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Is a TLE2426 enough to drive 9 opamps in this scenario

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  • Pict
    In the process of building something. I have an OPA134 in unity gain feeding the signal on to 4 x OPA2134 also in unity gain. Both amps in the 2134 are
    Message 1 of 10 , Dec 16, 2012
      In the process of building something. I have an OPA134 in unity gain feeding
      the signal on to 4 x OPA2134 also in unity gain. Both amps in the 2134 are
      utilised one for an in phase output and another for an inverted phase
      output. It is a distribution buffer for guitar or 4dBU line level sources.

      I see the quiescent current for these amps is 4mA each so I expect quiescent
      draw to be in the region of 36mA. Regarding signal current draw the OPA2134
      I presume will draw extremely minimal current from the input OPA134. The
      outputs of all the other 8 opamps are feeding the signal out through a DC
      decoupling 10uF NP connected to a 1K resistor and The highest load I expect
      is when then outputs are muted when I will tie the output side of the
      resistor to virtual ground

      I intended running the opamps from a +9/0/-9V split rail supply generated by
      a TLE2426. I see from the datasheet that it can sink/source 20mA.

      I am thinking that if the input signal is say 4 volts peak to peak and gain
      is unity everywhere the heaviest load is when the output is muted to virtual
      ground via the 1K resistor. I am thinking this should result in a peak
      current of 2/1000 =2mA, so I am not expecting the virtual ground output to
      have to supply more than 10mA.

      Really not clear about this so tried to research whether the TLE2426 has
      enough capacity. I've seen several posts that include the quiescent drive
      current in the TLE2426 load calculation but I was under the impression that
      that current is sourced directly from the power rails and has nothing to do
      with the virtual ground current. Confused as to how to calculate the load to
      determine whether the TLE2426 is up to the job without buffering or not. Can
      anyone shed some light for me please?

      Thanks,
      John






      [Non-text portions of this message have been removed]
    • Howard Hansen
      Instead of worrying about whether a TLE2426 has adequate capability to supply +/- current for your design you may want to use one of the circuits in the
      Message 2 of 10 , Dec 16, 2012
        Instead of worrying about whether a TLE2426 has adequate capability to
        supply +/- current for your design you may want to use one of the
        circuits in the following pdf for obtaining more current.
        <http://tangentsoft.net/elec/vgrounds.html>

        The other Howard


        On 12/16/2012 3:06 PM, Pict wrote:
        >
        > In the process of building something. I have an OPA134 in unity gain
        > feeding
        > the signal on to 4 x OPA2134 also in unity gain. Both amps in the 2134 are
        > utilised one for an in phase output and another for an inverted phase
        > output. It is a distribution buffer for guitar or 4dBU line level sources.
        >
        > I see the quiescent current for these amps is 4mA each so I expect
        > quiescent
        > draw to be in the region of 36mA. Regarding signal current draw the
        > OPA2134
        > I presume will draw extremely minimal current from the input OPA134. The
        > outputs of all the other 8 opamps are feeding the signal out through a DC
        > decoupling 10uF NP connected to a 1K resistor and The highest load I
        > expect
        > is when then outputs are muted when I will tie the output side of the
        > resistor to virtual ground
        >
        > I intended running the opamps from a +9/0/-9V split rail supply
        > generated by
        > a TLE2426. I see from the datasheet that it can sink/source 20mA.
        >
        > I am thinking that if the input signal is say 4 volts peak to peak and
        > gain
        > is unity everywhere the heaviest load is when the output is muted to
        > virtual
        > ground via the 1K resistor. I am thinking this should result in a peak
        > current of 2/1000 =2mA, so I am not expecting the virtual ground output to
        > have to supply more than 10mA.
        >
        > Really not clear about this so tried to research whether the TLE2426 has
        > enough capacity. I've seen several posts that include the quiescent drive
        > current in the TLE2426 load calculation but I was under the impression
        > that
        > that current is sourced directly from the power rails and has nothing
        > to do
        > with the virtual ground current. Confused as to how to calculate the
        > load to
        > determine whether the TLE2426 is up to the job without buffering or
        > not. Can
        > anyone shed some light for me please?
        >
        > Thanks,
        > John
        >
        > [Non-text portions of this message have been removed]
        >
        >



        [Non-text portions of this message have been removed]
      • Pict
        I am familiar with this page thanks, but don t have a suitable buffer opamp or transistors to hand. I can order them if need be but I d rather not if I don t
        Message 3 of 10 , Dec 16, 2012
          I am familiar with this page thanks, but don't have a suitable buffer opamp
          or transistors to hand. I can order them if need be but I'd rather not if I
          don't need to. I am not familiar with how to calculate max current required
          for the supply. Is it only the current in the virtual ground that is limited
          by the TLE2426 20mA limit on source/sink or do I have to factor in the rail
          current as well? If it's only the virtual ground current was my guess of
          around 2mA per opamp reasonable given the stated output loads and input
          level?

