Looking for the Circuit drawing of this LM4780 Board

[aparatusonitus]

Without knowing exact speaker reactive behavior I would use 2x22VAC(max.)/250-300VA as a sensible choice.

 

[asuslover]

Czech Lm4780

I knew I had it somewhere, but it has a very low resolution .

 

[avtech]

Thanks for all the suggestions (and apologies to the OP for threadjacking a little - hope it helps you too).

The schematic for mine is attached. Would I need to change any values for running at a lower voltage or would it be good to go?

 

[aparatusonitus]

As far as I can see the schematic as given has nothing to do with a proper operation of an LM4780...it is just a decoy schematic for confuse others Chinese copycats.

 

[avtech]

The schematic appears to follow the PCB and the values don't seem far away from the application notes. A few are different such as 22k instead of 20k for the Rf and Rf2.

As far as I can see the schematic as given has nothing to do with a proper operation of an LM4780

Are you able to elaborate a little? What is badly wrong with the schema?

I have realised that the silk screen on the board is labelled 28v-GND-28v at the AC input - but the schematic shows the rectified voltage as being +/-28VDC

So using a modified version of Tom's calculator, as you both mentioned, a transformer of 22-0-22 250-300va will cover a 4 ohm load or 180-160va if restricted to 8 ohm loads (assuming Vcc=28, Cf=dB).

I'm guessing the output power from 16 ohms wouldn't be worth bothering with?

 

[Mark Whitney]

Ground missing at C1 - C3 and C17 - C19?
R4 and R9 would normally not be 10 ohms.

 

[avtech]

Thanks Mark. I've been staring at the schematic for ages trying to figure out how it works (it doesn't seem like it should) and that's because the schematic is a pretty bad inaccurate copy.

I managed to find the correct schematic on another thread from 5 years ago and it makes a whole lot more sense now.

Regarding the 10R resistors, I found the following quote that might be of interest:

REACTIVE LOADING
It is hard for most power amplifiers to drive highly capacitive loads very effectively and normally results in
oscillations or ringing on the square wave response. If the output of the LM4780 is connected directly to a
capacitor with no series resistance, the square wave response will exhibit ringing if the capacitance is greater
than about 0.2μF. If highly capacitive loads are expected due to long speaker cables, a method commonly
employed to protect amplifiers from low impedances at high frequencies is to couple to the load through a 10Ω
resistor in parallel with a 0.7μH inductor. The inductor-resistor combination as shown in the Figure 6 isolates the
feedback amplifier from the load by providing high output impedance at high frequencies thus allowing the 10Ω
resistor to decouple the capacitive load and reduce the Q of the series resonant circuit. The LR combination also
provides low output impedance at low frequencies thus shorting out the 10Ω resistor and allowing the amplifier to
drive the series RC load (large capacitive load due to long speaker cables) directly.

 

[Mark Whitney]

R13 - 14 should be 2.7 ohms. And, if you use the stability parts in the feedback then Cc (220pF) is also missing.

 

[zoky2]

Does anyone have a BOM for the board from the first post ? I have 2 boards from audiomodule.eu ,but no values for elements.I was searching net but no luck for this bords.

Regards
zoky2

 

[avtech]

R13 - 14 should be 2.7 ohms. And, if you use the stability parts in the feedback then Cc (220pF) is also missing.

Thanks for the pointers Mark.

I'm getting myself confused with the calculations:

If I were to make this board parallel (connect the jumper between 'R OUT' and 'L OUT') for LH channel and then do the same again for a RH channel, what would the effect be in terms of power supply requirement?

For instance if I have 22VAC/~30VDC supply for a 4 ohm load expecting to be 250-300VA, what would the VA requirement be if I join the outputs on the board and add another board?

Does anyone have a BOM for the board from the first post ? I have 2 boards from audiomodule.eu ,but no values for elements.I was searching net but no luck for this bords.

Regards
zoky2

Sorry, I can't find any details of what's fitted to that particular board.

 

[PRR]

...If I were to make this board parallel (connect the jumper between 'R OUT' and 'L OUT')....

Don't do that without thinking!!!!

Say you have a 1% difference between amplifiers. At some moment, one is at 20V, the other at 20.2V. This is a 0.2V difference across the jumper. The amplifiers' output resistance may be less than 0.01 Ohms. Say the jumper is similar. 20V across the 8 Ohm load is 2.5 Amps to load, but 0.2V across 0.03 Ohms jumpering is 6.6 Amps of waste current and heat in the chip!

 

[avtech]

Thanks for the warning PRR.

The schematic says to 'short out the jumper' and the PCB is designed with a shorting link..

But looking at the datasheet, it suggests 2x 0.1 ohm 3W resistors in series with the output in the centre.

I take it that this would work as it would reduce the 6.6A/1.3W to 2A 400mW?

edit: the audio sector boards appear to use 0.15R 3w power resistors (output).

 

[Mark Whitney]

Are they not the large green resistors with R10 printed on them? R10 and R19 printed on the board.

 

[avtech]

The boards I am going to be using have a series inductor at the output.

 

[Killersupra]

Hello everybody,

I just stumbled across this thread for the same reason as OP, as it turns out, audiomodule.eu is down and I never received the PCB schematic and BOM via email, but I did get them printed out with my purchase.

After a long search I managed to find them, hope this does the trick.

Best regards.

 

[pitbul]

LM4780 I am using in parallel output with one important thing, resistors which defines amplification should be 0,1% accuracy, after that amp is working without issues which PRR mentioned.

 

[zoky2]

Killersupra,

thank you very much for the schematic and BOM,now I can put together the board .


Regards
zoky2

 

[Salar]

Hi to all, thanks a lot indeed! It is also very nice that some basics were discussed again,
really appreciate it.
All the best,
Salar

 

[Salar]

What volume pot to use with this board? Is it dependent on R11&R18? Sources are
strictly line level DACs. Is 10k the value to go?

 

[Salar]

One thing I still do not understand. The board has for each channel 3x2200µF caps on the voltage input.
Input is DC, not AC, no rectification on the bord, this has to be done externally.
So what capacitance value do I choose after rectification?