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Old 10th March 2013, 03:18 AM   #11
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If you have earths at the TT and the amplifier you could be getting a ground loop.
Try disconnecting one or the other.
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Old 16th March 2013, 04:04 AM   #12
Carlp is offline Carlp  United States
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I rebuilt this PS switching to full bridge rectification (giving me 34v to start) into the first two caps of the original, and adding a LM317 set to pass 24v or so. Final cap was 22uF followed by 5R then into 3300uF final cap. Note that the jfet amp board has some PS decoupling on-board.

Hum is a bit reduced but still there. Then I went back to battery supply (two 9v in series), and lo and behold, the hum is still there at pretty close to the same level. So at this point the PS isn't adding any hum, but I still have hum!

The phono stage using the 9v has no AC, but the TT of course does. (Again, the amp is pretty quiet). Could the TT be passing AC on to the phono stage? Even without the TT turned on? The TT is Luxman PD-277, which has a power switch and a tonearm switch. Platter starts to turn when tonearm is moved to platter. Hum exists even when power to the TT is turned off, but it's louder when turned on, and even louder still as the tonearm moves toward the center and the platter motor.

Tomorrow I'll try disconnecting the TT leads, and if I can rig something up, try shorting RCA plugs on the inputs to see what that does.

I'm guessing this hum problem is bigger than just the PS...
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Old 16th March 2013, 04:29 AM   #13
agdr is offline agdr  United States
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Quote:
Originally Posted by Carlp View Post
Then I went back to battery supply (two 9v in series), and lo and behold, the hum is still there at pretty close to the same level. So at this point the PS isn't adding any hum, but I still have hum!

Tomorrow I'll try disconnecting the TT leads, and if I can rig something up, try shorting RCA plugs on the inputs to see what that does.
You do need to short the inputs to the jfet pre-amp to ground and see if you still have hum. Most likely the hum is being picked up on the input since you heard it while on batteries. You should hear no hum at all with the inputs grounded.

The input wiring going from your turn table to your jfet amp should be twisted, each signal input wire and a ground wire twisted together at 3 turns per inch or tighter.

Last edited by agdr; 16th March 2013 at 04:37 AM.
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Old 16th March 2013, 10:34 AM   #14
AndrewT is offline AndrewT  Scotland
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Moving magnet and moving coil pickups both depend on coils.
Those coils by design are sensitive to receiving magnetic signals.
Good design of the magnetic circuit around and in the pickup would I think be part of what rejects external hum fields interfering with the wanted signal.
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Old 16th March 2013, 02:36 PM   #15
Carlp is offline Carlp  United States
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Thanks, AGDR. I'll try to rig up something to test shorting the inputs soon. Is it OK just to jump a wire across the RCA connectors for each input? As for the wiring, someday I may replace it. What's there now is stock, and nothing special. But I'm just finishing up re-wiring another TT (Thorens TD-165) so it'll have to wait a little on this one.

AndrewT, can you elaborate? In particular, given the jfet circuit and board I'm using (you can see the circuit here: Boozhound Laboratories: JFET Phono Preamp Kit - similar to the Salas jfet phono here on DIYAuduo) what kind of circuit design could I use to help deal with the hum? An Aikido style approach where you inject inverse noise to counteract the noise? (That's beyond my skills to design.) Something else?

I agree with agdr that it seems likely it is being picked up on the input side, and clearly the AT440MLa is picking up AC hum from the motor as the hum increases as it moves toward the center of the LP (which is closer to the motor). I'll report back after shorting the inputs, and I need to look at the wiring of the TT and check grounding there.
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Old 16th March 2013, 02:55 PM   #16
agdr is offline agdr  United States
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Quote:
Originally Posted by Carlp View Post
Is it OK just to jump a wire across the RCA connectors for each input?
That link is very helpful to see what you are building. Yes, you can just jump across the inputs, ground to signal on each. If you are using RCA shorting plugs, just push them on. If you a are using a wire it would be a good idea to connect the ground end first since those jfet inputs will be pretty static sensitive.

A photo of your setup that shows the board-to-jack and jack-to-turntable wiring would be really helpful.

Last edited by agdr; 16th March 2013 at 03:00 PM.
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Old 16th March 2013, 03:01 PM   #17
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I would check your 0V / GND cabling. If the gauge is too thin you might get ground pick-up. I appreciate that the amplifier only uses milliamps but try using 10 or 15A hook-up wire for the ground connections.
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Old 16th March 2013, 04:06 PM   #18
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
........... magnetic circuit around and in the pickup.............
Quote:
Originally Posted by Carlp View Post
..............the jfet circuit and board I'm using..............
Not the pre-amp. Look at the pickup.
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Old 16th March 2013, 07:38 PM   #19
Carlp is offline Carlp  United States
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Thanks for all the input.

OK, here's my update: Shorted or no inputs has NO buzz or hum. I also removed the ground wire and the hum was now a buzz. Bot seemed the same frequency, but with inputs it's a definite kinda muffled hum and with inputs but no ground it's a definite sharper buzz. A very different sound.

I'm in the midst of boiling maple sap for syrup but will try to grab a photo of the breadboarded PS. But no fair ribbing me about the rat's nest.

Edit:
Oh, and AndrewT, I figured you meant the cartridge, but thought maybe the circuit would help enlighten a solution. Short of trying things with grounding of the TT, I'm not sure what to do next. I won't be toying with the cartridge coils b/c I'd be sure to fork it up. That leaves "the magnetic circuit around...the pickup." that's the part I'm wondering about. I've not read anything about this and don't understand enough about coils, right-hand rules and magnetism (and all the fields dancing around the space near the pickup) to really work this out on my own.

Last edited by Carlp; 16th March 2013 at 07:47 PM.
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Old 16th March 2013, 11:56 PM   #20
agdr is offline agdr  United States
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Quote:
Originally Posted by Carlp View Post
Shorted or no inputs has NO buzz or hum.
OK, so that narrows it down to hum coming in through the input wires. Either powerline hum (fluorescent lights, etc.) picked up by the wiring itself, or as AndrewT says magnetic fields by a moving coil type of pickup.

If the wire, things now get interesting on grounding. Hum pickup is the same in both input wires, so if twisted it is up to the amp to cancel those common signals out ("common mode signals"). But your single-ended (single power supply, V+ and ground) amplifier design won't do that. It would take a differential input using +/- supplies. So with a single ended amplifier shielded mini coaxial cable is usually used instead, with the shield grounded, to keep the noise out. That only works for the distance from the RCA jack out to the tonearm though, where eventually the mini coax is too stiff and you have to go back to twisted pair, but then that is inside a metal (grounded) tonearm.

I just did an search and came up with a picture of the typical thing:

https://home.comcast.net/~tubes/Audi...Bal_Wiring.png

From here:

Turntable Forum • TD-160 wiring question

4th post down. The second link actually has two ways to do it. The top picture is especially slick if you have coax that has two conductors in the middle. You connect the shield to the chassis on the phono and chassis on the amp, but the amp ground goes to one internal wire and the signal line to the other.

And that maple syrup sounds wonderful! I'm a maple syrup fan, but only had the store bought variety. Mmmm.... fresh off the tree.

Last edited by agdr; 17th March 2013 at 12:02 AM.
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