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help with Op Amp function in Tone Amp circuit

roger2

roger2

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Subscriber
seeking help understanding Op Amp function in Tone Amp circuit

I am trying to understand what all is going on in these circuits. For purposes of discussion I will refer only to the top half of the schematic attached...

I don't know if I am even close...but it appears to me that terminal 6 of the Op Amp feeds the Tone ON/DEFEAT switch. This switch returns signal to either post 3 of the Tone board (tone DEFEAT) or post 1 of the Tone board (tone ON). But I am confused by Ci5 which seems to be feeding the output and bypassing the signals that return from the Tone switch.


More specifically:

1) Of Ci3 and Ci5, which are in the signal path? Always or just when tone is switched ON? (Ci1 obviously is always in the signal path)

2) Related to that question, why are terminals 6 and 7 of the Op Amp tied together?

3) Is audio signal always routed through the Op Amp even when tone switch is in the "Defeat" position?


Schematic attached. BTW, is this jpg large enough to read?
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Attachments

  • !tone amp w defeat switch enlarged 400 in pdf.jpg
    !tone amp w defeat switch enlarged 400 in pdf.jpg
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Pin 7 of the op-amp is the output of the op-amp. Pin 5 is the input. And pin 6 is also an input (the inverting input). The resistor (Ri7) applies feedback from the output of the op amp to the inverting input. This is negative feedback.

To answer your question, Ci5 is always in the signal path. Ci3 is also in the path always. With the tone defeated, the path is through Ri23, then Ci3. With the tone enabled, it is through Ri9 and all of the other components connected to it, and Ci3. In effect, Ri7 is paralleled by either the series combination of Ri23 and Ci3, or Ri9 ....
The combination of components in the feedback path of the op amp shape the gain vs. frequency characteristic of the amp.

With the tone switch in defeat, the gain of the op amp is set to be mostly flat, and the gain is set by (1+Ri23/Ri5) or about 22, although Ri7 is in parallel with Ri23, so this lowers the gain to about 20. Ri7 also makes the gain at DC go to 1, to keep DC offsets from being an issue. Ci3 sets the lower corner frequency.

The signal always goes through the op amp.

Terry
 
Thanks a lot Terry. That does help. At my level I understand basic component function but circuits, feedback in particular, are not within my grasp at this point. I need to study the books...

But your answer did clarify for me the fact that the op amp and all thee of the caps tied to it are in the signal path. Which, for practical purposes (I am rebuilding this board), is what I needed to know.

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I have a couple further questions about this circuit, if I may, for Terry or anyone willing:


1) if 14vdc was applied to post 6 (the input) of the tone board (yes, i did), could that possibly damage either the cap Ci1 or the op amp?


2) I hear a noise in the left speaker only when switching the tone ON/DEFEAT switch to the ON position. (This was happening before I mistakenly applied 14vdc to the input as described above).

Also, I replaced all the electrolytics after which this noise is way less.
Before putting in fresh caps this noise was more like a brief increase in volume that was much louder than normal volume with the switch in either position.

Would this issue be caused by a bad op amp or by some individual component that becomes live when the tone circuit is engaged?

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Applying 14v to pin 6 of the board wouldn't hurt the opamp,but it is applying reverse voltage to the cap, so it could be damaged. But it is a 50v cap, and usually they can tolerate a reverse voltage at much less than their rated voltage, so I doubt it is bad. But to be safe, you should change it.
As far as the noise, my guesses in order of likeliness:
Maybe the switch is noisy? A good cleaning with Deoxit may fix it.
A leaky capacitor can be noisy.
Another source of noise is a dirty potentiometer. Again, Deoxit is the cure.
You say the noise sounded like an increase in volume while moving the switch? It is the job of Ri7 to keep the gain of the opamp at 1 while the switch is open (between positions). Pretty unlikely that the resistor is bad, but maybe....

Terry
 
Thanks again Terry...

I am going back into this board so replacing Ci1 will be easy.

RE the noise:

--The switch has already been swapped out for one known to be good
--All the electrolytics are new. As mentioned, this did seem to affect a change but not totally fix the issue. Could one of the small value film caps be bad?
--I don't believe that this board has pots. The rotary controls have 11 discrete positions. That isn't a pot is it?
--It will be much easier to change Ri7 than the OpAmp...maybe I could swap Ri7 and Ri8 and see if the noise follows?

Yes, before I put in new electros, the noise, when moving the tone switch from DEFEAT to the ON position, sounded like an increase in volume of the music. Much louder than when the switch was resting , and operating normally, in either position. But this was for less than a second.

After the new electros, all I hear is a brief static noise only when moving from DEFEAT to ON position and only in the left channel.

Seems the easiest thing would be to swap those two resistors and see what happens...will either find the cause or eliminate one possibility.

Thanks :thmbsp:
 
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Swapping the resistors isn't a bad idea. Those larger value resistors, especially if it is carbon composition do go bad sometimes.
The rotary controls that have many positions are almost always just plain old pots with a mechanical detent setup to emulate a multi position switch. They are drawn as pots in the schematic.
Op amps rarely go bad in such a way as to just get noisy. They usually fail catastrophically so that they don't work at all.
It is pretty rare for any of the lower value caps to go bad.

Terry
 
Kenwood PN's on the boards. That would make the majority of the resistors a carbon film type, with the remainder being either metal film or perhaps a few thin film. Unlikely that there's any carbon comp resistors in the unit.
 
fixed

TerryS - thanks again for your explanations and suggestions

Turns out that the switch responded favorably to a good DeOxit treatment.

Odd because this switch was moved over from the MUTE function where it had not shown any issues. I assumed that it was OK. Even odder because the original problem, which apparently was fixed when the new electrolytics were installed, and the subsequent problem were both related to the same function.

But all's well...:thmbsp:
 
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