100Hz hum in preamp

[scott2000]

As it stands the hum is practically inaudible at normal recording levels so it's very usable,

I have plenty of room now to swap the 47uF filter cap on the EL84 for another 470uF which should drop things again - although I don't know if the 50Hz will fall with additional filtering.

I thought you already had a 470uf at the el34?

 

[untune]

I thought you already had a 470uf at the el34?

I do - I added an RC filter stage in addition to what I already had, to drop some voltage.  So it's a 22uf/choke/470uF pi filter, then there's a 2K2 resistor and the 47uF (that used to be where the 470uF is) supplying the EL84.

What I can now do is make that a 2K2/470uF for more filtering

 

[scott2000]

Makes sense.....



do you have an updated drawing?


You really shouldn't want to hear hum at recording levels. No reason it can't be way down.....

 

[untune]

Makes sense.....


do you have an updated drawing?


You really shouldn't want to hear hum at recording levels. No reason it can't be way down.....

I will update my schematic when I get a chance

I fear I may be overstating it a little there, it's difficult to describe subjectively - I say practically inaudible only because I know it's there based on the frequency analysis.  My monitors run into a volume control that I have about 9 o'clock for comfortable listening levels.  I have to crank that all the way up and insert another 12dB of gain in Reaper and then its audible, but by then the hiss from the valves/residual noise is just as apparent... but by that point, signal would be loud enough to knock a wall down.  So really, I could stop where I am and still be satisfied with the noise performance (especially considering the tight chassis layout) but I'll exhaust the other possibilities before calling it quits

 

[scott2000]

I fear I may be overstating it a little there, it's difficult to describe subjectively -

lol....Sounds like it a little.....

 

[untune]

lol....Sounds like it a little.....

That's the problem when you listen with your eyes ;D As soon as measurement and analysis come into play, it becomes an obsession

Funnily enough, I've not yet really examined what role (if any) the actual valves are playing.  I'm not sure how much tonal difference different brands make, I never (personally) heard any evidence of it when I tested it in the past... but I there are armies of audiophiles who would oppose me, I'm sure ;D I've rolled everything except the EL84 so that might be an interesting experiment

 

[untune]

Small update to this... I swapped the first filter cap at the EL84 from 47uF to 470uF and it didn't change the hum levels.

50Hz is still there, extremely low level and shifting the OT makes very minor differences but I can't get it below about -77 dB with no gain.

I've shuffled some ground wires around and tried to correct some things - following Ian's published advice and grounding XLRs at the entry point to the chassis as they are transformer isolated and shielded up to the transformer.

I'm waiting for some mild steel to try making some additional dividers/shielding.

Strangely, I'm now getting a slight reduction in the hum at the first input stage (mic/line) compared to the second input stage (instrument) despite them being previously identical.  The only thing I can think that may be an issue there, is that the filter cap for the first stage is grounded at the base of the first stage (cathode cap/res) whereas the second stage filter cap is grounded at the base of the *third* valve, the EL84.  Going to experiment with that later.

Might also try a low value resistor right before the choke to drop a few more volts.

Cheers

 

[scott2000]

Could leaky caps (at certain voltages) cause any issues? Or  tubes ???

 

[ruffrecords]

I think your HT supply is now OK. Your remaining issues are ones of pick up. either electromagnetic, between transformers, or electrostatic to high impedance unscreened wires

Cheers

Ian

 

[untune]

Could leaky caps (at certain voltages) cause any issues? Or  tubes ???

I bought a couple of Teleunken valves and have a bunch more on the way.  Swapping them frequently could have caused issues at the socket, it feels a bit loose, I'll look at retouching them

Ian; I think you're right, screening is my last thing to try.  I'm going to get some screened single core.  In other places I've used screened balanced cable and just linked one of the twisted pair ith the shield, that might do.

Quick aside as I've not done it before - terminating the mic XLR input in order to check the noise levels - do I use a 200ohm resistor, or 2x100ohm, attach each to hot and cold, are they grounded etc?  Or do I just short both pins to ground?

 

[ruffrecords]

Quick aside as I've not done it before - terminating the mic XLR input in order to check the noise levels - do I use a 200ohm resistor, or 2x100ohm, attach each to hot and cold, are they grounded etc?  Or do I just short both pins to ground?

You connect the resistor between pins 2 and three. There is no connection to pin1 because this is a screen. It is not a signal carrying conductor.

Cheers

Ian