Hi Jeff, I do not have a 5000, but it sounds as if you can set the bias on the PA transistors with software. If that is the case, have you checked that the quiescent current is really what it is supposed to be for the transistors that are in the PA. If you have more distortion at lower drive levels, it sounds as if it could be cross over distortion which would show up more with less drive. That is corrected by biasing each transistor further into class A from class B as you probably already know. Since the problem seems to occur only on 5000's that were made before a certain date, maybe it is in the bias measuring circuitry.
73 Tom W0IVJ Jeff Anderson wrote: >Hi Brian, > >I've run THD (on the analog signals) and IMD (on the RF signals). What >I hear correlates strongly with IMD levels at lower powers (where much >of our voice energy is) when compared between 5K driver, 5K final, and a >1k. Correlation does not mean causality, but...no one has yet come up >with a better idea. > >Other notes... > >I recalibrated the bias and power (default bias seems to be 1.0A for the >drivers and 2.0A for the finals). I can't tell if there's a change in >distortion or not, but IMD did not change appreciably (a dB or so). >Will listen again tomorrow - my ears are tired. > >Also, I tried 10-band EQ settings from Dale. No joy. > >Does anyone who is familiar with the 5K circuitry (and understands PA >design - this isn't my field) have any ideas for improving the linearity >of the PA final stage? (Apart from adjusting bias, that is.) This >would be a great experiment to try. > >- Jeff > >Brian Lloyd wrote: > > >>On Jul 8, 2008, at 12:18 PM, K6JEK wrote: >> >> >> >> >>>It not just Jeff. I'm one of the guys who've been listening to the >>>various audio tests. We first noticed something funny in someone >>>else's 5000 before Jeff even got his. We ran experiment after >>>experiment. This guy finally returned his for a refund. Then another >>>fellow (not in the local rag chew group) broke in with his 5000 to >>>say he's just waiting for someone to figure this out. His 5000 had >>>the same problem. We've been listening and recording with a variety >>>of equipment ranging from 1000's to direct conversion home brews. >>> >>>It is subtle. It might be rare. But it is not unique to JCA >>> >>>I think Jeff's nailed it. The audio up through the driver stage is >>>perfect. It gets funky in the final. Why and what to do about it is >>>another matter. >>> >>> >>> >>Has anyone run single-tone and two-tone tests and looked at the >>distortion products? (THD and IMD). If you can hear it you can >>probably measure it. The spectrum will tell you something about what >>might be causing it. >> >>Oh, has anyone looked at the gain of the PA? If the PA uses feedback >>to linearize it, there may be a problem with the feedback loop and the >>PA may be running open-loop. >> >>-- >> >>73 de Brian, WB6RQN >>Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com >> >> >> >> >>_______________________________________________ >>FlexRadio Systems Mailing List >>FlexRadio@flex-radio.biz >>http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >>Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >>Knowledge Base: http://kb.flex-radio.com/ Homepage: >>http://www.flex-radio.com/ >> >> >> >> >> > >_______________________________________________ >FlexRadio Systems Mailing List >FlexRadio@flex-radio.biz >http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz >Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ >Knowledge Base: http://kb.flex-radio.com/ Homepage: http://www.flex-radio.com/ > > > > > > _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kb.flex-radio.com/ Homepage: http://www.flex-radio.com/