          Regards,
          John

          From: Howard Hansen <hrhan@...>
          Reply-To: <Electronics_101@yahoogroups.com>
          Date: Sun, 16 Dec 2012 17:46:51 -0600
          To: <Electronics_101@yahoogroups.com>
          Subject: Re: [Electronics_101] Is a TLE2426 enough to drive 9 opamps in
          this scenario






          Instead of worrying about whether a TLE2426 has adequate capability to
          supply +/- current for your design you may want to use one of the
          circuits in the following pdf for obtaining more current.
          <http://tangentsoft.net/elec/vgrounds.html>

          The other Howard

          On 12/16/2012 3:06 PM, Pict wrote:
          >
          > In the process of building something. I have an OPA134 in unity gain
          > feeding
          > the signal on to 4 x OPA2134 also in unity gain. Both amps in the 2134 are
          > utilised one for an in phase output and another for an inverted phase
          > output. It is a distribution buffer for guitar or 4dBU line level sources.
          >
          > I see the quiescent current for these amps is 4mA each so I expect
          > quiescent
          > draw to be in the region of 36mA. Regarding signal current draw the
          > OPA2134
          > I presume will draw extremely minimal current from the input OPA134. The
          > outputs of all the other 8 opamps are feeding the signal out through a DC
          > decoupling 10uF NP connected to a 1K resistor and The highest load I
          > expect
          > is when then outputs are muted when I will tie the output side of the
          > resistor to virtual ground
          >
          > I intended running the opamps from a +9/0/-9V split rail supply
          > generated by
          > a TLE2426. I see from the datasheet that it can sink/source 20mA.
          >
          > I am thinking that if the input signal is say 4 volts peak to peak and
          > gain
          > is unity everywhere the heaviest load is when the output is muted to
          > virtual
          > ground via the 1K resistor. I am thinking this should result in a peak
          > current of 2/1000 =2mA, so I am not expecting the virtual ground output to
          > have to supply more than 10mA.
          >
          > Really not clear about this so tried to research whether the TLE2426 has
          > enough capacity. I've seen several posts that include the quiescent drive
          > current in the TLE2426 load calculation but I was under the impression
          > that
          > that current is sourced directly from the power rails and has nothing
          > to do
          > with the virtual ground current. Confused as to how to calculate the
          > load to
          > determine whether the TLE2426 is up to the job without buffering or
          > not. Can
          > anyone shed some light for me please?
          >
          > Thanks,
          > John
          >
          > [Non-text portions of this message have been removed]
          >
          >

          [Non-text portions of this message have been removed]









          [Non-text portions of this message have been removed]
        • Jan Kok
          Your second paragraph appears to be missing some text: The outputs of all the other 8 opamps are feeding the signal out through a DC decoupling 10uF NP
          Message 4 of 10 , Dec 16, 2012
            Your second paragraph appears to be missing some text:

            "The outputs of all the other 8 opamps are feeding the signal out through a DC
            decoupling 10uF NP connected to a 1K resistor and The highest load I expect
            is when then outputs are muted when I will tie the output side of the
            resistor to virtual ground"

            ...so I don't fully understand your circuit. I especially don't
            understand what you are saying about muting. Why do you need to tie an
            _output_ to ground?

            Am I correct in assuming that the OUT pin of the tle2426 is tied to
            the circuit's "ground"? And your power supply is floating (no internal
            connection to ground) with 18V across its two output wires? (For
            example it might be a couple 9V batteries in series, or a "wall wart"
            power supply. BTW, beware that some wall warts can introduce
            significant noise into your system - but that's another topic.) If
            not, please explain.

            If all your op amps are driving high impedance loads, then the tle2426
            doesn't have to supply any significant current at all.

            The quiescent supply current of the op amps is irrelevant to this
            discussion because the current flows out the + terminal of the power
            supply, into the V+ terminals of the op amps, out the V- terminals,
            and into the - terminal of the power supply. None of that current goes
            in or out of the tle2426's OUT pin.

