Trouble removing 60Hz buzz in phono preamp - Page 1
Forum section: electronics / projects

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 03:54:02 am
content: Hey everyone, I'm having trouble tracking down a frustrating issue. I've got this. The pre-amp works great, haven't had any real issues with it...except one.The turntable I'm using has unshielded lines from the phono cartridge to the board. This naturally will pick up some noise. However, there is another issue of 60Hz hum. There is an AC/DC converter on the PCB but it does not contribute to the noise. The noise is still there even if I skip the AC/DC and power it directly from a clean DC power supply. The phono lines are separate from any other noise generating components, I'm fairly confident this is being picked up from the surroundings.I've tried a couple of things to avoid this. At first, I thought that the high impedance of the phono input made inducted noise more prominent, so I added a ~60k pulldown on the cartridge-side of the DC blocking cap. No change.I thought that maybe - since the VCOM voltage comes from the PCM2900 and that has an impedance of roughly 30k, that this was a high enough impedance to disallow induced voltages from draining quickly. So that must mean I need a voltage buffer with a low impedance. To try out this theory I set a quality power supply (HP E3611A) to 1.655V (VCOM voltage measured at 1.656V). The + lead was connected to VCOM, the - lead connected to AGND. The outputs are isolated and earth ground wasn't connected. Voila, the noise greatly diminished with no degradation of audio quality.Then I noticed that it didn't matter if the power supply was on or off. In fact it didn't even matter if the leads were both plugged in. It was enough foreitherof them to be connected to the power supply to cut the noise down substantially. However, if the (turned off) power supply is unplugged from the mains, the noise returns.I'm a bit miffed by this. It seems that earth ground and VCOM/AGND have some interaction. What I don't understand is why earth ground is special vs . I had assumed that it was enough for the entire signal path to be referenced to a clean, separate ground and powered by low noise linear regulators. It seems this isn't the case. And since this is a 2-prong turntable there is no chassis or earth ground available once it's all together.Can anyone who knows what's going on here point me in the right direction?EDIT: Attached a recording of the hum while connecting/disconnecting the aforementioned lead. Volume warning for some pops.

author: james_s
author_rank: Super Contributor
timestamp: February 09, 2018, 03:58:11 am
content: Just to start with the easy thing, is the ground wire from the turntable to the preamp connected? Without that it will hum.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 04:03:45 am
content: From the phono cartridge? Yes, there are two and they both are connected to the center pin. However I hadn't thought to check that inside the tonearm...maybe it got disconnected? I'll check it out right now.EDIT: Disconnecting the phono cartridge produces some monstrous buzz, and alternately disconnecting the left/right grounds from the cartridge head produces the same monster hum on the left/right respectively. So it seems as though it's all connected.Attached recording of the hum to OP.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 09, 2018, 04:48:19 am
content: There's typically another wire that connects the frame of the table/arm to the chassis of the preamp. It's almost impossible to kill hum without it, though a few tables do manage with just the RCA cables.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 06:12:40 am
content: This is a pre-amp on a PCB internal to the turntable, not a separate pre-amp.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 09, 2018, 06:41:06 am
content: So you have only a USB output?No analog audio output here?How did you decide that the USB connection was not implicated here?How are you listening to this and hearing the hum?If you are plugging this into a notebook/laptop (or tablet) operating from a mains-source power "brick", what happens if you completely unplug the power cord from the computer and operate on batteries?Unless the turntable was damaged and repaired improperly, it seems unlikely that it would suddenly develop such a massive hum problem out of thin air. We have to assume that it was designed and manufactured to be much better than that.There are at least half a dozen important details missing here which prevents any better analysis.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 04:12:42 pm
content: My apologies, I'll try to add more relevant details.This is a PCB I made. USB output only. There was hum from the USB previously, but I eliminated that by disconnecting the USB cable VBUS and powering the DAC from a separate linear regulator. The USB noise was a much higher pitched whining when it was present. I'm listening to it through my laptop, using headphones to better hear the noise. The whining is gone, all that's left is the 60Hz.Unplugging the laptop removes a very slight "fuzz" that's present, but it's not at 60Hz and is relatively minor.Putting my hand near or on the unshielded cable going from cartridge -> pre-amp input increases the volume of the hum substantially without changing the character of the sound. I have a little shorting plug I made that shorts both channels to ground in the pre-amp. Rather it shorts them to the virtual ground used as the center voltage for the op-amps. Nearly total silence. If there was noise coming from within the board I've been assuming I would hear something on the output even with the inputs shorted.It may be that having an unshielded cable coming from the cartridge to the PCB is the culprit, the cable is probably about 1.5 feet long and it's just 4 loose wires. It came that way; I haven't disassembled the tonearm yet but that might be the next step in order to replace this with a shielded cable.It also doesn't help that the turntable body is wood and plastic so there is no shielding. Maybe there is some metallizing spray that exists I can try out as a makeshift shield...

