Thank you for the input. Slowly getting smarter on this stuff. Looking at Rob Robinett’s website ”How to Draw Tube Load Lines”. Plan to draw up the load lines for the monos as is based on his tutorial to enhance my understanding. Between working in high speed aluminum tubing (old pilot joke) having me gone half the week and stomping out life's little fires it’s slow going.
Power supply update.
Both monos “done”. Doing one at a time for comparison.
Notable improvement in the sound.
AC ripple at speaker measured 2.8 mV prior. Now 1.9mV. Improvement, but still seems high.
C1 24 uF Film
Choke 7H 150mA 100 Ohm
C2 115 uF (82+33)
What is a reasonable amount of AC ripple at speaker from 45 DHT amp with AC filament?
Both monos “done”. Doing one at a time for comparison.
Notable improvement in the sound.
AC ripple at speaker measured 2.8 mV prior. Now 1.9mV. Improvement, but still seems high.
C1 24 uF Film
Choke 7H 150mA 100 Ohm
C2 115 uF (82+33)
What is a reasonable amount of AC ripple at speaker from 45 DHT amp with AC filament?
I would say you are at a reasonable amount of ripple now, getting down closer to 1.2mV is possible but it takes practice. Good 10-turn hum pot. If you are using individual filament transformers then fiddling with placement. And looking at the frequency of your hum to make sure you are working on the right area of the amp. 120Hz your power side needs work, 60Hz your AC filaments need more work.
Matt
Matt
Thanks.
layout is done and looks reasonable (modifying a pair of mono blocks… My own build will come later) with one possible anomaly:
If you look at the schematic on my first post, the heaters for the 6SN7s have their voltage elevated by tapping into the center wiper of the Hum pot. I’m new to this, but have looked at quite a few schematics, and never seen anything like it.
The wire from the hump pot traverses the chassis back to front to a pair of 270 ohm resistors on the driver heater pins. How much noise would be on that wire? Is it worth reconfiguring to a more conventional set up?
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Measuring hum and hearing hum are different things. If you don't hear it you are done with that bit. If it irritates you there is always DC or more fiddling with your AC setup.
Lifting the 6SN7 filament by tying to the wiper of the hum pot doesn't look like it would necessarily be an issue. You can easily rig up a voltage divider right after your Pi filter to elevate the 6SN7 filament and see if that makes any difference. Or you can disconnect the wire from the hum pot wiper and remeasure hum. Your heater cathode voltage worst case looks to be 55Vs, which is within spec for all types of 6SN7s. Elevating AC heating of indirect heated tubes can lower hum but I've never found AC heated 6SN7s hummy, its more of a thing with higher Mu tubes in phono preamps.
Longish runs of wire are not necessarily a problem, just potentially so. As long as the wire is away from anything carrying AC, or crossing it at 90-degrees, it should be okay. In the days of strong AM radio transmissions if you were in the right area you might pick up a radio signal but that does not sound like hum. Again just to check you are sure the hum at the speaker terminals is 60Hz?
Matt
Lifting the 6SN7 filament by tying to the wiper of the hum pot doesn't look like it would necessarily be an issue. You can easily rig up a voltage divider right after your Pi filter to elevate the 6SN7 filament and see if that makes any difference. Or you can disconnect the wire from the hum pot wiper and remeasure hum. Your heater cathode voltage worst case looks to be 55Vs, which is within spec for all types of 6SN7s. Elevating AC heating of indirect heated tubes can lower hum but I've never found AC heated 6SN7s hummy, its more of a thing with higher Mu tubes in phono preamps.
Longish runs of wire are not necessarily a problem, just potentially so. As long as the wire is away from anything carrying AC, or crossing it at 90-degrees, it should be okay. In the days of strong AM radio transmissions if you were in the right area you might pick up a radio signal but that does not sound like hum. Again just to check you are sure the hum at the speaker terminals is 60Hz?
Matt
Hum only audible ear to speaker with MAOP 10 in FHXL… but only 89dB efficient. Have 100 DB efficient speakers to swap in. Predominantly 120 Hz with harmonics present up and down.
Meanwhile a power tube swap brought ripple down to 1.2mV at speaker so overall very pleased with my power supply update. Doesn’t mean I won’t tinker some more!. 👍
Thanks!
While I’m trying to figure out 6SN7 load lines and such, tinkered with a capacitor swap. Changed out the 45 cathode resistor bypass cap (100V/100uF electrolytic) with a pair of paralleled 47uF polymer aluminum. Mixed results.
With a well worn set of RCA 45s a subtle increase in detail. With new EML 45s maybe a bit shrill. Maybe need more time on the EML 45s… Or maybe time to adjust my hearing. Will give it some time, but I’m thinking about pulling the polymers back out.
With a well worn set of RCA 45s a subtle increase in detail. With new EML 45s maybe a bit shrill. Maybe need more time on the EML 45s… Or maybe time to adjust my hearing. Will give it some time, but I’m thinking about pulling the polymers back out.
…back to power supply.
The air conditioning motor run caps I ordered arrived. Unfortunately, I will be sending them back.
I was under the assumption that all motor run caps are the same. They are not.
The better ones are made of polypropylene. The lesser ones are made of who knows what. Polyester maybe?
