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In the circuit diagram on this page, there appears to be an unspecified op-amp. Could anyone guess at a component I could use for this and/or explain why there is no specification?

Background: I am mildly clueless about electronics. I want to build this piezo preamplifier,

enter image description here

which uses a pair of matched JFETs. It lists possibilities for special matched JFET components which do indeed seem to be difficult to source, and alternatively suggests single JFET components which would have to be matched by hand. It recommends the above page as instructions for JFET matching. I'm trying to figure out what components I should get if I go down the hand-matching route.

Side-questions: (Apologies if these are too numerous, please ignore at your convenience.) Can I build the matcher on a breadboard or will it be better to solder together? Given the purpose of the circuit I'm matching the JFETs for (eg. it's run off 48V phantom power, not a 9V battery) should I make any adjustments to the matching circuit (ie. use a voltage source other than 9V battery; a different Rset value)? Or can anyone recommend a matched JFET component available in the UK?

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    \$\begingroup\$ @Bimpelrekkie You are violating an explicit copyright notice by copying that schematic into the question. \$\endgroup\$
    – Finbarr
    Nov 6 '17 at 14:38
  • \$\begingroup\$ Oops ! :-( Ill remove it \$\endgroup\$ Nov 6 '17 at 14:39
  • \$\begingroup\$ Instead of trying to find matched JFETs consider making one of the gate biasing resistors partly variable to tune out the offset or use a different circuit. If the circuit is so sensitive to mismatches in the JFETs then it really isn't a well designed circuit. \$\endgroup\$ Nov 6 '17 at 14:43
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    \$\begingroup\$ To answer the actual question : pretty much any opamp will do the job. It's used at DC and unless you care about the last mV of mismatch, its input offset voltage and current won't upset the match very much. \$\endgroup\$ Nov 6 '17 at 14:48
  • \$\begingroup\$ @BrianDrummond: Please put answers in the answer section below, not in the comments. \$\endgroup\$
    – Dave Tweed
    Nov 6 '17 at 15:47
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As Brian says, pretty much any opamp that can operate with a total supply voltage of 9V or less (i.e., NOT a 741!) will do the job. It's used at DC and unless you care about the last mV of mismatch, its input offset voltage and current won't upset the match very much.

To address your additional concerns:

Can I build the matcher on a breadboard or will it be better to solder together?

It doesn't much matter. If it's something you'll be doing fairly often, you'll want to pick a sturdier construction technique.

Given the purpose of the circuit I'm matching the JFETs for (eg. it's run off 48V phantom power, not a 9V battery) should I make any adjustments to the matching circuit (ie. use a voltage source other than 9V battery; a different Rset value)?

The circuit sets the JFET up as a current source. Increasing the voltage will simply force it to dissipate more power and change its temperature, possibly confusing your results. With this in mind, you do want the transistors to be best matched under the same conditions under which they'll be operating in the actual application, so use your own judgement.

Or can anyone recommend a matched JFET component available in the UK?

Product recommendations are off-topic here.

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  • \$\begingroup\$ Thanks for addressing all the questions, and sorry I wasn't aware about product recommendations. \$\endgroup\$
    – Igid
    Nov 6 '17 at 20:38

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