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 09, 2018, 05:04:26 pm
content: You can glue aluminum foil around the inside of the wood and/or plastic parts. Spray on contact adhesive (3M Super 77 is a popular kind) Of course you must connect your shield to circuit ground.Did this NEVER work properly? It seems very strange that a nominally-operating piece of consumer gear should start behaving so badly. Was it broken at some point and improperly repaired? Was it working properly before you started hacking at it?

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 06:13:13 pm
content: The internal electronics were replaced by this PCB I made. That's a good idea though, I'll plug the OEM board back in and see how it compares. The hum was always there a little bit. My workspace is too small to fit a turntable and all the other stuff so I extended the phono lines about 12" to allow me to keep the turntable flat on the table and still reach the PCB laying on the bench.If it's a matter of shielding the phono lines (and keeping them short) that's easy enough. I'm still confused by why connecting either AGND or VCOM to a bench power supply that's turned off cuts the noise so much. Is there some capacitive/inductive coupling to earth ground going on in the power supply that shunts the induced noise? I'll get some pictures tonight to better illustrate.

author: Audioguru
author_rank: Super Contributor
timestamp: February 09, 2018, 07:16:51 pm
content: A phono cartridgeis alwaysconnected to the preamp with shielded audio cables. Yours isn't. So of course you have hum, the wires you have are antennas that pickup the mains electricity hum that is all around us.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 09:02:40 pm
content: Makes perfect sense. Why this tonearm didn't come with a shielded cable is another question...Any idea why connecting leads to the power supply as mentioned would cause the noise to diminish so much?

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 09, 2018, 09:11:36 pm
content: It is not clear how this was originally powered? Internal batteries? AC wall-wart (or line-lump)? USB-powered?Extending the phono pickup leads externally and connecting to various bench supplies, etc completely changes the original design parameters.If you are going to make changes that extensive, it will likely require a complete re-design of the whole shielding and grounding design from scratch.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 09, 2018, 10:02:09 pm
content: I tried to explain that in the OP. The board is powered by an onboard AC/DC converter. This converter doesn't contribute to the noise. Even if it's bypassed by unplugged it and powering the main DC power rail directly from a bench supply the noise does not change or increase.The power supply I am referring to in the OP is not actually "powering" anything. The intent was to see if a low impedance center voltage source (VCOM in the schematic linked in the OP) might be better at reducing the effect of noise, since currently the center voltage for the entire pre-amp is taken solely from the VCOM pin of the PCM2900 DAC which is at 1.656V.During this test the board was powered by the onboard AC/DC converter directly from the mains. It turns out that connecting the external supply between VCOM (1.656V) and AGND (0V)doeshelp. It was connected in parallel with the internal voltage source coming from the VCOM pin which has a ~30k output impedance. But it doesn't matter if the power supply is turned on or if both leads are connected. It is enough for either side (VCOM or AGND) to be connected to the external supply. As long as one of them is connected to the (turned off) bench power supply, and the mains cable is plugged into that power supply, the noise diminishes. Nothing else changes here and the board is being powered off of the internal AC/DC converter and operating normally.In short, while the board is running, connecting a single alligator clip to the PCB at either VCOM or AGND and plugging it into any of the jacks on the bench supply greatly reduces noise. I'm just not sure why.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 12:21:48 am
content: The open leads not shielded going to the needle cartridge are not the problem they would appear to be . All record players were made this way back in the day with open wires. It was dome this way to change the needle cartridge if needed.The wires going straight to an A/d is questionable. The output is only 5 to 10 m volts. Signal needs to be amplified to at least line level , 100 m volts. It does not end there. Can not put a bass note on a vinyl record as the needle would jump off the record to the next track. It is for this reason bass is turned down and treble up before printing a vinyl record. For this reason the signal must be frequency weighted turning the bass back up and treble down to compensate for this. For these reasons there must be a few op-amps before the A/D for an A/D to have a chance at converting the audio to digital within it's dynamic range.This being said there could be some easy fixes. The wires going to the needle cartridge are color coded. If you look close there is also colors marked on the cartridge. Do they match the colors of the wires? Second easy fix. Is the ground wire making it from the cartridge to the PCB board ground ?

