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PostPosted: 16 Oct 2017, 10:42 
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Joined: 27 Dec 2014, 14:15
Posts: 62
Fair enough. Some would argue that a PCB brings better consitency between units, but I understand your argument.

Anyway, I’m far from an expert, so I usually just sit back and read/enjoy people’s opinions. I appreciate you taking the time to elaborate.


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PostPosted: 21 Oct 2017, 05:16 
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Joined: 27 Dec 2014, 14:15
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I've done some minor experiments with the board, without any luck.

At the end, I've had an idea about using an output transformer. I had a pair of Murata DA102MC transformers, so I installed one on the left channel. No more hum... but almost no music either (only the top frequencies went through). I guess I should've expected that give it's a _Digital_ Audio transformer.


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PostPosted: 03 Nov 2017, 21:06 
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Joined: 23 Feb 2017, 02:02
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Wihout reguards to whatever you do, you should always have a very large cap 10+uF at heater ceter tap or directly at heater, connected to amp main return ground where rca and everything is tied together. From my exprience, this greatly reduces humm, comparsison to directly grounding center tap and having dc heater. I can run 6v ac while elevating heaters to 48v but having a 1000uF 63v and 0.1uF to suppars high freq noise from center trap heter to ground, resulting in less noise than dc heater elevated and 2.2uF center tapp to ground, and less nosie than directly grouding ceneter tap.
I get zero humm, only a little bit of white noise.
The plorbem with directly grouning your cetner tap is that it will have larger ammounts of white noise because the heaters are not at same potential as cathode and theres leakage. You will have zero humm but more white noise.

You should also never directly ground you amp case, then have your star ground return path connected to case. The star ground return parth should directly be grounded to mains, you can then put a ground from the star return to case, or directly ground your case to mains. I had massive hums for directly groudning case then connecting star ground retun to case.

From a bit of reading from past posts, you get hum regradless of what you do, seems like that high voltage, heater and circuit isn't the plorbem here.
If thats the case all thats left is the groudning causing hum. Or your input wires connected too close to your transformer. The transofmrer causes massive hum, always keep your amp+tubes away from the transfomer as much as possible. The ground connection from transfomrer also improtant, I recomend directly grounding your main big filter cap to transfomer, then running a single wire somewhere to closer of pcb away from transfomrer, closer to pcb this point should be your star return of all grounds, there should also be at least 1uF(+0.1uF for high freq bypass) cap from high voltage to ground in at this point.

If your amp and music source isn't conencted to 220v grounds but your pre is, then that will also cause humm. Easily fix this by connected all equipment to 220v ground.


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PostPosted: 04 Nov 2017, 07:22 
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Joined: 27 Dec 2014, 14:15
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I tried lifting the grounding from the case alltogether, but that didn’t change anything. I also tried moving the trafo farther from the tubes/circuitry... no luck.

In any case, my cheapo chinese scope came in today. I’ll be able to look at the power rails now and see if they’re good.


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PostPosted: 04 Nov 2017, 15:10 
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Joined: 23 Feb 2017, 02:02
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A lm317T wihout heat sink can be used to regulate the b+. It will require high wattage resistors from adjustment to ground, by dropping about 5-10v accross it you will have very pure voltage. It also says on datasheet aslong as the output is not shorted the lm317 is floating and can regulate several hundereds of volts.

In the case where tubes will never be reomved, the voltage supply should be constant. This will work, if load decreaces/tube removed the input voltage increaces and will make the lm317 exceed 40v input output voltage and kill it. This case you can use a zener reference to the base of a power transistor then this will supply a constant voltage to 317.

If you want to know more I'll tell you more.

Have you tried groudning your pcb ground DIRECTLY to 220v supply grounds ? You said you lifted the ground and let it flow if I am not mistaken. Your source and amp may be grounded, and your pre is floating this will cause a large humm.
It sounds like you grounded your case directly to 220v then you connect pcb ground to the case, this will cause humm.


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PostPosted: 04 Nov 2017, 17:50 
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Joined: 27 Dec 2014, 14:15
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Addind a LM317 to the PCB would require some modifications. I wouldn’t try that... unless it’s a last resort kind of thing.

Like I said in previous posts, lifting the ground had absolutely no effect on the hum, but it did add some high freq noise, if I remember correctly (not sure anymore, as I’ve tried so many things).

I’ll try to do some measurements tomorrow with the scope and try to see what’s the cause of it.


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PostPosted: 20 Nov 2017, 15:18 
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Joined: 27 Dec 2014, 14:15
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A solution! Finally! http://www.diyaudio.com/archive/blogs/alexcp/index.html

Apparently, if there's a mismatch between the two halves of the HV secondary, you can get a 50hz hum. If there's a mismatch between C18 and C19 you can get a 100hz hum.

I tried the simplest solution and I disconnected the center tap. Bam! 50hz hum gone. Now I have a 100hz hum due to the cap mismatch... But this is much lower. I could even live with it.

Good thing I didn't throw it away.


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