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Changing the voltage.

Blue Belle

pfm Member
Hello. I have a pair of Mission 767 speakers that I purchased for my collection, but they were without the LFAU bass control amplifier. However, I found one in the USA, and took the plunge. I've had it on a voltage transformer, it powers up, and allows the High and Mids through, but no bass signal is coming through. I've bridged the speaker crossovers and the speakers work all drivers in that way, but no movement what so ever via the LFAU. It's possible that the LFAU was used in combination with the PSX external power supply, and is wired to do so, hence the lack of activity. I have 3x Cyrus One PSX units to complement a Cyrus One amp, Tuner and the LFAU, I'd love to be able to get it all serviced, and then refinished, as you can imagine the components have taken a battering over the years. So, the main question, can I have the voltage changed in the LFAU so it will work on our 240v system, and then obviously allow me to plug it into the 240v PSX that I have set aside for it. Even without any servicing, I was truly stunned as to how good the Cyrus One amp was when playing through the 767's, it really is a lot of amp in a small box. Apologies for the long question, and hope it's not a stupid question too.
 
All of the early Mission/Cyrus amps included dedicated transformers for the country of destination. IOWs a replacement toroid is required to convert to another regional voltage (plus an appropriate input power fuse change).

This needn't be a problem, as your intended PSX outputs DC, therefore, it will power the amplifier section of the 767 LFAU, assuming the latter has actually been internally altered to run from PSX.* However, without installing a replacement UK voltage transformer into the LFAU, you would need to continue powering the LFAU's pre-amp stage via the voltage transformer. IOWs, just like with Cyrus Two/PSX, the pre-amp section continues to be powered from within, whilst the amp section (in this case, that which drives the bass units of 767) receives power from the PSX. This is why no bass signal is coming through via powering the LFAU from the voltage transformer alone.

* As with using Cyrus Two with PSX, it is necessary to remove the two DC fuses from the main PCB at FS1 and FS2. If these have already been removed then you are ready to go.
 
Apologies if I'm getting this wrong. So if I open up the LFAU, and both fuses are missing, then it has already been used with a PSX, and if I attach the 240v PSX without any voltage conversion on the LFAU, it's immaterial, because the PSX bypasses the voltage anyway? Thanks for the reply, Craig.
 
You're welcome.

Yes, if the fuses are absent then the LFAU is ready to be used with the PSX. It will not operate sans PSX, unless the fuses are replaced.

Although the PSX runs on 230-240VAC it outputs 40VDC. Despite the bass amp sharing the same chassis as the pre/filter section, the two run as though they are separate pre/power units, connected only by a pair of signal runs. The North American version LFAU still need be plugged into the voltage transformer, as it requires 120VAC for the pre/filter section to operate. As with Cyrus Two/PSX it doesn't matter if the LFAU power switch is set to on or off, as the pre is continuously powered when the unit is connected to the mains (the switch will toggle the LED on and off).

With a 500VA transformer and 70,000uF of filter capacitance behind it, that amp becomes a wee beast!

Enjoy.
 


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