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I wonder if the stock 24V DC wall wart is a "switching" power supply. These are very common. Have read they generate noise that can actually "backwards" contaminate the house AC. A Linear Power Supply (supposedly an inherently lower noise design) is preferred in some applications. If the low noise AC adapter you got off eBay doesn’t work and you don’t feel like spending for a LPS, I wonder if an iFi Power Purifier (see link) would help. Good luck.
The power feeding a turntable motor can have a profound effect on the resulting sound. Most wal-warts are switching supplies, and as Ghosthouse notes, can inject a lot of noise into the AC....which affects everything in your system. You should really consider a linear power supply which will likely pay some nice dividends, and beyond just the sound of your analog rig. Yup, way more than $25, but if you are a DIY type person there are some nice kits that can get you going for around $200.
Also, be sure the wal-wart you just purchased can handle the amp draw of your motor. If not, either it or the turntable motor itself are at risk of an early demise.
Thanks for the ideas. I don't know if the existing wall wart is switching or linear. I did do some internet research today - my conclusion was that it's not the architecture, it's the implementation. (although, I was quite surprised at how much the adapter affected the system, even when connected to an outlet in another room). So, I opted for a quick, low cost solution. If it works, yeah. If not, no harm, no foul.
As far as option B, I think the next option would be a rechargeable 24V battery.
BTW, the existing wall wart has an amp rating of 600mA, the new adapter (switching architecture) is rated at 2 amps.
Ghosthouse, that is an interesting little device, could be just the ticket. I will keep it in mind in the event that the new adapter is noisy.
Also, as an aside, I did have another 24V adapter, rated at 220mA. It has no connector, just raw wires, but a check with a digital multimeter shows it is working. When I plugged this adapter into the system (using the the same outlets as above) I could not hear any difference in the hum.
Also, I hear the hum when I plug the AC adapter into the power outlet - even if the output wire is not plugged into the turntable motor. And the turntable motor itself has no electrical connection to anything else in the system.
I think the adapter is injecting some noise into the system, as bcowen suggested.
Thus, my next option of a rechargeable 24V battery.
SMP anything is just a HF noise generator. It pollutes everything in your cherished hifi sytem.
Here a test, get an AM portable radio (without auto mute between stations) tune it down low around 800khz but off station.
And go near any thing that has an smp power supply, even Class-D amps and listen to noise it makes through that radio.
I have an ifi dc pur, and it is nice, helps the sound quality whether feeding my startech rex or uptone audio regen when fed from their wallworts. I do still plan to buy LPS for all of my dc powered devices for the reasons stated. I've also learned that R-core transformers are the best. I highly recommend the LPS links at head-fi and the ultra usb link at usaudionmart. Lengthy reading and different on the bleeding edge, but lots of useful info there. My system has never sounded better, and I expect many more cost effective upgrades in the near future.
I don't think the DC Pur is the solution to your hum problems though. If the PS replacement doesn't solve it, maybe your turntable motor has issues.
Well, some interesting developments on the hum issue. I got the new "low noise" power supply today (switching mode), and yes, it adds AC noise to the line, even more than the old wall wart. I got a little frustrated, and went on a power mission. I went through the house, and unplugged as much as I could find of AC/DC devices, including everything in my office, battery chargers in the kitchen, devices around the TV, you name it. I then unplugged everything in the audio system except the amplifier, (a Sony STR-DA5600ES), and the phono preamp (an Eastern Electric Minimax). On every input selection, the system is dead quiet, but when I set the input on the receiver to the phono preamp, there is just a taint of a hum. The hum is faint. (and yes, I have tried all kinds of variations of grounding wires between the turntable, the preamp, and the receiver, and while some are better than others, there is always a faint hum. You can only hear it when your ear is say 6" in front of the speaker, but it's there). When I connect the wall wart that powers the turntable motor, there is a jump in the hum. There is also a slight jump in the hum when I plug in my Olive Musica server - not as much as the wall wart, but a jump nonetheless.
Again, the hum is only present in the phono circuit. So, my conclusion is that there is either some AC noise in the line, something in the Minimax, something in the cartridge (Shure V15VMXr), or something in the cables - and that something is getting amplified.
I don't have any kind of power conditioning or power filtering, just a WireMold power strip. I do have some good cables in the system, but at this point, I don't think cables are the issue.
Open to thoughts.
THE HUM IS GONE!
Took a little effort. First thing I did was buy a Ebtech Hum Exterminator. I plugged the power cord on my Eastern Electric Minimax into it. 90% of the hum was gone.
There was still a very faint hum at higher volumes. Worse yet, when I would touch certain things (the motor housing, the tonearm, the phono adapter) the hum would increase. Made me think grounding wasn’t right.
Turns out there are two ground screws on my Cardas phono adapter. Even thought they both connect to the main body (aluminum, black anodized), I don’t think they are perfectly grounded. I moved the ground wires to the same post and everything is fine. Dead quiet.