Wednesday, May 13, 2015

Bedini 25/25 Class A Power Amplifier

Bedini 25/25 Class A Power Amplifier

I wanted to share a little bit about this Bedini 25/25 Class A power amplifier. Rated at a mere 25W/CH it still has an impressive THD rating albeit fairly low dampening rating. I do want to note, apparently this is a very similar design to the 100/100 without updated 93/94 output devices.

This particular came in with some serious distortion issues, you could literally watch the wave form table-top with my Tektronix 2215 scope. Some previous work had been performed on the unit so unfortunately the quality fiberglass PCB had ignorant tracing issues. As well a axial general electrolytic was replaced with a trimmed audio grade Nichicon FG. I've noted across most major brands of the last few decades that axial tend to perform poorly compared to a radial and usually exhibit some form of excessive wear. In most cases the foot print only allows for a axial as you don't want much lead spacing exposed, but in this case it work perfectly.  Another update was to the complimentary differential pairs, vs a standard a/ab operation where we see a set of differential pairs, complimentary pairs are typically utilized in Class A operation due to the ON-ON type operation of the circuit. Standard KSC1845/KSA992s are damn near a perfect replacement, watch the pinout VS the original EBC.

Their is very little information available online, and the customer was incredibly useful in tracking done technical data, one leg that had to be worked out was a biasing issue.The threshold for the driver stage  is an impressive 550mV+ then you can achieve draw across the emitter resistor sets. Some models may not have a 2K potentiometer installed apparently. 

After figuring the Biasing issue and new components the floor was dropped considerably, audible distortion eliminated and a nice little amp was left.

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