How big of a power transformer do i really need for a headphone amplifier?

[kazuviking]

I built a super simple class a hp amp and wondering how big of a trafo do i really need. The BOM states that a 25 or 50VA transformer is needed but when i measure the current draw its 110(+-10)mA on both positive and negative rails. Currently i run it from a 150W trafo(14-0-14V) that i salvaged from an old surround amp but its way to big in size and the other windings are not in use. Everywhere i read people suggest that you need way oversized one even if it barely draws any current. From my simple calculation it draws 3.2W on both rails so would a 4.5VA or 7VA 2x12V suffice?
Other info: The amp have its own super simple zener+transitor reg and the transformer have a full bridge after it. The current transformer is way too big and it don't fit into my small enclosure.
Any help is appreciated.

 

[AnalogSteph]

Got a link for the kit / design or something?

Not sure your math checks out... either that or the circuit is not very Class A. I would be expecting at least 75-100 mA per channel.

 

[kazuviking]

Got a link for the kit / design or something?

Not sure your math checks out... either that or the circuit is not very Class A. I would be expecting at least 75-100 mA per channel.

Its the sapphire 4 by rjm. I expected more current draw but the amplifier works normally and gets way too loud, it turns my sacrificial hyperx headphones into a mini speaker at full volume. In simulation the powerdraw fluctuated between 51 and 56mA per board per rail which is what i measured.
Notes from the BOM:

The Sapphire 4-series headphone amplifier / line stage features a discrete transistor current feedback amplifier (CFA). In the default configuration the class A output buffer provides 30 mW minimum output power into all headphone loads between 30 and 600 ohms. The output impedance is about 4 ohms, maximum, while distortion and output noise levels are class leading. Configured for 37.5 mA bias current, Q13 and Q14 dissipate about 0.5 W, so only a small heatsink about 30 C/W is needed. R11,12 control the bias current in the output transistors.

This one is not a powerhouse but i don't mind it since even a few mW of output power is more than enough for me.

 

[AnalogSteph]

25 VA seems plenty generous for this one even in the high output configuration. I'd be looking for about 10 VA with what you currently have, 7.5 VA would probably do as well. Note that getting +/-18 V requires a relatively lightly-loaded transformer.

 

[kazuviking]

Alternatively i can get the same rating transformer but with 2x15V instead. The min voltage the regulator accepts in 16.5V so i might be safe but isn't then a 15V trafo would work.

 

[solderdude]

A 15V transformer is only 15V at the rated output power. Usually the transformer is not loaded much (except for class-A amps) so the unregulated output voltage (depending on the type of transformer) can be anything between 15V and 18V AC so rectified 20Vdc to 24Vdc and with 2V drop out voltage of a regulator there is plenty to spare.

The power rating thus depends on the max. continuous output power + losses in the amp + extra power (relays, indicators, processors etc) + transformer losses + some headroom to allow for voltage drops on mains.

 

[kazuviking]

So if i apply the same thing(correct me if i'm wrong) for a 7VA 12V transformer then the voltage will be between 12V and 15.8(datasheet)V. One winding can provide 292mA, the load is 75mA with headroom then since the winding is quarterly loaded then the voltage should be around 13-13.5V which is 18.5Vdc after the rectifier(the formula i found (13-1,2)*1,4*1,15-0,5)=18.5. From my conclusion the 7VA should be enough.
PS Love your passive filters solderdude.