VA RATING

Shelah · 2012-04-12 9:25 am

to the diyaudio community i have a toroidal transformer i got from an old powered mixer.it had no specification with it. the only things i could measure from it were as follows:
primary: 220v 50Hz, 5 amp fuse.
Secondary:
1. 60v-0-60v
2. 19v-0-19v
3.24v-0.
the transformer was used on an op-amp based Class AB amplifier rated 325W 4 ohms. on the damaged pcb, these were some readings printed.

10 amp fuse before rectifier of amplifier (60v-0-60v)
1 amp fuse before rectifier for mixer section (19v-0-19v)
the diodes of the amp rectifier: "AO8, 6amps"
diodes for the mixer section rectifier: "1N4007

i dont intend on repairing the amplifier section cause of the amount of voltage used. i would like to use it on a 700w leach amp i got from this link http://www.te1.com.br/2011/03/amplificador-de-660-watts-rms-4-ohms-mono/ designed with the following specs.

Total Power 660 W RMS into 4 Ohms.
Amplification class AB
Frequency Response: 20 Hz to 20 kHz, +0, -0.3 dB
THD + N (Total Harmonic Distortion + Noise: at 4ohms = <0.05% 20Hz to 1kHz, increasing linearly to 0.1% at 20kHz
IMD (Intermodulation Distortion): 90dB at 8ohms
Supply voltage: Symmetrical Amperes 63 Volts x 12
Nominated 50 +50 Trafo x12A
Load impedance: min. 2R
Speed over (SR) 35V/us
Input sensitivity: about 1.1 V for full arousal

with the information given, can someone help me find out what the VA rating would be both theoretically and in the real world and if it would be suitable for the 700w leach amp. http://www.te1.com.br/2011/03/amplificador-de-660-watts-rms-4-ohms-mono/

DF96 · 2012-04-12 10:09 am

If it was used in a 325W amp then it is unlikely to be be able to support a 700W amp, except at a reduced duty cycle.

Tekko · 2012-04-12 10:19 am

I'd say the transformer is roughly 500-600 VA. For your 700W leach amp your gonna need atleast a 1kVA transformer.

AndrewT · 2012-04-12 1:14 pm

+-60Vdc PSU is unlikely to get any more than 50Vpk into your speaker terminals.
That predicts a maximum power output from this design @ ~312W into 4r0

Where did 660W to 700W come from.
Did you check the numbers?

Shelah · 2012-04-13 9:54 am

@tekko thanks for your estimation. i theorectically got somthing close to that. one other thing, the leach amp specs on 320volt.com and hobbyelectronics.blogspot.com says that a 54x2 transformer was used. but how would mine not work in this case?

@Andrew T, i did check the numbers and please check the web link i gave and also 320volt.com andhobbyelectronics.blogspot.com
i also forgot to put something on the specs.
output power: 260w 8R, 450w 4R, 680w 2R.

AndrewT · 2012-04-13 1:53 pm

Yes I looked at your linked page, yes I read some of what it said.

I'll ask again, did you check the numbers?
450W into 4r0 requires 60Vpk into your 4r0 test load. How do you get that from the details given?
260W into 8r0 requires 64.5Vpk !

it also said

Amplificador de 660 ~ 700 Watts RMS 4 Ohms mono

Do you just accept anything you read?

TonyTecson · 2012-04-13 2:03 pm

@Shelah,

how big is this traffo? what is the weight in kgs? are you using this traffo for a 2 channel leach or just one channel?

if you can answer my question i will be able to help you...

Shelah · 2012-04-13 5:30 pm

@ Andrew, sorry sir, maybe i didnt read.

am just in need of help on the same.
@tony the trafo could be about 5 to10 kilogammes, 12 to 15 centimeters in diameter, and about 7 centimeters thick.