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 01:05:56 am
content: There is a pre-amp which follows the RIAA curve pretty closely, close enough for my purposes anyway. I've attached a picture of the L/R response charts overlaid.If you check the schematic linked in the OP you'll see the pre-amps and the DAC.EDIT: Just checked continuity and both the ground leads from the cartridge and the tonearm are indeed connected to the board. However they are not connected to ground per se, but to VCOM which is the "virtual ground" in the schematic, providing the center voltage for the op-amps. Since this is a high impedance (~30k) path maybe this is the issue?I'm not sure where the grounding happens inside the tone-arm, without taking it apart. There are 4 wires, but clearly at least one of them is connected to the metal of the tonearm. I can't connect the (-) side of either signal line directly to AGND as this changes the frequency response of the pre-amp because of that center voltage. I would think this doesn't matter since there are DC blocking caps on both inputs anyway...but maybe I'm doing something wrong here.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 01:29:41 am
content: It is a nice green line ,, I would go farther and say a pretty green line with the black background and all. Give me a break. Details!! , picture of turn table , diagram with component values marked , usb audio converter.... Trust me a mechanic can not fix your car armed only with a picture of a pretty green line. To give this balance I have to admit it is a nice looking green line but hey , where is the beef.

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 02:05:29 am
content: Pre-edit: Just saw latest post. I put up that chart just to show that there was a pre-amp, I'm not taking the phono straight to the DAC. I posted a schematic in the first line of the OP which seems to be too inconspicuous so I'll repost it as an attachment here and modify the OP.--------------------------------------------------OK so I somehow forgot that when I combined the (-) side of the L/R signals that I didn't merge 2 wires (green & blue) into one but3wires (also black). That turns out to be the tonearm casing. After poking around some more here are the results:1) Leaving the tonearm shield wire disconnectedincreasedthe magnitude of the hum/buzz as expected.2) Connecting the tonearm shield wire to either isolated output on the bench supplyincreasedthe noise.3) Connecting the tonearm shield to AGND made no difference vs. VCOM.The following all stack with 1, 2, and 3:4) ConnectingeitherVCOM or AGND to any of the jacks on the power supply (+/-/EARTH)decreasedthe noise substantially.5) If the bench supply is connected in any manner described in (3), then connecting the tonearm shield to earth ground furtherdecreasednoise. If however the bench supply is totally disconnected from the board, then connecting the tonearm shield to earth ground substantiallyincreasedthe noise.6) If the phono lines are completely disconnected from the PCB (so the L/R/VCOM lines are floating at the connector) there is still some noise but it has a much less "buzzy" quality, closer to a sinusoidal hum. Then, while disconnected, connecting the bench supply (any jack) to AGND almost totally eliminates hum. Connecting to VCOM also drastically cuts it down but not quite as much as AGND.This seems like it should be a simple problem for someone more experienced in small signal audio but for the life of me I can't quite figure it out.It would seem that the role of the bench supply is to somehow couple the PCB to earth ground which reduces the noise, but there is no earth ground cable present on the AC input (2-prong).

