Choices for a simple transparent 5 to 10 band EQ (for vocals)

[Rob Strand]

OK, I got 220 & 1000uF, but both are 16V.  Should I go all the way to 1000?  I do not have 470uF...

Sounds like a good idea.  I do not want a "bang" on powerup, so better to increase.  So increase all 4 caps, right?  Not only C7 and C8, but C5 & C6 as well, right?

C5 and C6 can be either 220u or 1000uF, up to you, slight advantage with 1000uF.   Making C7 and C8 1000uF will help tame the turn-on thump/bang.

16V is pushing the caps close to the limit, especially C5 and C6.   It will work but it will reduce the life of the cap.

I just want to be clear that it is quite difficult to remove *all* the power-up thump when using those doubler circuits.  You can certainly reduce it.    Completely removing it will need extra circuits with transistors.    Most single supply circuit thump a small amount.

So not just connected to ground, but LITERALLY between the 2 caps?

You want  the transformer wire to go to the centre of two caps C5, C6 on it's own track, not through the ground plane.  How you did it looks OK.  You can then connect another track from the center of the caps along to C6 and C7 and then from that point to the ground.

You still have a bug on your power supply  12VAC to ground is still going to the diodes.  You need to remove that line.   One side of the transformer connects to both diode and the other side of the transformer connection to C5, C6.

Also you could connect the LEDs to C5 and C6 instead of C7, C8.  The idea is it helps stop the LED1 current changing the +ve supply voltage to the opamps due to it loading the power rail.

LED polarities look OK.

That's as far as I got this evening.

[jfrabat]

C5 and C6 can be either 220u or 1000uF, up to you, slight advantage with 1000uF.   Making C7 and C8 1000uF will help tame the turn-on thump/bang.

OK, 1000 it is then for all 4 of them.

16V is pushing the caps close to the limit, especially C5 and C6.   It will work but it will reduce the life of the cap.

I will start then with 16V until I can get higher voltage.

I just want to be clear that it is quite difficult to remove *all* the power-up thump when using those doubler circuits.  You can certainly reduce it.    Completely removing it will need extra circuits with transistors.    Most single supply circuit thump a small amount.

Understood.  I will have the bypass switch (hopefully no THUMP there!) to work with as well...

You want  the transformer wire to go to the centre of two caps C5, C6 on it's own track, not through the ground plane.  How you did it looks OK.  You can then connect another track from the center of the caps along to C6 and C7 and then from that point to the ground.

You still have a bug on your power supply  12VAC to ground is still going to the diodes.  You need to remove that line.   One side of the transformer connects to both diode and the other side of the transformer connection to C5, C6.

I am having a real hard time getting Eagle to separate the 12VAC2 from GND, because, to Eagle, they are directly connected and thus are the same thing.  In the schematics, it does not really matter, but when doing the board, it is a REAL PITA!  So I added a 0 Ohm resistor to dived the segments...  I can use it or use a jumper in the final build.

Anyway, here is what the new board looks like:



I tried going from the 12V AC on the right, to the middle of C5 (1) & C6 (+), and from there a point where C7 (-), C8 (+), D1 (cathode), D2 (anode), Switch 1 and R10 (which I had to add to trick Eagle) all are connected.  I really had to wiggle my traces in there!

Also you could connect the LEDs to C5 and C6 instead of C7, C8.  The idea is it helps stop the LED1 current changing the +ve supply voltage to the opamps due to it loading the power rail.

LED polarities look OK.

Here's the new schematic showing this as well:



Please review to make sure I did not screw it up anywhere.  Also, about R10, should I use a 0 Ohm resistor, or just a jumper?

[niektb]

0 ohm resistor or jumper doesnt really matter 

Deng bro, that is nót the nicest pcb drawing i've ever seen 
You should check if the NE5532's and the jacks can actually be fitted physically on your bord...
I think I would use a full-wave rectifier (so 4 diodes), not sure whether yours will work as expected actually

[jfrabat]

0 ohm resistor or jumper doesnt really matter 

Deng bro, that is nót the nicest pcb drawing i've ever seen 
You should check if the NE5532's and the jacks can actually be fitted physically on your bord...
I think I would use a full-wave rectifier (so 4 diodes), not sure whether yours will work as expected actually

You realize you are looking at both sides at once, right?  Pots go on one side, ICs on the other...

I agree it is not my best board, but it is not THAT bad...

Jacks and double gang pots go offboard, by the way.  And yes, I got plenty of space.  It is all checked.

As for the rectifiers, I had them, but took them out as a result of this board using AC.  I could use them in V+ and V-, but that will depend on how the breadboard works.