            The resistors that create the inverting buffers also don't create
            create a load for the tle2426. Trace the current paths and you'll see.
            But make sure the input op amp can supply enough current to those
            resistors. The "typical" output drive of the opa134 is +-35 mA. Who
            knows what typical means. I'd keep it under 20 mA to be safe and avoid
            possible distortion.

            Some situations that _would_ cause a large current in or out of the tle2426 are:

            - A shorted output. An opa134's "typical" output short circuit current
            is +-40 mA. Assuming the short is to ground, the current would return
            through the tle2426's OUT pin and exceed its current drive capacity.

            - Is the typical load on each output 600 ohms? If each of the 4
            positive outputs are loaded with 600 ohms, and the negative outputs
            are unloaded, then the outputs are driving a total load of 600/4=150
            ohms. An input that reached 3V at some instant would then cause 20 mA
            total to flow into the loads and return through the tle2426's OUT pin.

            Hope that helps...

            - Jan
          • Pict
            From: Jan Kok Reply-To: Date: Sun, 16 Dec 2012 23:34:55 -0700 To:
            Message 5 of 10 , Dec 17, 2012
              From: Jan Kok <jan.kok.5y@...>
              Reply-To: <Electronics_101@yahoogroups.com>
              Date: Sun, 16 Dec 2012 23:34:55 -0700
              To: <Electronics_101@yahoogroups.com>
              Subject: Re: [Electronics_101] Is a TLE2426 enough to drive 9 opamps in
              this scenario






              <snip>
              "...so I don't fully understand your circuit. I especially don't
              understand what you are saying about muting. Why do you need to tie an
              _output_ to ground?"
              If I mute an output I wanted to have the output tied to ground because the
              output is still connected to an amp via screened cable TS or TRS lead so I
              want to make sure I silence the amp by tying its input to signal ground so
              it isn't floating with a length of lead attached to it. Perhaps it would be
              better to have the mute switch (spdt) disconnect the guitar amp input from
              the OPA2134 output, and switch to connect the amp input to signal ground
              while leaving the OPA2134 output floating. The switch is break before make
              however so I thought it better to short the amp input to ground (connected
              > grounded) rather than switch to ground (connected>floating>grounded).
              There is 1K between the mute switch and the OPA2134 output which I don't
              think should load down the typical amp input impedance significantly.

              "Am I correct in assuming that the OUT pin of the tle2426 is tied to
              the circuit's "ground"? And your power supply is floating (no internal
              connection to ground) with 18V across its two output wires? (For
              example it might be a couple 9V batteries in series, or a "wall wart"
              power supply. BTW, beware that some wall warts can introduces
              significant noise into your system - but that's another topic.) If
              not, please explain."

              That's right. Power supply is floating ­ 2 9V batteries in series. TLE2426
              output is signal ground.


              "If all your op amps are driving high impedance loads, then the tle2426
              doesn't have to supply any significant current at all."

              All loads when outputs are active will be high impedance >20K to 1M

              "The quiescent supply current of the op amps is irrelevant to this
              discussion because the current flows out the + terminal of the power
              supply, into the V+ terminals of the op amps, out the V- terminals,
              and into the - terminal of the power supply. None of that current goes
              in or out of the tle2426's OUT pin."

              This is what I thought. Thanks for clarifying,

              "The resistors that create the inverting buffers also don't create
              create a load for the tle2426. Trace the current paths and you'll see.
              But make sure the input op amp can supply enough current to those
              resistors. The "typical" output drive of the opa134 is +-35 mA. Who
              knows what typical means. I'd keep it under 20 mA to be safe and avoid
              possible distortion."

              The input opamp load will be very minimal I think. 10K resistor network in
              the inverting buffers.


              "Some situations that _would_ cause a large current in or out of the tle2426
              are:

              - A shorted output. An opa134's "typical" output short circuit current
              is +-40 mA. Assuming the short is to ground, the current would return
              through the tle2426's OUT pin and exceed its current drive capacity.

              - Is the typical load on each output 600 ohms? If each of the 4
              positive outputs are loaded with 600 ohms, and the negative outputs
              are unloaded, then the outputs are driving a total load of 600/4=150
              ohms. An input that reached 3V at some instant would then cause 20 mA
              total to flow into the loads and return through the tle2426's OUT pin.""
              The maximum load occurs when the opamps are muted effectively tying the
              outputs to ground via 1K. The biggest threat to increased current draw seems
              to be underestimating the input voltage. Re-reading the TLE2426 datasheet I
              see the source/sink value of +-20mA is labelled 'Typical' (whatever that
              means). Absolute Maximum is listed at +-80mA so I hope that should give
              enough headroom to cope with transients without the virtual ground being
              pulled off centre and/or meltdown.