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 03:53:08 am
content: Now we are cooking with gas. I like these green lines way better than that other green line.It looks great to me? Low frequency emphases is in place with some nice gain. Why the hum? For my 2 cents it has something to do with a ground issue. With a record does it play any music and if so how does it sound?

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 04:39:29 am
content: It plays music just fine, and you can't really hear the buzz except on quiet parts. For some reason when I tested this board a month ago the noise performance was great. I'm not sure what the problem is, goblins in the wall maybe.Any objection to the way I've biased the op-amps to VCOM? I also have a jumper on board that lets me switch the (-) phono leads between VCOM and AGND, which I'll have to try again to check noise. I prefer the low-frequency response when referenced to VCOM even though it *shouldn't* make any difference due to the DC blocking caps on the input (right?).I saw some advice on another site to put two linear regulators in series for the op-amps to reduce sensitivity to induced noise. I might try that.This threadalso shows a schematic with a different bias setup than I've used, although I've seen my method out in the wild as well so I'm not sure which is better - or if it matters.I could easily swap the op-amp for one with a higher PSRR to see if that changes anything. I have a hard time seeing mains ripple get through an AC/DC converter, a switching converter, and an additional linear regulator (2 in the case of VCCCI) but you never know.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 05:34:13 am
content: It only has a slight buzz in the quiet parts of the record. That sounds like every record player that I have ever owned , ha.The op-amps are recommended for phono pre-amps in it's pdf file. That along with .01 decoupling condensers carefully placed screams mass produced phono pre-amp board that has been fine tuned over the years to be as good as it can. I would not fiddle with it as it has no place to go but down if changes are made.The buzz. I am hearing a first octave higher harmonic from hydro hum in your recording? This in unlikely to get through the preamp as it has higher gain at lower frequencies than highs. The only nasty that I can think of with first harmonic sound of hydro would be the full bridge rectifier of the power supply. It is a 12 volt power supply according to your diagram. I do not see this 12 volt power supply on your diagram. Where is this trouble making 12 volt power supply that is dumping buzz into our audio? Why is it missing from the diagram? If you are just playing modern music then who cares if it is buzzing. But if you are playing good stuff from the 1960s then we have to fix it. Where is this 12 volt power supply?

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 05:59:01 am
content: It's on a different schematic sheet, an IRM-10-12 from MeanWell. I designed this PCB using the best available material I could find. Most literature refers to split-rail supplies but as this is a single-supply I used the schematic given in the PCM2900 datasheet and another design from one of the big chip manufacturers. Forgot which one, I'd have to check my notes. Even if the 12V AC/DC supply is totally bypassed (unplugged with a clean DC bench supply providing the +12V) the buzz is still there. That leads me to conclude it's coming from the environment. I just can't seem to get rid of it without connecting that external supply as previously mentioned, the effects of which confuse me.The buzz definitely has higher harmonics, it sounds exactly like a60Hz square wave.

author: Damianos
author_rank: Frequent Contributor
timestamp: February 10, 2018, 06:22:37 am
content: The VCOMpin is connected to an internal reference and is provided to connect a decoupling capacitor to improve the operation (additional filtering).In the schematic of this project, there is a "cheat" from the VCOMpin, of few pA to bias the external Op-Amps. That seems OK.But this is also used as ground/reference for the entire circuit and an attempt to "bias the planet earth" from it!What I mean is in the circuit keep only the R21 and R26 connected to VCOM, everything else has to be connected on the "ground".

