Do i understand this pre-amplifier circuit correctly?

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
i'm building a simple preamp (for a diy TDA8932 chip amp).
diag0.PNG
I'm having trouble figuring out the grounding.
the + signal wire goes to the + input terminal on my amp board, and the -(gnd) signal wire connect to the negative (signal ground) terminal on my amp board. So therefore, the "bottom" wire on the diagram is grounded, as it connects to the signal ground (-)terminal, correct? i do not need an additional connection to a gnd terminal, right?
diag1.PNG
This modified diagram is how I understand I should build the circuit, correct? Or do i need an additional connection to ground/earth( i have one on my power supply for the amp, but that's literaly just the ground from my house power socket, conecting this circuit to that seems extreme somehow).
If this circuit is meant to function with a batery (be portable), there's no way to ground it, right, so the negative batery terminal functions as ground, right?

So the GND connection/symbol, just ilustrates the need for that wire to be grounded. basicaly, there's 6 connections to this circuit (2 input (+/gnd), 2 output (+/gnd), two DC (+/-)). or is there a seventh, ground connection to somewhere, if so, where.

I'm planing on using a 9V dc charger as power supply, so don't want to accidentily fry my amplifier chip.
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Thank you, it's confucing, why they have the negative terminal and the ground ilustrated in the circuit. It makes it seem like it needs to be grounded twice.
Other pre-amp circuits only have the DC+ and ground(the DC-).
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Additional question - if i put the pre-amp circuit in a metal casing, and connect that to the signal ground on my amp chip, it will function as a faraday cage (for EM and RF interferance elimination), right?
If the DC- functions as the ground, would connecting the casing to it be the same as grounding it?
 

kubeek

Joined Sep 20, 2005
5,623
I guess the ground point is there just as a reference point to talk about voltage at various points, without it it could be a bit ambiguous.
 

AnalogKid

Joined Aug 1, 2013
8,139
Ground is a relative term. It is the reference potential against which all other voltages in the circuit are described. A connection to a physical (earth) ground is not required for all circuits. In fact, it can inje t noise *into* a circuit in some applications.

Your drawing is correct, but I would keep the ground symbol.

ak
 

crutschow

Joined Mar 14, 2008
23,504
To minimize confusion you should use different ground symbols for earth ground and signal ground, as shown below:
In many published schematics the left symbol is incorrectly used, when the right one is what is meant.
upload_2019-9-23_11-30-39.png
 
Last edited:

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
Thank you, built the pre-amp, and it functions great.
Here's a simmilar question - i built a preamp, based on a different schematic, and the negative power supple connection wasn't shown.
I have it like this.
ampv2.PNG
the preamp is experiencing issues. speakers are buzzing, hissing, poping, they don't like this thing in the circuit.
So what could be wrong? i assume i could figure that out easily with a multimeter.
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
the resistors are correct values. i couldn't meassure R5, because the resistance kept slowly increasing, for some reason, just ticking up, starting from 0. could it be because of stored charge in the transistors, or something like that?

i used BC547C instead of BC549, but they're interchangable, as far as i understand.

The caps are correct orientation. I used spare wire from resistor leads to make some connections, could that be the problem? i assume it's the same as a wire or solder bridge.
 

AnalogKid

Joined Aug 1, 2013
8,139
i built a preamp, based on a different schematic, and the negative power supple connection wasn't shown.
Not sure what you mean here. The circuit in post #10 does not use a negative power supply. It needs only one supply, like the one in post #1.

Note that the input and output signals are not the + / - types (like a DC signal). They are an AC signal and its return.

ak
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
apologies for incorect terminology, the +18V wire is marked +, the wire marked with a red - is the bare wire/shielding of the power supply, so i marked is as the negative power terminal.

I know signal is not an actual current.
I was told in a different forum, that i need power decoupling capacitors. how do i know how many i need, and what value? hard to find info for some reason.

The first preamp i built (ended up using a different schematic than in the first post here), didn't have power decoupling caps either, but it works.
 

crutschow

Joined Mar 14, 2008
23,504
The first preamp i built (ended up using a different schematic than in the first post here), didn't have power decoupling caps either, but it works.
You can sometimes get by without decoupling capacitors, but it's never good practice to leave them out, as you don't know when they are needed.
Sort of like driving without a spare tire. ;)
 

Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
I've put in a 100u polar and 100n non-polar, paralel to the power supply, so will be fine, right?

The output cap is 2.2u, not k.
it was recomended to me to increase the load resistor from 1k to 470k.
if you know, would it be fine to leave out the 22p resistor, or replace it with a 47p? don't want to make a fourth trip to the electronics store:D
 

Audioguru

Joined Dec 20, 2007
11,251
A BC547 us an ordinary "general purpose" transistor. A BC549 is a low noise audio transistor. Look at the datasheets to see things like this.

The image in your last posts does not work:
1) Your previous circuits do not have a 2.2k or 2.2u output cap

2) Your previous circuits also do not have a 1k load and do not have a 22p "resistor".
22pF is a small value capacitor, not a resistor.
 

Audioguru

Joined Dec 20, 2007
11,251
This preamp circuit has a gain that is much too high for an electric guitar pickup. Also its input impedance is much too low (it should be a minimum of 1M). Frequently a single Jfet is used since it has a high input impedance and low gain.

A BC547 general purpose transistor has more than double the maximum noise (rumble and hiss) of a low noise audio BC549 transistor but the output level from an electric guitar pickup is fairly high so you might not hear the noise.

Why did you scribble all the grounds together? Of course they all should be connected together.
 

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Thread Starter

Aivaras Andrijauskas

Joined Aug 26, 2019
31
I connected the ground just to make sure i was correct on what to do. And i realise, i might have made a mistake, by connecting the ground of my preamp to the signal ground input on my amp circuit. Should only the preamps output wire be connected to the amplifier circuit?

Thank you for the fet preamp design. Does it not need an input cap?
Could i not alter the input on my preamp? i still want to finish it.
 
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