2. amps-preamps
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Phase inverting problem

Hello,

I have a Conrad Johnson PV-12A pre-amp. It is phase correct for the phono stage, phase inverting for the line stage.

My power amplifier is a conrad johnson MF2100. It is phase correct.

So my first idea was to connect the speakers to the power amplifier the wrong way (black to red, red to black) and then connect the cartridge the wrong way around as well (R: + and - reversed, L: + and - reversed). Then the phase should be correct for everything.

But there lies the problem. When I switch the connections on the cartridge, I get a really loud hum, makes the music barely hearable. Perhaps it has something to do with the fact that R- is connected to the cartridge body, it is some kind of earth? Anyway, switching the connections on the cartridge is not an option. So, what to do?

My only idea so far is, seeing that I only have one line input (cd), is cutting open the RCA cable and switching + and - of the line, and connecting the speakers to the power amp the correct way. So, I'll do just that. But maybe there's a better solution that I'm missing. Any ideas?
swaf
ahendler
416 posts
 
Try comparing your line stage inverted or not inverted. You may not here a difference and then it would not be a problem. Many people cannot here absolute phase differences
almarg's avatar
almarg
9,670 posts
 
My only idea so far is, seeing that I only have one line input (cd), is cutting open the RCA cable and switching + and - of the line, and connecting the speakers to the power amp the correct way. So, I'll do just that.
Don't! If the cd player and the preamp both have 3-prong power plugs, or if any other connection path were to be present between the chassis of the two components (now or in the future), doing that would short the cd player's output to ground.

Not sure why you would have a hum as a result of swapping cartridge leads. Are you sure you did that correctly, which usually means interchanging red and green with each other, and interchanging blue and white with each other?

Also, I'm assuming that when you say "phase correct for the phono stage, phase inverting for the line stage," you mean that the preamp output is phase correct with respect to the phono input, and that the preamp output is phase inverting with respect to the line level input. That would mean that the phono stage section of the preamp, in itself, is phase inverting. Otherwise you could simply switch the speaker connections and both sources would be phase correct, since the phono stage output goes through the line stage.

Regards,
-- Al
hifitime
715 posts
 
Definitely don't switch the interconnect wires. Way to many risks in doing that. The only way I can think of doing this without getting a different preamp, would be to add a switch to each speaker wire. That way, you can flip a pair of switches and reverse the speaker polarity. It can be done with a pair of double pole, double throw switches.
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