I am testing a test circuit based on a standard configuration of an op-amp summing amplifier whose source comes from a tone stack and I have noticed an anomalous behavior from the tone stack, so I wonder if it is possible for the two subcircuits to interfere with each other without a buffer.
My configuration is as follows, J1 = tone stack, J2 = mp3, J3 not present.
By adjusting the highs to the maximum, I get a very metallic sound, the lows decrease significantly and the highs also decrease slightly, the overall tone changes a lot. Below 10%, the total volume starts to increase again, the lows receive a lot of boost but the tone is more realistic. Adjusting the lows and mids does not show much difference.
Furthermore, by varying R1 between 0 and 10K of the OP, there is no change in the output gain. If I bypass the TS, R1 correctly modifies the gain, decreasing the gain increases it.
What could be the reason for this? Would a buffer between the two be necessary?
Thank you.
My configuration is as follows, J1 = tone stack, J2 = mp3, J3 not present.
By adjusting the highs to the maximum, I get a very metallic sound, the lows decrease significantly and the highs also decrease slightly, the overall tone changes a lot. Below 10%, the total volume starts to increase again, the lows receive a lot of boost but the tone is more realistic. Adjusting the lows and mids does not show much difference.
Furthermore, by varying R1 between 0 and 10K of the OP, there is no change in the output gain. If I bypass the TS, R1 correctly modifies the gain, decreasing the gain increases it.
What could be the reason for this? Would a buffer between the two be necessary?
Thank you.
That tone stack is passive
and values are from typical tube circuits.
So it is expecting to see typical 470K to 1 Meg impedance.
The opamp mixer is inverting
So with inverting stage just consider R1, R3, R4 the input impedance
So in this case 10k
which is way too low and likewise be changing as gain is adjusted.
needs high impedance non inverting buffer.
could be simple unity gain or can add gain if wanted
after tonestack.
might sound like wiring errors with tonestack as well.
but either way its wanting 470k to 1 Meg
not 10 K which would be way too low
just add buffer
are you using single supply for power or dual power as shown.
because changes need to be made for single supply
everyone seems to keep copying high impedance tonestacks from
tube amps, when not needed with opamps.
Can convert values to get same curve and be more suited for opamp.
If still passive high impedance still needed, but 50k to 100k more than fine
1Meg tonestacks are for tubes.
Maybe look at Marshall shred master, you wont beat it or reinvent it
and countour control is very useful for addtional filtering.
But can see inverting stage after tonestack only 47K needed.
Driving opamp inverted for distortion is simple and gets so called
magical "asymmetrical" clipping with no mismatched diode nonsense.
symmetry can be changed by change virtual ground or adding inline resistor with dual
rail to control offset. Marshall figured this stuff out 30/ 40 years ago
you wont beat Shred master or Guvnor clipping or tone controls
and values are from typical tube circuits.
So it is expecting to see typical 470K to 1 Meg impedance.
The opamp mixer is inverting
So with inverting stage just consider R1, R3, R4 the input impedance
So in this case 10k
which is way too low and likewise be changing as gain is adjusted.
needs high impedance non inverting buffer.
could be simple unity gain or can add gain if wanted
after tonestack.
might sound like wiring errors with tonestack as well.
but either way its wanting 470k to 1 Meg
not 10 K which would be way too low
just add buffer
are you using single supply for power or dual power as shown.
because changes need to be made for single supply
everyone seems to keep copying high impedance tonestacks from
tube amps, when not needed with opamps.
Can convert values to get same curve and be more suited for opamp.
If still passive high impedance still needed, but 50k to 100k more than fine
1Meg tonestacks are for tubes.
Maybe look at Marshall shred master, you wont beat it or reinvent it
and countour control is very useful for addtional filtering.
But can see inverting stage after tonestack only 47K needed.
Driving opamp inverted for distortion is simple and gets so called
magical "asymmetrical" clipping with no mismatched diode nonsense.
symmetry can be changed by change virtual ground or adding inline resistor with dual
rail to control offset. Marshall figured this stuff out 30/ 40 years ago
you wont beat Shred master or Guvnor clipping or tone controls
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You were right, adding a 1M pot instead of R1 and adjusting the value between 500k and 1M the behavior is as expected, although the TS is slightly affected in the adjustment. Would a JFET buffer or a JFET booster like the fetzer valve be a better solution? I would like to avoid adding more sections to avoid complicating the circuit too much or having to filter the power supply noise.
If I increased the impedance with a 1M resistor between the TS and C1 of the opamp, would it be an acceptable compromise in your opinion?
If I increased the impedance with a 1M resistor between the TS and C1 of the opamp, would it be an acceptable compromise in your opinion?
That would be a large source of Johnson noise, and lead to 40dB attenuation, so hopeless.... Simple opamp buffer on the output of the tone stack is the answer.
Putting extra buffer stages in is not complicating the circuit, it actually makes it simpler (mathematically), and usable.
BTW why is R2 larger than R1 - that implies the summing circuit has 20dB gain, which is not usually a good place for lots of gain (from a noise perspective).
C1/2/4 are redundant if all the inputs are fed from that tone stage as its already capacitively coupled. Adding a unity-gain buffer isn't going to add much DC offset either.
The diagrams I attached were to show the circuit I am working on, the values do not correspond to the ones I used. R2, do you mean the feedback resistance? It should be higher to have a gain.
I will try to add a JFET to isolate the TS from the op.
inverting stage with such high resistance would be noisy.
why I showed you the shred master tone stack.
values been changed for lower impedance.
or use a non inverting unity gain buffer.
for the higher value tonestack
for the most part they have quad opamps.
and that was what I considered to be a minimal
design 4 op amps.
why I showed you the shred master tone stack.
values been changed for lower impedance.
or use a non inverting unity gain buffer.
for the higher value tonestack
for the most part they have quad opamps.
and that was what I considered to be a minimal
design 4 op amps.