It absolutely is an inherent fault of the 834P @lewm . No quality piece of equipment should be that susceptible to RFI. It is broken or poorly designed. If other phono-pre don't do it and this one does, that is all the clue you need for that. That it uses tubes is no excuse.
Tube PHONO preamp interference - RFI, EMI, bad grounding?
Hello!
My tube phono is picking up interference most probably from the air. It's EAR yoshino 834p, using three 12AX7 tubes. It's sounds pretty amazing and I willing to try everything to keep it.
Here is a sample of the sound -
The rest of the setup is ARC LS16 mk1, Classe CA200, Chord Qutest, Technics SL1200 with Nagaoka MP200, Tannoys D700
I have tried many things already -
- grounding the phono to the preamp, grounding the phono to a socket, covering the phono with a pot, saucepan - no change
-plugging the phono preamp alone into an integrated (Bryston B60) and removing other stuff.
- the important part is I have taken the phono to two other places and it worked perfectly fine, even with the cheapes cables.
- I haven't had any problems with previous phono preamps which were all solid state.
- if I unplug the turntable the signal fades to about 50%
- if I try different RCA cables, there's not much of a change even they are shielded (audioquest mackenzie, supra etc.)
- the signal also fades when I grab the cables. Also works if I grab or squeeze the output cables.
- I have tried to wrap the cables into aluminum foil, I have noticed a difference but it's still unlistenable.
- I have tried pluging in a 5 meters long RCA output cable and walked with the phono preamp around the room. It's simply like carrying an antenna. Placing the phono on the floor helps but again, the interference is still present.
Do you have any suggestions what else to try? Is there some kind of grounding that would prevent the phono preamp acting like an antenna?
I haven't tried a new set of tubes yet.
I think the 12AX7 are simply too sensitive to all the mess in the air. The ARC LS16 preamp was catching the same signal very quietly when I took it's cover of.
Thanks!
Filip
Showing 13 responses by theaudioamp
Noise sounds like communications and/or a processor ramping up/down to do something. Any chance you are using USB to connect to something connected to your stereo or sharing a power supply for anything related to the turntable? Phone into a USB charger near the stereo?
If none of the above, then I would consider you phone pre broken and/or poorly designed. There are some ideas above to reduce RF noise. The MFR should be doing that, not you. If SS preamps don't do this and this one does, I would ditch it. |
Learn something new every day! Thanks @esarhaddon |
@esarhaddon , that is not HAM or CB. You can tell it is packetized data and/or for packetized processing. The keeps are the demodulation of the envelope. I will see if I can extract the frequencies of the "beeps" as that may provide a better clue of the source. Some items have well defined packet frequencies.
@audphile1 , the change that is described by the op, appears to be a rapid change increase raising above the floor, then the change stops happening about 5 feet up. The floor is likely acting like a shield, but it could also be ground plane effects, or an inductive current loop.
@filipm Amazon has clamp on ferrite bead kits.
Unfortunately it can be tough to get proper specifications of anything on Amazon. Different clamps work at different frequencies. Digikey will cost you more, but you know what you are getting. For $20 though, start with Amazon. Based on your comments about the output cables, put clamps on all wires input, output, and AC. Due to stray capacitance, every wire can act as an antenna.
@filipm , are you in a high-rise apartment building? The reason I ask is they often host large cell tower arrays on the roof as well as antennas for other radio types. |
@filipm , making a phono pre-amp is not rocket science. Good low noise design, good quality transformers (if they are MC and use them), precision parts for the RIAA curve, and some other basic engineers concepts. When it comes to tubes as well, your cartridge likely has enough distortion that the distortion of the tubes almost does not matter.
In my experience, most of the "sound" from a phono preamp comes from the matching between the cartridge and the preamp, and not some inherent advantage or disadvantage of the preamp. If it does not at a minimum have several matching resistor choices for MC and MM, I would not want to use it. Even then, for an MC, a mix of capacitor and resistor matching will yield the flattest response. I am not saying that is your personal goal, just it can be.
Why this matters is don't get hung up on your particular preamp. I would be taking it to a friends house and verifying there is not noise in a completely different location so that you can sell it. Then I would buy a preamp with a good range of matching resistor selections so that you can tune the sound for your cartridge. |
I go back to my original statement. Flawed design or broken. It should not due this. If this happens on both channels, then it is a flawed design. A small capacitor between the signal input and signal ground (or case ground) and the noise will probably disappear. Cables may make a difference .... or not. |
@filipm ,
The dominant signal in the recordings you posted is 50Hz. It is much larger than any other frequency component. I don't know if that is due to noise in your system, the recording, or it is a demodulated frequency from the noise you are experiencing.
The bursts are a packet rate of what looks like 2KHz, which is close to what you would get from GSM, but not close enough. (1.7KHz versus 2KHz). It is bang on 2KHz.
Maybe there is a wickedly noisy power supply in your vicinity and the RFI in in the building wiring. |
@filipm ,
I am suggesting that your phono stage is picking up RF that is being radiated through the building wiring. using the inverter would not change that. Though I assume that your turntable was unplugged from the AC and/or unplugged from your phono stage when you did that test. |
@filipm , an underlying 50Hz, with a 2KHz packet rate is not WiFi. WiFi is rarely an issue with interference into low frequency analog electronics. The 2.4GHz signal is fast enough that most "things" cannot demodulate the frequency, and the modulation method also helps. I strongly suspect it is something else, though it could be 2 somethings, the 50Hz from building wiring, and the 2Khz from something else. Some home automation could be close enough to a 2KHz tone.
@filipm can you please do the following if you have a multimeter. Measure the resistance from the signal ground on the Phono input to the metal case of the box.
CABLES: I don’t know all the cables you tried, but you don’t need BS marketing crap like how the AQ Mackenzie is described. You need good old fashioned doubled shielded coax. Ignore all the marketing crap and use something that is designed using proper engineering principles like the Blue Jean LC-1. https://www.bluejeanscable.com/store/audio/LC1-design-notes.htm A cheap $10.00 coaxial 75 ohm video cable is likely to work better than that AQ crap for rejecting RF noise.
If adding the RCA cable (without the turntable) brings back the noise completely that tells me that your probably not using coax and/or (could be both), there is no connection from the signal ground to the metal enclosure. There should be a connection, at least capacitive between the two, so that the "cage" is unbroken and so that the potential (voltage) between the two is the same at RF frequencies so that no noise can transfer between the two.
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