Hey y'all,

Here's a widely known SDD-3000 circuit (thanks @analogguru) with a few personal changes (still working through some kinks). I haven't messed with adding tone stacks really in any pedals that I build. I understand the basic concept, but don't have the proper knowledge of how/where to place it.

My idea is pretty simple. Bass and treble control. Both knobs all the way up are full bass and treble dictated by whats in the circuit. As you start turning counter-clockwise, they cut their respective frequencies. My reasoning behind this is it can allow bass players to use the circuit with the bass all the way up, and guitar players can cut the bass where they need it. Treble to taste. I think this is similar to the way the early Timmy pedals worked.

Anyone have any thoughts on what circuit/where I should place it? Honestly want it to be dead simple.

Thanks so much!

Off-topic, but..

Get rid of C10..
Or, if you insist on its redundant existence, place a low value (47 to 100 Ohm) series resistor..
(between pin1 and cap's positive plate)

Quote from: ToneRangerAudio on October 23, 2024, 01:52:58 PMAnyone have any thoughts on what circuit/where I should place it? Honestly want it to be dead simple.

Since you've already got an inverting op-amp for IC2, adding Baxandall Bass+Treble controls to that stage would be very simple. And if you want cut-only, that's also easy to arrange with a Baxandall circuit.
You won't lose anything since there's no actual tone shaping going on in that stage. 470n/100K coming in gives a highpass at 3.4Hz, lower than you'd ever need. Simialrly, 360K/4.7pF gives a rolloff around 22KHz, which is plenty high enough to never be noticed. So the *only* effect of the stage is the x3.6 gain.

+1 agree about Antonis comment. C10 really shouldn't be there.

I think the idea of C10 is that you filter out the noise introduced by the opamp on the half voltage supply. At that point, you don't need as large a value and you should never present a capacitive load to an opamp output (in general to any amplifier's output), because this will result in oscillation. If you really are that worried about noise (~2uV noise over a bandwidth of 20kHz... not really consequential) make it 10uF and add a series resistor to the opamp's output, at least 100 ohms (or make it even larger, like 220 - the usual capacitive load an opamp will complain about may be measured in PICOfarads, you are presenting it with about 100K-1M times that ).

Off-topic, again..

IMHO, you don't need input bias current cancelation resistor (R6) for TL072..
Also, flip 180^o C3 and delete C2/R4..
As for C7/R9 sub-sonic filter, no comment..

When choosing blocking capacitors, you should account for the lowest impedance you will presumably connect your circuit to. 10kohm is a sensible guess (there are many guitar pedals with low input impedance).

My (well, Rod Elliott's ultimately) recommendation is that you never specify electrolytic capacitors for frequency, because their capacitance is not stable over time. Go with something at least 3x larger than what you would need. 10x is even better. 10uF as "insurance" in place of 1u or 3.3u is a sensible choice. Then choose a pull-down resistor that will be negligible with the smallest (parallel) load you will present to the circuit. If your guess is 10kohm, then 100kohm would be a good choice. For what it's worth, for guitar a good f3 is around 60Hz (and some may actually find it excessively "fat"), which means that you only need around 220-330nF as a blocking capacitor (and that can be chosen to be poly rather than electrolytic).

Thanks for the recommendations everybody! Most of this schematic is not mine. I believe this was pulled from the Korg SDD 3000 manual from the 80s. Who knows what they were doing back then!

I'll make some of those changes and if anybody has any more recommendations, would love to hear them!

Quote from: ElectricDruid on October 23, 2024, 05:56:31 PM

Quote from: ToneRangerAudio on October 23, 2024, 01:52:58 PMAnyone have any thoughts on what circuit/where I should place it? Honestly want it to be dead simple.

Since you've already got an inverting op-amp for IC2, adding Baxandall Bass+Treble controls to that stage would be very simple. And if you want cut-only, that's also easy to arrange with a Baxandall circuit.
You won't lose anything since there's no actual tone shaping going on in that stage. 470n/100K coming in gives a highpass at 3.4Hz, lower than you'd ever need. Simialrly, 360K/4.7pF gives a rolloff around 22KHz, which is plenty high enough to never be noticed. So the *only* effect of the stage is the x3.6 gain.

+1 agree about Antonis comment. C10 really shouldn't be there.

