I am modifying a phono preamp kit to adjust it to a design of my own that we discussed some time ago on a thread here on this forum.
My concern is how to separate the silent and noisy grounds in the schematic to send them separately to the star ground.
How should I proceed?
My first idea is to isolate the input ground with a 10R resistor, but I am not sure about the gain and filter grounds if they should be isolated too from power ground.
My concern is how to separate the silent and noisy grounds in the schematic to send them separately to the star ground.
How should I proceed?
My first idea is to isolate the input ground with a 10R resistor, but I am not sure about the gain and filter grounds if they should be isolated too from power ground.
Attachments
i see 7 ground symbols.My concern is how to separate the silent and noisy grounds in the schematic to send them separately to the star ground.
which are silent and which are noisy? and why?
(this thread is in wrong forum)
NJM2068 has bipolar input, and for MM cartridge FET-input OpAmp is much better (for instance, OPA2192).
I also recommend short R451 R452 (it only makes unnecessary noise) and use lower C453 C454 (no more than 100 pF to prevent hump @ 8-12 kHz with typical MM).
I also recommend short R451 R452 (it only makes unnecessary noise) and use lower C453 C454 (no more than 100 pF to prevent hump @ 8-12 kHz with typical MM).
Avoid using dreaded GND symbols, rather draw all the "return" current paths explicitly and then try avoid sharing common paths where it counts.How should I proceed?
In a circuit with that huge amount of gain your biggest enemy will likely be magnetically coupled hum, not GND layout. Key to minimize it is above strategy and reduce physical loop area of the most sensitive loops.
Tape head demagnetizer is the weapon of choice to stress-test a real build/prototype.
As a practical approach, forget the idea of "star" ground schemes as those have EMC issues. Star GND is a DC/LF concept. Make a 2-layer or 4-layer board with solid GND pours (and don't forget to put lots of stiching vias) and call it done.
Your idea is good. I actually use the same concept. But it is not absolutely needed, only if you experience input related hum.I am modifying a phono preamp kit to adjust it to a design of my own that we discussed some time ago on a thread here on this forum.
My concern is how to separate the silent and noisy grounds in the schematic to send them separately to the star ground.
How should I proceed?
My first idea is to isolate the input ground with a 10R resistor, but I am not sure about the gain and filter grounds if they should be isolated too from power ground.
Attachments
To prevent too big DC @ your output I recommend replace LT1792 with 800 uV DC offset to OPA192 with 5 uV offset [ I have only 1.5 uV with it - see https://www.patreon.com/posts/64958853 ].Your idea is good. I actually use the same concept. But it is not absolutely needed, only if you experience input related hum.
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And you can hear the noise of njm2068, right?NJM2068 has bipolar input, and for MM cartridge FET-input OpAmp is much better
Nick, you are suggesting a chip that Mouser sells for more than $500 each. Isn't that a bit too much? This is to be an affordable phono preamp.To prevent too big DC @ your output I recommend replace LT1792 with 800 uV DC offset to OPA192 with 5 uV offset [ I have only 1.5 uV with it - see https://www.patreon.com/posts/64958853 ].
If there's offset I will add a servo or an output cap.
Well, this is the question I'm asking.i see 7 ground symbols.
which are silent and which are noisy? and why?
(this thread is in wrong forum)
In what forum this thread should be? Be sure I want it to be where I get my answers.
I see the OPA192 listed with a price of €3,75 for 1 piece.Nick, you are suggesting a chip that Mouser sells for more than $500 each. Isn't that a bit too much? This is to be an affordable phono preamp.
If there's offset I will add a servo or an output cap.
Of the 7 shown in the schematic, 5 are definitely quiet ones. The two on the 15V voltage sources are potentially noisy, depending on decoupling arrangements (not currently shown on schematic). There are also 'hidden' grounds which are potentially noisy too, those being the 0V connections to input and output - these are best assumed to be carrying CM currents.i see 7 ground symbols.
which are silent and which are noisy? and why?
(this thread is in wrong forum)
since the circuit is not complex, a point to point prototype could be made.How should I proceed?
Attachments
That is pretty bad.
Have a look at that post No 4... the relevant ground returns are separately routed to a single point on the RIAA PCB....
In addition to answering all the questions asked here on this thread, the RIAA PCB layout correctly connects the chassis ground to the RCA ground.
Heck... it even shows how to tie the rest-of-the-amp ground to the RIAA PCB ground.
Textbook perfect.
Have a look at that post No 4... the relevant ground returns are separately routed to a single point on the RIAA PCB....
In addition to answering all the questions asked here on this thread, the RIAA PCB layout correctly connects the chassis ground to the RCA ground.
Heck... it even shows how to tie the rest-of-the-amp ground to the RIAA PCB ground.
Textbook perfect.
so why are C451 and 452 used at all??? correct me if i'm wrong here but looking at the schematic in post 4 i thought they where attempting to keep those inputs from seeing a DC connection to chassis ground via capacitive decoupling....and clear me up where in the schematic are those coils L 451, 452???
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