Trying to solve 60hz ground loop humm in phono pre-amp.  - Page 1
Forum section: electronics / projects

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 03:15:21 pm
content: Hi I'm attempting to build a phono pre-amp that outputs into an inverting preamp with a switch between the 2 line/phono input. Like this I can input line level audio and phono level audio.Initially I started with with a line level pre-amp. It's a simple inverting amplifier circuit attached here:It works quite well. No noise issues...Board stack up:Red (Top): Audio SignalYellow (Power): +-15VPink (Ground): GroundGreen (Bottom): Audio SignalSo then I wanted to push my design skills a bit do a board to board type of thing where I can attach a phono card on the back of the input module and be able to switch between the 2.Which gave me this design for the input:It's the same circuit, plus extra switch, and inputs. Same stack up as above.And finally the phono:Same stack up but note, components are flipped in the back.The test condition is as follows the circuit is powered by my +15V, 0, -15V PSU and audio connected to amplifier. To simplify things and exclude external factors as much as possible, there is no input equipment connected. I.e: No turntables, no CD or dac or computer nothing. The circuit and amplifier are plugged into the same extension bar.1- When the line module is selected through the switch, it is super quiet, I can jack up the volume of the amp and there is no noise apart the standard high frequency hiss.2- When I select the phono module through the switch then the 60Hz humm becomes apparent. I don't have an oscilloscope but I have recorded the output and ran it in my DAWs spectrum analyzer and I can clearly see a bump at the 60Hz.But I found something interesting. If I plug shorting RCA caps on the phono input, the humm goes away completely. RCA cap is basicaly a male RCA shorted to ground with 450ohm resistor:Is there anything specific I can maybe hack on my boards to get rid of the hum?Thanks

author: jmelson
author_rank: Super Contributor
timestamp: March 21, 2021, 04:06:03 pm
content: Quote from: djtazzmtl on March 21, 2021, 03:15:21 pmIt sounds like you have the classic ground loop here, at the turntable. Is the phono cartridge grounded at ANY point in the turntable?If so, remove ground. It could be right in the cartrige, where it is attached to the tone arm, or somewhere in the cables back to the RCA phono plug. it could be an intentional ground or something that has happened when the insulation of the cable was pinched.Jon

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 04:43:27 pm
content: Hi Jon, Thanks for the reply but I'm afraid you missed the part where I noted no input devices were plugged in so it's not the turntable...

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 21, 2021, 04:56:44 pm
content: You may want to post a schematic.One thing to note from your description is that your "phono" preamp picks up noise when its input is left floating. Without a schematic, all I can say is that a typical phono preamp (which is called RIAA) has relatively high gain and high input impedance. So the fact it picks up mains hum when its input is floating is kind of normal.

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 05:12:55 pm
content: Hi, here are the schematics.

author: gf
author_rank: Super Contributor
timestamp: March 21, 2021, 05:31:27 pm
content: If the phono amp is quiet with 450 Ohm terminated input, then it is OK, and it will likely be quiet as well when the (low-impedance) phono cartdrige is connected (granted that the phono cartridge does not pick-up hum either - but that's a different issue then).

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 05:37:54 pm
content: Yeah it is quiet when the phono is connected. But I was wondering if there's a way that fix this for the sake of it?As other preamp I heard don't do do this...

author: bob91343
author_rank: Super Contributor
timestamp: March 21, 2021, 06:32:04 pm
content: I don't think the problem is within the circuit. It's extraneous noise coupled in via stray capacitance, perhaps. The higher the input impedance, the less it takes to pick up noise. When the shorted input is clean but the open input is not, that's what it has to be. Try various values of input terminating resistance to see where the threshold is and then calculate the amount of capacitance that would approximate that resistance. That will tell you how much shielding you need. In other words, it's shielding that will reduce this effect.

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2021, 06:49:15 pm
content: There used to be RCA sockets that had a 3rd grounding contact that grounds the signal to ground when the plug/cable is removed. These were used in older Phase Linear and Fisher amps if I remember correct. I believe the following link shows something similar:https://www.allelectronics.com/item/rcj-65/rca-jack-w/nc-switch/1.htmlThere is no harm in grounding the input of the Phono amp.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 21, 2021, 06:52:00 pm
content: As I told you, it's a high impedance (relatively) and high gain preamp. It WILL pick up noise if you leave the inputs floating.There's a number of things you can try, such as connecting your circuit ground to earth (to be done with caution though depending on your installation). Or some shielding. Not guaranteed to be effective. You may also check your preamp boards are not too close to a transformer, and that any wiring that would carry AC is also as far away as possible.

