Turntable Hum - I know, I know, there's 1000 threads but...

Discussion in 'Audio Hardware' started by rob303, Oct 8, 2014.

  1. SandAndGlass

    SandAndGlass Twilight Forum Resident

    If you experience hum and that hum stops any time you touch different parts of the TT, tone arm, metal deck, etc., even though you may and should have your TT grounded, run some additional wire or wires from the point(s) that you touched when the hum abated.

    By this, I actually mean, properly secure your TT and invert it, giving you access to the underside and attach spade connectors to a piece of wire and loosen a screw on the underside of the TT, slup the spade connector in and re-tighten the screw(s).

    Run this second or third wire to the grounding post of your phono preamp and that should eliminate any hum that might have been originating from the TT.

    Sometimes people use carbon fiber tone arms, as specially longer arms, and the wires that are contained inside of the tonearm are not insulated, may pick-up RF or electrical magnetic fields, because the carbon tone arms are transparent to these forces and do not provide the shielding that a metal tone are would have provided.

    Secondly, some areas or installs may be more subjected to these forces than others are, or some interconnect wire runs may be longer than others and you are using RCA connectors (not balanced cables), you should look at double shielded cable, which you can obtain at blue jean cables.
     
  2. Macman007

    Macman007 Forum Resident

    I have 2 of the Muffsy Phono Preamps I built as a kit then installed each in a project box. For those who don't know, it is based on the well regarded CNC phono stage. You can vary the load and gain for the cartridge you are running as well as capacitance. The unit uses DIP switches for gain and load, changing components allows you to raise or lower the window of adjustment, if needed. I've installed small post connectors recommended in the capacitor locations, allowing me to quickly change capacitance without having to power up the solder station. Once you settle on proper capacitance load required (usually between 50pf and 400pf total) for cartridge and cable combination you are done. Though initially designed for higher output MM and MC carts, there is another kit available to build a SUT for LOMC 's as well.

    My hum issue began with the first phono stage. After many hours of checking and rechecking the unit, all my components, cables and where they run, eliminating the house circuits, moving components, adding grounds, powering up from scratch, I still had a 60hz hum on the left channel. Both turntables make the same noise no matter what carts are installed and all wiring and grounds to the turntable and arm are verified OK. Cable routing makes no difference, nor does, component location, switching cables etc. When swapping the left channel output from the phono stage to the right and right to left, the noise follows to the right channel. Swapping phono cables right to left makes no difference. The hum is not there when I install phono shorting pins in the turntable input connectors and plug the phono pre out to the preamp. This is the only time the unit is dead silent with my preamps volume pot on 99% running MC 501 mono amps. It is dead quiet.

    The only thing I have not tried is different wal warts, the unit each use a 120vac to 15vac step down transformer. Swapping the unpolarised plug around makes no difference. I am to the point where I may buy another wall-wart to see if that is the issue, although I'm not hopeful.

    The units once broken in sound really good with the Op-amps I chose for them. Both units have over 100 hours on them so they are broken in. Both behave identically the second unit is updated with more grounding internal and different component and cable management so this type of issue should not exist. The right channel is dead quiet with my preamp gain up to 99%, but the left channel starts to get louder around 70% I cannot hear the hum when either unit or turntable is playing. I've had all my turntables hooked up tried different interconnects, no dice.

    The whole thing is more of an obsession with me, I never have the volume levels up high enough where you hear it without a record playing. With a record playing, you can't hear it even past 70%, at which point things start to fall off walls in other rooms and my wife freaks out on me. I have a C2500 tube pre as well and you can hear hum on both channels, especially when set on Phono and really cranked up with nothing playing. It is typical tube noise that is never there with music playing. Only my Mx135 phono stage is dead quiet until. You get volume up around 85%, then you can hear the cartridge acting like a guitar pickup for room noise etc.
    2 phono stages, identical noise issue totally duplicated in every way. Wall Wart or power supply issues, chances of anything else are remote, but I may try running it with 2 9 volt batteries to verify it's not the power supply /wall wart for each then see if the sound is still the same or gone.

    I suppose it could be a normal phono stage issue, and I'm being totally OCD about this. After all, you can't hear it with a record playing, at insane levels, even recording records to tape. so why am I worrying? IDK, but in my mind, the hum should be on both channels if it's design issues or not at all.

    I could just put a record on before cranking the volume and listen to music instead of obsessing and wasting listening time over 60hz hum you'd never hear under normal operating conditions....

    Thoughts folks?
     
  3. Macman007

    Macman007 Forum Resident

    Back at 'cha..

    After doing some deep thinking and investigation, my only area left to check was the internal cabling bringing the signal from the turntable inputs side of the phono stage. Since the voltage from the turntable varies between 1mv and 5mv approximately, it makes sense that any spurious noise being picked up and amplified with the signal would be between the tonearm interconnect and shielded cable to the audio board. The output side is over 250mv, so just like any regular line level signal, provided the run between output and next component is short, there is significantly less chance of any signal interference. Were talking under a meter altogether, so using XLR cables would be overkill. The cable from the turntable to the phono pre has never given me any issue of this type, and when plugged into other non Muffsy phono stages the hum noise is not there.

