by dcgillespie » Sat Aug 22, 2009 7:42 am
While keeping a load on the amplifier may prevent the oscillation, it would be even better to find the cause of the problem and eliminate it at the source. The nature of the oscillation can be a very helpful tool in tracking it down. It would surely seem however, that the root of the problem lies in some form of inappropriate feedback, as the oscillation goes away when a load is connected. Assuming a good layout and proper build etc., the feedback network would be a good place to start.
As a reference, the original ST-70 as designed is a highly stable feedback amplifier -- that is, it will not oscillate under any conditions of use, whether no load, shorted load, or even with just capacitance connected across the output.
Assuming the circuit and components you are using is a known stable design, try disconnecting the NFB circuit and see if that eliminates the problem. If so, you at least know where to start. If the oscillation is of a very low frequency nature, coupling cap values, decoupling caps and the like would be a good place to start. If it is ultrasonic or near ultrasonic, then the HF stability is at issue. The feedback cap and any HF shunting networks between stages control this aspect of stability. Even lead dress can play a roll in it.
Since it is load related, the power supply is not likely to be the cause. If both channels do it identically and are built so, that tends to indicate a duplicated build or part issue, or a basic design issue.
Some where in all of this you will likely find the cause. An external safety load may be a solution, but finding the cause would be far better. Good luck in your search!
David G.