Phono Preamp Noise - At Wits End

Discussion in 'Audio Hardware' started by VictoryHighway, Apr 15, 2019.

  1. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    Hello All,
    I need some advice. A while back (last August), I decided that I wanted to dip my toes into the world of vacuum tubes and bought myself a Hagerman Labs Cornet 3 phono preamp. I've been using it since and I've always noticed some hum when I've been playing records. My setup is as follows:

    Music Hall Ikura Turntable ---> Cornet 3 Preamp ---> Marantz 2215B Receiver ---> Speakers

    My equipment is on a vertical AV stand, with the turntable on the top rung, an SACD player and the phono preamp of the second rung, the receiver on the third rung and some cassette storage cases on the fourth (and bottom-most) rung.

    NOTE: The turntable's ground connection is wired to the preamp's ground terminal. The turntable and preamp are both DC powered (via wall warts).

    Recently, I've noticed that the hum was really only a problem with the right channel (there is some hum on the left channel, but it is barely noticeable). I did some troubleshooting, and discovered that the hum followed the right channel output of the preamp, regardless of what cables I tried.

    So, I decided to take apart the preamp, and I noticed that there are two large coils near the power input. Adjacent to the coils are two 470nF capacitors. The circuit board is mostly symmetrical. On the left side (which corresponds to the left channel), there is a copper foil shield between the coil the two capacitors. The right channel did not have a similar copper foil shield. I figured that this must be the problem since I know that coils are normally noisy.

    So, I went to a "nearby" electronics store (You-Do-It Electronics in Needham, MA) and bought some adhesive copper foil. When I got home, I cut the foil and soldered a small lead to connect the foil shield to ground. I was convinced that this was going to fix my problem. I reconnected the turntable to the preamp and the preamp to the receiver and powered the system up. Unfortunately, this did not fix the problem or even reduce the noise on the right channel.

    So, now I am in a state where I don't know what to do next (and as the thread title mentions, I'm very frustrated). Should I wire the turntable ground to both the preamp and the receiver? Should I try wiring the turntable ground to the AC service ground? I noticed that the RG and LG terminals of the phono cartridge are not connected to the turntable's ground terminal. Should they be?

    Does anyone have any advice?

    Thanks in advance.
     
  2. jbmcb

    jbmcb Forum Resident

    Location:
    Troy, MI, USA
    Do you get hum when the turntable is not plugged into the phono stage? Do you have access to a multimeter with RMS?
     
  3. Strat-Mangler

    Strat-Mangler Forum Resident

    Location:
    Toronto
    Did you try a cheater plug (3-prong to 2-prong converter) on your phono preamp?
     
  4. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    There is hum on both channels when the turntable is not connected to the preamp. Yes, I have a digital multimeter at home. I also have a digital oscilloscope.
     
  5. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    That is N/A in this case, since the wall warts for the turntable and preamp are two-prong plugs.
     
  6. Davey

    Davey very clever with maracas

    Location:
    SF Bay Area, USA
    The coils are part of a switching power convertor to bump the incoming 9VDC to 210VDC for the tube power supply. There's not really any low frequency AC in the chassis, and the coils are both in the single supply, no separate left and right. It could be one of the phono cables too close to AC power sources, or not enough shielding in your phono cables, or something else in the grounding. You could always swap cartridge leads and make sure it stays in the same channel to rule out the cartridge, just go one step at a time and narrow it down..
     
    vconsumer and SandAndGlass like this.
  7. Strat-Mangler

    Strat-Mangler Forum Resident

    Location:
    Toronto
    Are they plugged straight into the wall socket or into a power bar?

    Plug them into a power bar (3-prong) which would introduce ground.
     
  8. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    My whole audio setup is plugged into a multi-outlet power strip, which is plugged into a three-prong wall outlet. The preamp wall wart does not have a three-prong plug. Come to think of it, none of my audio equipment have three-prong plugs.
     
  9. JohnO

    JohnO Forum Resident

    Location:
    Washington, DC
    Try the time honored method, to try to reduce tube equipment hum, of rotating the AC plugs 180 degrees into the AC socket for each of the preamp and the Marantz. You have four combinations to try. Eight combinations if you then do the same for the TT.
     
  10. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    I can certainly try that. I’m not sure if it will help, but it certainly can’t hurt.
     
  11. jbmcb

    jbmcb Forum Resident

    Location:
    Troy, MI, USA
    Well that shouldn't be happening.

    OK, check to see if there is any voltage between the chassis and the shield on the RCA outputs of the preamp. Also check the ground lug for the turntable against the shields on the RCA inputs. There shouldn't be any voltage - if there is something is leaking, you could have a bad cap or bridged solder joint somewhere.
     
