bass brutalist : TL72 works fine but no distortion with RC4558

[eh la bas ma]

Hello,

I built a Bass Brutalist circuit some time ago.

schems :

https://kurtballou.com/docs/

(bottom of the list)

I wonder why I can't have anything but weird cracklings and clean signals when I swap the IC for an RC4558, or a NE5532 as suggested in the instructions. Only the TL72 sounds right, but this is supposed to be a variant of a variant of the Providence stampede sdt-1 (Brutalist Jr) using JRC4558 (schematics are close).

https://www.freestompboxes.org/search.php?keywords=Providence++Stampede+SDT&sid=82daf995613d6b8e574321af253f627c

So I am curious to listen how RC4558 or NE5532 are doing on this circuit, I don't understand why one chip is ok and the other one doesn't produce any distortion. In the first half of the Dist. pot rotation, there is some saturation produced by IC2 the single op-amp (I guess). In the second half, it disappears, the signal is almost clean with a few cracklings.

Any help would be welcome.

[antonis]

Try to place a 1μF or so cap just right before or after R7 (2k7) resistor..
(wherever is more convenient..)

For bipolar op-amps (like RC4558 or NE5532) you might need to lower R1 (2M2) resistor value down to 470k or lower..

[anotherjim]

There could be something leaking tiny current to 0v either to or from an opamp input. The electrolytic cap on the tone section would be my first suspect.
The TL072 has JFET inputs that should pretty much block DC current, The other types have BJT inputs that do allow some DC current to flow - if there is a path.
DC voltages on the opamp outputs are a MUST check. If they don't equal the Vref voltage, something is probably wrong with the build.

[eh la bas ma]

Thanks for your replies,

here are my readings on IC1, circuit switched off :

TL72

1= 4.60    8= 9.23
2= 4.60    7= 4.60
3= 3.69    6= 4.60
4= 0v       5= 4.53 V

RC4558P :

1= 4.95    8= 9.23
2= 4.97    7= 4.62
3= 3.98    6= 4.63
4= 0v       5= 4.56 V

Edit : if Vref is connected to R9, I read 4.60v on R9 pads.

[eh la bas ma]

Try to place a 1μF or so cap just right before or after R7 (2k7) resistor..
(wherever is more convenient..)

For bipolar op-amps (like RC4558 or NE5532) you might need to lower R1 (2M2) resistor value down to 470k or lower..

I tried this modification with an RC4558 and it's a success ! The cap alone (between R6 and R7) didn't work, I tried a 330k for R1 : the distortion appeared. I tried with 100k with better results. 220k sounds even better... How does this resistor interact with the op-amp in the circuit ?

[antonis]

I tried this modification with an RC4558 and it's a success ! The cap alone (between R6 and R7) didn't work, I tried a 330k for R1 : the distortion appeared. I tried with 100k with better results. 220k sounds even better... How does this resistor interact with the op-amp in the circuit ?

Cap placement aimed to prevend DIST pot crackling in combination with R1 value lowering..

RC4558 and NE5532 are bipolar input op-amps meaning they exhibit input bias current to be reckoned with.. That results into lowering (for inputs n-p-n BJTs) actuall bias voltage due to voltage drop across bias resistor..
(the above doesn't stand for FET input op-amps due to practically negligible bias current..)

IC1A output exhibits a DC voltage level (VR") equal to that appearing on its non-inverting input (lower than VR due voltage drop mentioned above) and is directly coupled to IC2 inverting input while its non-inverting input is biased at exactly VR..
IC2 output tries to make both inputs voltage equal (via NFB loop) resulting into an amount of DC flowing through DIST pot, R15, & R7 into -practically GND- IC1A input..
It also forces IC2 output to amplify VR & VR" difference to a level which reaches VR2 when added to VR"..

P.S.
I might confused you rather than enlighten so consider it a NOT DC roll-off to unity..

edit: Jim already told you in a more condensed way..!!

The TL072 has JFET inputs that should pretty much block DC current, The other types have BJT inputs that do allow some DC current to flow - if there is a path.

[eh la bas ma]

Thanks for the explanations.

Now I have many combinations possible. 3 single op-amps (TL71, UA741CP, OP07CP) and 3 dual op-amps (RC4558, NE5532, TL72) available.

It feels like TL71 and TL72 are very transparent, efficient but cold. OP07 and NE5532 are a bit more audible; RC4558 and UA741 add a little grit to the sound texture, it gets quickly muddy and fat. 

So far, OP07 and NE5532 sounds more defined, more subtle.