[Rob Strand]

I am having a real hard time getting Eagle to separate the 12VAC2 from GND, because, to Eagle, they are directly connected and thus are the same thing.  In the schematics, it does not really matter, but when doing the board, it is a REAL PITA!  So I added a 0 Ohm resistor to dived the segments...  I can use it or use a jumper in the final build.

That's a normal problem.   You shouldn't really add a 0R just to get around that.  Yes, is does break the two NETs but there's no real reason circuit-wise to add that 0R resistor.  I'd just use the GND for that TX wire and leave it.   Use separate text to label the connector.

In your updated schematic you have a new problem the AC input at the diodes is shorted to the AC input to ground.   Follow the original schematic and you will see what I mean.

Also R2  (to LED2) should go to C5.

[jfrabat]

It is also better to have regulated power supply with linear regulators.

I think I would use a full-wave rectifier (so 4 diodes), not sure whether yours will work as expected actually

You know, I do have some W01G 100V 1.5A rectifiers...  should I go in that direction? 

I think I will try it on the breadboard as it is and see what's what.  After all, it is almost ready to try (of course, i need to wait for the power source, and I am using 47K resistors instead of the dual gang pots).



But let me know your ideas.

[jfrabat]

That's a normal problem.   You shouldn't really add a 0R just to get around that.  Yes, is does break the two NETs but there's no real reason circuit-wise to add that 0R resistor.  I'd just use the GND for that TX wire and leave it.   Use separate text to label the connector.

I tried, but when making it to the board, instead of having the ground plane be ground, it was V- or 12VAC.  This is the only way I managed to get it to work.  I can always just put a jumper there, but this is the only solution I had to make the board work...

In your updated schematic you have a new problem the AC input at the diodes is shorted to the AC input to ground.   Follow the original schematic and you will see what I mean.

Also R2  (to LED2) should go to C5.

I see what you mean.  The LED original circuit threw me off there.  I kept the same connections, but without a diode and a resistor, I guess it works different!   

This is the latest schematic incorporating the solutions mentioned.  Correct me if I misunderstood your meaning, please!



And here is the board with the corrections of the schematic:



I also corrected the breadboard, as it had that same short...

[Rob Strand]

I can always just put a jumper there, but this is the only solution I had to make the board work...

It's more to do with the software than an electronics problem.

Correct me if I misunderstood your meaning, please!

All looks good.

Some bigger picture issues are:
- Do you want to add some input and output coupling caps?   
- Also are you happy with the fairly low input impedance?   If you plug a guitar or bass into that circuit directly it will load the pickups.

[jfrabat]

Educate me a little here..

Some bigger picture issues are:
- Do you want to add some input and output coupling caps?   

These are the caps we put at the input and output to filter the signal, right?  Blocking DC and allowing AC to pass (at the cost of low frequency).  Is that right?

What is the upside and downside of using them vs not using them for this application (voice mostly)?

- Also are you happy with the fairly low input impedance?   If you plug a guitar or bass into that circuit directly it will load the pickups.

What do you mean by "loading up the pickups"?  I intend to use this mostly for voice (microphone in any case).  Chain will be Mic to PreAmp to Compresor to EQ (this pedal) to Audio Interphase (or mixer console if live).

Now, if I can make it switchable (increase impedance with a toggle) then that woult be an interesting addition...  then I have the option to use it for guitar! But I have no clue how to do that.

[Rob Strand]

These are the caps we put at the input and output to filter the signal, right?  Blocking DC and allowing AC to pass (at the cost of low frequency).  Is that right?

What is the upside and downside of using them vs not using them for this application (voice mostly)?

The goal is more about blocking DC than cutting low-frequencies.  Ideally you want to choose large enough capacitors so the signal is unaffected.

Why you would put them in is more of a bigger picture view of reducing problems in different scenario's.   In an ideal scenario you could leave them out and the equalizer will still work.    An example of non-ideal scenario is you connect it to another device, where someone has left out the very same caps, or the caps are faulty, and there is DC on the output.  When you connect that device to an EQ with no input caps  the DC feeds into the EQ and that could reduce the headroom of the equalizer and cause unexplained early clipping.  Another scenario you connect it to a faulty device and the DC output from that devices blows up parts in your device.    If all devices use input and output caps it reduces the risk of weird stuff happening.

What do you mean by "loading up the pickups"?  I intend to use this mostly for voice (microphone in any case).  Chain will be Mic to PreAmp to Compresor to EQ (this pedal) to Audio Interphase (or mixer console if live).

