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My attempts at a design of a 3 stage amplifier
My attempts at a design of a 3 stage amplifier
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Old 16th July 2019, 05:31 PM   #601
edbarx is offline edbarx  Malta
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Join Date: May 2018
I have not yet performed a Tian Test as I do not know what is the definition of variable 'u'. I also tried to do google searches to read about Tian testing but did not succeed to find anything. I would like to know what are the objectives of this test and why a theoretical constant current source and a voltage source are connected in series with the feedback resistance chain. As I may understand, the purpose is to inject some sort of frequency to evaluate the rejection effect of the feedback loop and Miller Capacitance.
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Old 16th July 2019, 06:08 PM   #602
mjona is offline mjona  New Zealand
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Headphones used to come equipped with that sort of plug commonly referred to as a Jack abeit the socket on a domestic amplifier was an output.

I assumed that stage equipment had moved on to XLR connectors - I know this style of Jack was used for mixer and amplifier connectors back in the 70's. Stuff that old of passing interest only is not in my instant recall.
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Old 16th July 2019, 06:11 PM   #603
mjona is offline mjona  New Zealand
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Join Date: Jul 2007
Location: Upper Hutt
Headphones used to come equipped with that sort of plug commonly referred to as a Jack - albeit the socket on a domestic amplifier was an output.

I assumed that stage equipment had moved on to XLR connectors - I know this style of Jack was used for mixer and amplifier connectors back in the 70's. Stuff that old, of passing interest only, is not in my instant recall.
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Old 16th July 2019, 08:54 PM   #604
mjona is offline mjona  New Zealand
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Join Date: Jul 2007
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Quote:
Originally Posted by edbarx View Post
I have not yet performed a Tian Test as I do not know what is the definition of variable 'u'. I also tried to do google searches to read about Tian testing but did not succeed to find anything. I would like to know what are the objectives of this test and why a theoretical constant current source and a voltage source are connected in series with the feedback resistance chain. As I may understand, the purpose is to inject some sort of frequency to evaluate the rejection effect of the feedback loop and Miller Capacitance.
See Plotting Loop Gain Using the Tian Method - Spring 2011
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Old 17th July 2019, 07:51 AM   #605
edbarx is offline edbarx  Malta
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Two Grounding Omissions Found!

a) two large current loops with areas exceeding 100 square centimeters were found. These are formed by the output conductors feeding the output PCB. One of them is almost touching the transformer! Both amplifier channels have these loops with the one having its loop almost touching the transformer exhibiting the loudest hum/buzz/noise.
b) the grounding of the amplifier modules is trough a large aluminium plate at the bottom of the amplifier box. Lifting this plate off the box's base, I found there is paint insulating it from the box. Checking with an Ohmmeter, I found it is completely isolated from ground! This means, the only ground path from the amplifier modules was through the capacitance formed by the amplifier box's base, paint and aluminium plate.

I have the intuitition, which can be wrong, that these omissions may have forced grounding currents to take strange abnormal paths, causing the hum/buzz/noise issues.

My not so efficient memory tells me the output conductors feeding loadspeakers are to be included in the elmination list of loop areas.

Last edited by edbarx; 17th July 2019 at 07:55 AM.
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Old 17th July 2019, 07:08 PM   #606
mjona is offline mjona  New Zealand
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So having remedied these anomalies does this indicate final success? The output of your amplifier is also connected to the inverting input.

A radiated electromagnetic field at 50 Hz will induce a current in a conductor if the former is large enough and the latter is critically close.

Seeing the cramped space inside your chassis I suggested containment of the transformer stray field by fitting a copper strap around the transformer.

Whatever the stray field intensity is, it will be amplified. Your best hope is that distance helps mitigate the risk twisting leads also helps. You could re-route your wiring accordingly.
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Old Yesterday, 12:31 PM   #607
edbarx is offline edbarx  Malta
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The aluminium plate is now earthed with a 2.5 sq mm copper wire connected to earth. At both ends of the wire I used terminal lugs that were fixed using small bolts. I made sure the length of the wire is the shortest possible. The amplifier modules will be fitted with their main heatsinks, which are connected to 0V, in copious contact with the underlying aluminium plate, which is now securely earthed.

The input wiring will be isolated from power earth so as to be fed only from signal earth from the amplifier modules. I will have to make sure signal grounding does not give rise to loops as there are two channels.

Next task is rerouting the output wires as close as possible to the original PCB tracks feeding the output stage.

The stray mangetic flux will be minimized using an aluminium tape wound a couple or 3 times around the transformer's outer edge.

Hopefully, this resolves all these issues, as this journey has now taken far too long. As suggested earlier by Mooly, quick blow fuses can be used for speakers. I will use this amplifier at home, so huge powers are not necessary. Nevertheless, the adventurous journey exploring complexities as a result of allowing for very big powers, was a nice learning experience. I can only say, when I first tried the amplifier with large professional speakers, the sound was beautiful, powerful and detailed. Thanks to this forum, I would have never been capable to achieve this much.

Last edited by edbarx; Yesterday at 12:35 PM.
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