Does it really make a difference? I have no idea. But why put polyester, or whatever, caps in when I am after the perceived benefits of polypropylene. May as well stick with electrolytic. Again…just having some fun.
The air conditioning motor run caps I ordered arrived. Unfortunately, I will be sending them back.
I was under the assumption that all motor run caps are the same. They are not.
The better ones are made of polypropylene. The lesser ones are made of who knows what. Polyester maybe?
Does it really make a difference? I have no idea. But why put polyester, or whatever, caps in when I am after the perceived benefits of polypropylene. May as well stick with electrolytic. Again…just having some fun.
Polyprops do sound ever so slightly cleaner to my old ears. https://nl.mouser.com/ProductDetail/80-C4AQCBW5750A3NJ I ordered some of these...and bought some of their smaller cousins in the past.
For the bypass caps you could also try those if you have space or something more exotic like the Audionote Kasei. (I don't think I can actually hear a difference between bypass caps)
For the bypass caps you could also try those if you have space or something more exotic like the Audionote Kasei. (I don't think I can actually hear a difference between bypass caps)
I had been measuring at normal listening levels on the volume control. When I turn volume all the way down it measures 0.7mV. All good, except:
The hum varies considerably with volume control setting. Even though the hum was louder at idle before the changes it did not track with the volume control as much. Would this be more indicative of an interference hum, say, from the newly installed choke, or is this more of a filtering issue? Any clues or thoughts much appreciated!
Not being anal so much as trying to learn, which is intent of this project.
Thanks all!
Hum, if caused by placement of a choke, would be constant. Also the frequency of the hum would help. I assume the same behavior on both channels.
That the hum tracks with your volume pot suggests the noise is incoming. You could have a bad pot and or the ground connection, and maybe other connections, due to a cold solder. Have you pulled all your source interconnect cables before you measure? Are you using a shorting plug--a RCA jack that has central pin connected to outside shield? If you have pulled all your interconnects and are using a shorting plug are you still having this identical problem?
Matt
That the hum tracks with your volume pot suggests the noise is incoming. You could have a bad pot and or the ground connection, and maybe other connections, due to a cold solder. Have you pulled all your source interconnect cables before you measure? Are you using a shorting plug--a RCA jack that has central pin connected to outside shield? If you have pulled all your interconnects and are using a shorting plug are you still having this identical problem?
Matt
They are mono blocks so both channels are completely isolated and both responding similarly. Was late night so I will recheck Just to be sure.
Test accomplished attached to the DAC. Will pull the interconnects. Don’t have a shorting plug but I could short the RCA connector internally with a jumper.
A good starting point. Thank you.
With a shorted input, if hum is still noticeable and changes with VR rotation, that means that the hum is being introduced right at the first grid, coupled to its high impedance. Needs to be tackled in two ways, first by studying and maybe experimenting with heater wiring location and orientation. And just as important, you want the "loop" from VR wiper to grid to cathode to signal ground to bottom of VR, to be as compact as possible. This input loop is the receiving antenna for impressed noises and hums. A good start is to connect cathode resistor + bypass cap directly to the bottom of VR. If separated by a coax cable, directly to cable shield. If separated by open wire, don't do that.
All good fortune,
Chris
All good fortune,
Chris
To be clear, if doing this reduces the hum, then I have to shorten/correct the VR “loop”?
Yes, this is the "receiving antenna", and other locations inside the amplifier are the "transmitting antenna". Coupling between antennas is square law, so a transmitting antenna nearby (the heater wiring) is disproportionately important. You want to tackle both, to make both as poor antennas as possible.
All good fortune,
Chris
All good fortune,
Chris
OK. Just back from the workbench. The coax run from RCA connector had the center and shield separated about 2 inches prior to the volume pot connection. Plenty of excess slack, so I trimmed that back, now under half an inch separation. Made sure the coax cable was tucked up into the corner of the chassis. The pot is right underneath the input tube and is connected with only the length of the trimmed resistor leads. When I added dropping resistors to the heaters I redid the wiring, drill twisted and tucked in the corner of the chassis on the opposite side of the input coax. Twisted as close to the heater pins as I could get it.
This combined with shorting the RCA inputs brought measured idle AC down to 0.6 to .0.7mV at the speaker and tracking with the volume control is minimal. Connected to the DAC it bumps up to about .9mV so some external influence.
To be clear, none of this has ever been audible from the seating position, not even within a meter of the speaker. But improvement is improvement, and more importantly, I am learning.
Thanks Again!
Did as suggested. shorting the RCA does make a difference, maybe 1/10th mV so not too much. A little wiring cleanup, combined with shorting, made the volume control tracking negligible. I can turn volume pot a little past halfway before any appreciable increase in hum ear to the speaker. At 0.6 to 0.7 mV idle i’m guessing I’m at the point of diminishing returns!
As stated in my prior post, the hum has really never been a problem from the listening position. But I will take the improvement! Thanks again!
Was a bit premature on this—Brand new caps and brand new tubes.
With less than five hours of runtime there is nothing “shrill” at all! These EML tubes are freaking amazing.
Will give it some more time, but may stick with the polymer bypass caps after all. I’ve only done one of the monoblock so far. More time to compare.
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