WAD phono 3 motorboating with Raphaelite 300b

[adrianhall]

Hi

I recently bought a Raphaelite 300b amp from Hong Kong. A scematic can be found on the following page: À*·Æ¶ûµç×Ó¹ÜÒôÏ쳧

The amp sounds great with CD but motorboats when I plug in the WAD phono 3. The phono 3 has worked perfectly with a Miniwatt tube amp.

The motorboating increases as the volume is increased and disappears when only one channel lead is connected.

Does anyone have any suggestions to fix this problem?

Many thanks

Adrian

[kevinkr]

Do you have a scope and some tools? Schematics for both the amp and pre-amp would be helpful. (Please do not post copyrighted material here! Redrawn is OK as is posting with permission from the copyright owner) I suspect that there is significant subsonic output from the phono pre-amplifier and due to the design of the 300B power supply or its GFB implementation (if present) there is subsonic instability occurring.

One possibility would be to reduce the value of the coupling caps between the driver and output stages in the 300B amp, another would be to rework the psu to improve isolation between stages. Knowing what is best to do really depends on understanding the problem. For example if the coupling caps used in the 300B amp permit useful amplification below 1Hz they really ought to be made smaller, OTOH hand if the phono stage does not start to roll off a little below 20Hz that indicates a potentially poor design choice as well.

I think a little more information is required.

[adrianhall]

Hi and thanks for your response. I'll try and dig out a schematic for the WAD phono3 and explore your suggestions.