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Discussion Starter · #1 ·
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
 

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I really doubt that you can have too much of an "imbalance." A lower source impedance into a higher input impedance assures maximum voltage transfer. A problem in getting all the power out of the amp may simply be that pro amps often have a much lower gain than home-audio amps and require more voltage than home-audio preamps/sources can supply.


So, look up the data on your units. What is the output impedance of the receiver line outputs and what is their voltage output? We know the input impedance of the amp (52K ohms, which is unremarkable) but we do not know the gain or the input sensitivity.


Some users have inserted an extra line stage to boost the voltage in situations like this.
 

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Discussion Starter · #3 ·
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?
 

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Discussion Starter · #4 ·
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
 

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Quote:
Originally Posted by marvinbeaverdip /forum/post/18245830


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?
Quote:
Originally Posted by marvinbeaverdip /forum/post/18245845


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

Looking at the input sensitivity of 500mV of the amp, it is likely that any line output will drive it. Have you used those line outs for anything else to determine they are working OK?
 

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Discussion Starter · #6 ·
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?
 

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Quote:
Originally Posted by marvinbeaverdip /forum/post/18246057


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?

Dunno. I have been assuming that the 500mV input sensitivity is for full output but, perhaps, it means something else. Is there a link to a data sheet?
 

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The gain is certainly sufficient. Other thoughts: (1) Are you using the correct output terminals among the several options and (2) have you turned up the rear-panel gain control?
 

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Discussion Starter · #10 ·
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.
 

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I believe you will find an octal socket for the input transformer if used. If not used with a transformer, there should be 2 shorting links or a shorting module between 2 sets of socket connections. These jumper across what would normally be the transformer coils.


Without the jumper(s) you get the performance you are experiencing.


Also be sure to connect the COMMON speaker output terminal to the Ground screw next to it. There should be a shorting jumper there already.
 

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Discussion Starter · #12 ·
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?


 

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That is your answer. Bogen offered 2 transformers for this amp. They are listed in the data sheets you posted. One is for the 600ohm balanced use, the other is for the HI-Z inputs.


The easiest way to figure out the pins is to look at the back of the PCB and the socket terminals. You will see 4 connections minimum and posibbly 5. One will be a ground connection for the transformers enclosure can. Two connections will go to the input terminals and HI-Z jack. The other two will be the single ended input to the amp itself and its associated ground.


You can identify the hot input line by connecting the amp to a speaker, turn it on and turn down the volume almost all the way. Take a small probe or screwdriver and with your finger on the shaft, touch each contact in the socket. There is no voltage on this socket so no danger. When you hear the buzz, you have found the pin connected to the amps input. Mark it with a sharpie. Turn off the amp and unplug it.


Use a meter to trace the HI-Z (RCA) inputs center conductor to the corresponding pin on the socket. Once ID'd use a jumper to short the two sockets together and there you go! Your amp will now connect to the outside world via the RCA. You should now have beau coup volume.


If you still do not have normal operating volume, there may be other malfunctions in play requiring actual servicing. Bogen's gear was extremely reliable, durable and easy to repair IF it did fail.
 

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Discussion Starter · #15 ·
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.
 

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There is a single ended feed of signal that bypasses the transformer socket. This is sometimes possible when the transformer is not completely balanced with both the high and low legs of the input fully isolated from ground.


I could see the socket was a nine pin but if you look at the back side not all leads from the socket will be connected to traces on the board. Some will just be a thru hole solder lug for physical stability of the socket.
 

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OK the input gain control is R-101. The wiper is the hot feed into the amp. It functions to control the output of the input transformer (if used) and the HiZ inputs. The 2 RCAs are in parallel and should both work without a transformer or jumper in this version of the model HT250A.


Do the same touch test with a screwdriver to the center pin of the jacks and while you are touching it adjust R-101 gently clockwise. If you do not get a substantial buzz, there is definitely another issue.


Can you hear white noise from the speaker?
 

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Discussion Starter · #18 ·
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.
 

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Discussion Starter · #19 ·
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.
 

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How are you hooking up your speaker? It should be between the commen and 4 or 8 ohm terminal depending on your speaker impedance.


The 25v and 70v are high impedance outputs intended to drive many speakers with their own matching transformers like in an airport PA system - not what you want.
 
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