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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 8th January 2019, 08:47 PM   #1
rhthatcher is offline rhthatcher  United States
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Location: Beachwood, Ohio (Cleveland Area)
Default Taming Hum on two pre-owned Aleph Mini's

I have recently been changing my system from a 2 way passive crossover horn system to 4-way active system using a DEQX processor. Amazing technology, but that’s a topic unto itself.

Over the years I have used a Wellborne Laruel 300B amp I built 20 years ago, a 45 amp, and a Pass Aleph 3. I never should have sold that Aleph 3. I bought some amps to go active, plus I just built an Amp Camp 1.6 Stereo amp over the holidays. It was a fun build, and it was nice to fire up the soldering iron. Great build guide, too!

The purpose of this post is the other 2 amps I got: used Aleph Mini Clones. Both Alephs make some noise, one more than the other. I started down the research bunny holes on here and it looks like I bought some used Rawson builds. What the amps do well, they do extremely well. They sound great. However, there are some problems to be solved, and that’s where I’m looking for insight.

I’ll call them Tall Boy and Shorty because one is in a taller chassis than the other.

Symptoms – Noise & Hum

Tall Boy
Short the inputs – no speaker hum
Open inputs – hum
Inputs connected to Sonos source – much less hum, but not dead quiet.

Shorty
Short the inputs – no speaker hum
Open inputs – hummmmm – I don’t have a scope, but measured using OmniMic on my old horns. Biggest spike is at 240 Hz, with spikes at 60, 120, 360.
Inputs connected to Sonos source – less hum, noisier than tallboy

Diagnosis

I opened up both amps and looked around. I also did some searching on here to get a sense of what I’m looking at.

Power Supplies – big deviation from Pass design.

Both Amps use Hammond Transform 25VCT and one rectifier bridge. “As-Built” pix are attached below. Neither had the ground on the plug connected, I fixed that. Wires looks like they’re from the scrap heap. No bleeder resistors. In fact, no resistors at all. So it’s not a CRC, it’s only C. Tall Boy uses 6x 22,000 uF 25V Nichicons. Shorty uses 6x 15,000uF 25V caps. Shorty has 1 thermistor, Tall Boy has none. Both amps have +17V / -17V rails.


Amp Circuits

Tall Boy uses 327791 Boards.
This is a headscratcher - There is a 2.7 Ohm 5% 3.75W Resistor between board and negative speaker output terminal. Transistors look to be
Qty 2 - 2SJ74
Qty 1 – ZTX550
Qty 2 – MPSA18
Qty 2 – IRFP240
The only thing supporting the boards is the MOSFETS. There are no standoffs.
The leads on the smaller transistors are must be bent to go into the appropriate holes on the board. This is different from the drop-in “plug and play” boards on the Amp Camp Amp I just built. Probably just fine, but I found it a bit odd to see the legs twisted a bit, not stick straight.

DC Offset – Measured at speaker terminals = 43mV (Left), 104 mV (Right)
Voltage across 0.47Ohm R =0.648V
Therefore Bias = 1.378A


Shorty looks like it uses BrainGT boards. That’s what I can guess from searching threads here.
Transistors look to be
Qty 2 - 2SJ74
Qty 1 – ZTX550
Qty 2 – MPSA18
Qty 2 – IRFP240
The only thing supporting the boards is the MOSFETS. There are no standoffs.
Same thing on the smaller transistor leads as above.

DC Offset – Measured at speaker terminals = 34mV (Left), 37mV (Right)
Voltage across 0.47Ohm R (R27/28) = 0.637V
Therefore Bias = 1.355A


Questions / Potential mods:
Total Power Supply Rebuild – looking for suggestions / tips
DC offset adjustment - replace R8 with SP064W-500-ND (500 Ohms 0.5W, 1/2W multiturn pot)
Bias Adjust – replace R13 with SP064W-200K-ND (200 kOhms 0.5W, 1/2W multiturn pot)


