? for ya, Al


Hey, Al, I figured you would know the answer to a question that I have, so here goes. My system sounds best when I conect my sub to my rca pre outs for my left and right front channels, and then connect my amp to the xlr pre outs for my front right and left channels also. I'm wondering if this is bad in any way. When I shut my sub off, I can hear the amp in the sub doing that "fart" shut down noise through my front right and left speakers, so I'm wondering if the amp in my sub is cross talking with my power amp for my speakers in any way. I'm also wondering if havinv both rca's and xlr's connected for my front right and left channels at the same time is degrading the signal coming out of my pre amp in any way. If you are wondering why I don't just hook my sub to the sub pre out on my pre amp, the reason is because then I am forced to cross my speakers over at 80hz and this doesn't sound as good as running my speakers full range and using the crossover in my sub set at 40hz.

My pre amp is a classe ssp 25 and my amp is a classe ca-150, if this helps.

Also, anyone is free to answer this question but I'm sure Al would know, that's why I directed it towards him!
b_limo
Who is Al?
Hi B,

It depends on the specific design, and especially on whether the RCA and XLR outputs are separately buffered (driven from separate output stages), as opposed to the RCA output signal being driven from the same circuit point as one of the two signals in the balanced signal pair. Unfortunately, based on the information in the manual it appears that the latter is the case for the SSP-25, and so the sound quality you are getting from the main speakers MIGHT be slightly degraded (probably VERY slightly).

I'm basing that on this statement on page 11 of the manual, which is referring to the output connectors:
You may use any of these connections in any combination that does not duplicate channel assignments. For example, you may use the Front Left and Right channel balanced XLR outputs to connect the SSP-25 to the power amplifier(s) for those speakers and either the RCA connectors or the DB25 connector for all other
channels.
So what will happen is that any noise or interference that is picked up or introduced into the RCA cable going to the sub will appear on one of the two signals in the balanced signal pair going to the main power amp, but not on the other signal in that pair. The noise rejection that would normally be provided by the power amp's balanced input would therefore not be able to reject that noise. That would include noise at ultrasonic and/or RF frequencies, which may not be audible in itself but may have audible consequences due to intermodulation effects within the amplifier.

On the other hand, depending on your particular setup all of that may be so slight as to be inconsequential. An experiment that may be useful would be to compare the sonics of your main speakers when the sub is disconnected with the sonics they provide when the sub is connected but its level control is turned all the way down. Although that comparison wouldn't be fully conclusive because electrical noise produced by the sub will probably be greater when it is producing an output.

If your sub happens to provide XLR inputs, another experiment to try would be to use an XLR y-adapter, such as one of these, to split the preamp's XLR outputs and send them to both the sub and the power amp. That would keep the balanced signal pair balanced, and I suspect that the output impedance of the preamp is low enough to minimize or eliminate the side-effects that can sometimes result from using a splitter to send a signal to two destinations.

And, yes, the shutdown noise is very conceivably the result of a turn-off transient generated by the sub coupling to the input of the main power amp via the path that exists from the RCA cable through the preamp to the XLR cable. The way around that would be to turn the sub on before turning on the main power amp, and to turn the sub off after turning off the main power amp (after waiting some seconds until the main power amp can no longer produce sound using internally stored energy). Although doing that could conceivably cause the sub to produce a noise when the power amp is turned on or off, but hopefully not.

Best regards,
-- Al
I had an SSP25. Its not balanced. The xlr's are for convenience (or to fool someone that doesnÂ’t know better). Be careful with the rca's, though. They break very easily.
Zd542, thanks for pointing that out. But to clarify, do you mean that there is only one signal on the XLR connectors (the "hot" signal on pin 2), and pin 3 is grounded instead of being provided with the inverted or "cold" signal that would normally be provided on a balanced output? Or do you just mean that the internal signal path in the preamp is not balanced?

