Help with 120hz hum in Tube mic preamp

[Nelson Paschoal]

Hello friends, I´ve read a lot and do somethings in my project but can get away with this annoying hum, the schematic and the power supply are attached, I did some mods because I can´t get 12ay7 here so I used 12at7 instead, and I made heater elevation and virtual ground with 2 resistors for the filaments. With max gain I have -46dB of noise, it´s "loud" for vocals for example, I´m here to ask for some advices and tips for upgrading this little project. Thanks a lot for the attention!!
I´m from Brazil, sorry for my english if there´s some mistakes.

 

[tony hunt]

Can you be more specific: is it humming at 60Hz, or buzzing at 120Hz?

 

[Nelson Paschoal]

Can you be more specific: is it humming at 60Hz, or buzzing at 120Hz?

It´s humming at 120hz, I checked with a spectrum analizer, even have harmonics at 240hz. It´s not ripple, my supply have a very good filters, 10H choke and 220uF cap, there´s something I´m missing but I don´t know.

 

[ruffrecords]

120Hz hum would indicate an HT smoothing problem. In your power supply schematic do you actually have R3 fitted or is that just a dummy load for simulation purposes?

Cheers

Ian

 

[solkatten]

When you have a choke input supply the choke is exposed to all the AC ripple and can radiate 120Hz and its harmonic component I have experianced that. Putting some C before the choke can fix the problem, but will increas the voltage... You could always increas R to drop some voltage down the line... If you have a small C pre choke you still get small a ripple current...

When I look at your picture it seems like the choke is next to the xlr connectors, IPT and OPT that is a bad place to put it... If you cant relocate it put a larger C (ca. 10-220uf) pre choke so that it only sees DC, it may help...

 

[Nelson Paschoal]

Hi, it´s just for simulations.

 

[Nelson Paschoal]

When you have a choke input supply the choke is exposed to all the AC ripple and can radiate 120Hz and its harmonic component I have experianced that. Putting some C before the choke can fix the problem, but will increas the voltage... You could always increas R to drop some voltage down the line... If you have a small C pre choke you still get small a ripple current...

When I look at your picture it seems like the choke is next to the xlr connectors, IPT and OPT that is a bad place to put it... If you cant relocate it put a larger C (ca. 10-220uf) pre choke so that it only sees DC, it may help...

I´ll relocate the choke first, if nothing happens I´ll try you example of power supply, thanks for help, soon I post the result.

 

[Nelson Paschoal]

Another question, the ground from input and output should be tied together? I relocate the choke and the hum went down a little, but when I touch a metal part of the chassi the noise went down more, my house don´t have a earth, it´s 127v with a live and a neutral, I think this neutral is earthed, this could be something?

 

[bjosephs]

The chassis needs to be tied to something or it will act as an antenna. I believe tying the chassis to neutral through a very high voltage cap was the method before houses had earth grounds as standard. Take care because this is getting into scary stuff. A floating chassis can be electrified by an internal fault and a 2 prong plug can be put in backwards (Hence the high voltage cap).

What is your PS grounding strategy? The power transformer's center tap reference should go directly to the negative side of the first filter cap to keep the return path short. The PS ground line should come off of the cap to the next stage. Inserting a small amount of resistance in the ground line between stages can help discourage PS ground noise from polluting the audio stages. This article changes my approach (from all star grounds ) and get quieter systems.

If your input and output are transformer coupled then, no, they don't need their grounds tied together. Tying them defeats one major benefit of using the transformers in the first place. Definitely don't let either touch chassis (especially since it's floating, but also after it's grounded some other way)

 

[solkatten]

Another question, the ground from input and output should be tied together? I relocate the choke and the hum went down a little, but when I touch a metal part of the chassi the noise went down more, my house don´t have a earth, it´s 127v with a live and a neutral, I think this neutral is earthed, this could be something?

Xlr pin 1 input is normally connected direct to chassi. So your xlr cabel dont act as an antenns. Cabel screen in a balanced connection is part of chassi ground. In the best case scenario it will be grounded at one end. Audio ground is connected at one point to avoid ground loops…
Your hum problem most likely is a ground problem

It will be very hard to have quite preamp without a grounded chassi

 

[solkatten]

Be aware that your B+ voltage will increas from 0,8 of ac voltages up to 1,4 of ac voltages with cap pre choke

 

[Matador]

In addition to bjosephs's 0V reference comments: that circuit is going to have terrible PSRR: the B+ needs to be absolutely pristine, so I would recommend breaking up that 4K7 PSU resistor into three, 1K5 resistors, each with its own 220u to ground.

 

[solkatten]

In addition to bjosephs's 0V reference comments: that circuit is going to have terrible PSRR: the B+ needs to be absolutely pristine, so I would recommend breaking up that 4K7 PSU resistor into three, 1K5 resistors, each with its own 220u to ground.

And you could still have magnetic coupling between PT, choke, OPT and IPT

 

[Tubetec]

A choke input filter means the entire AC current from the rectifier plus DC is across the choke , if you've only considered the DC your choke could be well under spec for the job , if the core is in saturation it could radiate extra noise into your circuits /chassis .

Proximity of one iron cored component to another will have an infulence on induced noise , but if your chassis doesnt have a solid clean ground your fighting a battle on two fronts .