              "Hope that helps...
              - Jan"

              Yes I appreciate the pointers Jan. I have the confidence now to go ahead and
              complete the build and see if there are any problems in use. I think it
              should be OK but we'll see. Many thanks,
              John








              [Non-text portions of this message have been removed]
            • Derek
              I agree, one should never tie the output of an opamp to ground. Best is to use a MOSFET or BJT on the input and switch the transistor on to the ground node.
              Message 6 of 10 , Dec 17, 2012
                I agree, one should never tie the output of an opamp to ground. Best is
                to use a MOSFET or BJT on the input and switch the transistor on to the
                ground node. This would avoid the "pop" of a break-before-make circuit.

                To put the original question simply, quiescent current is the current
                drawn from the power supply rails to bias the internal circuitry of the
                opamp. Otherwise, other currents involved are:

                1. Output current drawn from the supply rails and is based on the load
                and feedback current.
                2. Current to the input nodes is based on the drive circuit and
                resistance to the virtual node.

                Derek Koonce
                DDK Interactive Consulting Services

                On 12/17/2012 11:05 AM, Pict wrote:
                >
                >
                >
                > From: Jan Kok <jan.kok.5y@... <mailto:jan.kok.5y%40gmail.com>>
                > Reply-To: <Electronics_101@yahoogroups.com
                > <mailto:Electronics_101%40yahoogroups.com>>
                > Date: Sun, 16 Dec 2012 23:34:55 -0700
                > To: <Electronics_101@yahoogroups.com
                > <mailto:Electronics_101%40yahoogroups.com>>
                > Subject: Re: [Electronics_101] Is a TLE2426 enough to drive 9 opamps in
                > this scenario
                >
                > <snip>
                > "...so I don't fully understand your circuit. I especially don't
                > understand what you are saying about muting. Why do you need to tie an
                > _output_ to ground?"
                > If I mute an output I wanted to have the output tied to ground because the
                > output is still connected to an amp via screened cable TS or TRS lead so I
                > want to make sure I silence the amp by tying its input to signal ground so
                > it isn't floating with a length of lead attached to it. Perhaps it
                > would be
                > better to have the mute switch (spdt) disconnect the guitar amp input from
                > the OPA2134 output, and switch to connect the amp input to signal ground
                > while leaving the OPA2134 output floating. The switch is break before make
                > however so I thought it better to short the amp input to ground (connected
                > > grounded) rather than switch to ground (connected>floating>grounded).
                > There is 1K between the mute switch and the OPA2134 output which I don't
                > think should load down the typical amp input impedance significantly.
                >
                > "Am I correct in assuming that the OUT pin of the tle2426 is tied to
                > the circuit's "ground"? And your power supply is floating (no internal
                > connection to ground) with 18V across its two output wires? (For
                > example it might be a couple 9V batteries in series, or a "wall wart"
                > power supply. BTW, beware that some wall warts can introduces
                > significant noise into your system - but that's another topic.) If
                > not, please explain."
                >
                > That's right. Power supply is floating ­ 2 9V batteries in series. TLE2426
                > output is signal ground.
                >
                > "If all your op amps are driving high impedance loads, then the tle2426
                > doesn't have to supply any significant current at all."
                >
                > All loads when outputs are active will be high impedance >20K to 1M
                >
                > "The quiescent supply current of the op amps is irrelevant to this
                > discussion because the current flows out the + terminal of the power
                > supply, into the V+ terminals of the op amps, out the V- terminals,
                > and into the - terminal of the power supply. None of that current goes
                > in or out of the tle2426's OUT pin."
                >
                > This is what I thought. Thanks for clarifying,
                >
                > "The resistors that create the inverting buffers also don't create
                > create a load for the tle2426. Trace the current paths and you'll see.
                > But make sure the input op amp can supply enough current to those
                > resistors. The "typical" output drive of the opa134 is +-35 mA. Who
                > knows what typical means. I'd keep it under 20 mA to be safe and avoid
                > possible distortion."
                >
                > The input opamp load will be very minimal I think. 10K resistor network in
                > the inverting buffers.
                >
                > "Some situations that _would_ cause a large current in or out of the
                > tle2426
                > are:
                >
                > - A shorted output. An opa134's "typical" output short circuit current
                > is +-40 mA. Assuming the short is to ground, the current would return
                > through the tle2426's OUT pin and exceed its current drive capacity.
                >
                > - Is the typical load on each output 600 ohms? If each of the 4
                > positive outputs are loaded with 600 ohms, and the negative outputs
                > are unloaded, then the outputs are driving a total load of 600/4=150
                > ohms. An input that reached 3V at some instant would then cause 20 mA
                > total to flow into the loads and return through the tle2426's OUT pin.""
                > The maximum load occurs when the opamps are muted effectively tying the
                > outputs to ground via 1K. The biggest threat to increased current draw
                > seems
                > to be underestimating the input voltage. Re-reading the TLE2426
                > datasheet I
                > see the source/sink value of +-20mA is labelled 'Typical' (whatever that
                > means). Absolute Maximum is listed at +-80mA so I hope that should give
                > enough headroom to cope with transients without the virtual ground being
                > pulled off centre and/or meltdown.
                >
                > "Hope that helps...
                > - Jan"
                >
                > Yes I appreciate the pointers Jan. I have the confidence now to go
                > ahead and
                > complete the build and see if there are any problems in use. I think it
                > should be OK but we'll see. Many thanks,
                > John
                >
                > [Non-text portions of this message have been removed]
                >
                >
              • Pict
                ... But as it is the output of the input buffer opamp directly feeds all four output buffers (the input buffer output is directly connected to the noninverting
                Message 7 of 10 , Dec 17, 2012
                  On 17/12/2012 13:49, "Derek" <derek@...> wrote:

