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

[Rob Strand]

What do you guys think? 1.4K~13.3K or 1.4K~15.9K?  Low end is almost the same (1,421Hz vs 1,447Hz), but I do gain a lot in the highs...

Since it's for voice and those parametrics have a low Q (wide bandwidth) the change would help more than it hurts.   Like you could just use 10k resistors on all ranges.

FWIW, at the low frequency side the pot resistance tolerance comes into play so the frequency limits of the real unit could be higher or lower than the calculations.

[jfrabat]

What do you guys think? 1.4K~13.3K or 1.4K~15.9K?  Low end is almost the same (1,421Hz vs 1,447Hz), but I do gain a lot in the highs...

Since it's for voice and those parametrics have a low Q (wide bandwidth) the change would help more than it hurts.   Like you could just use 10k resistors on all ranges.

FWIW, at the low frequency side the pot resistance tolerance comes into play so the frequency limits of the real unit could be higher or lower than the calculations.

I decided to use 12K in all the frequencies.  My reasoning is that, since I ran out of 10K resistors, I would use 12K resistors.   

At the end of the day, I will use a tone generator to tune the frequencies if necessary (I doubt it will be necessary, but at least to know the limits!).

I am still waiting for the C100K dual gang pots to arrive, but here is the breadboard minus the pots:



I so want to try it!  By the way, the CUT / BOOST pots right now are linear taper; I wanted to go with audio taper, but I did not have any on hand (I thought I did, but I don't!).  For this purpose, is it better linear or audio taper  (my logic already failed me with the dual gang ones!)?

[ElectricDruid]

By the way, the CUT / BOOST pots right now are linear taper; I wanted to go with audio taper, but I did not have any on hand (I thought I did, but I don't!).  For this purpose, is it better linear or audio taper  (my logic already failed me with the dual gang ones!)?

I'd say they have to be linear taper, because they have a centre zero. Any other pot type will finish up with the zero position being off to one side or the other.

[duck_arse]

while you have that much of it on the bb, TEST IT! sub 2 x 47k resistors for each missing section of the missing pots - test what you have while you wait, y'no, just in case .....

[jfrabat]

while you have that much of it on the bb, TEST IT! sub 2 x 47k resistors for each missing section of the missing pots - test what you have while you wait, y'no, just in case .....

Not a bad idea, actually...  I will do that. 

[jfrabat]

Well, this is what we got so far:

Power Stage seems to be working fine.  The power supply I am using (says is 12V 100mA) actually puts out 16.4V when plugged to the system (16.9V when unplugged).  My regulated power supply with linear regulators seems to be working fine, as the voltage prior to D2 is 12.1V (not sure about noise yet, just testing voltages).  V+ measured at 10.45V, and V- measured at 8.5V (I am still a newb, but those values seem perfectly reasonable to me, at least).

Sound Stage:

Obviously, I do not have the caps yet, so I could not test much of what the circuit is actually intended to do, BUT...  I get no sound coming through.  I will have to check all the equalization circuit again.  Not that I expected anything different, and at least I have something to do over the weekend while I wait for the pots to arrive.  I will bust out my audio probe and chase the sound signal.

[PRR]

> I'd say they have to be linear taper

For perfectly even spacing they want to be an "S-taper". {Edit: W-taper?}

It REALLY is easier to buy than to build. But we need to keep Felipe off the streets and healthy.

[jfrabat]

> I'd say they have to be linear taper

For perfectly even spacing they want to be an "S-taper".

Ah, the infamous W taper...  I hate it when they are required.  Not easy to get those locally.

It REALLY is easier to buy than to build. But we need to keep Felipe off the streets and healthy.

Buying is nowhere NEAR as distracting for my mind...  and not NEARLY as satisfying to play!

[Rob Strand]

For perfectly even spacing they want to be an "S-taper".

You can do OK with linears for the boost/cut.   There's probably some tweak you can do to the 47k to ground to tweak the dB's for the mid-way boost and mid-way cut positions (around 9 O'Clock and 3 O'Clock)).

