Majestic/Cal 45 2-in-1

[ntuncer]

It seems RC4558 is a unity gain stable which unused op-amp could be used as a voltage follower instead of terminating it.

 

[Chuck D. Bones]

Leaving the inputs of unused opamps or CMOS floating is an absolute no-no. Unpredictable & generally bad things can happen. I can understand why a builder might use only part of a dual (or quad), but leaving unused inputs floating is bush-league.

If you want to put the spare opamp to good use, wire it as a unity gain buffer and stick it between the TREBLE & VOLUME pots. you'll have to relocate C3 after the buffer.

The other thing J Rockett did that is problematic is connecting a capacitor from an opamp output to ground, which is what happens with C8 when TREBLE is dimed. A capacitor between the output & ground can cause instability if the capacitor is too large. How large is too large? Hard to say because it varies from mfgr to mfgr & lot-to-lot. It also depends on the feedback network. Sometimes the datasheet provides guidance. Better to avoid the problem altogether. A 100Ω resistor between the opamp's output and the load capacitor is sufficient to ensure stability, so J Rockett could have done that.

 

[HamishR]

For the simplicity (over-simplicity!) of this circuit it does sound remarkably good. Of course since I built the combo pedal I have moved on to other ODs, but none of them are all that different from this. I really like the Timmy platform because it's very malleable and you can easily tweak it to get a different sound. I have also learnt that using the second triode/gain stage (whatever you want to call it) you can bolster the sound, add body, add mids, add grunt - all good guitar friendly things. All of this I have learnt through trial and error and help from here, especially Mr Bones.

It seems a waste not to use the second half of the IC. I am a super-fan of minimalism but by using just a few extra caps and resistors you can improve the Rockett circuit significantly, both sonically and electronically.

 

[ntuncer]

Leaving the inputs of unused opamps or CMOS floating is an absolute no-no. Unpredictable & generally bad things can happen. I can understand why a builder might use only part of a dual (or quad), but leaving unused inputs floating is bush-league.

If you want to put the spare opamp to good use, wire it as a unity gain buffer and stick it between the TREBLE & VOLUME pots. you'll have to relocate C3 after the buffer.

The other thing J Rockett did that is problematic is connecting a capacitor from an opamp output to ground, which is what happens with C8 when TREBLE is dimed. A capacitor between the output & ground can cause instability if the capacitor is too large. How large is too large? Hard to say because it varies from mfgr to mfgr & lot-to-lot. It also depends on the feedback network. Sometimes the datasheet provides guidance. Better to avoid the problem altogether. A 100Ω resistor between the opamp's output and the load capacitor is sufficient to ensure stability, so J Rockett could have done that.

Thanks for detailed information! I will study and digest your message.

 

[ntuncer]

Leaving the inputs of unused opamps or CMOS floating is an absolute no-no. Unpredictable & generally bad things can happen. I can understand why a builder might use only part of a dual (or quad), but leaving unused inputs floating is bush-league.

If you want to put the spare opamp to good use, wire it as a unity gain buffer and stick it between the TREBLE & VOLUME pots. you'll have to relocate C3 after the buffer.

The other thing J Rockett did that is problematic is connecting a capacitor from an opamp output to ground, which is what happens with C8 when TREBLE is dimed. A capacitor between the output & ground can cause instability if the capacitor is too large. How large is too large? Hard to say because it varies from mfgr to mfgr & lot-to-lot. It also depends on the feedback network. Sometimes the datasheet provides guidance. Better to avoid the problem altogether. A 100Ω resistor between the opamp's output and the load capacitor is sufficient to ensure stability, so J Rockett could have done that.

Hello Chuck, if I understood correctly you mean the modifications like below. With these modifications, with the help of unity gain buffer we will have low impedance at the pin 7 and solve stability issue using 100R resistance. But tonewise, what do we expect to have? I guess less noise because no more unused op-amp and 100R resistor, but the sound of the pedal remains same?