My LM3886 Stereo Amp. w 12-0-12VAC Transformer

[Andreq]

So I made some calculation :

Rin = 10k
RB = 100k
Rf1 = 100k

Ri = If Gain = 15 --> 7.14K
If Gain = 20 --> 5.26K

CS = 470uF (I have 1x 4700uf on the PSU for each rail)

RM = ???K
CM = 470uf


Does that make sense ?

Ive calculated those values with :

Vopeak = ~16-17V
Ipeak = ~2A
PO = ~16 watt
Av = 11.31 (that's the minimum gain?? am I right??)

[ratza]

In that schematic you should add another resistor, between non-inverting input and ground; it should have the same value as R2 and it will be the input impedance of your amplifier. R3 will have to be the same as R4. Rmute should be calculated in a way to allow the chip to draw at least 0.5mA to disable muting. The value will therefore be lowest power supply voltage divided with this current => 12V/0.5mA = 24k; 22k will be even better.

[pacificblue]

Originally posted by Andreq
So I made some calculation :

Rin = 10k
RB = 100k
Rf1 = 100k

Ri = If Gain = 15 --> 7.14K
If Gain = 20 --> 5.26K

CS = 470uF (I have 1x 4700uf on the PSU for each rail)

RM = ???K
CM = 470uf


Does that make sense ?

Ive calculated those values with :

Vopeak = ~16-17V
Ipeak = ~2A
PO = ~16 watt
Av = 11.31 (that's the minimum gain?? am I right??)

Rf should be as big as Rin. Rb should be as big as Ri. Cm depends on the switch on/off delay you want to achieve and Rm. You should add Cin and Ci. Cs should be several capacitors. 100nF X7R or better, right next to the IC, 10 µF (high ESR) near the IC, 220-1000 µF (low ESR) still on the PCB not more than 100 mm from the IC and bigger capacitors off board in the power supply.

Originally posted by ratza
The value will therefore be lowest power supply voltage divided with this current => 12V/0.5mA = 24k; 22k will be even better.

(12 V - 2,6 V) / 0,5 mA = 18k8 -> 18k.

[Andreq]

Thanks guys!

I'll try to work out some of the additional stuff on the current PCB design and let you confirm everything is OK

Also, I think I'll add the input cap & resistor directly on the input plug.

[AndrewT]

Originally posted by Andreq
I think I'll add the input cap & resistor directly on the input plug.

better to keep the RF filter at the power amp input and to add an RF cap only across the RCA (or other type) input socket.

[Andreq]

Update!

As I had some time this week, I've rebuild the PSU with a nice PCB and 4x2700uF cap (2 per rail).

here's some picture of the build and the PCB layout.







As you can probably see, I'm using an old Computer PSU enclose, this will probably be changed when I will build a wood enclose for the whole amp.

I didn't have time to look closely to the amp pcbdesign, but I think I'll do this soon, this week or the next.

edit :

And by the way, I've tested the PSU output and I get ±19.3V without load.

[AndrewT]

Hi,
the ground plane joining the two caps nearest the output is almost continuous.

Look at the ground plane joining the two caps nearest the AC input.
Find the route that the current must follow?
The two AC connections have ruining the easiest and shortest route.
The two AC connections could have been hardwired with a twisted pair, leaving the plane intact.

You can partially repair the existing PCB by adding a hardwired link between the Zero volts terminals of the two caps.

[Andreq]

I don't really think this will have much of an impact on the end result, but I'll hardwire the 2 "input side" cap to the 0V if needed.

Thanks for the tips!

[Andreq]

A little question today,

As this power amp will be connected directly to my tv output... with no tone control, would it be "ok" to add a simple passive tone control to my board ?

Something like to this :



I'll have to tweak the amp gain as I'll probably need a bit more gain with a passive tone control, but that's not really a big problem as I'm still "designing" it.

Also, any of you know a good website (in Canada if possible) to order electronic component? I know digikey and mouser, but I still find the shipping way to high compared to visiting my local electronic store (which is limited in part diversity...)

Thanks

[Andreq]

It seem that I've lost my helpers

Anyway, I've been working on the "Master control" today.

I'm developing it using the Arduino plartform, on a breadboard...

The final build will be directly flashed to an Atmega328.

It will use a standard Sony TV IR code as this was the simpliest to implement in the software. I might design a simple remote control schematic, pcb & software if someone need it. For my own use, I'll be using my JP1 remote control with a simple Sony 12/15 device programmed into it.

When everything is done, Ill post the final PCB layout and software code so anyone will be able to build his own.

Attached is the schematic

My current proto board is somewhat different to this as it has the Arduino boot loader which use PD0 and PD1 to transmit and receive the code, but this part will not be needed when development is done.

Later, I'll add Relay control (to power on and off the amp) and the tone control if needed.

IC1: Atmega328-PU
IC2: 78L05
IC3: DS1807 (Maxim Dual Audio Digital Potentiometer)
IR1: TSOP1238 (or 1738).