[jfrabat]

Well, first thing first...  Let me get the line to actually go through first.  Then I will ask you how to do this.

I got 3 pedals on the build table; this one, a phaser (which has been a HEADACHE to troubleshoot, but we are almost there!), and a tube screamer clone (waiting for parts on that one).  Also had an OCD Overdrive, but that one I finished this week.  So I need to go one step at a time to avoid getting ahead of myself.

[Rob Strand]

So I need to go one step at a time to avoid getting ahead of myself.

You're not the only one.  I've got a whole heap of stuff that suddenly died as well.

[jfrabat]

By the way, just to check my logic, I need to place a 47K resistor in place of each part of the pot, right?



So 2 resistors per band, right?  For example, in band 1, I would put one between R7 and C13 (on the non grounded side), and another from C13 (same side) to C14, right?

[Rob Strand]

So 2 resistors per band, right?  For example, in band 1, I would put one between R7 and C13 (on the non grounded side), and another from C13 (same side) to C14, right?

Yes.  You connect a resistor in place of a pot.  One resistor for each 'gang' of the pot.   It's like the pot is stuck on one setting.

[jfrabat]

OK, checked my board.  Found a few small errors (last OpAmp V+ was not connected, and, for some reason, I soldered the signal of the output jack to the ring part of a stereo jack; I also had D3 connected to the wrong row).  I now get signal coming through the circuit, but I am hearing no amplification (the signal comes out very faintly).  The signal loses strength through the circuit.  Also, I feel that the cut part of the system is working, but the boosting is not.

Question; in the scheme I posted, the OpAmp's port 4 is connected to V-, which in this case, is actually 5V (the other day I measured and it was 8.5V, but today it is 5V now; guess D3 had something to do with that.  V+ is now 8V, BTW).  Shouldn't pin 4 of these OpAmps go to GND?

I also need to check the first pot, as the signal goes through only with that pot at minimum.  I checked it against the schematic, but it seems to be connected OK.  Pot itself also checked out.  Not sure what it is, but I will check it tomorrow.

[Rob Strand]

By the way, just to check my logic, I need to place a 47K resistor in place of each part of the pot, right?

[your sch]

Question; in the scheme I posted, the OpAmp's port 4 is connected to V-, which in this case, is actually 5V (the other day I measured and it was 8.5V, but today it is 5V now; guess D3 had something to do with that.  V+ is now 8V, BTW).  Shouldn't pin 4 of these OpAmps go to GND?

The power section in that schematic has some problems.

In the original schematic,



The power supply works by taking the AC from one winding of a transformer then it creates a DC supply with both +12V and -12V.   The two diodes and the two 100uF form a doubler circuit.   The following two 150R and 100uF caps provided extra filtering to remove ripple/hum from the DC rail.    The main point here is the supply is a bipolar supply with +12V and -12V.

In your schematic: a *single* *DC* rail is applied to the DC jack then it is passes through the 12V regulator.  In short you won't get +12V and -12V.   Also the diodes and caps in you schematic can never produce +12V and -12V.  With a DC rail that diode + cap doubler actually makes not sense as it *only* works with an AC input.  So you will need to remove all that stuff - but that won't make it work.

As far as a solution goes.  It depends if you want to input 12V AC or 12V DC on the power jack.   So the options are:

1) 12V AC in.  Use diode + cap doubler circuit like the original schematic.  If you want to use 12V regulators you will need to replace the 2x 150R + 100uF with a LM7812  and a LM7912 to get +/- 12V;  will need to add some caps for the regulator

2)  12V unreg DC in  --->  12V regulator (LM7812)
                                     ---->  negative supply generator circuit   --->  -12V regulator (LM7912)
      A slight variation is to connect the negative supply generator to the output of the 12V regulator.
      The negative supply generator will be a whole other circuit.

3)  Using only a single 12V supply rail.  And convert the whole circuit to single supply.

