I have compact fluorescent lights in almost every fixture in the house. I also have a dual 4 foot above my workbench. When I turn off all the lights in the basement, the noise doesn't change at all. I'll try to get a decent capture of the 85kHz noise... figures, I just spent 5 minutes and I can't see it anymore. If I see it again I'll get a photo.
Here is a scope capture that shows the heater AC with noise from stage 3 overlaid. It's clear that the 150mV spikes align with the "high frequency" edge on the distorted heater sine wave.
What I'm hearing is 120hz buzz. There is some higher frequency hash when the gain is cranked but I think that is to be expected. Hang on, I'm putting together a short video to stick on YouTube... ...thanks for waiting!
Here is the video.
Here is a scope capture that shows the heater AC with noise from stage 3 overlaid. It's clear that the 150mV spikes align with the "high frequency" edge on the distorted heater sine wave.
An externally hosted image should be here but it was not working when we last tested it.
What I'm hearing is 120hz buzz. There is some higher frequency hash when the gain is cranked but I think that is to be expected. Hang on, I'm putting together a short video to stick on YouTube... ...thanks for waiting!
Here is the video.
I guess I took too long to post, you guys sneaked in between. 🙂 Well, you can see that my trace is very similar to the one in the link trobbins provided. I don't have a 100 ohm pot handy, but I do have some 250 ohm resistors I can try.
Duh, no effect... 100 ohms / 2 = 50 ohms per leg. I'll throw together two 45 ohm resistors from some 15's I have. P=(6.3/45)*6.3 = 882mW, so 3 1/2W resistors in series is safe. Be right back. I know you guys are on the edge of your seat, rooting for the amp building freshman over here. hahaha...
The humdinger is used to 'tune' the heater voltage (with all its harmonics) signal being coupled to the grid - the pot allows a null to be found - a simple humdinger with fixed resistors won't provide any change per se.
45 ohms from each leg of the heater supply to the 70Vdc offset, at the valve farthest from the xfmr, no effect. 🙁 Still seeing 120-150mV of noise at stage 3.
Is the goal of the humdinger to provide a high current path to ground for the spikes? I've provided a current path to the CT of the heater winding, but the 70Vdc is being created through a simple voltage divider using 100k and 470k resistors. There is no high current path to ground...
Is the goal of the humdinger to provide a high current path to ground for the spikes? I've provided a current path to the CT of the heater winding, but the 70Vdc is being created through a simple voltage divider using 100k and 470k resistors. There is no high current path to ground...
Darn.. so I'm not going to fix this today. So the humdinger pot, is it just a normal pot? Nothing special except that it should be able to dissipate 1 watt?
Okay, then you have two separate issues going on. One is the 85 kHz carrier which is externally sourced from your lighting. The other is the 120 Hz buzz from the steep wavefront.
I would install a tiny little split bobbin transformer to feed the AX7. It can be tucked away in a remote area. Just keep the leads twisted.
Humdinger is still a band-aid, but it can reduce the effects to tolerability. Those instrument amps never paid much attention to RF and hum suceptibility.
I would install a tiny little split bobbin transformer to feed the AX7. It can be tucked away in a remote area. Just keep the leads twisted.
Humdinger is still a band-aid, but it can reduce the effects to tolerability. Those instrument amps never paid much attention to RF and hum suceptibility.
I use a 200R 10T 0.75W Bournes trimpot or a 2W WW - as I have them lying around. 100-500R range should be ok.
The humdinger pot technique acts as a 'filter', although it is really a neutralising circuit per se. It alleviates one form of hum ingress, and even if you didn't have higher frequency rectifier noise added to the mains frequency, is still a worthwhile addition to any amp imho.
All amps with one common PT will exhibit this effect to some degree when using AC heaters. Your heater wiring layout may have exacerbated the coupling. Similarly, the use of extra main filter capacitance.
The humdinger pot technique acts as a 'filter', although it is really a neutralising circuit per se. It alleviates one form of hum ingress, and even if you didn't have higher frequency rectifier noise added to the mains frequency, is still a worthwhile addition to any amp imho.
All amps with one common PT will exhibit this effect to some degree when using AC heaters. Your heater wiring layout may have exacerbated the coupling. Similarly, the use of extra main filter capacitance.
Can you elaborate on this? You mean a separate transformer for pre-amp heaters? Why is split bobbin significant?
trobbins said:
I'll have to dig around at work tomorrow and see if we have any lying around, otherwise I'll be heading to the local electronics shop some time this week.
I'm starting to think that DC regulated heaters might be in order for the pre-amp, or at least for the first two 12AX7's. If the humdinger doesn't help, I'll look into doing that.
Something like a Hammond 166F12. Not sure about the amp specifics (how many 12AX7's) but chose for the proper current draw.
Split bobbin minimizes capacitance between primary and secondary windings, and typically has high leakage inductance. This all equates to a relatively clean output, and will be immune to that 85 kHz trash on your AC line. Still would be recommended to elevate the heaters about 40V.
Split bobbin minimizes capacitance between primary and secondary windings, and typically has high leakage inductance. This all equates to a relatively clean output, and will be immune to that 85 kHz trash on your AC line. Still would be recommended to elevate the heaters about 40V.
Okay, I'll look into it. Luckily there's a Hammond office right down the street from where I work. I'm running 5 12AX7's, right now they're wired parallel so the current is about 1.5A, but I could rewire them as 12.6V half the current. I will still keep the elevation since I have a couple of cathode followers. Looks like 166J12 (1A) would be perfect.
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Just called Hammond and the little transformer is only $14, so I ordered it. Hopefully they have it in stock (their computers were down so she was going to have to call over to Guelph). Also ordered a 125GSE output transformer for my next SE EL34 project. 🙂
I just installed the new transformer, wired to the pre-amp heaters (main power xfmr is still powering the quad 6V6 heaters) and it's all good. Very sweet and quiet. It killed a lot more noise that just the buzz. Too bad everybody else is in bed, I really want to crank it up and test the sustain without the buzz interfering!
Thanks to everybody for the help and suggestions. My next project will have a 5AR4, zero silicon in the design, so hopefully I won't have the same headaches with switching noise!
Thanks to everybody for the help and suggestions. My next project will have a 5AR4, zero silicon in the design, so hopefully I won't have the same headaches with switching noise!
Good for you. Would love to see from scope captures when you have the chance. So the 85 kHz is also under control ?
Chances are its still getting through the main transformer, but since it only affects the output stage the noise level is small.
Shielded and split bobbin xfmrs are your friend.
Sorry to take you down the snubber path, but it at least needed to be checked. Process of elimination.
Chances are its still getting through the main transformer, but since it only affects the output stage the noise level is small.
Shielded and split bobbin xfmrs are your friend.
Sorry to take you down the snubber path, but it at least needed to be checked. Process of elimination.
No worries, I appreciate the help.
I'll do some more scoping to see if I can find the 85kHz again. I'll also post both heater supplies overlaid, for comparison.
I'll do some more scoping to see if I can find the 85kHz again. I'll also post both heater supplies overlaid, for comparison.
Here is a shot of the two different heater supplies, overlaid.
I still couldn't find the 85kHz noise. I have no idea where that was coming from... oh well, it's gone, and my amp sounds fantastic now, so I'm happy. 🙂
I still couldn't find the 85kHz noise. I have no idea where that was coming from... oh well, it's gone, and my amp sounds fantastic now, so I'm happy. 🙂
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