Synth-diy-test

I think other JFET op amps (TL071 et al) should work - just need to 
adapt it the TO-99 can of the LF13741.

Weird situation - I've used a pair of class-2 wall-wart powered Harman 
Kardon multimedia speakers for various monitoring. Never had a problem. 
Sounds like the Creative speakers power supply is not to well isolated 
from the line.

- Oren

On 1/31/26 6:40 PM, Richie Burnett wrote:
> 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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