My Amp Camp Amp is pictured, too. That was a really fun project!
Attached Images
File Type: jpg Tall Aleph Mini Power Supply.jpg (33.9 KB, 517 views)
File Type: jpg Short Aleph Mini Power Supply.jpg (35.2 KB, 523 views)
File Type: jpg IMG_9661s.JPG (531.3 KB, 517 views)
File Type: jpg IMG_9662s.JPG (492.7 KB, 498 views)
File Type: jpg IMG_9664s.JPG (505.3 KB, 480 views)
File Type: jpg IMG_9649s.JPG (548.3 KB, 160 views)
File Type: jpg IMG_9652s.JPG (493.1 KB, 149 views)
File Type: jpg IMG_9659s.JPG (421.7 KB, 147 views)
File Type: jpg IMG_9657s.JPG (427.0 KB, 132 views)
File Type: jpg RT ACA16 Build.jpg (431.1 KB, 152 views)
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Old 8th January 2019, 09:35 PM   #2
Dennis Hui is online now Dennis Hui  Canada
Hoopy Frood
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Taming Hum on two pre-owned Aleph Mini's
Just some random thoughts/comments...

Good thing you've made the safety earth connections. That was the first thing to do.

The legs are twisted because their pinouts are different from the mosfets, for example,
used originally on the BrianGT boards. Be careful you don't short out those parts.

I would rebuild the PS completely on PCBs into a CRC supply. You can reuse the
caps.

Please consider adding standoffs to the PCBs. I would also add the changes
to allow trimming of DC offsets.

Nice ACA build!
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Old 8th January 2019, 10:00 PM   #3
Zen Mod is offline Zen Mod  Serbia
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Taming Hum on two pre-owned Aleph Mini's
agree with D

study schmtc of usual FW CRC supply , take good care of physical layout of cap bank - where input wires are connected , where output wires are connected and where is takeout for GND

besides that , ensure proper pcb fasteners , triple check all thermal interfaces , be sure to have big washers on mosfets , same as to have split washer on every bolt on every thermal interface , including Graetz bridges ...... I did change several of these , due to loosened bolt and bridge loosing cooling ....


you can see my (always twisted) logic here

Click the image to open in full size.
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Last edited by Zen Mod; 8th January 2019 at 10:03 PM.
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Old 9th January 2019, 07:50 AM   #4
albertNL is offline albertNL  Australia
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Taming Hum on two pre-owned Aleph Mini's
Quote:
Originally Posted by rhthatcher View Post
Power Supplies – big deviation from Pass design.

Both Amps use Hammond Transform 25VCT and one rectifier bridge. No bleeder resistors. In fact, no resistors at all. So it’s not a CRC, it’s only C.
Tall Boy uses 6x 22,000 uF 25V Nichicons. Shorty uses 6x 15,000uF 25V caps.
The original Pass Labs Aleph 3 also used a single bridge rectifier and a C (not CRC) psu without bleeder resistors. It used 8 x 22000uF, so a bit more than these have.
Quote:
Tall Boy uses 327791 Boards.
Qty 2 - 2SJ74

Shorty looks like it uses BrainGT boards.
Qty 2 - 2SJ74
The fact that they are using 2 x 2SJ74 makes them more like an Aleph J (JFet front end rather than 9610 mosfet used in original Aleph series).
Quote:
Questions / Potential mods:
Total Power Supply Rebuild – looking for suggestions / tips
Generic FW psu is a good choice as suggested by Dennis and Zen Mod.
Quote:
DC offset adjustment - replace R8 with SP064W-500-ND (500 Ohms 0.5W, 1/2W multiturn pot)
If you look at the Aleph J schematic http://www.diyaudio.com/forums/attac...j-skema-bw.png the DC offset can be adjusted by R8 but the total resistance is R6 (562R) + R8 (0-500R). The fixed resistor is used together with a trimmer. The trimmer is as small as possible (let the fixed resistor dissipate most of the heat) but large enough to cover the range of values you might need.
If I remember correctly, there was only room for 1 resistor and no trrimmer on the BrianGT boards. If you replace the resistor that is on there now (what value is it?) by a 500R trimmer you will probably not have enough range. I would recommend the approach used in the store Aleph J, which is resistor + trimmer (probably best to use values in the Aleph J BOM). If you want to try trimmer only, you will probably need to go to 1k.
Quote:
Bias Adjust – replace R13 with SP064W-200K-ND (200 kOhms 0.5W, 1/2W multiturn pot)

On the Aleph J, bias is adjusted using trimmer R7 (no fixed resistor in series). It is specified as 2k. Why would you use a 200k trimmer?

