Synth-diy-test

I'm quite sure a TL071 will be fine.  Can't see anything special in what is a very old design.

As for your speaker PSU, what country are you in ?  The problem with class 2 (to my mind) is that it assumes neutral is connected to earth somewhere, but in some countries earth sits half way between line and neutral, whilst in others the whole concept of line and neutral is a bit irrelevant as they are often swapped over on the wall socket.
________________________________
From: Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of Richie Burnett <rburnett@richieburnett.co.uk>
Sent: 01 February 2026 00:40
To: synth-diy mailing list <synth-diy@synth-diy.org>
Subject: [sdiy] Blown LF13741 and suitable replacement?

Hi all,

An unfortunate tale, a lesson learnt, and a question for the analogue
experts...

Today I was giving my old Roland SH-09 a quick test with some powered PC
speakers that I had handy on my work bench plugged into the line output
socket.  (Creative T10 multimedia speakers powered from their own Class-2
wall-wart power supply.)  I played it for about an hour through these little
speakers and all was fine.

Then I decided to plug it into my mixing desk to record some sounds. When I
pulled the jack cable connecting it to the powered speakers there was a loud
crackle (despite me having turned down the volume on the powered speakers.)
The SH-09 then played out of tune when I connected it to the mixing desk!
It was initially about 8 semi-tones flat, and playing notes an octave apart
resulted in less than an octave span in pitch.  This behaviour had me
stumped, so I pulled the audio cable connecting it to the mixing desk, and
plugged the multi-media speakers back in again.  Now the VCO plays the same
frequency (about 6Hz!) regardless of what note I play.  Not a good day for
the SH-09, or me :-(

On investigation it turns out that the LF13741 JFET op-amp in the keyboard
CV sample-and-hold circuit of the SH-09 has been cooked.  Playing C notes an
octave apart on the keyboard presents 1V, 2V, 3V to the input of the op-amp,
but always about -3.5V coming out.  For what it's worth the VCO still
responds to changes in the synth's VCO octave range switch, LFO mod, and the
pitch-bend lever, so I don't think anything else has been damaged.  Fingers
crossed!

Can anyone recommend a modern replacement for this part, before I pay an arm
and a leg for what might turn out to be a counterfeit LF13741 on ebay?  The
device in my SH-09 is actually a circular metal can package, but the pinout
matches a standard single op-amp DIP-8 package, so I'm guessing a plastic
DIP version would suffice?

I'm just cross that I didn't think of the *potential* for damage
connecting/disconnecting Class-2 speakers to a piece of Class-1 earthed
equipment.  My multi-meter shows 102VAC between the shield on the audio
cable coming from the Creative powered speakers and mains earth!  Modern
gear is probably more tolerant than 1980's technology, but definitely
something for others to be aware of to reduce the risk of damage to other
vintage gear.

-Richie,


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