AC heaters ultimate hum limit

[ruffrecords]

Sorry, yes, I guess I was meaning 12v. This is what happens when you post 5 minutes after you wake up.

LOL

I guess I could add in the pads/links so it could be wired either way. I have been concentrating on improving the later. Don't want to undo that.

Cheers

Ian

 

[ruffrecords]

Also perhaps flood the top layer above the heater traces to act as ground screen.
Heater traces could be closer together?

I went though a couple of iterations. First I had the heaters tracks above each other on either side of the board but then you need spurs to connect the AC input and humdinger. Then I had both tracks on top with ground plane beneath but that caused a ground plane loop around each tube (I guess I could fix that with exclusion zones). lastly I had the two underneath which broke the ground plane loop.

I might revisit the tracks on top of each other scenario.

Cheers

Ian

 

[CJ]

you could dictch the traces and run ext wires to thru holes,

 

[trobbins]

Always lots of design compromises - ac voltage capacitive coupling, ac current coupling, 12V, 6V, spacing to other traces, spacing to parts in a 3D sense, inter channel spacing.

 

[Whoops]

Hi Ian,
sent you some emails with info that might be relevant to you

 

[Whoops]

some useful readings on Heaters:

http://www.valvewizard.co.uk/heater.html

http://el34world.com/charts/fenderservice4.htm

http://el34world.com/charts/CommonHookups.htm


some amps with DC heaters:

http://el34world.com/Hoffman/images/2El84skem.gif

http://el34world.com/Hoffman/images/26V6skemdc.gif

http://el34world.com/Hoffman/images/2034Layout.gif

 

[ruffrecords]

I have finished the final PCB layout. Mostly minor tweaks but the main change is to route the heater tracks between the tubes on top of each other on opposite sides of the PCB. I have snt of for some prototypes. I will build one when they arrive and check its performance.

This has taken longer than expected because I have been messing around with REW which is an excellent piece of software for measuring audio spectra. I finally got it calibrated to my Focusrite Scarlet 2i2 and built a simple interface box protect it from phantom power and provide some attenuators on the input and output. I even had a hum issue with that which was due to a Carnhill transformer I was using. replaced it with a Sowter one in a mu-metal can and that is fixed. I can now reliably measure incredibly low levels of hum (and its harmonics). I will post some comparative pics in a future post which will give me something to compare with when I get the new PCB built.

Cheers

ian

 

[ruffrecords]

Here is the noise spectrum of the classic mic pre housed in a Fischer cassette with dc heaters.

Cheers

Ian

 

[kambo]

just curiosity
attached ...

 

[ruffrecords]

just curiosity
attached ...

It is an artefact.  The noise spectrum varies continuously with little peaks like that all over the place. I used four averages to smooth thinks out a little but deoending on when you take the snap shot you get little peaks like that.

Here is the same amplifier this time powered by AC heaters. No mistaking the peaks on this one!!!

Cheers

ian

Edit: Note that with the standard A or IEC filters for noise measurements, this 50Hz peak drops by about 20dB.

 

[Deepdark]

those peaks looks pretty normal for AC heaters. If we were in a DAW, it would means about -76dbfs peak. I'm really thinking about trying dc heater

 

[ruffrecords]

And lastly for comparison, the same amplifier with the gain turned right down so what you see now is just the noise and the hum from the output stage. Largest 50Hz peak is now -75dBu.

Cheers

Ian

 

[ruffrecords]

To put this all into context, attached is the loop back spectrum. This is REW generator via 2i2 to my test box (output looped back to inputs) and back into 2i2. Generator adjusted to give 0dBu at test box output. As you can see this results in a -7dBFS input to REW so for calibration purposes we need to add 7dB to all the REW readings to convert them to dBu. This means the readings in dBu I gave in the previous post are all 3dB better than I thought.

Cheers

Ian

 

[Deepdark]

Good to know

If I remember well, my apogee is about 12db

 

[kambo]

confused with your dbfs - dbu conversion !
what standard r u using?

 

[ding]

...built a simple interface box protect it from phantom power and provide some attenuators on the input and output.

Can you share some info on this? I've been looking at this http://www.pmillett.com/ATEST.htm but its a little too expensive of a build for me right now.

Thanks

 

[PRR]

> I used four averages

I have used several-minute running average to discount that type artifact and bring out the long-term trend which will annoy more. (That was for acoustic rumble, semi-random; I doubt a preamp needs minute-long averaging.)

 

[ruffrecords]

confused with your dbfs - dbu conversion !
what standard r u using?

It is not a standard it is an actual measurement of my Focusrite Scarlet 2i2 USB box.

Cheers

Ian

 

[ruffrecords]

...built a simple interface box protect it from phantom power and provide some attenuators on the input and output.

Can you share some info on this? I've been looking at this http://www.pmillett.com/ATEST.htm but its a little too expensive of a build for me right now.

Thanks

No problem. I shared the design in this thread:

http://groupdiy.com/index.php?topic=62238.msg789547#msg789547

It was originally intended for use with my QA400 but when I discovered the REW software (which runs on Mac/Widows/Linux) I switched to using my scarlet 2i2. I am currently using the second schematic in that thread. The most expensive part is the transformer. Originally this was intended to provide a balanced output from the QA400 but when I moved to the 2i2 that was not necessary. However, I kept it in because is also protects the 2i2 from phantom power when it is plugged into mic inputs. If that is not an issue for you then you can dispense with the transformer.

Cheers

Ian

 

[kambo]

sorry to bug you again
i think i got the 2i2 part, but i dont get the adding 7db for converting dbu to dbFS...
you are already reading in dbFS!