4780 component values for +/-30Vdc ? Need verification :)

[tsiros]

Ok so i got the 4780 PCB from hifidiy.net . I also got a 2x20 VacRMS 200 VA toroidal. I got two 1000 V 35 A bridges and a pair of 6800 uF 50 V

from the transformer output and the 4780 datasheet you can see that i am looking for ~40 W output per channel at 1% THD which hopefully will give me 30 W at lower THD (not that i will drive my speakers that high, AN12CF, nor that i care that much about power numbers, but I find it helps to have an estimate of such figures).

so now i have to figure out what values of resistances (and capacitors? not sure if i have to change them, too) i need to use.

Please tell me if the following makes any sense:

for 30 W at the speaker, and considering they are 8 Ω i need P=IV => P=V^2/R, R=8Ω, P=30W => V= ~15.5V

now, since we're not amplifying DC, this is an RMS value. for a sinusoidal signal at the output this means a 22 V peak.

so what i thought is: we need to amplify the 100 mV signal from the source to 22 V to the speakers, so i need a gain of 220. which means the ratio of the output resistor to the input resistor should be 220. So if i choose an Ri of 100 Ω and an output ("feedback") Rf of 22 KΩ... is that the correct solution?

[AndrewT]

220 is an enormous gain.
Most builders use from 20 to 30times gain.
Very low source voltage does require more gain and we do sometimes see 50 to 100times for high gain power amplifiers.

Are you sure your source has a maximum output of only 100mVpk?
A CD player and most "full size" stand alone optical players have a maximum output of around 3000mVpk.

[tsiros]

the 100 mV (200 mV Vpp) figure i mentioned was only as an example value.

Indeed, the sound card (i am not going to use that, of course, i plan on buying a simple dac) i have has an output of 4 V Vpp (2 V amplitude). I checked it with my silly little oscilloscope.
I suspect a decent DAC or a better soundcard (not sure if diyaudio.com has any beef about soundcards, i think there exist soundcards that sound quite good) will have a similar range.
with a 4 V Vpp The gain of 22 is reasonable...

what i wanted though, is the way i calculated it, correct?

in other news, i can't find any of the components (5 W .18 Ω resistor, 1K metalfilm resistors, etcetcet) locally

thank you for replying

[AndrewT]

inverting amplifier gain = Rupper leg / Rlower leg
non inverting amplifier gain = 1 + [Rupper leg / Rlower leg]

If a capacitor is added in series with the Rlower leg then the DC impedance of the lower leg increases to near infinity.
That reduces the DC gain to 1 in a non inverting amplifier.

metal film resistors have become the "standard" resistor.
They are very easy to find in 1% tolerance and usually 50ppm or 100ppm.
Generally available in 250mW, 400mW, 500mW or 600mW.

The higher dissipations of 1W, 1.5W and 2W are much more difficult to find and much more expensive.
Rapid-online will sell you 100off pack of 600mW 50ppm 1% tolerance metal film for ~£1.60, about €0.02 each.
Diodes and Zeners are similar in price.

[tsiros]

So i am right?

why do i feel i know you?

[tsiros]

well, rapidonline doesn't carry 5W resistors :|

edit: andrewT, i don't mean to sound mean, but you are not helping at all. What you say is relevant to what i am trying to build, but you did not answer my question and started giving information about things i do not care much right now

[AndrewT]

5W and other high power resistors are generally available in wire wound construction.
If you want/need metal film then you will have to use combinations of lower dissipation resistors.

Look at the DPA, he uses metal film exclusively and you see multiple parallel arrangements to get low resistances and/or higher power dissipation capability where this is required.

The big advantage of using 8off 1% 50ppm 600mW resistors (4.8W total) is the tight tolerance without the need for hand matching that the 5% component virtually demands.

[tsiros]

the 5 W .18R is ceramic.
the 1K is metal film.
in any case, this is not the topic of the thread, i am more worried about how to calculate appropriate values for Rf and Ri!

[AndrewT]

5W 0r18 is not ceramic. It is almost certainly wire wound. The enclosure for the resistor is probably a ceramic. Similarly the core around which the wire is wound is probably a ceramic. I have no idea what the "paste" that is used to glue the resistor into the "coffin" enclosure is made from.

1W metal film are available but at extra cost. I habitually use two parallel resistors to give 1.2W when this is required. Costs me ~3.3p

[tsiros]

why are you still talking about this?

i do not care about these. i will order these parts as they are described in the BOM.

what i care about (And started this thread) is whether i am using the correct method to figure out the Rf and Ri values.