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Old 1st April 2014, 07:53 PM   #11
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Join Date: Dec 2005
Location: Loganville, GA
OK all,
This is the current state of my pre-amp. Now that I know it will work to my satisfaction, I will rebuild it to make it look more presentable. I may even try making a PC board version. I took the 6CA4 out and put a pair of 1N4007's in.
The background hum did not change. I then put the 100 ohm resistors on the heater lines. That made a major improvement. I took the 1N4007's out and put the 6CA4 back in, it was still quiet. Just as quiet as with the 1N4007's. I tried putting the heaters on a DC power supply, the hum was way too high so I went back to AC on the heaters.

I WELCOME all comments as to component values and mods to this circuit.

Thanks to all
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File Type: jpg pre-2.JPG (178.7 KB, 108 views)
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Old 1st April 2014, 09:12 PM   #12
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You aren't using distributed star grounding from the photo - that should be first and foremost the main change to make.

The spigot of each 12AX7 should be made as the local star ground.

You should use a tuned humdinger, with a 100 to 200 ohm pot, instead of a fixed humdinger.

Pedantically, you should have a separate twisted pair heater cable to each 12AX7.

Are you using a shorting RCA plug on each channel when you check for hum level?

Last edited by trobbins; 1st April 2014 at 09:15 PM.
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Old 1st April 2014, 11:42 PM   #13
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Your heater wire should be laying flat against the chassis, not flying in the air above the components, sockets and other wiring. Have you had a look at the sticky at the top of this forum?
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Old 2nd April 2014, 12:39 PM   #14
DF96 is offline DF96  England
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Yes, you won't want to hear this now, but AC heater wiring should be the first to go in as near to the chassis as possible. Then signal wiring can be left floating in the air.

All cables carrying AC should be at least loosely twisted with a cable carrying the same current the other way. You appear to have no twisting on the heater secondary or the mains primary.

The loop containing charging pulses should also be kept as small as possible. The sort of wiring you have done could be got away with in a power amp or a radio, but not a phono preamp.
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Old 2nd April 2014, 02:11 PM   #15
MelB is offline MelB  Canada
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The shielding on your input leads should be grounded at one end only.
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Old 7th April 2014, 11:19 PM   #16
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Join Date: Dec 2005
Location: Loganville, GA
Hello everyone,
I think I have all the bugs worked out.
I rewired the heaters for 12 vdc and laid the twisted pair next to the chassis.
The amps current state is such a mess (from all the mods) that I will rebuild it completely. At that time I will take care to get better lead dress and proper star grounding.

Thank you for all your help and suggestions. This was truly a learning process.

The attachment shows the noise level when I lift the cartridge off the record.
I know, it's not a sophisticated test but I should do for now.

Oh yea, it sounds very nice.
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