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Frarun 5th April 2013 03:10 PM

transformer power for gainclone amp
I have doubts regarding transformer power for Gainclone amp. I made a calculations based on
the datasheet for LM3875 page 20, but instead of assuming a output power and R load
assume the constant 2x19 AC voltage from transformer, and calculate a power required for
particular R load. This is what I get:

1. 30W / 8ohm -> 2x19V 105W
2. 40W / 6ohm -> 2x19V 140W
3. 60W / 4ohm -> 2x19V 210W

What if I use a 2x19V _140W_ transformer and connect 4ohm load? Will it broke a
transformer because the current will not be sufficient?


AndrewT 5th April 2013 06:22 PM

The usual rule that works for virtually all power amplifiers is to use a transformer with a VA rating that is 1times to 2times the total maximum output power of the amplifier/s.

For 30W of output power use 60VA to 30VA.
for 60W use 120VA to 60VA.

However, due to the high regulation of very small transformers I recommend no smaller than 160VA.
That would support upto 160W of total power output.
This could be two 80W channels or three 50W channels or four 40W channels.

Frarun 5th April 2013 07:10 PM

I should add, that in my calculations I totally skipped the 15% mentioned on page 21 of LM3875 datasheet.... and yes. My units was wrong :) Should be 'VA', not 'W'...

In deed this 150VA is my limit because of size of transformer (about 95mm). My problem is I need to choose a correct / optimal voltage for this transformer maximizing output power (or I mess something here?). Is there a simple rule to do that? Should I maximize this for specific load impedance?

AndrewT 6th April 2013 07:21 AM

The Voltage requirement is very different from the VA requirement.

The output from the amplifier is determined by the current and voltage delivered to the load, usually a reactive load.
The maximum voltage determines the maximum power that can be delivered to a resistive load.
That maximum output voltage is very dependant on the PSU voltage and in turn on the transformer voltage.

Look at the datasheet to find the maximum power you require.
Use that Pmax to determine the Vpk available to the load.
From Vpk and the losses through the amplifier determine the PSU voltage when the amplifier is delivering that maximum power.
Adjust the PSU voltage to the unloaded condition.
Determine the Vac of the transformer for that unloaded PSU voltage.

Frarun 8th April 2013 01:08 AM

Thanks for your response. Looks like simple calculation.

I plan to use a 150VA transformer (more popular in my country than 160VA). This fits my requirement for about 95mm diameter for transformer (toroidal). As you said I could get something like 50% of that power. It will be 2x40W output power. The speaker impedance will be 6ohm...
Now the voltages.
Upk = sqrt (2x40x6ohm) = 21.9V


Originally Posted by AndrewT (
... losses through the amplifier ...

I am not sure what is that. Is this makes a heat? I will add here a +5V described in datasheet as 'dropout voltage' (still don't know if is the same):
Upk + 5V = 26.9V
and add 15% voltage (transformer regulation) to have an unloaded voltage:
U = 26.9V + 26.9V*15% = 30.9V DC

It gives a +-30,9 DC / 1.41 = +-22V AC transformer
Is that correct?

AndrewT 8th April 2013 11:13 AM

That looks pretty close.
Look at the datasheet for confirmation of supply voltage (loaded voltage while delivering maximum power) to output power.

Frarun 8th April 2013 12:07 PM

Do you mean 'Output Power vs Supply Voltage' chart?

For about 31V supply voltage It points to 55W for 6ohm. Is that mean I can get all below this 55W and only limit is my PSU?

For RL =4ohm, there is a power drop for 31V supply voltage on the chart :/ I will not be able to use such resistive load?

Frarun 8th April 2013 12:15 PM

Grrrr. I get the wrong voltage. The loaded voltage is probbably my 26.9V. So from the chart it points to 40W! :) The 4ohm load gives 50W output power.

TonyTecson 8th April 2013 12:30 PM

do not despair, you probably won't notice is the rail voltages at full power that matters more than the standby rails...

Frarun 8th April 2013 01:32 PM

The 27V is below a standby 31V (15% margin). Hope this is a safe assumption.

What is really interesting, the impedance of speakers may vary from freq. So even If I connect nominal 6ohm speakers it may drop to like 4ohm(?). I fill this affects the PSU, and could generate some bad conditions for the design.

I saw lots of gainclones where the very big transformers are used (like 250VA). I was not sure it is really required. This is probably for some better peak output power...

Anyway. Thank for any comments/info.

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