OPTI-MOS using 50-0-50 450VA TRAFO

Calebay · 5th February 2005, 11:51 AM

Hello

i got 2 trafos 50-0-50v and 450VA from an old SONY ES power amp.

i like to build the OPTI-MOS amp seen here
http://www.sealelectronics.com/kits/index.htm

but it sems my railvoltage will be to high. for the VA rating. is there any way to lower the rail voltage? using regulation? or something else? so i will get closer to 50 volt rails.

tnx

EmZee · 5th February 2005, 03:24 PM

According to the book the OPTIMOS Amp can handle up to 85V DC

You have then to replace the output trannies to buz901cdp/buz905cdp
lower the emitter resistors to 0.22

This will give you a 400Wrms amp

regards

Calebay · 5th February 2005, 08:31 PM

okay ty

but i will end up whit + - 70V Rails
do i need to change anything then? and will the trafos be to small?

AndrewT · 6th February 2005, 10:28 AM

Hi,
your +-70v rails will give about 200w into 8r and just under 300w into 6r. The 450 VA rating should be OK at these loadings even driving a full range speaker.
Into 4r the voltage will sag badly but you can ameliorate this with extra capacitance in the DC filter. This will not cure it.
It will not suit a stereo amp although many manufacturers may well supply a similar size as yours and quote short term outputs.
Check with your supplier about changes to higher voltage transistors.
regards Andrew T.

Calebay · 6th February 2005, 12:46 PM

Quote:

Originally posted by AndrewT
Hi,
your +-70v rails will give about 200w into 8r and just under 300w into 6r. The 450 VA rating should be OK at these loadings even driving a full range speaker.
Into 4r the voltage will sag badly but you can ameliorate this with extra capacitance in the DC filter. This will not cure it.
It will not suit a stereo amp although many manufacturers may well supply a similar size as yours and quote short term outputs.
Check with your supplier about changes to higher voltage transistors.
regards Andrew T.

tnx agin

this is for 1 450VA trafo per Channel correct? how many watts in 4 ohm? and how do you calculate it?

AndrewT · 6th February 2005, 01:49 PM

Hi,
you can expect a peak output voltage of about 5v to 10v below Vrail with BJT and 10v to 15v below Vrail with FET.
The power is Vpk * Vpk / 2 / R.
Your Vrail will drop slightly for 8r load maybe 5v, but will drop much more into 4r maybe 10 to 15v below unloaded Vrail
Vpk (4r) could be as low as 45v giving 253w, way below potential 400w for a stiff PSU.
regards Andrew T.

Calebay · 6th February 2005, 02:09 PM

okay

i will be using them for driving 1 8ohm woofer eatch so guees its okay then.

5th February 2005, 11:51 AM	#1
Calebay diyAudio Member Join Date: Jul 2003 Location: Denmark	OPTI-MOS using 50-0-50 450VA TRAFO Hello i got 2 trafos 50-0-50v and 450VA from an old SONY ES power amp. i like to build the OPTI-MOS amp seen here http://www.sealelectronics.com/kits/index.htm but it sems my railvoltage will be to high. for the VA rating. is there any way to lower the rail voltage? using regulation? or something else? so i will get closer to 50 volt rails. tnx

5th February 2005, 03:24 PM	#2
EmZee diyAudio Member Join Date: Jan 2005 Location: Luxembourg	OPTI-MOS using 50-0-50 450VA TRAFO According to the book the OPTIMOS Amp can handle up to 85V DC You have then to replace the output trannies to buz901cdp/buz905cdp lower the emitter resistors to 0.22 This will give you a 400Wrms amp regards

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5th February 2005, 08:31 PM	#3
Calebay diyAudio Member Join Date: Jul 2003 Location: Denmark	okay ty but i will end up whit + - 70V Rails do i need to change anything then? and will the trafos be to small?

6th February 2005, 10:28 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, your +-70v rails will give about 200w into 8r and just under 300w into 6r. The 450 VA rating should be OK at these loadings even driving a full range speaker. Into 4r the voltage will sag badly but you can ameliorate this with extra capacitance in the DC filter. This will not cure it. It will not suit a stereo amp although many manufacturers may well supply a similar size as yours and quote short term outputs. Check with your supplier about changes to higher voltage transistors. regards Andrew T.

6th February 2005, 01:49 PM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, you can expect a peak output voltage of about 5v to 10v below Vrail with BJT and 10v to 15v below Vrail with FET. The power is Vpk * Vpk / 2 / R. Your Vrail will drop slightly for 8r load maybe 5v, but will drop much more into 4r maybe 10 to 15v below unloaded Vrail Vpk (4r) could be as low as 45v giving 253w, way below potential 400w for a stiff PSU. regards Andrew T.

6th February 2005, 02:09 PM	#7
Calebay diyAudio Member Join Date: Jul 2003 Location: Denmark	okay i will be using them for driving 1 8ohm woofer eatch so guees its okay then.

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