I did not look up the product numbers you mentioned but noticed they were not what is specified in the Aleph J BOM. Why deviate from the Aleph J BOM?

Hope this helps!

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Old 9th January 2019, 10:33 AM   #5
Dennis Hui is online now Dennis Hui  Canada
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Location: Toronto, Canada
Taming Hum on two pre-owned Aleph Mini's
It's not clear how the part designation on his boards match up with say, the
standard Aleph-J schematics.

Agree that they are suppose to be mini AJ's. My suggestion to OP is to
refer first to the original AJ schematics and see how well things match up
(minus the obviously few output mosfets).

Question for OP: How much work are you willing to put into this? For example,
are you amenable to salvaging parts from the amp board and rebuilding on
new PCBs, and also do some mechanical work (drilling and tapping some new holes)?

Cheers,
Dennis
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Old 9th January 2019, 12:03 PM   #6
rhthatcher is offline rhthatcher  United States
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Quote:
Originally Posted by Dennis Hui View Post
Agree that they are suppose to be mini AJ's. My suggestion to OP is to refer first to the original AJ schematics and see how well things match up (minus the obviously few output mosfets).
Yes, I think they're supposed to be Mini-J's. I'll study up to see how they compare. Good call.


Quote:
Originally Posted by Dennis Hui View Post
Question for OP: How much work are you willing to put into this? For example, are you amenable to salvaging parts from the amp board and rebuilding on new PCBs, and also do some mechanical work (drilling and tapping some new holes)?

I'm willing to gut the thing and rework it. The PS should be straightfoward. Having just built Amp Camp Amps, and looking at these, the boards look to be similar in terms of complexity. The main difference here is drilling and tapping.



Right now my gut says build the Power Supplies and appropriately tweak DC Offset and Bias (if needed) and give them some run time. 10W range is plenty for my horns. If I could get my hands on the transistors I'd consider a 30W amp for my woofer.
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Old 9th January 2019, 12:24 PM   #7
rhthatcher is offline rhthatcher  United States
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AlbertNL,


Thanks for the insight - good read! I'm putting together bits and pieces as I comb through these pages. Before I order any trimmers I'll get a closer look at schematics and make sure I get the right values. Thanks for the reminder of fixed plus pot, not just relying on the pot by itself.
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Old 9th January 2019, 01:54 PM   #8
Dennis Hui is online now Dennis Hui  Canada
Hoopy Frood
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Join Date: Aug 2002
Location: Toronto, Canada
Taming Hum on two pre-owned Aleph Mini's
Shorty's offsets, while not great, are minimally acceptable. May I suggest you
start with a new PS there, with thermistor between PS gnd and chassis and safety
earth, and see what improvements are achieved.

Can you check the value of the fuses? (Just in case it's something completely
inappropriate.)

Cheers,
Dennis
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Old 9th January 2019, 02:33 PM   #9
rhthatcher is offline rhthatcher  United States
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Quote:
Originally Posted by Dennis Hui View Post
Shorty's offsets, while not great, are minimally acceptable. May I suggest you start with a new PS there, with thermistor between PS gnd and chassis and safety earth, and see what improvements are achieved.

Can you check the value of the fuses? (Just in case it's something completely
inappropriate.)

I'll check fuse values. Good idea. I checked that they're in there, but I may need a magnifying glass to read them.



Different question. I'm putting together a BOM for power supplies.



Thoughts on keeping the existing Hammond 25VCT transformer vs. going to a 2x15V 200VA AnTek? I need to make some measurements on chassis depth to see about a board plus toroidal xformer. 2x$32 + shipping isn't a killer, but I'd like to reuse what I can where possible.



I see some Pass Power Supply PCB's on Ebay that look like the exact design, I'm thinking of going for those.
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Old 9th January 2019, 03:46 PM   #10
rhthatcher is offline rhthatcher  United States
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I did a little homework. Looks like my idea to keep the Hammond won't work. I need 2 secondary windings.
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