after I will complete the Prototype boards.
So, any comments ? 😀
P.S. All the computations are done with the help of this public site.
Given that this is my first post here I appreciate all opinions 🙂
I started designing my own 3.1 preamp because I did not find a simple, solid and expandable design anywhere. By simple I mean few parts and by solid I mean low THD and noise.
By expandable I mean usable as a stereo mixer in the future (hence the Rx04 resistors where the mixer will be split), by adding a virtual ground and an inverting opamp or adding more pots for variable filters like this. Also I plan on adding a microphone preamp but first things first.
The inputs are designed to be fed mainly from x.1 PC soundcard, and because of the jumpers someone could use full-range speakers as well.
My main concerns before actual building the prototype are the auto oscillation of the opamps and the unforeseen issues I may have overlook 🙂 Nobody is perfect.
I started looking first at simplest designs such as this and this but I did not want to compromise neither the quality or the simplicity. I think Sallen-Key filters are enough.
The +/- 15v power supply is based on LM317/LM337 split rails and it's star grounded. Decoupling capacitors are 1uF MLCC (I plan on soldering another one on the backside to get to 2uF maybe I can 'sense' something 🙂)
I will use sockets on all preamps because I plan on testing TL072, NE5532, and LM4562.
I don't think I will need Electrolytic capacitors on the actual preamp pcb given that after the regulators I have 4700uF tanks and a whole lot of pi filters everywhere using 1uF/2uF MLCCs. But I will plan on testing this anyway.
The outputs from 100ohms resistors / volume potentiometers are fed directly into a parallel 1uF||1uF capacitor network to the amplifiers (lm1875 / lm3386 but that story is for another time).
Resistors Rx05/Rx06 are used instead of a buffer as a result of esp-project-155 design notes. I used them in the subwoofer section just as for the unit look of the design, although I plan on testing the non-inverting buffer as well.
First is the twitter Left Channel preamp (Right and Center are a just a copy):
An externally hosted image should be here but it was not working when we last tested it.
Second is the Subwoofer Channel:
An externally hosted image should be here but it was not working when we last tested it.
Last but not least the power supply using a PI filter (I saw this somewhere and liked the idea):
An externally hosted image should be here but it was not working when we last tested it.
Using 2uF instead of 1uF gives ripple rejection Cutoff at 7.9Khz.
As you see the jumpers are used as a bypass switch but I can always replace them with switches connected by LYCY cables.
I will also post pictures and waveforms (not sure if my UT81B handheld oscilloscope is up to the task) after I will complete the Prototype boards.
So, any comments ? 😀
P.S. All the computations are done with the help of this public site.
Hi,
Given that nobody commented on this, either nobody saw anything in that schematic or I made no mistakes or this is not that interesting..
Here are the first measurements :
Some issues I found:
- The oscilloscope picked up some noise apparent in the source signal but it might be because I don't have a proper probe. (I attached the actual signal below, measured at input but the HF shows up everywhere).
- Lm1875 does enter in clipping at about 12v (+25v/-25v supply) with the above preamplifier.
- Not sure if the potentiometer is ok before the coupling capacitor in the ampliifer, or after ?!
So any thoughts ? 🙂
Given that nobody commented on this, either nobody saw anything in that schematic or I made no mistakes or this is not that interesting..
Here are the first measurements :
An externally hosted image should be here but it was not working when we last tested it.
Some issues I found:
- The oscilloscope picked up some noise apparent in the source signal but it might be because I don't have a proper probe. (I attached the actual signal below, measured at input but the HF shows up everywhere).
- Lm1875 does enter in clipping at about 12v (+25v/-25v supply) with the above preamplifier.
- Not sure if the potentiometer is ok before the coupling capacitor in the ampliifer, or after ?!
An externally hosted image should be here but it was not working when we last tested it.
So any thoughts ? 🙂
Some random thoughts…
#1: The crossover frequencies seem very high. Typically, subwoofers should not emit frequencies that are easily localized, so that the ".1" channel can be placed somewhere that will not affect localization from the satellite speakers, and not cause needless mono summation of program material that ought to remain outside of the single ".1" speaker. Usually, this means little sub output above 100Hz or so. 338Hz is squarely inside of what Ma Bell thought was the essential part of the speech band for their ancient analog phone system. 338Hz is way too high!
Thought #2: I'm not sure what that power supply Pi filter is all about. What signals are on these circuit nodes? Why use a carefully crafted filter on a node that really should have no signal on it whatsoever? Typically, people use regulators to establish isolation from a power source and the consumer of that power. They also use shunt (bypass) capacitors to steer transient power supply currents, possibly away from other subsystems, or away from potentially slow regulators and into a 'fast' ground, or away from other subsystems within the device. I'm actually designing a box now with a really unconventional but effective power supply arrangement, so I'm not biased against new approaches, but I just can't see what problem this Pi filter is trying to solve. A great aircraft designer once said "If it ain't lift, it's drag." Simplify!
Thought #3: I know you want flexibility, but usually, a surround system can take stereo and digest it into the satellites and the ".1". However, you have separate inputs for these channels. Maybe add some extra stuff to allow someone to input stereo and decode it to your 3.1 outputs, as well as take a 3.1 signal and send it forth?
Thought #4: Preamps usually have a volume control and switching. You've shown us an odd crossover whose signal sources have to be pieced together. Think of the overall function of this box, and try to make circuits that help that along. Sure, it's fine to play around, but it's more fun when you imagine a device's function at a little higher level and then get circuits to make that work out.
