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Interesting PCB jack input help needed.

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Interesting PCB jack input help needed.

Postby Animalbeats » Sat Jan 16, 2021 5:29 pm

Can someone explain to me this input setup?

I think it has to do with impedance.....any help is greatly received.

Please see photo.

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Re: Interesting PCB jack input help needed.

Postby zenguitar » Sat Jan 16, 2021 5:33 pm

Welcome to the SOS forums.

Your picture doesn't show, but right clicking on it will allow anyone to open it in a new tab/window.

Andy :beamup:
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Re: Interesting PCB jack input help needed.

Postby Hugh Robjohns » Sat Jan 16, 2021 7:25 pm

I have no idea what your picture is supposed to be showing... but if you're referring to the 0 Ohm resistor, they are often used in place of wire links to cross tracks on a pcb, or where other components are not required. They can be placed easily by machine as part of the normal board construction.
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Re: Interesting PCB jack input help needed.

Postby Animalbeats » Sat Jan 16, 2021 7:33 pm

It's supposed to be an 1/4 inch input jack on a PCB with

1 small smd resistor....I believe for RF frequencies...?

2 smd Capactors linked in series.....Which I think are to do with impedance?

2 smd Resistors linked in Series.... they have double black lines so I believe is zero Ohms?

And I can't see any further track.....I believe the signal goes into an electrolytic cap then a WIMA before going off to an op-amp?
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Re: Interesting PCB jack input help needed.

Postby Hugh Robjohns » Sat Jan 16, 2021 9:23 pm

Impossible to say what's going on without an accurate schematic and the op-amp type number -- if it is an opamp? With six legs it's possibly a balanced RF choke... Clearly, a lot of connections are missing from your sketch.

What is this input stage from?
And why the interest?
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Re: Interesting PCB jack input help needed.

Postby BillB » Sat Jan 16, 2021 10:27 pm

Hi AnimalBeats, welcome to the Forum. I think Hugh is saying, if you want a meaningful answer, you need to provide a meaningful question. ;)
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Re: Interesting PCB jack input help needed.

Postby Animalbeats » Sat Jan 16, 2021 11:24 pm

Thank-you for all your welcomes.

I’ve loved SOS since I saw it on my teachers desk whilst doing music and audio tech 20yrs ago, and I’m finally realizing my ambitions now.

I’m hoping to do some reverse engineering to develop my own product.

Sorry for the missing connections, I can’t see how the go either. Here’s as best I can draw.

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Re: Interesting PCB jack input help needed.

Postby Hugh Robjohns » Sat Jan 16, 2021 11:52 pm

The 33078 is a low-noise dual op-amp, and it appears to have a couple of transistors feeding the inputs of one half -- which is a classic budget mic pre configuration. The PCB is obviously at least double sided and may be multi-layer, so you'll need to see where the tracks go on both sides of the through-holes.

If you want to reverse engineer it, you'll need to work out a full schematic, either visually, or with metering, or both.
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Re: Interesting PCB jack input help needed.

Postby James Perrett » Sun Jan 17, 2021 12:08 am

Hugh Robjohns wrote:If you want to reverse engineer it, you'll need to work out a full schematic, either visually, or with metering, or both.

Though it might be easier to find a service manual with the schematic already drawn out. If this is a mic preamp then there are plenty of decent alternative designs online.
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Re: Interesting PCB jack input help needed.

Postby N i g e l » Sun Jan 17, 2021 2:19 pm

Those cylindrical melf resistors look a right fiddle. Why choose them? what are their special properties (apart from rolling off the bench) ?


-----------

an example of an input stage with opamps & transistors is the Mackie VLZ. This is an old version.....


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Re: Interesting PCB jack input help needed.

Postby Hugh Robjohns » Sun Jan 17, 2021 2:38 pm

N i g e l wrote:Those cylindrical melf resistors look a right fiddle. Why choose them? what are their special properties (apart from rolling off the bench) ?

Better noise performance in parts of the audio circuitry carrying (relatively) higher currents -- such as through the front-end transistors.

Impossible to know without seeing a wider view of the PCB, but it looks like the OP's device has a very much simpler front-end than the Mackie design, which is quite a sophisticated arrangement involving a three op-amp instrumentation amplifier with an elaborate long-tailed pair in front of it.
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Re: Interesting PCB jack input help needed.

Postby Animalbeats » Sun Jan 17, 2021 4:31 pm

Sorry guys, I appreciate your level of detail, which actually exites me :bouncy:

However, can you dumb it down for me please

1 small smd resistor....I believe for RF frequencies...?

2 smd Capactors linked in series.....Which I think are to do with impedance?

2 smd Resistors linked in Series.... they have double black lines so I believe is zero Ohms?

And I can't see any further track where the signal goes into an electrolytic cap then a WIMA before going off to an op-amp, why could this be?
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Re: Interesting PCB jack input help needed.

Postby zenguitar » Sun Jan 17, 2021 4:37 pm

The problem is that you are asking people to guess. It would be a LOT easier for everyone if you would let people know the specific piece of hardware. Make and model, not a generic reference.

And if they can identify a schematic, they can help you with accurate answers to your questions.

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Re: Interesting PCB jack input help needed.

Postby James Perrett » Sun Jan 17, 2021 4:57 pm

To be honest - you haven't got the faintest idea of what you are talking about.

You've mis-identified most of those components and are talking complete rubbish.

If you really want help you are going to have to tell us exactly what this piece of gear is and why you are interested in the circuit.
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Re: Interesting PCB jack input help needed.

Postby Hugh Robjohns » Sun Jan 17, 2021 5:20 pm

Animalbeats wrote:However, can you dumb it down for me please

No... Because you don't have sufficient circuit detail to draw any conclusions at all

1 small smd resistor....I believe for RF frequencies...?

In what way 'for RF frequencies'? What makes you believe that? What would a small SMD resistor be doing wih RF frequencies! How would RF frequencies be reaching that resistor?

2 smd Capactors linked in series.....Which I think are to do with impedance?

Affecting impedance in what way? Why!

2 smd Resistors linked in Series.... they have double black lines so I believe is zero Ohms?

Possibly... I'm not entirely clear which resistors you're talking about... But I suspect your ring identification might be incorrect.

And I can't see any further track where the signal goes into an electrolytic cap then a WIMA before going off to an op-amp, why could this be?

Because its a multilayer board? It certainly isnt common to have an electrolytic feeding a smaller capacitor, so I would suspect you've misunderstood the circuit topology.

Can I clarify, please, what is your level of understanding audio electronics, and exactly what are you trying to achieve?
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