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Taming Hum on two pre-owned Aleph Mini's
Taming Hum on two pre-owned Aleph Mini's
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Old 17th February 2019, 05:44 PM   #51
6L6 is offline 6L6  United States
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Taming Hum on two pre-owned Aleph Mini's
Holy crap the transformer secondary is made a C.T. directly on the chassis.

Don't plug that one in anymore.
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Old 22nd February 2019, 04:38 PM   #52
rhthatcher is offline rhthatcher  United States
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Back to Shorty.

I thought I would try some things from this thread/board and some ideas from some calls with Jim T (6L6) before making a parts order. I’ve been too busy all week to sit down and make a shopping list and look up parts at Digikey. And let's work with what I have first. Maybe the fix will be free? Wishful thinking.

First wave of changes:
  • Flipped the transformer around and twisted secondary wires, played with flip/flopping secondary wires
  • Added more ground wires between PSU boards (now 6 total)
  • Swapped 1k resistor and trim pot between R7/R8 positions
  • Test and turn DC Offset down. ~3mV on one channel, ~2mV on the other when cool
  • Added 2 more ground wires to play with some grounding at RCA’s (left them taped up).
  • Put Thermistor right on the PSU board and down to chassis. Shortended earth ground wire. Both connect at one screw between PSU boards now.
  • Checked the grounding with the CL60 place.
o Power supply ground to chassis = ~13.8 Ohms
o RCA Negative to Chassis = ~13.9 Ohms
o Speaker Negative to Chassis = ~13.9 Ohms
o IEC gnd pin to chassis = 0

Still hums.

Next: remove RCA input ground wire use the pigtails to go to PSU ground directly. No change.

Next – cheap and dirty (and temporary!) CRCRC filter. I used the ground pigtails and cut the V+ and V- wires. I got a couple 22000uF caps from the original amp board and some 0R22 5W sand resistors from another amp and put it together. Double sided foam tape to hold it together. Voltage dropped from +/- 19.9 to +/- 19.37 at the rails. Check DC Offset and Bias. Plays music. All good.

Still Hum.

Crack a(nother) beer and search more threads with “aleph” and “hum” as key words. Wow, I’m not the only one!

Lift one RCA input ground at the connector = louder hum
Tried 14AWG wire between RCA Input Grounds with and without one of the RCA grounds connected. Hum

I read something about shielded interconnects and problems. I make my own interconnects using Belden 1505F (RG-59/U Coaxial Cable 22 AWG Stranded Copper Conductor Dual Shield 75 Ohm), so this got my interest. The ones on my bench are about 1M, and are connected a Sonos Connect direct to the amp. That’s how I’ve been testing. Sonos to amp to some cheap Yamaha surround speakers I pulled out of my living room.

I found a vintage pair of Radio Shack RCA’s a nearby box and decided to try those. When I unplugged an RCA things changed – hum volume was not the same. This got me thinking. Perhaps I have a ground loop related to the inputs?

Take a step back, let’s take a different approach. First I played with this on my bench, then I confirmed it on my horns:

Both RCA inputs shorted – quiet
1 RCA Shorted, one RCA Open (nothing connected at all): Short channel = quiet, open = hum
1 RCA Shorted, one RCA connected to preamp. Both quiet
Both RCA’s to preamp = hum
One RCA to Preamp, one RCA open. 1 quiet, 1 hum.

So – based on that… What should I look at next?
Something in the amp?
Jensen Isomax or some DIY equivalent?

I bought a Quasimodo board from MPAudio today. I put together the parts BOM. And I’ll need a scope.
I was also going to get some Schottky’s to replace my diode bridges and play with the power supply some more.
Before I buy the scope, any ideas based on the inputs?
Attached Images
File Type: jpg Secondaries Twisted.jpg (925.4 KB, 138 views)
File Type: jpg Grounding.jpg (841.0 KB, 138 views)
File Type: jpg CRCRC.jpg (987.0 KB, 134 views)
File Type: jpg Testing on Amp Stand.jpg (473.3 KB, 131 views)

Last edited by rhthatcher; 22nd February 2019 at 04:44 PM.
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Old 22nd February 2019, 08:30 PM   #53
Eric is offline Eric  United States
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Taming Hum on two pre-owned Aleph Mini's
Wow - you've been busy here! A few thoughts about places to look - I'm making a bit of a stab in the dark here...

Are you using the original bridge rectifiers or have you tried using known/new ones? I've had some issues with a rectifier that was on the edge of failing and it introduced lots of transformer groan that was coming through the speaker outputs.

Are you certain that the output mosfets are properly insulated? Might be worth removing the boards and checking the physical area where they are mounted to make sure they are smooth, no unintentional grounds, no holes/piercings in the silpad?

How about the RCA input jacks? If they are not fully isolated, you could be introducing ground hum.

I'm just trying to extend the generally poor construction techniques that you've highlighted to other areas of the construction...
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Old 22nd February 2019, 08:36 PM   #54
rhthatcher is offline rhthatcher  United States
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Eric,



Bridges are new as of last month.



I measured RCA's to Chassis and Amp ground. All as expected.



Nothing looked wrong when I had the MOSFETs out. Insulators and heatsinks looked fine. That was all torn out and reassembled yesterday. I'll be doing another teardown / rebuild this weekend to take out the extra RC in the CRCRC and fix up the wires I clipped. I'll have another look.
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Old 22nd February 2019, 08:39 PM   #55
silasmellor is offline silasmellor  Denmark
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Taming Hum on two pre-owned Aleph Mini's
As an aside, maybe place that terminal block on something non-conductive for safety. It looks like a bit of uninsulated wire poking out, if that gets in touch with your chassis the hum is the least of your problems


What happens if you lift the ground connection from one of the channels to the PSU with a resistor (just for testing)?

Last edited by silasmellor; 22nd February 2019 at 08:42 PM.
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Old 22nd February 2019, 09:00 PM   #56
rhthatcher is offline rhthatcher  United States
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Tak! The Terminal block wiring with be cleaned up when do the CRCRC to CRC surgery.



I'll try the resistor trick and see what happens. Any idea what value to try? I have some spares from my tube amp building days, so hopefully there is something that can work.
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Old 22nd February 2019, 09:56 PM   #57
silasmellor is offline silasmellor  Denmark
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Taming Hum on two pre-owned Aleph Mini's
well if a CL60 works, why not 10R?
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Old 22nd February 2019, 10:31 PM   #58
mpmarino is offline mpmarino  United States
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Thinking way back, my first mini A (the first amp I ever made that didn't have a fire ) had hum with similar characteristics. I don't think it was an 'aleph' thing though

I recall taking the psu ground to one channel, then bringing the ground from that channel to the other channel. 'series' ground I guess....

it did work .
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Old 23rd February 2019, 02:28 AM   #59
6L6 is offline 6L6  United States
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Taming Hum on two pre-owned Aleph Mini's
Quote:
Originally Posted by rhthatcher View Post
Both RCA inputs shorted – quiet

Ground loop
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Old 23rd February 2019, 08:40 AM   #60
Mark Whitney is offline Mark Whitney  Netherlands
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https://www.updatemydynaco.com/docum...lemsRev1p4.pdf

Figure 4 and 5 explain ground loops and a way to reduce there impact. You can see that L1, L3, L4, L5 and L6 work as a single transformer.
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