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Old 7th September 2014, 10:07 AM   #1
cwtim01 is offline cwtim01  Hong Kong
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Default Encasing my first power amp

Hi,

I've built my very first power amp (a 8W class A single rail amplifier based on a Juma design). The PSU and amp boards have been tested working, and the final task left is to put them into a case.

However, despite reading a lot in this forum, I'm not very sure of what should be the proper way to do so - and terms like ground loop, chassis ground, power ground, signal ground, audio ground, ground bus, star ground ... are confusing (to me) as ever. Worse yet, some gurus seem to disagree on certain topics.

Therefore, I drafted the attached diagram showing how I plan to install the wiring in the case, and I would like helps in pointing out mistakes I've made, as well as any comments.

Thanks for your help in advance.
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File Type: jpg Case Connection.JPG (158.4 KB, 141 views)
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Old 7th September 2014, 10:10 AM   #2
AndrewT is offline AndrewT  Scotland
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Does the assembly work properly on the bench?
That is the place to experiment with and hunt for wiring errors.
Don't look for wiring errors AFTER your have folded it up, to fit inside the box.
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Old 7th September 2014, 10:12 AM   #3
AndrewT is offline AndrewT  Scotland
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The inputs, the LV AC and the mains are correct.
Do you have a single polarity power supply? to a single polarity amp?

The outputs are wrong.
The return from the output should be close coupled to the Flow all the way back to the source of the current.
You have taken the return as close coupled back to near the amp pcb and then put in a BIG LOOP to the PSU !!!!!
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Old 7th September 2014, 10:16 AM   #4
AndrewT is offline AndrewT  Scotland
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Why did you use a 4A filter for a 200VA transformer?

200VA / 230Vac needs ~ 900mA fuse and ~ 900mA filter.
Use a T800mA fuse and use a 1A filter.
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Old 7th September 2014, 10:33 AM   #5
cwtim01 is offline cwtim01  Hong Kong
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Thanks Andrew for your comments.

Quote:
Originally Posted by AndrewT View Post
Does the assembly work properly on the bench?
So far they work OK on the bench, but I noticed that when the wiring were too messy, oscillation could occur.

Quote:
Originally Posted by AndrewT View Post
The outputs are wrong.
The return from the output should be close coupled to the Flow all the way back to the source of the current.
You have taken the return as close coupled back to near the amp pcb and then put in a BIG LOOP to the PSU !!!!!
Thanks, will correct that.

Quote:
Originally Posted by AndrewT View Post
Why did you use a 4A filter for a 200VA transformer?

200VA / 230Vac needs ~ 900mA fuse and ~ 900mA filter.
Use a T800mA fuse and use a 1A filter.
I was worrying about inrush current due to the large cap bank (176mF in total), hence used a bigger fuse. Would inrush current be an issue?
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Old 7th September 2014, 10:36 AM   #6
cwtim01 is offline cwtim01  Hong Kong
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Quote:
Originally Posted by AndrewT View Post
Do you have a single polarity power supply? to a single polarity amp?
Yes, it's a single polarity power supply to a single polarity amp. Attached is the schematic of the amp. (just that 24V became 22V due to the transformer I received).
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File Type: jpg Schematic.jpg (58.9 KB, 130 views)
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Old 7th September 2014, 10:41 AM   #7
AndrewT is offline AndrewT  Scotland
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A soft start limits the transformer start up current.

Big capacitance is not usually a problem.
Add a soft start and try a T1A fuse to see if it allows more than one restart after depleting the cap bank.
If the cap bank does blow a T1A fuse with the soft start fitted then look at adding a slow charge circuit. A bypassed Power Thermistor in the transformer secondary/ies
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Old 7th September 2014, 10:48 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
So far they work OK on the bench, but I noticed that when the wiring were too messy, oscillation could occur.
if you get oscillation or evidence of instability when wires are moved around then your WIRING is at fault. Either excessive radiating of interference or excessive pick up of interference.
Correct the wiring.
I wrote in another Thread a few minutes ago:
Quote:
EVERY SIGNAL connection is a TWO WIRE connection.
The Source supplies the current to the Flow wire of the pair. The Source receives back the Return current through the other wire of the PAIR.
The CURRENT that leaves the Source MUST come Back to the Source, exactly.
This applies to every wire pair in the amplifier, speakers, PSU, low level signal, feedback (traces), every PAIR!
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Old 7th September 2014, 11:17 AM   #9
cwtim01 is offline cwtim01  Hong Kong
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Wiring diagram updated.
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File Type: jpg Case Connection 2.JPG (162.3 KB, 122 views)
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Old 7th September 2014, 04:37 PM   #10
AndrewT is offline AndrewT  Scotland
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That looks good.

Wire that up on the bench and prove it works first for a mono assembly, then for the other mono assembly and finally for a stereo assembly.
Move all the pairs around. Prove that there are no interference effects. fold the assembly into roughly the shape and locations that will fit in the chassis. this should still work.

When that has been proved, move the whole assembly to the chassis, exactly duplicating each electrical connection.
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