I am having an odd bass issue with my Simple SE

[bigdh31]

I have been using my Simple SE for over a year with no problems and I really enjoy it.

Recently I replaced my Berseford 7520 digital pre-amp with a Grant Fidelity DAC-09. The DAC-09 seems to produces much more low bass energy than the Beresford.

The bass sounds great for couple of hours and then it starts changing and becomes some what fuzzy sounding. The upper frequencies do not seem to be affected. I was using a Mullard CV4024 as the input tube and changed it to JJ ECC81. It does not happen as much with the JJ tube but I can still hear it some. It does not seem to be related to the volume setting.

I have tried different tubes in the DAC-09 also. I connected a class D amp to the DAC-09 and listened for several hours and the problem did not happen.

What should I check? Could it be an issue with the constant current chips? I wired the RCA jacks directly to the board since I planned to use a preamp. Should I add some resistance in series with the RCA's? Could it be the coupling cap?

[GoranB]

Do You have a decoupling cap on the catode biasing ressistor of your input tube?

[rknize]

Do you have any pics of your build? Does the PCB have adequate ventilation? Since it takes a few hours for the issue to occur, it sounds like some kind of thermal issue. If you can run it with the PCB exposed, you can rule that out.

[bigdh31]

I had the parts on hand to add a shunt mode volume pot to my simple se. I have ran it now for over four hours and I have not been able to recreate the problem. I am not for sure why but this seems to have solved the problem.