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RE: how to remove feedback loops

Posted by Ejam on June 11, 2017 at 18:31:50:

Vinnie

I appreciate the desire to have a no feedback amplifier, they do if well designed sound good, but with the UA-1 you have three main problems to overcome. First, without overall feedback the damping factor will be very low and depending on your speakers this may lead to boomy bass. Second, overall feedback reduces the distortion of the output stage (which generates the most), again you may live with this or not. Third, the gain of the amplifier will increase as the feedback reduces gain, making the amp very sensitivity. The suggestion to bypass the input pentode with a cap will further increase gain and hence sensitivity. Your system may not be able to cope with this. Regarding the vacuum rectifier, it has its own benefits, such as slow warm up and lack of switching artifacts. If you replace with diodes then the voltage secondary will need to be lowered. In essence there are a lot of difficulties to overcome and it maybe be better to design your own amp.

I would suggest given the 6BQ5 if you want to a non-overall feedback design to start with a triode output stage and a single triode up front. Phasing splitting can be done as per Jack Elliano's design (included) or the 6BQ5 Oddwatt (on the net). Very simple to breadboard, no overall feedback and having built both, great sounding. Hope this helps.

PS. Don't skimp on the output transformers and use 10k if trioding