LM338 regulated snubberized PSU for audio amplifiers

[Upupa Epops]

2.5 A * 120 V is 300 W. How is, by you meaning, efficiency of this amp ? 300-120 = 180 . Look at datasheet, where you can read, that Ptot = 120 W.

 

[carlosfm]

did anyone calculated the resistors for voltage regulation (R3...R6) on the chip?
In my calculations, they only have to be 1/4W , max. 1/2 W. So why are these so great ones? For R3, R4 i come up to 0,01W, so 1/4 W would be large enough. R5 and R6 need 0,27W so here 1/2 W would be fine...

I ask this because in smaller dimensions it's easier to get the fitting Resistors.

I design my PSUs to last for a lifetime.

So did Carlos - why pick on me?

Yeah, I forgot... 1N4002 is what I use.
Normal diodes there, no gold-plated variety.

 

[slackman]

With a toroid with 2x30V i'll get about 42V. Voltage drop of MURs will be 1.5V per Diode, in bridge drop will be 3V. V-drop of LM338 is about 3V. So there enough reserves to rise up power supply to 35V. Am i right?

Mic

 

[carlosfm]

Notice that those diodes are completely unnecessary in this application.

Come again?
When you have a Cadj. cap of more than 10uf it is recommended to use that diode (from Adj. to output).
I use a 47uf cap, so...

The other diode is not mandatory but doesn't do any harm either.

Better go read the datasheet, Per...

 

[carlosfm]

Yes, you are right Michael. I think that I used IN4002's there, I'll update the site when I have time.

Nuuk, i'll send you a revised schematic tonight, with the diodes type.
But not the brand.

 

[peranders]

Come again?
When you have a Cadj. cap of more than 10uf it is recommended to use that diode (from Adj. to output).
I use a 47uf cap, so...

The other diode is not mandatory but doesn't do any harm either.

Better go read the datasheet, Per...

Yes..... and if you have a controlled environment you don't have to use these diodes but as you said, no harm in having them either.

 

[Nuuk]

Nuuk, i'll send you a revised schematic tonight, with the diodes type.

No Carlos, let's keep that a big secret for now!

 

[carlosfm]

The mysterious diodes revealed

 

[moamps]

2.5 A * 120 V is 300 W. How is, by you meaning, efficiency of this amp ? 300-120 = 180 . Look at datasheet, where you can read, that Ptot = 120 W.

Maybe the power factor (cos(fi)) does not equal 1.
Also, power losses in the transformer, rectifier etc. must be taken into account.

Milan

 

[Upupa Epops]

Milan, losses in rectifier are several watts. By transformer of this size is it up to ten watts. Practical efficiency of similar amp is around 60-65 %. For 120 W you don't need toroid transformer 300 W. BTW, by " musical " loading you can without problem overload similar transformer twice times, which is often used at amps for musicians.

 

[moamps]

I'm afraid your basic assumption that 2.5A x 120V gives 300W is wrong because current and voltage are most likely not in phase in this particular case.

Milan

 

[Upupa Epops]

VA, I know, but don't catch me by word and answer on straight question : Is neccessary to use for 120 W solid state amp transformer 300 VA ? Or : Use this transformer some designer of electronic in this case ?

 

[moamps]

VA, I know, but don't catch me by word and answer on straight question : Is neccessary to use for 120 W solid state amp transformer 300 VA ?

It depends on the particular application, as jackinnj already explained (post#140).

 

[Upupa Epops]

Which particular application ? Here we are talking about transformer for SS amp in class AB used for home reproduction, not about transformer for FM transmitter .

 

[carlosfm]

Well I shall say that with 1,000~1,500uf capacitance you need a big trafo, and it's never enough.

Go regulated and 150~200VA is enough for two LM3886 chips.
Bass is TIGHT, voltage doesn't sag and the "juice" is always available.

 

[Nuuk]

Carlos, you seem to have lost a lot of weight!

 

[Greg Erskine]

Carlos, you seem to have lost a lot of weight!

I think its a double headed sperm.

 

[moamps]

Which particular application ? Here we are talking about transformer for SS amp in class AB used for home reproduction, not about transformer for FM transmitter .

Any decent amplifier should have a transformer whose power is at least double the nominal output power. Higher transformer powers pose no real dangers (on the contrary). Diyers may experiment with different transformer power/output power ratios but should keep in mind that if they are going to drive their amps in difficult environments at nominal powers (e.g. active subwoofers), the ratio should be at least 2:1 to prevent transformer overload. Many commercial amplifiers do not abide by that ratio but, then again, this is a DIY endevour.

Modern FM transmitters have SS PP AB/B blocks and do not differ all that much from a diy amplifier. At least in terms of efficiency.

Milan

 

[Upupa Epops]

Yes, you have thruth - ratio 2:1 give to amateurs more enjoy . Is funny, while they searching for free samples, 'cos they can economize, on other side at the same time spend much more money for this ratio .

 

[moamps]

It's a question of costs and benefits, as with everything. However, we've been discussing the benefits so far. The actual budgeting is a matter of personal choices and priorities that everybody has to deal with on their own. I imagine not all diyers are downright destitute