MS-275 'The Switch'

[crazycody1993]

In a lot of places i've looked, everyone talks about the red and yellow switch inside the phoenix gold MS-275, and with the questions about it, most of the answers say 'it makes it run cooler on lower ohm loads, and lowers your output voltage'

But now that i have gained quite a bit of electrical knowledge about amplifiers over the last 2 months, i was curious if someone knew what it ACTUALLY did?

I want to understand what it does in order to change those things, and which way to have the switch for the setup i'm putting it in? if anyone has any answers it would be greatly appreciated

And an early thank you for everyones input,

Cody

[Perry Babin]

It changes the regulated rail voltage. At higher voltage, low impedance loads cause excessive heating and cause excessive current flow through the outputs. Reducing the rail voltage reduces heating and current flow.

Set the switch to match the speaker load you'll be using.

[1moreamp]

It switches into circuit an extra zener diode in the PWM control circuitry that sets the rail supply voltage.
That's it, plain and simple. nothing fancy just a different voltage drop across the circuit to control rail supply voltage level.
The two different voltages are for the 4 and 2 ohm power levels so the drop across the outputs will not cause rapid thermal shutdown operation while running low ohm loads. The difference is about ~+&-5 volts lower voltage to run lower ohms with. Many old school class AB amps drop the rails for lower ohm loads.

And Perry's right just set it for the load your driving and the amp will be more thermally stable.

[crazycody1993]

now with the lowered voltage, will it still drive the same ohm load at equivelant levels?

[Perry Babin]

If you set it to 4 ohms, it will do rated power into 4 ohms.

If you set it to 2 ohms, it will do rated power into 2 ohms but will not produce the 4 ohm power rating into 4 ohms.

If you set it to 4 ohms and connect a 2 ohm load, the amp will either overheat, fail or go into protect mode.

[crazycody1993]

okay, sounds perfect! thanks a ton guys,

[807tubenut]

in my MS-275 it sets the power supply to regulated, or unregulated, Red paint dot indicated regulated, the yellow dot indicates unregulated, or that's how it works on mine... in regulated mode, no matter what B+ is, the rails will always be 50vct, in unregulated iv seen up to 90vct with 15v input.

[1moreamp]

Originally Posted by 807tubenut

in my MS-275 it sets the power supply to regulated, or unregulated, Red paint dot indicated regulated, the yellow dot indicates unregulated, or that's how it works on mine... in regulated mode, no matter what B+ is, the rails will always be 50vct, in unregulated iv seen up to 90vct with 15v input.

All the switch does is jumper a second zener diode in or out of circuit with the opto feeding the PWM IC to raise or lower < switch position dependent> the secondary set point voltage to the four ohm level ~ +&- 30 VDC. The jumper-ed zener diode sets the two ohm load voltage ~ +&- 21 VDC measured at the output transistors, or the on board fuses, and ground lead referenced to the true center tap of the power toroid transformer. The driver stages see +&- 29 or +&- 40 VDC.
Switch closed the second zener is shorted by the switch contacts for 2 ohm operation, open its 4 ohm operation with the second zener in circuit to the opto. No caps in the secondary side of the supply are rated above 35 VDC each, so a 90 volt reading must be a summed reading across both rails, not to common ground Center tap of the toroid, and 90 volts would have to be above the recommended 14.4 max input voltage PG spec'ed in all their literature, and it surely must have been a summed reading of +&- 45 volts supplied to the driver stages only, the outputs operate at about 7 to 9 volts less on each rail then the driver stages do. Just a FYI from the old guy...

[807tubenut]

Understood, my voltages were VCT, only testing the + and - rail as you suspected, nothing referenced to the transformers CT, voltage measured at the fuses.


Thanks for the info, you are the PG man!

[1moreamp]

Originally Posted by 807tubenut

Understood, my voltages were VCT, only testing the + and - rail as you suspected, nothing referenced to the transformers CT, voltage measured at the fuses.


Thanks for the info, you are the PG man!

It's all good, I just wanted to clarify any info I may have left out so a clear picture of the supply design was on the table. I don't want any of my ramblings here on the DIY to go vague on anyone..lol...laters...C