DIY- and Holton's Lateral-amp problem
A while ago I made a N-channel amplifier that was intended for use with +-35V. Now I have made a similar amp for +-70V and I have some problem. With no input the amp is stable and no sign of oscillation at the output at any rail voltage up to +-70V. But when I connect an input I get some random 'noise' at the output at about half a volt or so. The 'noise' is not constant in frequency but I would guess less than 100kHz by looking at the oscilloscope. This is clearly audible and it sounds like the speakers have a broken voice coil. The problem appears whether I have a speaker connected or not. I had similar problems with Holton's lateral amplifier when using IRF610 as VAS as he suggested. When changing back to MJE340 all problems disappeared. I'm currently using IRF610 as VAS and i'm wondering if substitute to MJE340 would help. I have also made it possible to add resistors to my amp, R17 and R20, that are currently being bridged.
Does anyone know what causes this phenomen?
Here are the two amplifiers, first mine and than Holton's lateral.
Should C9 not go across the 22k top feedback resistor?
C9 across two stages (LTP & VAS) avoids the Miller cap problems. A nice solution chosen by many designers who claim this sounds better.
Try adding a pot in series with C9 & see if adjusting eliminates the spurious noise. try also adding a cap in parallel to this CR. how about a CR in parallel with M1 gate to -ve Vrail. Could the problem be noise input back into the NFB from the output leads? Adding the remainder of the zobel circuit would stop this.
Have you checked how your problem looks like with an oscilloscope? May I ask why you have chosen a MOSFET?
Can it be that you load the input stage too much with mosfet? Parasitic oscillations of the mosfet?
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