Turntable Preamplifier Noise Problem

[fazildiken]

I built Doug Self's MM/Mc turntable preamp. This circuit is the Elektor premaplifier 2012 circuit, but there is an increasing noise parallel to the volume pot. As I open the volume potentiometer, the noise increases. I took the power supply completely out of the box and the problem has not improved. I replaced the signal cables with shielded coaxial cable and the problem persists. I am trying it as MM only. I have attached pictures and a video of the noise. Please help.

 

[Freecrowder]

Need schematic as built. Have you tried moving the whole box away from all the other equipment. And you have tried it with the lid on correct? Have you disconnected the phone cart and connect shorting plugs does it go away?

 

[fazildiken]

First of all, thank you very much for your help. I closed the top cover. I moved it away from other equipment. I made the gain +10db. The noise is almost non-existent. There is only a hisssssss sound. I have to take it next to other devices. How can I solve this problem?

 

[rayma]

Is the noise gone if you use the MC/MM switch set to MM, which bypasses the first flat gain stage?
On MC you will get more noise, but on MM (with the MM turntable plugged in),
there should be little audible "white" noise at normal volume control settings.

Of course, with the volume at maximum, you will hear some noise, which is normal.
Similar levels of "white" noise in both channels would indicate normal performance.
Additional noise volume heard with the inputs unconnected is due to the abnormal operating condition.

Hum (50/60 Hz) and/or buzz (100/120 Hz) would be a different matter, due to construction or external pickup.

 

[Freecrowder]

Well everything you said above sounds like a normal phono preamp operation. A hiss sound at full volume is normal. If everything seems normal away from other equipment then some level of isolation away from other equipment is also normal. Having to strategically place a phono preamp away from noise sources is very common with any phono pre. Mine for instance sits on a shelf away from other equipment especially other equipment with big transformers. So you will have to experiment with placement and maybe use a little longer interconnect to hook up with.

 

[stocktrader200]

If your noise level is average, turning the volume to where you can hear noise results in 10w or so of output when the record is playing. @92db/w Speakers
Mine sits around 84db s/n With single NJR2068.

 

[Mark'51]

Your video clip's sound track sounds like you have power line noise coming in from somewhere. Your observation that it disappears when you move your preamp away from other devices is a good starting point for solving the problem. I would determine WHICH device is injecting that noise. Can it be moved further away w/o causing a problem? What about things like power cords, can the noise be reduced by moving them around? Routing signal lines parallel to power cords can be an issue, see if separating them has an effect.

You have a starting point, now you need to zero into the actual nature of the problem.

 

[volavola]

Just for a quick test, you could remove the ground yellow/green rope from the chassis.

 

[fazildiken]

1-I connected the whole box body to the psu gnd and made a faraday cage.it's not fixed.
2-I took the gnds of the RCAs to free. I made the gnd connection through the box.it's not fixed.
3- I connected the turntable GND directly with the preamp gnd.it's not fixed.
4-I kept the signal cables away from each other.it's not fixed.
5-I connected a 10r 1w resistor and a 100nf 250-400v capacitor in parallel and connected one end to one of the screws in the box and the other end to the psu gnd.it's not fixed.
6-I disconnected the mains ground line. It didn't work.it's not fixed.
7-I connected only the working system to the mains and unplugged all other devices.it's not fixed.

• I couldn't find a solution. The only option left is to remove the preamp.
• Thank you very much to everyone for their help.

 

[Drone R2D2]

Hello, friends,

Just wanted to share one possible way to reduce noise — maybe it’ll be helpful to someone.

The issue might be related to the way the "cooled resistance" at the MM input is working.
Also, there's a small inaccuracy in Douglas Self's circuit, around IC4 and IC8.
A good fix could be to try op-amps with J-FET inputs in that part of the circuit — they tend to handle this kind of task more quietly.

 

[fazildiken]

Drone R2D2​

Would you advise me to install opamps instead of IC4 and IC8.​

 

[Drone R2D2]

I don't usually work with chips myself, so it’s hard for me to give specific recommendations.
But try using whatever op-amps you have at hand — it might help confirm whether you're on the right track.
Even something as simple as a TL072 shows a reduction in noise in Micro-Cap 12 simulations.

 

[fazildiken]

I have TL072 and Opa2134, I will try them and write the result. Also artificial intelligence advises me to suppress rf signals by installing a 220pf capacitor between the input signal and gnd.

 

[Drone R2D2]

Also, keep in mind: adding a capacitor at the input will affect the frequency response due to interaction with the cartridge.
In most cases, it’s probably unnecessary — the input cable already adds about the same amount of capacitance.