Recent sound deterioration shrillness, ringing

Because of recent speaker changes I've been flooding the forums trying to accommodate these changes and have run into a problem. First, for purposes of this discussion, the chain of components that I'm dealing with is this: Apple TV - Peachtree Audio preamp - RCA TO XLR CABLES - Behringer Ultracurve DEQ2496 EQ - XLR cables to Peachtree 220 power amp - speakers. Using other sources, the problem is the same.

With straightforward RCA connection from preamp to power amp everything sounds great and when I first hooked up everything as described above, it also sounded good. However, that's no longer the case; when hooked up through the EQ the sound turns somewhat shrill with what I would call a somewhat "ringing" effect to it even with the EQ power off which defaults to signal pass through. I've checked all connections for proper polarity and found them all to be properly connected.

Leaving the EQ out of the chain would be the first obvious choice for resolution but using it is the only way I have for connecting a subwoofer. One thing that crossed my mind is that, during setup I accidentally connected the preamp output to line out instead of pre out and thought my walls were going to cave in before I got it shut off.

I hope I've provided enough info for, at least some speculation and will appreciate any ideas that come to mind.
The Behringer is a very powerful unit, but it is not considered very good sounding. Check to make sure all the effects are bypassed.
Broadstone...I've been following your dilemma on some of
your other posts.

Why can't you hook up your sub via the high level inputs
and use your amp to connect it? You just parallel the
speaker wires coming out of the amp. Or you could even
run a set of speaker cables from the speakers to the sub,
if that's easier. If your sub has both high level inputs and
then outputs to the speakers, that would be even better.

That would be the solution, unless I'm missing

What kind of sub do you have? Make and model.
Jim (Broadstone), how have you been connecting the sub to the Behringer? If by any chance you have been using an XLR-to-RCA adapter or adapter cable at an output of the Behringer, most such adapters short the inverted signal on XLR pin 3 to ground (XLR pin 1). The output circuits of some components will not be happy under that condition, and damage could conceivably even result in some cases. (There is no problem using such an adapter at XLR inputs, as you indicated you are doing).

Regarding the possibility of connecting the sub to amplifier outputs, assuming the sub can accept speaker-level inputs, your amp is class D, and apparently uses some version of the ICEPower class D modules. Some ICEPower versions have large DC voltages on both their + and - output terminals. I would therefore not connect a powered sub to the outputs of your amp without first getting an ok from Peachtree. Or if Peachtree indicates that the 220 has that kind of an output, using specialized adapters that have been available which utilize capacitors to block the DC.

Best regards,
-- Al
Al...thanks as usual for your insight. I always learn so much from your posts.

I have a comment and a couple of questions that I hope you can answer.

I find it hard to believe that in this day and age that a manufacturer such as Peachtree would sell a product that could potentially damage another unit. I bet there are a lot of consumers who hook up a sub using the speaker (high) level inputs.

If the amp did emit DC, wouldn't the speakers them selves be damaged? The sub he is using is a Polk Audio Micropro 4000 that has just a high level input, but no output to the speakers, (it also has a low level, but we're talking high level). I realize that the sub is powered, but wouldn't the voicecoil and/or crossover in the speakers react the same way as the sub would with DC present?

Seems to me this would be a really bad thing for Peachtree, (or any company) to produce.
Thanks, Mofi. And your questions are good ones. BTW, I myself learned about this issue with certain ICEPower class D modules (which as I indicated may or may not pertain to the Peachtree 220) just a few months ago in this thread.

A passive speaker won't care about the DC, because to a very close approximation it is present equally on the + and - terminals, and thus it will not cause any current to flow in the speaker. However a powered sub will typically connect its negative input terminal to its circuit ground, which will typically be connected to its AC safety ground through a low impedance, which in turn will be connected to the amplifier's AC safety ground through the AC wiring, which in turn may result in a near short being applied to the large DC voltage on the amp's negative output terminal. The result potentially including damage to the sub, the amp, and the listener's ears.

Also, connecting the sub's negative input terminal to a ground point on the amp, as is often done when connecting powered subs to amps having balanced or bridged outputs (i.e., amps having signals on both their + and - output terminals), would not be a solution in this case because with the sub's - input terminal being at zero volts the sub would then be subjected to the huge DC voltage on the + terminal.

My understanding is that those ICEPower modules were used by a significant number of different amplifier manufacturers. As indicated in the thread I linked to, adapters incorporating capacitors to block the DC were and perhaps still are available for connection of powered subs to those amps.

Best regards,
-- Al
Al..thank you for the link to that post and for the additional info. I was absolutely unaware of that potential problem with Class D amps!?!

Being in the high end audio business for as long as I was, I have personally seen, several times, what DC present at either the preamp or amplifier can do. All of those instances could be traced to faulty equipment. But to make gear that could cause that right out of the box is just...stupid :-)

Thanks again for your insight!!!
Thank you. I'll try to address each of your comments together.

1. I've been using the Behringer for well over a year so to whatever extent it degrades the sound, it becomes relative because the system has always sounded fine until now.
2. When I spoke with Peachtree they said the only way to hook up a sub is to use a splitter from the pre out to deliver the signal simultaneously to the powered sub and the power amp. That works but it substantially degrades overall sound to the extent that I described earlier. Also, separating the signal like that gives the EQ no control of low bass needed for auto room correction.
3. I spoke with Behringer who ok'd the use of the unbalanced aux out stereo connection for the sub. This works fine and I assume that this will accommodate auto room correction as well.

Bottom line is that it looks like this setup works but I still have this problem with degraded sound. I'll be back on it tomorrow and will report what I find.
I'm not sure about newer Icepower but older modules, including one in my Rowland 102, have this problem. It is because of usage of single supply while reversing direction of the speaker using 4 Mosfet (switches) bridge. That way, half of supply voltage appears on both speaker wires. This supply voltage can be as high as 80V in some modules. There are types of class D modules, like Hypex, that use dual supply with one speaker wire fixed at GND. Channel Island used such modules. If you have any DC voltmeter measure voltage (without the music) from one of speaker wires to chassis or GND of input connector on your sub.
First, it would appear that all of the above is much more important than what I am about offer; some have claimed using a simple zobel network on the speaker cable posts have alleviated the negative descriptions you have outlined.
Thanks, Kijanki, even though I only sort of understand your explanation as it might apply to my situation your reference to switching inspired me, for some reason to look look at my interconnects again as possible culprits. This may be because, being a new user of XLR cabling, phase issues have been in my thinking. Now, I realize that, even though connections in and out of the equalizer are balanced, because connections to from the preamp to the EQ and from the EQ to the amplifier, are through RCA adapters, balanced signals are not being delivered. Anyway, because I tried everything else and determined that the problem had to be between the preamp and the equalizer somewhere, I decided to check the interconnects.

I removed them from the system to check for simple continuity and found something that can only called strange. Although the RCA cabling all checked out, when checking the resistance between the pins on the XLR's I found a VERY low resistance reading between 2 of the pins on one of them. In checking between the same pins on the other cables there is no evidence of even the slightest continuity. This seems impossible; in my understanding and experience, there is either a short or not, not just sort of a short.

Anyway, I just happened to have an extra XLR cable which I installed and everything sounds great again. I went one step further and reinstalled the one with the anomaly and the crummy sound returned. So...I found the problem but am totally baffled. It even crossed my mind that there might be an impedance issue but the voltage applied to the cable by the meter is DC and because this anomaly didn't present in any of the other cables, this could not be the issue.

What I found here seems so unlikely or even impossible, I'm keeping the cable for others to evaluate. BTW, the cable is relatively new showing no damage of any kind.