Like Bjoseph Im hesitant to try an offer any kind of electrical advice as theres so many unknowns. Perhaps a mains isolation transformer , with a centre tapped secondary which you use to establish your own local clean ground might be an option , but please check with a local electrician to make sure that kind of set up is safe where you are .
Connecting your neutral and ground of course works , but in the event of a fault in the device it could expose you to hazardous voltages , again referenced by other contributors . What would be way more dangerous though is suppose the wire from the electricity pole supplying you and your neighbours falls down , someone with the best of intentions recconnects it but instead reverses live/neutral , now your chassis is at full mains potential , you could easily end up with a situation where your exposed not just to a dangerous voltage but a lethal one . Likewise if the mains plug was wired wrong or inserted into the socket the wrong way you could end up with a live chassis .

Respect the power of electricity , make sure you understand clearly any advice offered here, and especially remember your electricity supply could be subject to variation (swapped live/neutral) which is beyond your control .

 

[Cranehazard]

I have never been able to get the heaters quiet enough for a studio preamp with ac or simple dc. For me Regulated DC has been the answer specifically a voltage doubler into an lm317 with the chassis as the heatsink.

 

[ccaudle]

my house don´t have a earth, it´s 127v with a live and a neutral

For equipment without an earth (also called safety earth, protective earth, safety ground) connection you should make sure that there is no way a simple fault such as cracked wire connection or cracked insulation can cause a power line to contact the chassis. In the US equipment with only line and neutral is called "double insulated" because it is built in a way that any single break of an insulated connection cannot contact the chassis.
So the power transformer is usually mounted on an insulated plate, and plastic insulation is underneath and around all of the power wiring on the AC input side of the power supply, just so that if there is a break somewhere including in the power transformer windings there is no way the chassis can be connected to the 127V power. Note that the power transformers used for such equipment should also be double insulated, so that if the insulation breaks on the primary winding there is no way that the input power can be connected to the output winding.

Xlr pin 1 input is normally connected direct to chassi. So your xlr cabel dont act as an antenns. Cabel screen in a balanced connection is part of chassi ground. In the best case scenario it will be grounded at one end. Audio ground is connected at one point to avoid ground loops…

Note that if the cable shield is only connected at one end between equipment which does not have an earth connection, there could be a very large voltage difference between the two chassis, which will be a common mode voltage to the input stage. With transformer input that should be tolerated, but would not work with direct connected input.

Perhaps a mains isolation transformer , with a centre tapped secondary which you use to establish your own local clean ground might be an option

Not safe. Please do not make suggestions for power wiring modifications without understanding all the safety implications.

Bill Whitlock (MisterCMRR) has a really good presentation, I thought linked from somewhere on this forum, but I could not find that link.
The Central Indiana AES chapter has a link:
https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
Starting on page 22 discusses power distribution. Page 29 has a diagram of an isolated ground and shows why that is completely unsafe in a fault condition.
Starting on page 188 is a more in depth treatment of power line considerations.

The presentation is long, but fortunately includes the notes, not just the slides. It takes a little bit of understanding to understand the entire presentation, but it is like a textbook on audio system grounding. Highly recommended.

 

[Matador]

Isn't this the situation that Class Y capacitors were made for? For example, having CY's between neutral and the chassis would help shunt EMI and other electrostatic noise to the return of the primary.

 

[ccaudle]

having CY's between neutral and the chassis would help shunt EMI and other electrostatic noise to the return of the primary.

That seems like a good way to inject high frequency power supply noise onto all of your shields. Seems counterproductive if you don't have a specific reason for doing that.

 

[Tubetec]

Fair enough if my idea was inappropriate for the two wire system Caudle , I stand corrected
Still I did preface my comments with a solid caution before and after ,
Quoting the line in isolation like you did makes it look downright careless on my behalf , so perhaps you may choose to include the parragraph in the quote for context , upto yourself .

Thankyou for unearthing Bill's safety advice about mains electricity , its on top of my reading list .
Its an unfortunate situation many places have hap-hazzard electrical connections , probably without even basic fuse protection in many cases , not that fuses are even safe by modern standards . You would think in places where mains is inherently unsafe ,at very least the government should reccomend some kind of safety advice to prevent live chassis type situations occuring in the domestic setting , then again of course the safety precautions could be beyond the means of many in less well of places .

Here in Ireland if your working on a building site direct 240volt connection for tools ,even with double insulation ,is not used . The vast majority of builders tools run off 110volts here , this is usually supplied by a 3kw or so step down transformer with RCD protection on the output side and of course it carries its ground connection through to the mains supply (with ELCB protection) thats also connected to the centre tap on the output side . Now even if someone makes contact with a conductor on the secondary side of the transformer they only stand 55volts to ground across their chest for a millisecond or two before the breaker interupts the supply , that could still kill but much less likely too than 240 volts direct from a very low impedence source like your local pole or bigger ground based transformers .

 

[Nelson Paschoal]

Thanks very much everyone that replied in this discussion, I´ll not make any change in the eletrical connections as they are so now, it´s safe, with a little noise, but it´s safe, I never trade a little noise instead of my life, there are more ways to explore, like run the filaments in DC well filtered and regulated, and there´s a thing that I noticed in my osciloscope that there isn´t a AC coupling, it´s only DC and because of that maybe I did not see the ripple of the HT, I doubt that the noise come from the HT, but it´s better check.
Again, thanks everyone, and my next experience will be Vari Mu compressor!!!!