Finally, sorry if I'm harsh, but keep working with this stuff and see what makes sense. Also, take advantage of the fact that there are many many schematics on line for so much gear, current or classic. When I was learning audio electronics in the '70s, I was never able to see how 'real' devices actually work, only goofy circuits in hobbyist books that had little to do with anything. You really do have the history of audio circuitry in front of you - look at it, see what was done, take exception to it, and do something else!
Best of luck!
#1: The crossover frequencies seem very high. Typically, subwoofers should not emit frequencies that are easily localized, so that the ".1" channel can be placed somewhere that will not affect localization from the satellite speakers, and not cause needless mono summation of program material that ought to remain outside of the single ".1" speaker. Usually, this means little sub output above 100Hz or so. 338Hz is squarely inside of what Ma Bell thought was the essential part of the speech band for their ancient analog phone system. 338Hz is way too high!
Thought #2: I'm not sure what that power supply Pi filter is all about. What signals are on these circuit nodes? Why use a carefully crafted filter on a node that really should have no signal on it whatsoever? Typically, people use regulators to establish isolation from a power source and the consumer of that power. They also use shunt (bypass) capacitors to steer transient power supply currents, possibly away from other subsystems, or away from potentially slow regulators and into a 'fast' ground, or away from other subsystems within the device. I'm actually designing a box now with a really unconventional but effective power supply arrangement, so I'm not biased against new approaches, but I just can't see what problem this Pi filter is trying to solve. A great aircraft designer once said "If it ain't lift, it's drag." Simplify!
Thought #3: I know you want flexibility, but usually, a surround system can take stereo and digest it into the satellites and the ".1". However, you have separate inputs for these channels. Maybe add some extra stuff to allow someone to input stereo and decode it to your 3.1 outputs, as well as take a 3.1 signal and send it forth?
Thought #4: Preamps usually have a volume control and switching. You've shown us an odd crossover whose signal sources have to be pieced together. Think of the overall function of this box, and try to make circuits that help that along. Sure, it's fine to play around, but it's more fun when you imagine a device's function at a little higher level and then get circuits to make that work out.
Finally, sorry if I'm harsh, but keep working with this stuff and see what makes sense. Also, take advantage of the fact that there are many many schematics on line for so much gear, current or classic. When I was learning audio electronics in the '70s, I was never able to see how 'real' devices actually work, only goofy circuits in hobbyist books that had little to do with anything. You really do have the history of audio circuitry in front of you - look at it, see what was done, take exception to it, and do something else!
Best of luck!
Hi,
Thank you for the feedback, it's all exactly what I needed.
If it can be criticized than it's not perfect 😉
#1 Valid point, I wanted to give the subwoofer the ability to reproduce even more frequencies. But you are right, I never thought about localization 🙂 ..
#2 The idea of the RC network there was to lower the noise from the regulator even further. I am now using LM317/LM337 as shown here. I can't find the link anymore with that RC network on an opamp and believe me I tried. (P.S. I liked the resistors even more when it helped my PCB traces 😉 )
#3 Yes that is the plan 😉 I will extend this in both directions 2ch->4ch 4ch->2ch
(8channels maybe ?!) Schematics coming soon 🙂 But for now I just want to make it work and find the best values for those resistors without the amplifiers clipping. I thought about using trimmers and adjusting even as I go along but I dismissed that idea.
#4 Volume controls will go at the input side of the preamps as an array of digital potentiometers connected to either SPI/I2C to a microcontroller. I will see when I get there.
So the idea here is to make some sort of jack of all trades preampllfier good enough for anything and everything. (I know that it can't do both extremely well, but if I can get it to -96db noise I am declared happy)
That's a nice quote "If it ain't lift, it's drag.", I will remember it 😀
Right now I am trying to finish the aluminium case because the amp goes into temperature protection with an old case I had, so that's another thing on my plate I didn't anticipate.
Thank you again though. It gave me some food for my thoughts where I can improve (add, remove, and optimize).
Thank you for the feedback, it's all exactly what I needed.
If it can be criticized than it's not perfect 😉
#1 Valid point, I wanted to give the subwoofer the ability to reproduce even more frequencies. But you are right, I never thought about localization 🙂 ..
#2 The idea of the RC network there was to lower the noise from the regulator even further. I am now using LM317/LM337 as shown here. I can't find the link anymore with that RC network on an opamp and believe me I tried. (P.S. I liked the resistors even more when it helped my PCB traces 😉 )
#3 Yes that is the plan 😉 I will extend this in both directions 2ch->4ch 4ch->2ch
(8channels maybe ?!) Schematics coming soon 🙂 But for now I just want to make it work and find the best values for those resistors without the amplifiers clipping. I thought about using trimmers and adjusting even as I go along but I dismissed that idea.
#4 Volume controls will go at the input side of the preamps as an array of digital potentiometers connected to either SPI/I2C to a microcontroller. I will see when I get there.
So the idea here is to make some sort of jack of all trades preampllfier good enough for anything and everything. (I know that it can't do both extremely well, but if I can get it to -96db noise I am declared happy)
That's a nice quote "If it ain't lift, it's drag.", I will remember it 😀
Right now I am trying to finish the aluminium case because the amp goes into temperature protection with an old case I had, so that's another thing on my plate I didn't anticipate.
Thank you again though. It gave me some food for my thoughts where I can improve (add, remove, and optimize).
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