Earlier on you mentioned you might use it with an instrument.   In the case where you connect the instrument directly to the EQ the low input impedance of the EQ will load down the pickup.  As is the EQ is more of a 'line level' device as opposed to an instrument compatible device (like an EQ pedal). 

In order to make it an instrument compatible device you need to add a buffer to the front end.  You get the idea for the input buffer from  section "A" on this schematic, although I'd probably make the 470k's higher.   Notice the input stage here has an AC coupling cap.     The resistor across the input stops "pops" when you connect it to another devices.

https://www.schematics.com/site_media/media/project/previews/55855-preview2x.png

The output side of this circuit in section "B".  This circuit has an additional opamp, which you don't need.  The things to take note of are: the resistor in series with the output of the opamp, which helps stop the opamp oscillating when connected to cables, and the capacitor and resistor to ground which provide the AC coupling at the output.

If you hunt around on the web you will see a lot of circuits which use the same ideas for the input and output stages.

[niektb]

You realize you are looking at both sides at once, right?  Pots go on one side, ICs on the other...

I agree it is not my best board, but it is not THAT bad...

Jacks and double gang pots go offboard, by the way.  And yes, I got plenty of space.  It is all checked.

As for the rectifiers, I had them, but took them out as a result of this board using AC.  I could use them in V+ and V-, but that will depend on how the breadboard works.

Yes I totally noticed that but depending on the pots, you might not have space for the opamp pins on the other side... But if you're sure about that, then it's okay

I think the biggest thing that's bugging me is your routing. It's much neater if you use only angles of 45 degrees (and you should totally avoid angles of 90 degrees and higher, especially when using high-speed signals, as it degrades your signal )

If your board is using AC then you need something that rectifies right? (using a special rectifier or 4 diodes doesn't really differ a whole lot but a special rectifier can often handle more current/voltage)

[jfrabat]

The goal is more about blocking DC than cutting low-frequencies.  Ideally you want to choose large enough capacitors so the signal is unaffected.

Why you would put them in is more of a bigger picture view of reducing problems in different scenario's.   In an ideal scenario you could leave them out and the equalizer will still work.    An example of non-ideal scenario is you connect it to another device, where someone has left out the very same caps, or the caps are faulty, and there is DC on the output.  When you connect that device to an EQ with no input caps  the DC feeds into the EQ and that could reduce the headroom of the equalizer and cause unexplained early clipping.  Another scenario you connect it to a faulty device and the DC output from that devices blows up parts in your device.    If all devices use input and output caps it reduces the risk of weird stuff happening.

So it seems like a good idea to add them.  Is there any downside to adding them?  How big should I go?

Earlier on you mentioned you might use it with an instrument.   In the case where you connect the instrument directly to the EQ the low input impedance of the EQ will load down the pickup.  As is the EQ is more of a 'line level' device as opposed to an instrument compatible device (like an EQ pedal). 

If I do increase impedance, will it affect using it as a line level device?  That is the part I am not sure of.  If it has no negative effect, then I prefer to increase the impedance and be able to use it for guitars as well.  But if it does, then I will leave it as is, as the main purpose is to use it on a line coming from a Mic (after the preamp and compressor).

In order to make it an instrument compatible device you need to add a buffer to the front end.  You get the idea for the input buffer from  section "A" on this schematic, although I'd probably make the 470k's higher.   Notice the input stage here has an AC coupling cap.     The resistor across the input stops "pops" when you connect it to another devices.

https://www.schematics.com/site_media/media/project/previews/55855-preview2x.png

The output side of this circuit in section "B".  This circuit has an additional opamp, which you don't need.  The things to take note of are: the resistor in series with the output of the opamp, which helps stop the opamp oscillating when connected to cables, and the capacitor and resistor to ground which provide the AC coupling at the output.

If you hunt around on the web you will see a lot of circuits which use the same ideas for the input and output stages.

If my previous knowledge is correct, the buffer would be R2, R3, R4 and C1 (input), right?  I can't find the output one...  (by the way, there are 4 sections A and 4 section B)

Yes I totally noticed that but depending on the pots, you might not have space for the opamp pins on the other side... But if you're sure about that, then it's okay

Then it is OK.  I measured it and it works out.

I think the biggest thing that's bugging me is your routing. It's much neater if you use only angles of 45 degrees (and you should totally avoid angles of 90 degrees and higher, especially when using high-speed signals, as it degrades your signal )

OK, I took your advice.  I re-routed the board.  Here is the new board:



Still not pretty, but better.  I took out all the weird angles in the traces.

