diggy fresh

Api 325 D.i mod oscillation
« on: March 23, 2020, 08:24:18 PM »
Hi guys, hopefully all is well.

i've been toying with a diy Api 325 modded for D.i (high impedance preamp) by Bo Hansen (thanks!).

here's the schem:
http://www.hansenaudio.se/API%20325%20DI%20mod..jpg


It works but i have an oscilation that "distorts" the signal and change pitch as i change gain on my gain switch.  The tone of the oscillation seem to get lower as i boost gain, going out of the upper audio band at low gain.


What situations can lead to oscillation from the 2520 in this circuits? To help me trouble shoot.


My input and output leads are well appart and shielded.

I've used shielded wires to wire the whole thing exept dc wires.

I used a shielded cable for the gain switch lead also, can capacitance to ground there cause oscilation? I made a basic lorlin switch with point-to-point resistors for the gain, which is in the feedback loop i think.


It is a diy layout and pcb, i have made a double sided board with ground plains connected on both sides also.

The problem is probably in my layout assembly,  I would just like pointers on where to look for and what could cause the opamp to oscillate in this case.

Any insight ?

thanks
john
English is my second language, apologies

Autodidact audio geek, constantly learning.

Very interested in old technology, i mostly learn from the info you guys share on here, so thanks!


 

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