Using different amps in HT. Phase inverting and non phase inverting amps. IMPORTANT

[TLS Guy]

I have just answered a question from a member who is going to use a Conrad Johnson amp for his mains and an NAD for Center.

Now this has come up before, and I have been meaning to post on this very important topic before.

You have to know whether the amps you are using are phase inverting or not. Both are common, and the owners manual frequently does not tell you!

In my answer to the member you will see why this is an important vital consideration when, mixing amps, even different models from the same manufacturer. Here is what I wrote.

Now you have another problem you have not considered, and a lot of people don't when using different amps in HT. That is phasing. On a lot of amps, especially from that era they are phase inverting. That is to say the speaker output is 180 degrees out of phase with the input. If my memory serves me correctly, and it may not, I believe your NAD is a phase inverting amp. Your Conrad Johnson and your receiver I don't know. Now if your Conrad Johnson is non phase inverting and you NAD is phase inverting, you mains and center will be exactly out of phase even if wired correctly.

Now it is a funny thing but owners manuals seldom state if an amp is phase inverting or not.

The only way you can tell for sure is to put an audio oscillator to one channel of an amp and connect the oscillator output also to the x plates of an oscilloscope and the speaker output of the channel connected to the oscillator to the Y. If the pattern is a straight vertical line the amp is non phase inverting. If it is a circle it is phase inverting.

Now this is a very common error when using different amp brands. I have inverting and non inverting amps in my rig, and they are from the same company and designer. The manuals are silent on the issue. I can tell you if you are using different amps this has to be done to know for sure. You can try by listening, but with the scope test tells you for certain. Obviously an out of phase condition, commonly caused by this problem is disastrous to the audio results.

If you have a phase inverting amp being used with a non inverting amp, then the speaker connections to the inverting amp should be reversed.

I hope you or anybody else reading this really understands this. It is really important.

 

[HangtownMatt]

This is important and is very easy to overlook because it is somewhat difficult to understand. The owner's manuals for my amp and pre-amp go into fairly good detail about it, but I still get confused. I think the bottom line is quantified in this quote from my owner's manual ... "Each component in the stereo system is either phase correct or phase inverting. It is of no consequence if an individual component is phase inverting, as long as the system as a whole is correct. This will be the case as long as the number of phase inversions is even (or zero)." This is part I don't get. How does one determine if the the system on a whole is correct? It's not to hard on a 2-channel stereo, but when we venture into the world of home theater and surround sound I have no idea; too many components. So my question to TLS Guy is this: How can we determine if our system as a whole is correct?

Matt

 

[TLS Guy]

This is important and is very easy to overlook because it is somewhat difficult to understand. The owner's manuals for my amp and pre-amp go into fairly good detail about it, but I still get confused. I think the bottom line is quantified in this quote from my owner's manual ... "Each component in the stereo system is either phase correct or phase inverting. It is of no consequence if an individual component is phase inverting, as long as the system as a whole is correct. This will be the case as long as the number of phase inversions is even (or zero)." This is part I don't get. How does one determine if the the system on a whole is correct? It's not to hard on a 2-channel stereo, but when we venture into the world of home theater and surround sound I have no idea; too many components. So my question to TLS Guy is this: How can we determine if our system as a whole is correct?

Matt

I think some of these auto set ups recognize out of phase conditions. However the gold standard is to use a scope to check phasing. I know that is tough because few members have them. However, there are now more and more members using pre outs and amps of a variety of pedigrees for various combinations of channels.

I guess my best advice is to up your educational level and invest in suitable test equipment if you want to set up complex systems. My guess is that members who are not using either a receiver, amps in one case or identical amps,probably have serious phase problems impairing their systems, if they are not checking phase carefully.

Your manual discusses phase, but I can tell you this is far from the rule.