                  >I agree, one should never tie the output of an opamp to ground. Best is
                  >to use a MOSFET or BJT on the input and switch the transistor on to the
                  >ground node. This would avoid the "pop" of a break-before-make circuit.

                  But as it is the output of the input buffer opamp directly feeds all four
                  output buffers (the input buffer output is directly connected to the
                  noninverting input on all four output buffers). If I short the input of
                  one output buffer with a MOSFET I'll short them all to ground.

                  Regards,
                  John
                • Derek
                  All four output amps should have some resistance isolation between them. If there is none, then there is a potential issue at that point. Or go back one more
                  Message 8 of 10 , Dec 17, 2012
                    All four output amps should have some resistance isolation between them.
                    If there is none, then there is a potential issue at that point. Or go
                    back one more stage and short the input of the input amp - this should
                    be isolated as well.

                    I may have missed something earlier. Did you post a schematic? Is this a
                    design you are modifying or in process? If you have to modify, then
                    think about a small daughter card with some type of buffer amp in place
                    so that the shorting of an input can be easily implemented.

                    Derek Koonce
                    DDK Interactive Consulting Services

                    On 12/17/2012 12:59 PM, Pict wrote:
                    >
                    >
                    >
                    > On 17/12/2012 13:49, "Derek" <derek@...
                    > <mailto:derek%40dkoonce.com>> wrote:
                    >
                    > >I agree, one should never tie the output of an opamp to ground. Best is
                    > >to use a MOSFET or BJT on the input and switch the transistor on to the
                    > >ground node. This would avoid the "pop" of a break-before-make circuit.
                    >
                    > But as it is the output of the input buffer opamp directly feeds all four
                    > output buffers (the input buffer output is directly connected to the
                    > noninverting input on all four output buffers). If I short the input of
                    > one output buffer with a MOSFET I'll short them all to ground.
                    >
                    > Regards,
                    > John
                    >
                    >



                    [Non-text portions of this message have been removed]
                  • Pict
                    I m out of time at the moment to work more on this but will post a schematic when I get a chance to devote some attention to it again (work taking over life
                    Message 9 of 10 , Dec 17, 2012
                      I'm out of time at the moment to work more on this but will post a schematic
                      when I get a chance to devote some attention to it again (work taking over
                      life again).

                      Regards,
                      John


                      From: Derek <derek@...>
                      Reply-To: <Electronics_101@yahoogroups.com>
                      Date: Mon, 17 Dec 2012 14:16:21 -0800
                      To: <Electronics_101@yahoogroups.com>
                      Subject: Re: [Electronics_101] Is a TLE2426 enough to drive 9 opamps in
                      this scenario






                      All four output amps should have some resistance isolation between them.
                      If there is none, then there is a potential issue at that point. Or go
                      back one more stage and short the input of the input amp - this should
                      be isolated as well.