    The wire that came with the kits is a lesser expensive shielded variety with 2 internal leads for left and right channel with shield wire wrapped around both jackets with an outer jacket around everything. The run from the left and right single ended connectors to the audio board is well under 2 inches. I measure approximately 2.25 inches overall. There is some extra left over, I didn't want to have the cabling strapping tight with little room to move since we're dealing with very thin wire here. The cable is routed as far away from the power leads of the switch and connect and the 17 vdc wall wart. Moving the cable close does not increase the hum intensity nor does moving it away decrease it. Still this is where the weak link had to be. I desoldered the cable from the board side then decreased the length of the wiring to minimal, where it is not taught with the boards installed in the case and the silk screened connector side of the box installed. The hum had decreased by a factor of at least 1/2. Where before I could plainly hear the hum 5 feet away with the preamp level set to 60-70%, at the same level I needed to put my had up against the speaker to hear it. Well alright, some progress at last!

    Seems like the wiring that comes with the kit while up to the task of transferring normal line level signals back and fore, for the really low level signals on the phono side it is not shielded near enough. Even at minimal length with proper cable management of other wiring to the AC-DC power supply it still picks up and transmits some 60hz hum, though I'd guess many would never hear it. I don't think many people crank 2- 500+watt mono amps with 75% of the preamp gain looking for hum or other noises from their phono stage, unless they are hear noise while music is playing or between tracks. I wasn't, I'm just one those obsessive-compulsive kooks looking to see how quiet my home-built phono stage really is.

    My next move is going to be to pick up some well shielded cable, perhaps something with a drain wire along with the Left-Right-Ground leads in the jacket, then attach it to a point on the aluminum case. Adding a metal terminal to attach it to is as easy as drilling a small hole then soldering the wire to it. Another idea would be to use well shielded power cable instead of plain old insulated copper strand. It might benefit the design overall to design a proto board layout where the main panel audio connections (phono input, output, and ground lug) solder directly to a board behind the case panel, then the board will solder directly to the audio board. This would eliminate the internal audio wiring. I think that would be the smartest way to go. Owners of older versions could upgrade their units if desired. Of course, a better grade of em shielded cabling is cheap, easy and quick. If anyone has signal wire they recommend or other suggestions, please post them.



    This little phono preamp does punch well above it's weight. We hear that a lot, but in this case I find it to be an understatement. I've used many good phono stages before this, internal and external units, and I have to say nothing at this price point sounds better to my ears. For those with moving magnet or HO Moving coil carts, it's just the ticket. The unit is not hard to build at all, it comes with excellent online documentation, parts are clearly labeled, counted and separated. There is even enough solder enclosed to build the unit. There are multiple modifications and upgrades posted with hundreds if not thousands of online pages in the forums dedicated to both the CNC and Muffsy units. For those with low output cartridges, a SUT kit is also available as well as other kits. Component quality is very good, high quality 1% metal film resistors, Nichicon capacitors and more are included. The AC-DC power supply is very robust. Of course you can order the hit without the power supply and wall wart, using 2 - 9 volt batteries to power it instead. Full charge will give you about 40 hours of play time. I was using 9 Volt rechargeables for a while, just remember to turn the unit of when you are done listening, or the batteries may be dead by your next listening session! The aluminum enclosure box and wall wart are available in many places online, I got my wall wart from Jameco Electronics. My black B0905 project enclosure included a sexy silver aluminium end cap which came from a Ebay seller in China ( where they all ship from) cost about 20$ delivered and was here in around 2 weeks, right after the kit arrived. You can get them all black or all silver anodized. There are terminals on the Muffsy power supply board for LED status lamps on and off. I drilled a small hole in the correct area of the silver end cap and installed a Bi-Colored LED (Red and Blue, there are color choices), Red for power off , Blue for power on that made it look an nice as any store bought phono preamp. All the connectors and wiring are on one side, so all you see is the end with the LED, or it's small enough to be kept out of sight.



    For those of you using older units, receivers, integrated amps etcetera equipped with basic built in phono stages, you will really enjoy the sound quality. As I said before, I have 2 of them I built so far. I need to get the second unit its own project box. While it comes with 2 Op-Amps that sound great, you can buy better Op-Amps and change them out in a few moments as the unit includes solder in Op-Amp sockets. I bought 2 Burr Brown units from Mouser and 2 others recommended on the forums. Under 100$ for all 4, and there are many lower cost choices available if you want to roll Op-Amps the way others roll tubes. Plus you get props and bragging rights from other folks when they ask you where you bought that awesome sounding phono preamp. If you are looking for a high quality excellent sounding phono preamp that can grow with you, seriously you can't go wrong. I could probably sell one of mine for 3 times what have invested in it.

    I will post which wiring/cable I end up using and the results. Once over this last small hurdle, there is nothing between me and great sounding music from my Lp's. I use a HOMC Sumiko Blackbird cartridge and the Muffsy really makes it sing. I have other carts from Shure and Audio Technica (AT125LC, AT-LS-500LC , and Shure M91ED with Jico SAS) they all sound excellent with the Muffsy. I can't compare it to the phono stage in my C2500, thats a tube preamp that cost close to 10K. It doesn't sound better or worse than the phono stage it has, just different, that's the only way I can describe it to be honest.
     

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