Basic LM4780 amp

[tsiros]

yes, that is why it will be vented, top and bottom.

 

[tsiros]

as of yet:

i've ordered a 120VA 2x18V 3.33A toroidal, four 10mF 50V capacitors (2 per channel, if necessary i'll get more later), two rectifier bridges, the LM4780 and a pcb for it

i'll be giving it +-25V, that's supposedly 30W per channel at 1%THD, i won't be using more than 10W though and usually it will be at 1W.

now i need a <0.7C/W heatsink and the various thingies (caps, resistors, wires, connectors, fuses, fuseboxes, switches etc)

any comments suggestions or warnings would be most welcome!

 

[tsiros]

I need some help determining the values of Rf and Ri for the amplifier.

are those the usual feedback resistances that determine the gain of an opamp based amplifier?

if so, i might be able to determine their values.

 

[tsiros]

I request this thread be moved in the construction subforum

 

[tsiros]

Ok so maybe my previous questions were stupid or elementary, i don't know...

now i have an other question

What kind (single/multi strand, other), gauge and type (shielded, unshielded, twisted pair, other) wire should i use for the various connections?

1. from mains socket to power switch
2. from switch to power transformer
3. from transformer to bridge rectifier
4. from rectifier to LM4780 PCB
5. from pcb to speaker terminals

thanks for any and all input

 

[AndrewT]

1 & 2 double insulated mains wiring.
3 4 & 5 twisted pair.

 

[tsiros]

thanks! but what about gauge and ... strandiness?

 

[AndrewT]

1 & 2, maybe 0.5sqmm
3 & 4, maybe 0.75sqmm or 1sqmm
5, 1sqmm or 1.5sqmm
I would use stranded for all. Makes it much easier to do twisted flow and return.

 

[tsiros]

you've been of great help, andrew, thank you

 

[tsiros]

ok so i completed it
earth connector is connected to middle of symmetrical transformer and also connected to ground, between the smoothing caps :


-Vcc---)|---GND---)|---+Vcc

however i get buzz. i shouldn't be getting this much buzz. the usb is with ferrite core. the signal wire is high quality shielded.

this, (lm4780) amp has lots of buzz.

the other amp did not have the connection between plug earth and middle of transformer. it was DEAD QUIET.

question: what will happen to the lm4780 if i DISconnect the connection between PLUG EARTH ("E") and MIDDLE of TRANSFORMER, but KEEP the connection between middle of transformer and GROUND on PCB?

 

[AndrewT]

You talk about "earth".

That is the stuff we stand on when outside in our gardening boots.
Protective earth is the third wire in a 3wire mains cable.
Chassis is the metal case around some versions of electrical equipment.
What are you referring to?

 

[tsiros]

the third wire in a 3 wire mains cable :/

 

[AndrewT]

do not disconnect the chassis from Protective earth.

Look instead at the audio grounding.
Where is your audio ground connection?

 

[tsiros]

there is no chassis. not yet, at least. audio ground... i am not sure :|

 

[tsiros]

there is a wire going from the pcb ground to the transformer middle tap (incidentally. the transformer is dead cold ) . there is also a wire going from transformer middle tap to the Earth in the 3wire mains socket. If i disconnect that, the noise is minimized. There is some buzz (not hiss. not thermal noise.) i can still hear if i place my ear about 10cm from the speaker, but i can't hear it from listening position. then again it's not very quiet now, i shall try again at 4am when it is very quiet.

if i connect it, though, i get very audible buzz and in fact i can hear all the noises my computer is making. interference.

 

[AndrewT]

There is an error in the audio grounding. That error is causing the buzz.

 

[AndrewT]

I do not directly connect PE to the PSU.
I connect the PE to Chassis. This is the first stage of the Safety bit attended to.

I connect the Main Audio Ground, or just as effectively, the PSU Zero Volts to Chassis. This Chassis connection can be many tens of centimetres from the other Chassis connection. They do not need to be coincident, not even near each other.
I could just as easliy connect the Speaker Return terminal to Chassis. That would be a good connection that can carry mains fault current to PE.

 

[tsiros]

I just assembled the lm4780 pcb from hifidiy, this one pretty sure i assembled it correctly... what about interference from elsewhere? i will try uploading a picture later (going to a book bazaar with the missus) so you can see the wiring.

thanks for your help, andrew!

 

[tsiros]

This is the wiring.

hopefully you can make out the pcb ground/transformer middle that is not connected to mains earth

pls don't kill me

 

[AndrewT]

I can see two twisted pairs away to the right. I think these are inputs.
I see a double insulated set going from terminal strip to black square panel and then to PCB. I think these are the 3power wires.
The white pair from PCB look like speaker output.
What is the little orange wire from PCB to speaker terminals?

I don't have a clue where the Main Audio Ground is.