Need help with Triple Parallel LM1875 dynamics amplifier.

[Grega]

here you are.....i hope i interpreted you right andrew

[AndrewT]

Correct.
Look at the RC and F-3dB of the two input filters.

I think you have gone too far in changing from 150uF to 22uF. Try 33uF and 47uF. But listen to all three and you judge.

[gootee]

Grega,

You will want to make the star ground point at the grounds of the smoothing caps in the power supply, probably on a stub trace protruding off of the caps' ground trace.

Therefore, on this board you will need a completely separate ground for the input signal ground, not connected to any power ground on this board, and with its own wire running back to the star ground on the power supply board. And in that case the grounds of the input RCA connectors for the whole unit will need to be isolated from the case and chassis, with their grounds only connected on these boards.

Alternatively, you could connect the RCA grounds together where the inputs enter the case (bot don't connect them to the case or chassis) and run one wire from between them to the star ground point, and then not have a signal ground wire from this board to the star ground.

In any case, the input signal ground on this board should not be connected to the power ground on this board.

[danielwritesbac]

At post 27, connect Signal Star ground to Power Star ground with a 2.2 ohm resistor. . . because it is the small value resistor or a long thin trace that manufacturers Signal Star ground. While we might not keep the resistor, it is vital for assisting layout.

Let's take a look about what we know about the speaker cable. The same force, current, strength exists on speaker + as speaker - because if we disconnect either conductor, all of the current and signal is gone.

We can assume that there is a fiercely strong AC music signal on both speaker wires. If that gets into the the small signal ground, there is a high current noise bomb. The groundlift resistor (bright blue 2.2 ohms on my example) removes some of this current and none of the noise. However, the groundlift resistor "weakens" the extraneous current thus robbing the noise of its power.

Basically, this little 2.2 ohm resistor prevents bad layout and it gives you options:
1). Jumper for a more forwards sound to help dull speakers
2). Use 2.2 ohm groundlift on the amp board, for expected operation if the amp isn't a parallel amp.
3). Omitting the resistor gives you the option of cabling the signal star ground point to a centerpoint location via equally short length cables for the "master signal star" needed by parallel amplifiers, to facilitate hookup of the RCA jack and potentiometer. The resistor would also move to that location (master signal star) so that there is only one. One cable goes from this one 2.2 ohm resistor to 0v.

P.S.
How many speaker return cables are needed from running 3 amplifier boards as a parallel amplifier?
Only one cable--speaker return from the speaker jack goes to 0v output tap of power supply board.

[gootee]

The problem would come from using a signal-ground reference for the input resistor that is connected to a conductor that runs back to the power supply with currents on it that are not from the signal ground input.

Even assuming that the speaker ground return is not involved and goes back to the star ground in the power supply separately from everything else, we'd still have the decoupling caps' ground return currents. They will induce significant voltages across the inductance and resistance of the wire from the board to the power supply ground, which will appear... back at the amp board's now-bouncing "ground". That would make the ground end of the signal input resistor have a bouncing voltage. That bouncing voltage would effectively be directly arithmetically summed with the differential input signal voltage across the amplifier input pins. That would be "a BAD thing".

There should be one and only one "star ground". Audio ground should not be connected to power ground in any way, except at the single star ground point, back at the power supply. To do otherwise would result in some amount of noise and or hum being added to the amplifier input signal, and would also result in a "sneak feedback path" that could cause anything from more distortion to oscillation and chipamp destruction.

[danielwritesbac]

Of course the only guarantee of avoiding speaker return current, would be a rail splitter for signal ground.

[AndrewT]

Originally Posted by danielwritesbac

P.S.
How many speaker return cables are needed from running 3 amplifier boards as a parallel amplifier?
Only one cable--speaker return from the speaker jack goes to 0v output tap of power supply board.

No it doesn't.
The spkr return cable runs twisted with the spkr feed cable all the way back to the amplifier. Then the spkr return runs twisted with the Power Ground wire to the Main Audio ground.
The spkr return wire does not go to the PSU.

[Grega]

Correct.
Look at the RC and F-3dB of the two input filters.

I don't know what this means, but, if I read correctly the caps on the input need their values amending I think. I don't know what RC and F-3db means but I will try to read up and understand

I think you have gone too far in changing from 150uF to 22uF. Try 33uF and 47uF. But listen to all three and you judge.

OK - will do

You will want to make the star ground point at the grounds of the smoothing caps in the power supply, probably on a stub trace protruding off of the caps' ground trace.

sorry gootee, what is a stub trace?

In any case, the input signal ground on this board should not be connected to the power ground on this board.

ack that

No it doesn't.
The spkr return cable runs twisted with the spkr feed cable all the way back to the amplifier. Then the spkr return runs twisted with the Power Ground wire to the Main Audio ground.
The spkr return wire does not go to the PSU.

I interpret this as being the speaker ground is the same as the input ground. correct?
sorry for the questions but some of these is well above me.

[danielwritesbac]

Originally Posted by AndrewT

No it doesn't.
The spkr return cable runs twisted with the spkr feed cable all the way back to the amplifier. Then the spkr return runs twisted with the Power Ground wire to the Main Audio ground.
The spkr return wire does not go to the PSU.

I have no choice other than to agree, because of the parameters. . .
The thread title says "triple parallel" which is a power boost arrangement and extra power could not be useful with hard sound results of returning speaker current into the amplifier board ground plane.
However, the thread title also says "dynamics amplifier" meaning that the amplifier would be lively and therefore it is not productive to return the speaker voltage to the 0v tap of the power board.
Result:
Any option that is either an even balance or better than both is a welcome improvement.

[danielwritesbac]

Estimation table, bare minimum NFB cap size, sized small enough for some mild "warm sound" distortion, so let's not do worse.
1uF input cap, 3.3k feedback shunt, 100uF NFB cap
1uF input cap, 2.7k feedback shunt, 150uF NFB cap
1uF input cap, 2.2k feedback shunt, 220uF NFB cap
2uF input cap, 3.3k feedback shunt, 150uF NFB cap
2uF input cap, 2.7k feedback shunt, 220uF NFB cap
2uF input cap, 2.2k feedback shunt, 270uF NFB cap
2uF input cap, 1.5k feedback shunt, 330uF NFB cap

If, instead, you'd like to hear some undistorted extra low pitches plenty loud then first allow more bass to pass the NFB cap with either a larger feedback shunt resistor value and/or a larger value NFB cap, and then after that, you could use a larger input cap. It is funny with LM1875 because with extra low tuning is like a house cat with the voice of a lion, so much the opposite of the table above.