Hi,

I recently updated the TXO-2 I built to TXO-4 status. The new modules are mounted directly above the existing modules using 1.25" standoffs thus the wires connecting them are about 2" long. Just long enough to go between the boards and strip enough to get into the terminal blocks.

I'm having a problem with oscillation. At first power up it immediately blew the fuses. I normally power new projects up with the variac but since this amp has been so well behaved in the past I figured the new modules would be fine but of course Mr. Murphy raised up and smacked me. After putting in a couple of new fuses (both hot & neutral fused) and connecting it to the variac I slowly raised the voltage until I started seeing current draw on the ammeter and I could hear the power transformer starting to hum.

I pulled out the two new modules and slowly brought the voltage up again. No problem, it came up normally. I tried the two of the modules from the other channel, which were the originals and known good and they too work fine. I then connected them in parallel with the TXO-2 configuration and brought the voltage up again. Sure enough it started drawing a lot of current. Connecting a frequency counter up to the speaker terminals shows that at the point it starts pulling current it's oscillating at about 115-120kHz. Obviously this isn't good. I did not build the modules with the Zobels installed as the instructions said they are normally not required.

As can be seen in the photo below the modules sit end to end and the additional modules are installed directly above the existing ones and wired in parallel to the module below. I can't physically mount them any closer and the wires connecting them are as short a physically possible. The wire is 18GA stranded, not that this should make a difference.

I did replace feedback resistors in the existing modules with .1% parts to match the new boards and in fact matched them even closer using my LCR meter to .05%.

I can of course install the Zobel parts but I wanted to be sure I wasn't missing something simple. I've reviewed the documentation throughly and I don't see anything that I've missed. Are the TXO-4 setups prone to oscillation? Oh yes, it oscillates with and without a load. Doesn't seem to make a difference.



Thanks!


Mike

Russ,

Thanks for the quick response. I'll check my parts bin but I don't think I have any 330pf on hand. I'll PM you my shipping address if you don't mind sending them out. So, should I include the Zobel as well as the larger compensation caps? I'll go pull the opamps and see if that help as well.

Thanks,

Mike