Fw: Mixer opamp, which one?

[Arthur Harrison]

>MH:
>
>>All,
>>
>>I'm planning to add an 8 (or more) channel stereo mixer to my modular and
>>have been going throught the various schematics I have. In many of them
the
>>designer has used a TL071 as the opamp of choice. On checking a 4-channel
>>mixer I presently have, the builder has used a TL081. The datasheet I have
>>on the TL071 specifys the devices noise distortion, whereas there is no
>>mention of the subject on the TL081 datasheet, (both sheets are from
>>Motorola). Question, which is the better to use?
>
>AH:
>
>According to the original manufacturer, Texas Instruments, "The JFET-input
>operational amplifiers in the TL07x series are designed as low-noise
>versions
>of the TL08x series amplifiers."
>Typically, Vn=18 nV/Hz at f =1kHz for the TL07x parts.
>
>http://www-s.ti.com/sc/psheets/slos080d/slos080d.pdf,
>page 7 has table data for TL07x noise characteristics
>page 13 has a graph of "Equivalent input noise voltage vs Frequency"
>
>http://www-s.ti.com/sc/psheets/slos081d/slos081d.pdf,
>page 9 has table data for TL08x noise characteristics
>page 17 has a graph of "Equivalent input noise voltage vs Frequency"
>
>The chart and graph data is the same for both parts, so I do not know
>why TI claims that the TL07x parts are better for noise.
>
>I conjecture that these are identical die, perhaps tested for different
>criteria.  (There are some types of specifications given for one part,
>and not the other.)
>
>If you breadboard a channel with a socket, you may want to compare several
>different op amps for noise.  There are some parts with better noise
>specifications
>than the TL07x available, these days.  For example, see:
>http://products.analog.com/products/info.asp?product=AD797 for the AD797.
>Keep in mind that circuit impedances have a direct impact on noise
>performance
>for op amps, and that the actual design, not just the op amp type, must be
>optimized
>for obtaining the lowest noise.
>
>MH:
>
>>Additionally, the designs I have do not use more than six inputs. Are
there
>>any issues with the number of channels one just "bolts" onto the input of
>>the opamp"?
>
>>Any advise would be welcome.
>
>AH:
>
>Good layout in important.  A PCB with planes for ground, Vcc and Vee is
>ideal.
>
>Merry Christmas to all!
>
>-Art
>