      Connect the opamp +Vcc to 12VDC.
      Connect the opamp -Vcc to 0V (ground)
      Add a Vcc/2 circuit like most pedals have with  a resistive divider and cap to ground to create Vref = Vcc/2
      Change circuit *signal* grounds to 0V (ground)
      The more involved part is to detach some of the points which are currently connected to ground and wire those
      to Vref Vcc/2 so the opamps are biased correctly.
      Add coupling caps where you need to separate the DC and ground
      Add coupling cap to the signal input.
      Add coupling cap to the signal output.

      Maybe a few other points I haven't elaborated on but that the general idea.


I suppose you have to choose which way you want to go.   Solution (3) loses headroom since the supply is 12V = +/-6V but the true bipolar supply is +/- 12V.


I want to emphasize that when using 12V regulator you will need an unregulated input supply which really produces about +/-16 to 18V.   You need a DC input greater than 15V to ensure the regulators work.

[jfrabat]

Ah, that explains a lot...  OK.  So, my best course of action is to get a 12V AC-to-AC transformer and delete all that regulated power supply with linear regulators part; either that, or place it and a mirror of it on V+ and V- (and I would need a 7912 for that one, and figure out how to use B1 - B2 in this case - correctly for -12V).

What would you recommend? Leave out the regulated supply part?  I looked om Amazon, and the have 12V AC-to-AC transformers...  and 16VAC if I will use the regulated supply part (times 2).

Would you be willing to guide me in this process?  I am still learning all this...

[jfrabat]

So if I keep the circuit like this:



And use one of these power adapters, I should be OK, then?

PD: I just noticed I need to move the + side of the bypass LED to the 12+ signal...

[Rob Strand]

And use one of these power adapters, I should be OK, then?

If you follow the original, without the regulators, then I'd go for a 12VAC plug-pack for sure.   The 16VAC will produce quite a high voltage on the DC side.

Probably a good idea to beef-up the caps C5, C6, C7, C8 to 470uF 25V to 35V.

FYI: One thing about these doublers is they usually cause a power-up "bang" in the audio because one side of the supply always starts before the other.   With R3 and R4 in place and larger C7 and C8 it slows down the speed the DC side rises and hides some of the "band".

On your new schematic, 12VAC2 needs to connect to the ground between C5 and C6.   When you wire it up you need to physically wire it exactly that way as well.

R1 and LED1 aren't wired correctly.  The original wired the LED to AC but I'd probably power both LED's from the DC rail(s).   So R1 and LED1 go from ground to -ve C6, R2 and LED2 go from ground to +ve C5.   All I'm doing here is dividing the LED currents between the + and - rails.

[jfrabat]

Probably a good idea to beef-up the caps C5, C6, C7, C8 to 470uF 25V to 35V.

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

FYI: One thing about these doublers is they usually cause a power-up "bang" in the audio because one side of the supply always starts before the other.   With R3 and R4 in place and larger C7 and C8 it slows down the speed the DC side rises and hides some of the "band".

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?

On your new schematic, 12VAC2 needs to connect to the ground between C5 and C6.   When you wire it up you need to physically wire it exactly that way as well.

So not just connected to ground, but LITERALLY between the 2 caps?  So make the board like this:


I forced the trace between the 2 caps (C5 & C6)

Dual gang pots will be off board (I just needed the holes to be in the board).  Single gang pots will be in the other side, as usual.  For the On/Off Toggle, I am still debating if to use a ON/OFF/ON that is the same size as the the other switch, or use the ON/OFF I have (which has a little smaller toggle lever).  Either way, both fit...

R1 and LED1 aren't wired correctly.  The original wired the LED to AC but I'd probably power both LED's from the DC rail(s).   So R1 and LED1 go from ground to -ve C6, R2 and LED2 go from ground to +ve C5.   All I'm doing here is dividing the LED currents between the + and - rails.

So, replacing 100uF caps for 1000uF caps, and changing the LED layout (please check if polarity of LED is correct!), the schematic should look like this then:

[jfrabat]

DELETED (duplicated)