    TarnishedEars likes this.
  12. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    Well, the preamp is in a plastic enclosure, so there's no shielding there. Maybe I should wrap it in aluminum foil? Anyway, I performed the voltage measurements, and all I got was in the order of microvolts, so that doesn't seem to be a problem. I also did a continuity check between the ground terminal of the turntable and the ground terminal of the preamp as well as between the ground and the shields of all the RCA connectors, and that seems OK also.
     
  13. macster

    macster Forum Resident

    Location:
    San Diego, Ca. USA
    Contact Hagerman or check here and try some of the solutions listed.

    M~
     
    TarnishedEars likes this.
  14. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    Seattle, WA
    The only things that I can think of to try to verify that it doesn't have any DC offset on its outputs, and to make sure that this unit is located far away from the power Transformer inside of your receiver.
     
  15. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    That's what I saw first when looking for a schematic of this. Corry's copper slug and snail tape to the rescue.

    I would additionally extend the ground wire from the preamp to a ground lug on the receiver.

    This phono preamp has three tube stages, a lot of gain via tubes. The first two 12AX7 tubes can be swapped to see if a noisy tube moved away from the input can help, and one could even move the last 12AX7 to one of the earlier positions, which shouldn't upset the RIAA EQ or total gain much.

    I didn't quickly discover if the heater voltage is AC, or well-filtered DC, another way noise can enter tubes.

    Unfortunately, tubes themselves can be noisy randomly, one of the things that organ pulls are good for - the Hammond or Baldwin 12AU7s in plentiful supply were only the lowest noise tubes, prescreened by the organ manufacturer. This would more likely be hiss noise rather than hum noise.
     
    Last edited: Apr 16, 2019
  16. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    I've just connected the turntable ground, preamp ground, and the receiver chassis ground together and then bonded that whole stack to a nearby baseboard (we have forced hot water heating in the house). That seems to have dramatically reduced the noise. There still is a slight 60Hz hum, but it's probably the best that it's going to get for now. I might swap around the receiver to the bottom-most shelf on the A/V rack, so that it will be furthest from the turntable and preamp and redress the interconnects, but that will have to be another day.
     
    Strat-Mangler likes this.
  17. patient_ot

    patient_ot Forum Resident

    Location:
    USA
    Sounds frustrating, makes me glad I don't mess with tubes. Hope you get it sorted out.
     
  18. qwerty

    qwerty A resident of the SH_Forums.

    Good that the noise is reduced.
    Given the hum is in one channel, have you tried swapping the l+r tubes around to check if one of the tubes is contributing the hum?
     
  19. harby

    harby Forum Resident

    Location:
    Portland, OR, USA
    12Ax7 type tubes are two-tubes-in-one. They are used here so left and right are both amplified by the same tube enclosure.

    Above I meant to say "the last 12AU7", which is a lower gain 12AX7, used in last of three gain stages.
     
  20. mkane

    mkane Musics a Passion

    Location:
    Cloverdale,CA
    move the pre
     
  21. gguy

    gguy Well-Known Member

    Location:
    Wildomar, CA
    Does the noise increase when the volume goes up? If not, you May have a bad tube.

    If it’s not tube related, one thing I did to kill the ground hum with wall warts is run additional ground wire off of the preamp lug to a grounding plug in the wall outlet that the wall wart is hooked up to. This dropped my noise floor considerably.
     
  22. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    Well, last night I finally decided to tear down my system and rearrange the components and re-wire everything (well, almost everything). I've laid out the system so that the turntable is on the top shelf (kind of has to stay there), the preamp and a couple of cassette storage units on the second shelf from the top, the SACD player and cassette deck on the third shelf and the receiver on the bottom shelf. All component chassis are grounded together with the ground ultimately tied to the baseboard. There is still a slight 60Hz hum, but I'm basically calling it case closed now. I suppose I could go crazy and spend a lot of money to upgrade all of the interconnects or go down the power conditioning route, but I think that's pretty much overkill right now.
     
  23. mkane

    mkane Musics a Passion

    Location:
    Cloverdale,CA
    Move it further still, if possible. I have 3 tube phono pre's and all virtually silent.
     
  24. VictoryHighway

    VictoryHighway Well-Known Member Thread Starter

    Location:
    Hopedale, MA
    I'm not really sure where I could move it to that would be a better location. It's already at the best location on the A/V rack, away from all other active electronics.
     
  25. Stunsworth

    Stunsworth Forum Resident

    Location:
    Uk
    I recently bought a Chinese copy of the EAR834P and it had severe hum when it arrived (coincidentally from the right channel). This unit doesn’t use a wall wart there’s a mains input.

    I sent the amp to a tech here in the UK and the only way he could remove the hum was to put the power supply in a different box and connect the two with leads for high and low voltages (plus earth). However in your situation I’d be contacting the manufacturer directly and asking them for their advice.
     
    Thorensman likes this.

Share This Page