Pro Audio Design Forum

I suspect the Beta mismatch to make trouble of the same kind and amplitude as the Vbe mismatch. This has to do, about how much the servo will have to compensate to balance the circuit. I am using a similar design in mike preamps using two LM394 where balancing is done with a trimmer. They work perfe...

I think Hfe un balance should be considered as well as Vbe un balance. Assuming ZTX851 Hfe=150 5% accuracy, with R4=R5=499 and Ic=5.5mA: Delta Vb=499x( 5.5/150)x0.05 mV. That is about 1mV Delta Vb, to compare to a lower than 1mV Delta Vbe achieved by matching. Who knows ZTX851 Hfe tempco ? About sma...

Thanks for your welcome and fast answer. I forgot to mention the preamps I worked on use two LM394 cross parallelled. I am impressed by the ZTX851 and the most excellent work you made from it. The noise figure is great. I see matching can be better than the 500uV I figured from reading earlier posts...

I am very interested in this thread. I built a mic preamplifier using 4 modules based on the Grahem Cohen double balanced design. I am so found of this design, I wish to build more, but the major trouble is the availability of low noise matched bjts. I found this: AS194 AS394 Matched NPN transistor ...