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In Reply to: RE: Where from that hum?? posted by vinnie2 on September 11, 2016 at 14:13:25
Should have thought of this a long time ago.... I hooked my cd player directly to the new amp (took the preamp out of the picture) and low and behold the hum went much lower. That means I need to dive into my preamp because it isn't as quiet as I thought it was. I have dc on the 26 filaments of the preamp, but I might need to take another look at my filtering for the dc.
At least I know what the main problem is now. I will still have to do a little tweaking on the 845 too, but I will plus up the preamp first. Thanks for all the help folks, I appreciate it.
Edits: 09/14/16Follow Ups:
A spectrum analyzer will show 60 cycle (AC hum) and 120 cycle (PS ripple) in relation to a typical test signal ( usually a 1KHz sine ).
I playing around with my RH test amp last night and noted that the 60 cycle hum was down about 75dB or so from the test signal.
120 cycle ripple would go down by adding more PS filtering.
It is easier to see if a circuit change is meaningful or not with something visual to look at like a graph in real time.
That would be great if I had one. Expensive piece of equipment I believe.
If you have a PC and a decent 192KHz soundcard, you have a basic audio measurement system.
There are freeware programs like ARTA or others online.
Search the archived posts for what others have already done.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
I think for now I will just use my on line tone generator to determine if it is 60 or 120. Much simpler and I already have everything I need. Thanks just he same guys.
A DC filament supply filter that is cap input will have some nasty looking ripple that sometimes can be heard even if the ripple level is low.A choke input supply filter for the DC filament supply will have only sine wave ripple that is not as easily heard.
Because the current draw is high and the voltage is low the needed Henry value of the input choke is low and won't break the bank.
Calculation for critical inductance Henry value for a input choke.
((voltage/current in amps) + DCR of choke) / 1000 = choke value in Henry
For my 300b I use the CH-2. It's 70mHy and that's overkill.
For a 26 critical inductance is going to be about 5mHy or something like that. Of course the supply voltage will have to be high to start with.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/14/16
Thanks Tre'. I will keep that in mind as i look for a solution.
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