This is great. Just removed C10 and everything else everyone has said so far. Any recommended schematics out there for that Baxandall circuit? Still new to a lot of this so learning as I go!

Quote from: antonis on October 24, 2024, 04:53:33 AMOff-topic, again..

IMHO, you don't need input bias current cancelation resistor (R6) for TL072..
Also, flip 180^o C3 and delete C2/R4..
As for C7/R9 sub-sonic filter, no comment..

Thanks. Keep C7/R9? Whatcha think?

Quote from: ToneRangerAudio on October 24, 2024, 07:18:57 PMrecommended schematics out there for that Baxandall circuit?

It may help to call it by the right name. James, not Bax.
https://forum.pedalpcb.com/threads/tone-stacks-part-2-james-baxandall.8307/

Quote from: ToneRangerAudio on October 24, 2024, 07:23:36 PMThanks. Keep C7/R9? Whatcha think?

I'd delete R9 and make C7 4.7μF to 10μF (max)

P.S.
Plz interchange IC1 pins 5 & 6 polarity, as well as IC2 pins 2 & 3, 'cause we are not used to directly bias inverting inputs..
(I mean + & - signs..)

Let's try a more friendly schematic..
(some items deleted, one added and others changed order and/or value..)



P.S.
After R4 deleted, C3 polarity orientation doesn't matter a lot..
(IC1's pins 7 and 2 should have no DC voltage difference..)

Quote from: PRR on October 24, 2024, 07:57:00 PMIt may help to call it by the right name. James, not Bax.
https://forum.pedalpcb.com/threads/tone-stacks-part-2-james-baxandall.8307/

I did actually mean Baxandall, e.g. I was suggesting turning that IC2 gain stage into an active EQ. The problem with that is that it's awkward to do that *and* have the x3.6 gain, so perhaps it is better to have a passive James network, and then use the amp afterwards to provide make-up gain.

Quote from: antonis on October 25, 2024, 05:09:59 AMLet's try a more friendly schematic..

Don't you understand that the SDD-3000 pre-amp is sacred, Antonis?!? You can't just start changing parts or you might lose the TOAN!! 

More seriously, it looks like reasonable rationalization to me.

https://postimg.cc/TLpCmzMx

Quote from: ElectricDruid on October 25, 2024, 07:26:16 AMYou can't just start changing parts or you might lose the TOAN!! 

What TOAN..??

The one included between 0.18Hz and 160kHz..?? 

A minor point here, VR1 and R4 should change places since the capacitance to ground from the pot is higher than that of the resistor.  This capacitance tends to reduce feedback at high frequencies and the combination of reduced feedback meeting the falling response of the op amp may cause instability, even if it is above audio frequencies.  If it oscillates at frequencies above the audio spectrum, it may not be heard but it will reduce the headroom of the signals you can hear.

This is referring to the schematic of post #11.  Post #!4 makes it worse.

Quote from: amptramp on October 25, 2024, 08:31:57 AMA minor point here, VR1 and R4 should change places

Good catch, Ron..!!

Quote from: antonis on October 25, 2024, 05:09:59 AM

Quote from: amptramp on October 25, 2024, 08:31:57 AMA minor point here, VR1 and R4 should change places since the capacitance to ground from the pot is higher than that of the resistor.
<snip>
This is referring to the schematic of post #11.

Quote from: antonis on October 25, 2024, 08:48:31 AMGood catch, Ron..!!

Sorry Ron, Antonis, I'm not understanding you. Why does swapping over VR1 and R4 change anything at all? It's a series connection of two resistors - the order is unimportant. Please explain.

Thanks.

Quote from: amptramp on October 25, 2024, 08:31:57 AMA minor point here, VR1 and R4 should change places since the capacitance to ground from the pot is higher than that of the resistor.  This capacitance tends to reduce feedback at high frequencies and the combination of reduced feedback meeting the falling response of the op amp may cause instability, even if it is above audio frequencies.  If it oscillates at frequencies above the audio spectrum, it may not be heard but it will reduce the headroom of the signals you can hear.

This is referring to the schematic of post #11.  Post #!4 makes it worse.

I don't understand. The inverting input is (virtual) ground, OK. The resistor has a capacitance (small), the pot has a capacitance (larger). The resistor should dampen any oscillations anyway, "isolating" the output from the input, no matter where it is placed.

What ElectricDruid said.