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Old 22nd February 2008, 01:46 PM   #1
smbrown is offline smbrown  United States
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Default Advice on hum in phono stage

So I completed building an "Octal Cornet". Sounds very nice, everything I'd hoped for in the project. I'm using it into an existing line stage. I am experiencing some low level hum and I'm looking for advice on eliminating it.

In my build I added a choke, which is not in the original design (10H 65Ma) it comes right after the first PS cap and is on the far side of a 17" wide aluminum chassis from the phono section, as is the power transformer, much greater spacing that you'd find in a normal Cornet. So I don't think it's from that source. Also, I don't have a scope, but it sounds like 120hz, not 60hz.

The heater supply is built with the 40,000 uf of cap as Hagerman shows in his design, and all those ground leads are collected at one point and tied to the PS ground, also collectd at one point. That one point is then grounded to the banana jack for turntable ground, which is immediately next to the RCAs for input. This jack is solidly connected to the chassis.

All of the signal grounds are collected along a 16ga copper ground bus, which terminates at the same ground lug next to the RCA inputs. One other slight variation I made from Hagerman's design is to use 6.8uf Obligatos (one per channel) as the last PS decoupling cap, and those are located away from the PS, in the audio portion of the chassis and grounded to the signal bus. I did try adding capacitance and it didn't help.

I have tried this with the 120v AC input grounded to the chassis and not grounded, didn't seem to make a difference.

I have isolated the problem to the phono amp because the line amp is dead quiet on any other input. I have tried using in stepup transformer prior to the phono stage to reduce the gain needed from the phono amp, didn't make a difference. By the way, the hum gets louder as the volume is increased and is about the same in both channels. The last change I made to the Hagerman design is that I located a toggle switch at the input to allow me to select two different loading resistors, 47k and 100k. The position of the switch does not seem to have any inpact on the hum.

Appreciate any helpful ideas you all might have, thanks!
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Old 22nd February 2008, 02:57 PM   #2
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What i would try:

- does the hum vary depending on whether the input is connected to the TT or shorted? Surprisingly often the reason for hum is not the phono pre but the interaction with the TT.

- does the hum vanish if you suddenly remove ac power? The preamp will continue working for a few seconds, while the caps are charged and the tubes hot, but is there hum?

- this topology (absence of overall NFB) makes it easier to find out which stage contributes the most to the hum. You can either remove tubes, or diconnect caps between the stages
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Old 4th November 2009, 06:46 AM   #3
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Default Any updates?

Did you manage to figure out what your hum problem is?

I've got exactly the same issue with my octal cornet, down to the fact that it sounds and scopes like 120Hz hum. I'm stuck trying to figure out what the next step is as well.
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