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 06:57:21 pm
content: Ok the RCA ground pin is connected to the signal, ground which then that is connected to chassis and to then to earth.The 2 modules when attached board to board are 1 cm part.So are you saying that my boards and circuit look ok?

author: helius
author_rank: Super Contributor
timestamp: March 21, 2021, 07:01:07 pm
content: Quote from: djtazzmtl on March 21, 2021, 06:57:21 pmIn general to prevent ground loop problems, you do not want grounds wired in series. Rather, wire them all in parallel (the "star ground" configuration) so that current that flows from each node to ground is independent of the voltage at other nodes. This is the way to minimize hum pickup.

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 07:12:51 pm
content: So I just disconnected the RCA jacks physically from the phono board. I.e pull the connector out and the hum is gone...If you guys look at the 2 boards. There are 2x 4 pin Molex connectors for the inputs. I.e Left, Right and ground for each left right...Can I simply just disconnect the ground pins of the RCA of the phono?

author: gf
author_rank: Super Contributor
timestamp: March 21, 2021, 07:14:56 pm
content: Since it is quiet with shorted input or with the phono cartridge connected, the open input is very likely the gate where the hum enters the circuit, while everything else is likely OK. Like bob91343, I also guess that is is rather capacitive coupling (and rather not inductive coupling or a ground loop). As SiliconWizard pointed out, the phono amp is very sensitive as it has a gain of more than 60dB. It has to be, since the phono signal's amplitude is very low. But the phono cartrige has a low impedance, thus (almost) shorting the coupled-in hum when connected. The same applies to your 450 Ohm terminator.EDIT:Quote from: djtazzmtl on March 21, 2021, 07:12:51 pmDidn't you say the hum occurs with open inputs? (i.e. when no plug/cable is attached?)Do you mean that you de-soldered the sockets now, and it is gone?Quote from: djtazzmtl on March 21, 2021, 07:12:51 pmYou can't. The phono signal needs a return path.

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 07:46:08 pm
content: Ok so I need to fix the way the Molex connectors/ground is done between the 2 boards and the RCA plugs?

author: gf
author_rank: Super Contributor
timestamp: March 21, 2021, 07:56:53 pm
content: Sorry, I get more and more confused now. Which configuration exactly does hum?So far I did understand that the phono amp did hum when its inputs are unconnected, while shorted phono inputs are ok, and with phono cartridge connected is was ok as well. Is this correct?So I don't understand why you bring the connectors between the boards into the play now.[ Two RCA cables between the boards could indeed form a ground loop - but this loop were independent of the phono amp's inputs being open or shorted. So if there is no hum when the phono amp's inputs are shorted, then the connection between the boards can be ruled out as reason for the hum. ]

author: jmelson
author_rank: Super Contributor
timestamp: March 21, 2021, 08:07:28 pm
content: Quote from: djtazzmtl on March 21, 2021, 04:43:27 pmYup, I mjissed that detail! OK, you have environmental pickup. Try putting the preamp in a metal box and grounding the ground connection to the box. Think of Altoids tins. Every lab should have a few of different sizes. If that doesn't fix it, you need to filter your +/- 15 V supply. Any high-gain, high-impedance circuit is going to suffer fron this kind of sensitivity.Jon

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 08:11:11 pm
content: There is 2 modules. A line input and a phono, they are connect together board to board and there's a DPDT throw switch between the 2 inputs. There is also 2 pairs of RCA connected 1 to the line input, 1 to the phono inpit.Remember there is no external input devices connected so no turntables, no cd players. Nothing...When the switch is on the line input and I jack up the volume of the amplifier it's dead quiet.When I push the switch to the phono, there is a 60hz hum. Remember there are no turntables connected.If I plug the two 450ohm shorted RCAs into the RCA female of the phono module the hum goes away. Also if I plug the turntable the hum reduces significantly (acceptable, my Technics 1200 has its own issues lol).