VibroKing$ · 2012-04-13 6:24 pm

To estimate power of toroidal transformer it can compare weight with a similar product.
Transformer Manufacturer UK - Airlink Transformers
600VA for 350Wrms + 350Wrms is suitable for domestic use, 800VA or more for professional.
also Clone with 2sc5200 x 8 = 1200W perhaps is a waste if you do not have to drive loads below 2 ohms (imho)

Shelah · 2012-04-14 1:51 pm

@Vibroking$ what do you saggest in the correct number no 2sc5200 i can use at the output for 2 ohms?
and is mine trafo for domestic or professional use with regard to the data given in my first post?

Shelah · 2012-04-14 1:57 pm

the other thing i forgot in the trafo it measures 3.5 ohms at tyhe primary. is it useful in determining the va rating?

Tekko · 2012-04-14 2:01 pm

No, thats just the windings dc resistance.

To calculate VA u need core area, wire size, current and voltage ratings.

AndrewT · 2012-04-14 2:02 pm

Do you want to drive 2ohm speaker?

VibroKing$ · 2012-04-15 1:40 am

Shelah said:
@Vibroking$ what do you saggest in the correct number no 2sc5200 i can use at the output for 2 ohms?
and is mine trafo for domestic or professional use with regard to the data given in my first post?

imho theoretically with 60Vcc and 2Ohms loads are 900W with sine wave and 1800W with square wave.
maximum dissipation for each rail is 450W for safety roughly about 600W
four 2sc5200 transistor for rail ..

TonyTecson · 2012-04-15 2:06 am

@tony the trafo could be about 5 to10 kilogammes, 12 to 15 centimeters in diameter, and about 7 centimeters thick.

wish you can be more precise....but anyway, according to a table here: Toroidal Cores Stock

you could have an 800va traffo, so at 120volt secondaries can draw out 6.66amps. your actual dc current is 10% less....

your unloaded rails will be +/- 95 volts.......at full power of say 300watts into 4 ohms, expect your rails to sag to +/70 volts or thereabouts....

all this paper computation of power is meaningless.....unless you can do an actual testing with a real amplifier, not a paper amplifier....

your primary dc resistance of 3.5ohm says you got a good quality torroid there...

Shelah · 2012-04-15 7:49 am

@Andrew yes would like to drive the amp to 2R0
@Vibroking$ if I let the diagram as it is would it still be a wastage of 2sc5200x8 at 700W, 2R0?

AndrewT · 2012-04-15 11:36 am

2r0 test resistor or 2ohm reactive speaker.
I don't think you understand the difference.

VibroKing$ · 2012-04-15 1:10 pm

Shelah said:
@Andrew yes would like to drive the amp to 2R0
@Vibroking$ if I let the diagram as it is would it still be a wastage of 2sc5200x8 at 700W, 2R0?

imho theoretical with pure resistive load transistors need of 350w
but reality speakers have complex impedance then the required power can be greater.
more transistors for high volume professional use (disco club) It's greater safety.
(The original Leach use only 2xMJ15003 = 500W with +/-58 V)
(Ncc200 only 1xMJ15003 = 250W with +/-40V)

AndrewT · 2012-04-15 1:21 pm

The original Leach Lo Tim used 2pair of 250W devices to run from the 40+40Vac transformer.

That is 1000W of output device.

I built a 3pr clone (1380W of devices) and it has a maximum output of 170W into 8r0 and is not capable of long term heavy duty use into 4ohm speaker, even though it can drive a 4r0 test load to >300W.

VibroKing$ · 2012-04-15 2:45 pm

AndrewT said:
The original Leach Lo Tim used 2pair of 250W devices to run from the 40+40Vac transformer.

That is 1000W of output device.

I built a 3pr clone (1380W of devices) and it has a maximum output of 170W into 8r0 and is not capable of long term heavy duty use into 4ohm speaker, even though it can drive a 4r0 test load to >300W.

I agree with 2x40Vac transformer --> +/-54Vcc
theoretical power ...
8Rload = 182Wrms (78.5% efficency sine wave) ... 364W (100% efficency square wave)
4Rload = 364Wrms and 730Wsquare with max dissipation each rail of 182W (pure resistive load)
real power ...
Vce sat + Vas + Vemitter resistor perhaps lost at least 4V
load receives about 50Vp max 156Wrms on 8Ohms

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