                      I may have missed something earlier. Did you post a schematic? Is this a
                      design you are modifying or in process? If you have to modify, then
                      think about a small daughter card with some type of buffer amp in place
                      so that the shorting of an input can be easily implemented.

                      Derek Koonce
                      DDK Interactive Consulting Services

                      On 12/17/2012 12:59 PM, Pict wrote:
                      >
                      >
                      >
                      > On 17/12/2012 13:49, "Derek" <derek@... <mailto:derek%40dkoonce.com>
                      > <mailto:derek%40dkoonce.com>> wrote:
                      >
                      > >I agree, one should never tie the output of an opamp to ground. Best is
                      > >to use a MOSFET or BJT on the input and switch the transistor on to the
                      > >ground node. This would avoid the "pop" of a break-before-make circuit.
                      >
                      > But as it is the output of the input buffer opamp directly feeds all four
                      > output buffers (the input buffer output is directly connected to the
                      > noninverting input on all four output buffers). If I short the input of
                      > one output buffer with a MOSFET I'll short them all to ground.
                      >
                      > Regards,
                      > John
                      >
                      >

                      [Non-text portions of this message have been removed]









                      [Non-text portions of this message have been removed]
                    • Jan Kok
                      ... So it sounds like there are 4 output sections, each section producing an in-phase and an inverted copy of the input. And it sounds like you want to mute
                      Message 10 of 10 , Dec 17, 2012
                        > <snip>
                        > "...so I don't fully understand your circuit. I especially don't
                        > understand what you are saying about muting. Why do you need to tie an
                        > _output_ to ground?"
                        > If I mute an output I wanted to have the output tied to ground because the
                        > output is still connected to an amp via screened cable TS or TRS lead so I
                        > want to make sure I silence the amp by tying its input to signal ground so
                        > it isn't floating with a length of lead attached to it. Perhaps it would be
                        > better to have the mute switch (spdt) disconnect the guitar amp input from
                        > the OPA2134 output, and switch to connect the amp input to signal ground
                        > while leaving the OPA2134 output floating. The switch is break before make
                        > however so I thought it better to short the amp input to ground (connected
                        >> grounded) rather than switch to ground (connected>floating>grounded).
                        > There is 1K between the mute switch and the OPA2134 output which I don't
                        > think should load down the typical amp input impedance significantly.

                        So it sounds like there are 4 output sections, each section producing
                        an in-phase and an inverted copy of the input. And it sounds like you
                        want to mute each of the 4 sections independently, i.e. you want 4
                        mute switches, one for each output.

                        First, I wonder why you need mute switches for the outputs, since
                        there is usually a mute switch or at least a volume control on each
                        input to most mixers, amplifiers, etc. So mute switches on the outputs
                        seems redundant and might sometimes cause confusion: "Why aren't we
                        getting signal from... Oh, the output of the distribution amp was
                        muted."

                        But anyway, if that's what you want to do, I'd suggest having the
                        output of the opa134 go to 4 SPST mute switches. The other side of
                        each mute switch would go to the + input of the non-inverting buffer
                        and to the 10K resistor that goes to the - input of the inverting
                        buffer. When the switch is open, there is no current through the input
                        10K resistor, and the inverting buffer acts to bring its - input to
                        ground. The + input of the noninverting buffer is tied to virtual
                        ground through the input 10K resistor. So you just need one SPST
                        switch per section.

                        By the way, the clicks and pops you get from switching high impedance
                        inputs is due to those inputs getting charged up from stray leakage
                        currents. When the switch is transitions from open to closed, there is
                        a large, abrupt voltage step as the charge is discharged through the
                        switch into the low impedance signal source. A simple fix is to tie
                        the high impedance input to ground through a large (say 100K or 1Meg)
                        resistor. When the switch is open, the resistor drains off stray
                        leakage current and keeps the input at ground. With the mute switch
                        circuit described in the previous paragraph, a separate drain resistor
                        is not needed because the 10K input resistor and the 10K feedback
                        resistors serve that function.

                        Here's another reason why trying to ground the output of an op amp
                        doesn't make much sense and probably won't work well if at all. Note
                        that the closed-loop output impedance of the opa134 is 0.01 ohm! In a
                        struggle between 0.01 ohm and 1K ohm, which do you think will win? :-)

                        Cheers,
                        - Jan
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