connections to a "professional" amp issue

I posted a very similar post in another section but haven't got a response, and I'm wondering if the folks here could help me out.

Here's the situation. I bought (for practically nothing) two monoblock PA amps. They're Bogen HTA-250A's, rated at 250watts rms each. My thought was to use one to power a subwoofer I'm building, or maybe party speakers at some date. I wasn't concerned about super clean power, but that they'd have some decent power. I had heard of a few people doing this online, and they were surprised at how clean they DID sound, so I thought I'd try it.

The issue was inputs, and it has RCA unbalanced Hi-Z (52k ohm), and low-z balanced (600ohm) inputs. I wanted to use the low-z preamp outputs from my receiver. I asked Bogen if they had some sort of module to add to connect these correctly and they said plug the RCA from your receiver to the RCA Hi-z inputs". That's it.

Now here's the problem; plugging into the hi-z only creates a loud ground loop hum, and very little music. Just to test to see if it was the input creating the noise, I disconnected the preamp and plugged an ipod directly into the RCA, figuring there's no ground issue with a battery powered source, and the hum/buzz is completely gone, replaced by music. However, even at its max (the source output and the amps input knob. I was able to turn it up that loud, if that gives you any idea how NOT loud it was), it certainly wasn't delivering full power.

[update] I found a broken ground connection for the coax cable to the adjacent TV and the hum is nearly gone. The issue now is the lack of power. At a rated 250watts these amps should be pounding my speakers.

What can I do? Convert the RCA unbalanced to a balanced input? I've read online that a low-z output to high-z input is common, and it's usually on the magnitude of a 1/10 ratio of impedance. I'm not sure what a typical rca preout is rated at, but this imbalance in impedance seems to be too great and is limiting the power produced. Is there something else I'm overlooking? Any advice is appreciated

Thanks Kal- the specs say this
Input Sensitivity: High impedance, 500mV; Low impedance balanced, with optional
transformer, 150mV

Input Impedances: Hi-Z, 50,000 ohms unbalanced; Lo-Z, 600 ohms, balanced or
unbalanced, and 1:1 bridging with optional plug-in transformers

I've never heard of a line stage. If you have a sec could you direct me to a product to get a better idea of what they are and what they do. What is the input sensitivity of a more consumer oriented amplifier?

btw, I have tried looking up the preout impedance and voltage outputs, but can't seem to find them in the manual. It's something I had been looking for. It's a pioneer vsx-d914k

No, I haven't used them to power anything else. I tried the LFE output, as well as the preouts for several channels and it's all a similar result. There's sound produced, but not as much as I think there should be considering the advertised rating of the amp. Doing a quick scan of other forums I came across several line-level voltage boosters, but it sounds like this may not be the issue?

I've tried several of them, with similar results. They all produce music, just not with the kind of volume I'd expect. The results were similar when plugging an ipod with rca cable directly to the amp, so I don't think it's my preouts. I could turn the gain all the way up on the amp, and turn the ipod volume up to a level that should be equal to a CD component output, and I was certainly not being blown away (I was just a couple feet from the speakers. if the amp was putting out a full 250watts the speakers would have been dismantling themselves).
Am I overestimating the power of this amp? Is there something about paging amp power ratings that is different than consumer amps? I know crown makes a 70v paging amp rated at 350watts, and 400watts @ 4ohm, so I would think this one should be at least the rated power running an 8ohm speaker.

Gizmologist- I took the back off and took some pictures. It has no jumpers installed, neither does the other amp on the shelf. They (bogen) sell optional input transformers, but it's not clear to me what they're for. Do you know which connections should be jumpered? or should I be buying one of the input transformers?

OK, thanks. It appears (the pics I posted) that the transformer sits on a 9 pin connector, not 4 or 5. In any case I'll track down those two connections.
I'm curious though, what circuit are we bypassing by doing this? A signal is going through at this point, because sound is being reproduced, just not at full power. If you have time, enlighten me since I'd really like to learn more about it.
Because I've narrowed the issue down to one thing, (no ground loop issue anymore) I also wrote back to Bogen with this information and asked them again what their opinion is. Hopefully I can get this sorted out. I've found a few people selling these amps that used them as DJ amps, which implies they should power big speaker pretty da#n loud.

I just tried that, and got lots of buzz/white noise. Not head throbbing, but substantial. Perhaps my ears are just really bad, and the sound out of the speakers when cranked from my preamp is actually deafening but I can't tell? Probably not, since I still wake up every time I hear the dog rattle it's tags a floor below us. I haven't heard from Bogen's tech dept yet, but another friend suggested that I get their transformer for lo-z balanced input, and then buy a converter or preamp that will convert my RCA to balanced.

Kal had suggested just a line booster, and another forum suggested a product like this http://www.zzounds.com/item--BEHXENTX502 which does a multitude of things, including what we're talking about.

Reading through the specs again, I see that it contains a low cut filter. I found the switch, but haven't determined if it's on or off. Would I be correct in assuming that if this filter were on, it would explain the lack of bass I'm getting? when hooked to the LFE preout with the LFE x-over set to 100hz, I was getting nearly no sound, but got much more when connected to the L or R preouts.

The speakers are hooked up correctly. When testing on 8ohm home speakers they were hooked to the 8ohm outputs, and when connected to a set of 4ohm truck cab speaker boxes they were connected to the 4ohm. Similar results with both. I think we're beyond the "are you sure you have it plugged into an outlet" type of suggestions.

The problem with the cut switch is that I don't know if the cut is "on" when the switch is closed, or when it's open. Can you determine that from the schematic? there are no markings on the board to help me. There are 3 pins through the board with two of them being soldered. Currently, the switch is is open between those two pins.
I'm using a pioneer vsx-d914k as a preamp. From what I can test, it's operating correctly, and the preamp outputs are per spec.
My thought are leaning toward a line amp as well, but based on the specs (500mv for full power) what I have should be plenty.

The response from Bogen Communications;

How many 8-ohm speakers are you connecting to each amplifier?

The problem you are experiencing may not necessarily due to which input
you are using but the level of signal coming from your source - HT
receiver

The Hi-Z (RCA) input on the HTA-amplifier requires 1/2V (500mV) to drive
the amplifier to full output (250W). If this input does not reach the
500mV required, then the amplifier will not produce the desired output
and your speakers will be low

If you choose to use the Lo-Z (screw terminals) input which requires
only 150mV at the input, then you have to insert the matching
transformer - model TL600

You can also check the specifications for your HT receiver to see what
level of output signal is produced by that receiver



I have the receiver manual next to me and there is no section in the specifications that list the output/impedance for the preamp/LFE outputs. It notes that the input sensitivity is 200mv/47ohm and the output for DVR/VCR record etc is 200mV/2.2kohm.
I'm thinking about ordering a microphone preamp like the one I posted a link for.

The berenger may be overkill, but it's also only $45, and has the ability to later plug in a microphone or my acoustic electric guitar depending on what speaker I want to power. The Samson S-Convert was also recommended, similar price range. Did I miss something on that page, or was that little RDL unit really $200?
There's also this unit that sells all over ebay and does what the RDL one does for $50. I'll keep looking.

?

I've put the project on the back burner, but I think you're right though. I think what I need is something simple that does the job, nothing too intricate.