If your board is using AC then you need something that rectifies right? (using a special rectifier or 4 diodes doesn't really differ a whole lot but a special rectifier can often handle more current/voltage)

The original drawing uses the diodes and caps for that.  I tried "improving" it with rectifiers, but I ended up screwing up the circuit.  So I went back to the original.

[jfrabat]

By the way, I just noticed I made the resistor size a little too small in Eagle (physical size).  I have 3 options:

1. Delete all resistors and replace them with the correct size (best option, but time consuming)
2. Squeeze the 1/4W 7mm long resistors into holes 5mm apart
3. Use 1/8W resistors (which I have)

Would 1/8W be enough for this pedal?  Technically, it drains 100mAh, so 1/10W should be enough, right?  To be honest, I am leaning towards #2 because I do not want to waste all the resistors I already got on the breadboard (I would save them, of course, but they will be out of the carton holder, so they would easily get misplaced).  Everything else checks out size wise.

[Rob Strand]

Sorry about the A/B thing.   It's a bit weird the way they have marked the opamp segments as large A's and B's outside of the opamp.   So what they means is U1A and U1B.

So it seems like a good idea to add them.  Is there any downside to adding them?  How big should I go?

If I do increase impedance, will it affect using it as a line level device?  That is the part I am not sure of.  If it has no negative effect, then I prefer to increase the impedance and be able to use it for guitars as well.  But if it does, then I will leave it as is, as the main purpose is to use it on a line coming from a Mic (after the preamp and compressor).

No real downside, other than adding more parts.      You could argue adding a buffer adds a bit of noise, but really with so many opamps in the circuit it's not going to be noticeable.

Adding a buffer will have little effect on the line-level.

For the schematic link I posted:
For the input,  add buffer U1A use say:  R2=2M2, C1=100nF, R3 =2k2, R4=1M
For the output, around U14B,  use say R59=100 ohm, C18 = 10uF to 47uF is fine, R60=100k.
The values are not absolutely critical.

Even without the buffer a circuit with so many opamps will add noise, especially for the instrument case.   The way that is combated is using pre-emphasis and de-emphasis circuits.     Take a look at the Boss GE-7 schematic for example.   For the mic case you have a preamp so it's going to boost the signal and help the signal to noise.   For line-level signals you don't really need pre-emphasis and de-emphasis.

If my previous knowledge is correct, the buffer would be R2, R3, R4 and C1 (input), right?  I can't find the output one...  (by the way, there are 4 sections A and 4 section B)

That's correct.

The thing to realize about all this is you can just take a circuit and built it, or, you can tweak it.    However, when you tweak it you have to make design decisions and that means thinking about a whole lot of stuff you might not have considered, or even known about.

1. Delete all resistors and replace them with the correct size (best option, but time consuming)

That's probably the right thing to do but it's a total pain in the butt.  Options 2 and 3 are up to you.

Would 1/8W be enough for this pedal?  Technically, it drains 100mAh, so 1/10W should be enough, right?  To be honest, I am leaning towards #2 because I do not want to waste all the resistors I already got on the breadboard (I would save them, of course, but they will be out of the carton holder, so they would easily get misplaced).  Everything else checks out size wise.

For modern LEDs resistors R1 and R2 at going to be too low and will make the LED too bright.  You could probably use 4.7k resistors there.  The 330Rs are going to get hot but the 4.7k's will not.

The power supply resistors R3, R4 would be best left at 1/4W (maybe even 1/2 W to handle turn-on peaks).

[jfrabat]

No real downside, other than adding more parts.      You could argue adding a buffer adds a bit of noise, but really with so many opamps in the circuit it's not going to be noticeable.

Adding a buffer will have little effect on the line-level.

Sold!  Buffer is going in.

For the schematic link I posted:
For the input,  add buffer U1A use say:  R2=2M2, C1=100nF, R3 =2k2, R4=1M
For the output, around U14B,  use say R59=100 ohm, C18 = 10uF to 47uF is fine, R60=100k.
The values are not absolutely critical.

So, Something along these lines then?

Even without the buffer a circuit with so many opamps will add noise, especially for the instrument case.   The way that is combated is using pre-emphasis and de-emphasis circuits.     Take a look at the Boss GE-7 schematic for example.   For the mic case you have a preamp so it's going to boost the signal and help the signal to noise.   For line-level signals you don't really need pre-emphasis and de-emphasis.

1. Delete all resistors and replace them with the correct size (best option, but time consuming)

That's probably the right thing to do but it's a total pain in the butt.  Options 2 and 3 are up to you.