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 07:58:28 am
content: That makes sense, and agrees with a couple sources I've read. I assume in this case that it's OK to keep the 100pF caps (C17, C21) connected to VCOM as well, or should they just go to AGND since they block DC anyhow? The three bits on the very far right of the schematic are jumper pads that aren't being used so those lines to JP1/JP2 may as well not be there. The actual traces for those jumpers on the PCB are extremely short - less than the length of an 0402 passive - so no antenna to pick up noise.So it would seem that VCOM is only connected to the non-inverting side of the op-amps through R21 and R26, and otherwise only connected through caps that should block any DC currents. And of course to the (-) side of the phono inputs. I would think that would also mean very low power draw but you're saying that's not the case. Could you explain why? The phono inputs I can understand, since any induced current will affect the VCOM voltage as well, but what about the 22uF capacitor banks and the 100pF caps? What's the benefit of referencing those back to AGND? Not arguing, just trying to understand.It would certainly help to not worry about negative polarity on the LF filter caps, and switch back to tantalum.On the topic of DC blocking capacitors, I haven't found any specific explanations as to how those should be sized. I've looked through both "Small Signal Audio Design" and The "Art of Electronics" but I couldn't find any details for this application. C19 and C32 are both 33uF. I don't even remember how I came to that number, and it seems that every schematic I see has a different value for those caps. Some as low as 1uF, but never an explanation of how a particular value is chosen.I've attached a couple shots of the layout highlighting VCOM. The two phono input traces follow VCOM around that corner on the bottom layer, and terminate at the same place as the VCOM trace from J11 changes layers (about halfway up the edge of the board). It is quite a long and winding path for VCOM which is surely causing problems...

author: Damianos
author_rank: Frequent Contributor
timestamp: February 10, 2018, 07:46:04 pm
content: The two 100pF capacitors are there to prevent any RF interference, picked up from the source wires, to disturb the circuit operation. So, they must be connected in parallel with these wires at the point they enter to the circuit, before the interference spreads around. There is not any good reason to connect these "antennas" on the bias line.About the 22uF capacitor banks: think simply, what is their purpose? What is the ratio of their values to the decoupling capacitor of the VCOM? Also the signals coupled there are going to the inputs of all the amplifiers (internal and external) that are high impedance.Same for the wires from the source, there is not any reason to couple any interference ... ...For the input DC blocking capacitors, as they are part of a high pass filter with the input resistors, the value is determined by the desired low frequency limit. So, for around 10Hz the value can be less than 1uF.I am thinking that you can try (only for testing) to decouple the VCOMusing real large capacitance (more than few times the sum of the other capacitors), but I am not sure if there are any consequences.EDIT: Also take in account the comments ofJohn Heathabout the correct wiring of the cartridge.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 10, 2018, 08:11:14 pm
content: There is a trick that is used for microphone buzz. They use an isolation transformer . In your case this would be the equivalent of a differential input op-amp gain 1 buffer for the input. It would be a lot of extra trouble and no guarantee of a solution but if worst comes to worse you could give it a try.

author: DimitriP
author_rank: Super Contributor
timestamp: February 10, 2018, 08:30:29 pm
content: Is the USB GND left floating on purpose or accidentally ?http://www.yourfinalsystem.com/cable-design-f-a-q/147-how-do-data-only-usb-cables-work

author: SVFeingold
author_rank: Frequent Contributor
timestamp: February 10, 2018, 08:45:13 pm
content: Hmm I'm not sure why it's floating in that schematic. In the layout it is connected to ground on the digital side, but not connected to analog ground. It used to be but that introduced a loud whining noise.That 33uF cap with a 47k resistor gives a cutoff frequency of ~0.3Hz, which according to Small Signal Audio Design is too low as it's desirable to remove infrasonic noise caused by the mechanics. I think bumping that up to ~10Hz would be a good idea, I'll try it out tonight.Everything about coupling to AGND instead of VCOM makes sense. An interesting effect was that if the phono input was coupled to AGND the frequency response would change, as shown in the attached chart. In that case however the rest of the circuit was still coupled to VCOM as shown in the original schematic, which may have something to do with it.Great info everyone, I'm going to try these things out tonight! I feel like a dummy overlooking what in retrospect seem like straightforward things but what can I say.