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 08:19:19 pm
content: Hi GF,1- When the enclosure is completely closed and the turntable is not connected to the RCA. As you said open inputs. Hum occurs.2- When shorting plug is connected to the phono RCA, no hum (also enclosure closed)3- When turntable is connected, hum is reduced. I tested the turntable with another commercial preamp I have and it has hum but that's the turntable as the other preamp doesn't hum when the TT is disconnected.4- I physically severed the female input RCAs from the board. It's a pair of twisted cables with Molex connector so I just pulled out the cable and no hum either...

author: gf
author_rank: Super Contributor
timestamp: March 21, 2021, 08:44:15 pm
content: Quote from: djtazzmtl on March 21, 2021, 08:19:19 pmSo the RCA socket is not directly soldered to the PCB, but there is a piece of (twisted pair) cable in between?One thing you can try is to replace this cable with a shielded one and check if it helps.If it does not help:Is the enclosure made of metal?If it is metal:Has the RCA connector a conducting connection to the enclosure?Do any other ground connections to the enclosure exist, so that a gound loop could be formed?EDIT:If the case is metal and the RCA has a conducting connection to the case, then you can check wheter disconnecting the RCA from the case helps as well (so that the RCA connector is only connected via the cable to the PCB, but no longer in contact with the case).

author: djtazzmtl
author_rank: Contributor
timestamp: March 21, 2021, 09:09:02 pm
content: Correct there's is 2 stereo pairs of twisted pair cables per input. So 1 stereo pair for the line input and 1 for the phono. There's a total of 8 wires/pins involved in this...Line input L + G (twisted)Line input R + G (twisted)Phono input L + G (twisted)Phono input R + G (twisted)The case is metal and it's fully powder coated. I do not see any breaks in the paint as well the female RCA mounting point is plastic.The ground is attached to the chassis. I soldered a wire on one of the ground on the line input board to the chassis from the chasis all the way to earth. I tested continuity with my multimeter. If I remove the chassis ground. Then there's noise on the line input as well...

author: srb1954
author_rank: Super Contributor
timestamp: March 21, 2021, 09:49:35 pm
content: Quote from: djtazzmtl on March 21, 2021, 05:12:55 pmPut a small value ceramic capacitor, say 47-100pF, across the inputs to the phono preamp to suppress RFI pickup. This should preferably be placed directly right on the phono sockets, with very short leads, for maximum effect.I once had a problem with an old stereo system that had a persistent buzz that couldn't be eliminated by all the normal grounding measures. It turned out to be RF interference from a local WiFi system that was being rectified by the inputs of the preamp IC ,an NE5534. Some RF bypass capacitors on the inputs and improved shielding significantly reduced this noise source.What is the purpose of the 2K2 resistors in series with the op amp inputs? These won't help the noise performance of the input stage of your preamp.

author: djtazzmtl
author_rank: Contributor
timestamp: March 22, 2021, 01:30:24 am
content: @gf I tried the shielded wire. The shield as the ground and the conductor as the signal. Same difference still a buzz... Only the RCA cap does the trick...As for RF I'm not sure it's RF. It's 💯 60hz hum.

author: srb1954
author_rank: Super Contributor
timestamp: March 22, 2021, 02:47:03 am
content: Quote from: djtazzmtl on March 22, 2021, 01:30:24 amEven if it isn't RFI in your case it is still a wise precaution to incorporate some RF filtering. The important thing to note is that the RF signal can be rectified by the protection diodes in the amplifier IC and what you get is the rectified envelope of the RF signal, which may well be in the audio range. There is some much RFI around these days from various broadcast sources, cell phones, WiFI, Bluetooth, 2-way radios etc it is inevitable that you will suffer from interference at some stage unless you take reasonable filtering precautions.In my case, I thought I had a 50Hz (which is the local mains frequency in my part of the world) hum problem as the frequency of the noise appeared to be approximately 50Hz. However, close examination of the waveform with a very sensitive scope showed that the noise waveform was a pulse waveform not sinusoidal. As this pulse waveform couldn't be reasonably produced by the normal mains frequency fields it led me to suspect RF pickup, which I later confirmed as coming from a WiFi router.

author: magic
author_rank: Super Contributor
timestamp: March 22, 2021, 06:49:19 am
content: See what happens if the chassis is grounded to the phono board's ground or one of the phono RCA's.Quote from: srb1954 on March 21, 2021, 09:49:35 pmProbably protection for the NE5532's input clamps. It does add white noise, but still not even as much as the feedback network.

author: djtazzmtl
author_rank: Contributor
timestamp: March 22, 2021, 01:52:59 pm
content: The phono board common ground is grounded to chassis in fact if I disconnect the chassis ground it gets worse. There's no way my board to board connection is somehow causing a ground loop?

author: djtazzmtl
author_rank: Contributor
timestamp: March 23, 2021, 04:23:53 pm
content: So I can confirm...1- If the Phono RCA is left open/unconnected, there is noise.2- If the turntable is connected, no noise.3- If shorting cap is on, no noise.4- Using shielded wire, didn't seem to make a difference.5- Connecting/Disconnecting ground from chassis no difference.