Would 1/8W be enough for this pedal?  Technically, it drains 100mAh, so 1/10W should be enough, right?  To be honest, I am leaning towards #2 because I do not want to waste all the resistors I already got on the breadboard (I would save them, of course, but they will be out of the carton holder, so they would easily get misplaced).  Everything else checks out size wise.

For modern LEDs resistors R1 and R2 at going to be too low and will make the LED too bright.  You could probably use 4.7k resistors there.  The 330Rs are going to get hot but the 4.7k's will not.

The power supply resistors R3, R4 would be best left at 1/4W (maybe even 1/2 W to handle turn-on peaks).

Since I am adding the buffer, I will need to reroute anyway, so I deleted the resistors and added 2 more mm of spacing so that the 1/4W resistors fit fine.  Already cutting it close on the cap voltages, so I don't want to gamble with the resistors as well.  I will get to work on the board once I know the schematic is correct (Hint, hint!  Waiting for you!  LOL!).  For CLR, I will start at 4.7K on the breadboard, and work from there. 

[Rob Strand]

So, Something along these lines then?

That's just about it.   

You don't need to add U1B.  The output can come off  IC5B.
Just move R41, C2, R42 to the output of IC5B.
 
So, the changes would be, including shuffling the part designations down,
- Remove IC1B, R39, R40
- Make R27 100R
- Connect C2 to R27.  Note: need to make C2 a larger value 10uF to 47uF
- Connect  100k  (call it R39) from C2 to ground.

For CLR, I will start at 4.7K on the breadboard, and work from there.

Sounds good.  The LEDs brighness is easy to tweak.

[jfrabat]

- Connect C2 to R27.  Note: need to make C2 a larger value 10uF to 47uF

So C2 will need to be electrolytic (the highest film I got is 1uF)?  What would be the polarity then?  Negative towards output?

[jfrabat]

So, we are talking about something along these lines? 



NOTE: I renumbered automatically based on the board layout, so numbers have changed form last schematic.

If that is so, I will modifiy the breadboard (power supply is on its way, by the way).  Here is the board layout for that schematic:



If it all checks out, I will get this board printed...



I know, I still got to polish the silk screens, but I am getting excited about this project!

[Rob Strand]

So, we are talking about something along these lines? 

Yes, looks good.

So C2 will need to be electrolytic (the highest film I got is 1uF)?  What would be the polarity then?  Negative towards output?

1uF is a bit small,  I'd say that would be the smallest you would want to use in that position and I'd probably not go for a 1uF in that part of the circuit.

So the easiest solution is an electrolytic.   You might see people complaining about putting electros in but really there's zillions of pieces of equipment with Electros on the output.   The only only to do is make sure it's a large value to prevent the voltage on the cap increasing.    Say 10uF to 47uF.

As for the polarity.   In you case there is a 'more' correct orientation for the cap.  Put the negative towards the opamp and positive towards the output.

The reason for cap having  a correct orientation is:
- Opamp IC1A an NE5532 which has a transistor input stage which causes the output to sit at a negative voltage,
say -0.2V.
- the rest of the circuit is non-inverting overall so the output opamp ends up at -0.2V.
- so the conclusion is the -ve terminal on the cap is best connected to the opamp.


Here's more technical stuff making life difficult:

The reason why IC1A sits at a small negative voltage is because the opamp has transistors at the input.  The base current of these transistors causes a voltage drop across the 1M input resistor.  That voltage drops ends-up at the output of IC1A.

The best prevent the DC voltage is to no to use a NE5532 for IC1A.  Use a JFET input opamp like a TL072.   JFET input opamp do not pull current through the 1M resistor.  If it were building it for myself I'd probably go that way.

The cons of have the DC voltage through the circuit is the boost/cut pots have more of a tendency to go scratchy. Each of the EQ stages inverts so it will have -0.2V on the input and +0.2V on the output; alternating in sign after each stage.  So that means there's 0.4V across the EQ boost/cut pots.   When the pot gets old that DC voltage flickers and you get scratchyness.

So if you wanted to keep the NE5532 *and* reduce pot potential scratchyness there are a number of options.  The best would be to put another coupling cap between the output of IC1A and where R5, R6 and the first Boost/Cut pot join.     You can reduce the offset by half by dropping the 1M to 470k but that's only reducing the problem.   There's a few other ways but to be the best solution is to use a JFET input opamp like a TL072.

[jfrabat]

I do have TL072.  I even have TL082, which I understand is more HiFi than the 072 (and also JFET).  Should I drop the TL082 in there?  In this case, keep polarity with negative towards output?