KPA500 audio muffled?

[ke9uw]

I have gotten a few reports of sounding like my audio is muffled as compared to running the K3 alone. It is true that with several listeners in different states, there is not a consensus of opinion on that. Some of the guys think it sounds just fine, and some say they can hear a slight muffled sound with the KPA turned on. The point of comparing it to K3 only and K3 with KPA500 is that the mic is not in the equation. The report is clearly that the amp sounds muffled. Not unreadable, just slightly muffled.
I have a bias current of 0.4A...about the only thing I can think of to check. I think there was some discussion of setting the bias current here, but I can't find it again. What is the range of bias current for best linearity? I don't see any help in the manual.

[ke9uw]

Oh and my HV starts out at about 81 vdc and drops to about 63 vdc at 400 watts out...that is about where I run it with vox ssb.
I did just change to 240vac input which results in 81 vdc and drops to 70 vdc at 400 watts, but I haven't gotten any reviews on that...just did it to see. Bias still at about 0.4 A.

[Eric Swartz - WA6HHQ]

Both of these Voltage changes between RX and TX under load are normal.

73, Eric

---
www.elecraft.com


On 11/15/2011 8:00 AM, ke9uw wrote:

> Oh and my HV starts out at about 81 vdc and drops to about 63 vdc at 400
> watts out...that is about where I run it with vox ssb.
> I did just change to 240vac input which results in 81 vdc and drops to 70
> vdc at 400 watts, but I haven't gotten any reviews on that...just did it to
> see. Bias still at about 0.4 A.
>
> -----
> Chuck, KE9UW
> --
>

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[ke9uw]

Thanks Eric on the voltage being normal. I thought I'd change things to possibly get some more data on what it is not, and at least keep the pilot lights on the 120vac strip from blinking slightly.
 There was also a comment on my email (but hasn't appeared here) that I should look into RF getting into my mic input. I am doing that and I know that I have some RF floating around here having gotten into the pc monitor, etc. I like that as a possibility although these critics I mentioned are very good at recognizing RF in the audio. But maybe just a tad can produce the results they seem to have noticed. And the correlation with higher power is obvious.
 I think there is also some "transistors just don't sound good...get yourself a four by one and do some real QRO". I am aware of that bias as well.
 I really like this KPA500 amp.