Regards,
-- Al
Al, Thanks for your response! I do have xlr inputs and outputs on my sub. Maybe I'll try the splitter, as well as using the xlr inputs and outputs on thesub. I'm assuming that itwouldcross my front speakers over if I went that route. It seems that my little 5 1/2 inch drivers are actually happier at really loud volumes, being crossed over at 80hz; visually, less of that flappy driver thing that occurs when your speakers are turned up too loud.

Zd, That's interesting what you said about the xlr's not being balanced; bummer. The sound is better using xlr's though, and it was the only way I could get rid of a ground loop hum that I had. I didn't try removing the ground plug from my amp or pre amp power cord though, and that might of resolved the issue. I realized a month ago that the power cords I'm using have ground plugs that you can unscrew, I'm assuming for that very reason. Anyways, what did you think of the ssp 25 in terms of sound quality for a 2 channel setup? Any other thoughts about the ssp? I know what you mean about the rca's. Every time I disconnect a cable, they pull out.
One to be trusted.
Al is the poster Almarg, who tends to offer some of the best responses of any on this site.
"Zd542, thanks for pointing that out. But to clarify, do you mean that there is only one signal on the XLR connectors (the "hot" signal on pin 2), and pin 3 is grounded instead of being provided with the inverted or "cold" signal that would normally be provided on a balanced output? Or do you just mean that the internal signal path in the preamp is not balanced?"

I may be misreading your statement, but, wouldn't both of those choices in the above quote mean pretty much the same thing? Its been over 10 years since I had my SSp25 but I remember for sure that pin 2 is hot and if you want to use a XLR to rca adaptor, you need to get one that connects pins 1 and 3. Also, If pin 3 is grounded from the factory, wouldn't that imply that the internal signal path in the preamp is unbalanced, as well? If the OP has the manual, I remember reading that it does say somewhere in it that this is the case. It may be a little vague or hard to find in the literature because most companies like to say that their stuff has balanced inputs/outputs but don't like to point out if the component itself is not fully balanced, as well.

OP,

As far as sound quality goes, I thought it was fine for movies. For 2 channel, it depends on how much music you listen to. Its OK for casual listening but you would probably like a high quality 2 channel preamp much better for any serious listening.
Elizabeth and Abucktwoeighty, I appreciate the kind words. Thanks!

Zd542, thanks for the response.
If pin 3 is grounded from the factory, wouldn't that imply that the internal signal path in the preamp is unbalanced, as well?
Yes, but the converse is not by any means necessarily the case, and that is what is in question here. It seems safe to assume that the internal signal path of the preamp is not balanced, but that in itself says nothing about whether pin 3 is grounded or has a signal on it.

A good design that has an unbalanced internal signal path but provides an XLR output will generate a balanced pair of signals in its output stage, with one signal polarity (usually "hot") going to pin 2, and the other polarity (usually "cold") going to pin 3. I suspect that is the case here, which would make all of my earlier comments applicable.
I remember for sure that ... pin 2 is hot and if you want to use a XLR to rca adaptor, you need to get one that connects pins 1 and 3.
That would apply to the XLR input connectors, not to XLR outputs. Adapting an XLR output to RCA would not require connecting pins 1 and 3 (i.e., grounding pin 3) under any circumstances, in part because if a signal is not provided on pin 3 by the component, the component would already be grounding pin 3, assuming the design is not extremely deficient. And also because pin 3 is not needed when adapting an XLR output to an RCA input.

Again, my guess is that the preamp provides a balanced pair of signals to the XLR output connectors, making my earlier comments applicable. But if that guess is wrong, and pin 3 is grounded, the balanced inputs of the power amp would still provide some amount of noise rejection, but to a lesser degree than if a balanced pair of signals were provided on pins 2 and 3. (The reduced noise rejection if pin 3 is grounded would result from the differing output impedances on pins 2 and 3, and from the fact that any noise would be a factor of 2 greater in relation to the voltage difference between the two signal lines). So in that situation my earlier comments would still be somewhat applicable, but the likelihood of the sub's presence making a difference in what is heard through the main speakers would probably be a bit less, since the potential benefits of a balanced interface would be partially nullified even without the sub's presence.

Happy holidays to all! Best regards,
-- Al