power amp question

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Richard Monroe

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I have a Carver PM125 power amp, which while small, meets the studio's needs. I don't use it to drive my monitors, which are active. I use it with POD Pro, VAMP Bass Pro, and VAMP-2 to drive a Marshall 1X12 cab with a Celestion vintage 30 or an Ampeg 1X10 bass extension cab for a variety of guitar and bass sounds. Now the Marshall cab is an 8 ohm cab, so I use the power amp in bridged configuration for 8 ohm output. The bass cab is a 4 ohm cab, so I can use the Carver in normal stereo or dual mono configuration for 4 ohm output. Now, I understand the 8 ohm bridged configuration, and I've used it that way for quite a while. But this Ampeg is my first 4 ohm cab, so- What's the difference between "dual mono" and "normal stereo configuration? Sorry, used unit, no owner's manual. Thanks in advance for any insight.-Richie
 
Sorry Boing- same thing I got- "This page cannot be accessed" What I did find out is that the amp was designed by Bob Carver, who designed the Phase Linear 400, which we used for a bass and stereo amp in the 70's, a fairly respectable lineage. Unfortunately, the only link up is a guy who will be happy to *sell* me the owner's manual, which I fear *ain't* gonna happen. -Richie
 
CAUTION!!READ THIS NOW!! I just reread your post. The thing about amps is they need a certain minimum impedance on them or they burn up! When you bridge a power amp you aren't setting it to have an 8 ohm output. You can only put an 8 ohm cab on it because if put something with a lower impedance on that bridged output the amp will fry! Bridging uses both channels to increase the voltage swing which makes one output with more power. But the trade-off is that you must must must stay at 8 ohms! That's why the amp says bridged output-8 ohms only! To run your 4 ohm cabs you HAVE TO unbridge it because if you plug in that 4ohm cab to the bridged output the amp will be smoked. And realize that each channel will have about half the power of that bridged output-remember, bridging is adding both channels together to make one channel with more power.


Sorry bout that link. I got the same error page. I got to it from google, where I found it. Same deal. You gotta pay. Didn't realize that would be a problem. Hopefully someone can help, but sometimes you just gotta pony up.

I thought a bit and did some research.

Couple things first- if you don't need to run the Marshall and Ampeg at the same time, all you have to do is unbridge the thing, plug the Ampeg into one channel, and off you go. You can stop reading after this paragraph. Then you plug the Marshall in when you want, bridged or not. The amp should be able to handle 8 ohms if it can handle 4. If you want both at the same time read on.

Dual mono seems to mean a couple different things. It can mean that each channel has it's own power supply instead of sharing, which is the more common(cheaper) way to go. It also used to mean that by flipping a switch both channels could be fed the same(mono) signal from one input. This is now called parallel mono.
It may be a true dual mono-2p/s- amp. One of the problems with cheap power amps is they don't like unequal loads on the two channels, in part because of the shared p/s. It may be telling you in old language it can handle unequal loads using each channel however you want, ie 4ohms left and 8ohms right, whether or not it has dual p/s.
If it has a switch for dual mono/stereo, try patching a signal into each input separately in dual mono mode. If you get signal from both channels from one input it really means parallel mono. You will have to get the manual or speak to someone who knows if the amp can handle unequal loads in stereo mode.
If it doesn't have a switch, it seems to be telling you you can use and abuse each side separately, without keeping the load on each side the same. Again, you are golden. So you can use one side for the 8ohm Marshall, and the other for the 4ohm Ampeg.


Experiment and be careful. hope this helps.
 
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Thank you, Mr. Boingoman, sir.- Got it. No I am aware that sending the bridged 8 ohm signal to a 4 ohm cab is a bad idea, and no, the 2 cabs are not being used simultaneously, although apparrently they could be, in dual mono configration, although the reduced output into the 8 ohm cab probably wouldn't produce a hell of a lot of power. It is, after all, only about 62 watts per channel in 4 ohm configuration. It does work, though.-Richie
 
Yo Arcaxis! You are *the man*! Got it. No problem. It does 50w into 8 ohms, which is plenty for my purposes, and 62 watts into 4 ohms in dual mono or normal stereo mode, which is plenty, so I don't even need to use bridged operation, which as pointed out above, is a risk if it happens to be plugged into the 4 ohm cab. That means I can just switch the output cable from cab to cab and forget about it. And- If you want that bass loud- I just use the Avalon for a preamp, making sure the signal isn't hot enough to clip the Carver. The bass cab is rated at 200 watts, so I don't think it's in any danger. That's plenty of power to terrorize the studio cat. Thanks again.
And a note to Boingoman- the parallel mono operation is not the same as dual mono, but there is a jumper inside, so the amp can be configured for parallel mono if you want. I'm ignorant enough that I think I'll leave my grubby little hands outside the box, and stick to playing guitar.-Richie
 
Hope I didn't offend-forgot to put a smilie on my last post. It seemed like you thought you had to bridge the amp to use an 8ohm cab.

Last month I watched a guy (local sound "pro") fry two amps and one sub because he was running in p/m, didn't really get it, and thought he could get more power by adding boxes to drop the impedance. I just wanted to make sure you avoided the same fate.

Checked out your site. Nice review for your disc. I'm new to recording, hope I can see something like that for one of mine someday. What is that long mic above the AKG's in your photo gallery? looks cool....

Thanks for the link arcaxis. I just stopped when I hit an "official Looking" website. Any comments on the booming vintage manual/brochure market?

I have the original brochure from my lexicon pcm-41. Maybe it will be worth $10 in twenty years.

seeya
 
No Boingoman, I didn't think I needed to use bridged configuration to *use* an 8 ohm cab, I thought I needed to use bridged configuration to *drive* an 8 ohm cab. I underestimated the output into 4 ohms. The studio pictures are way outdated. We're waiting for the current upgrade to be complete before taking all new pictures. We've added a bass, a bass cab, a pair of Neumann KM184's, a Digi002, and the computer from hell is in the mail! I've also added Bass VAMP Pro, a small PA, and a Soney ECM-MS957 mid-side stereo mic. That tricked out SG has been morphed into a Les Paul and an American Telecaster. We've also added the M-Audio SBX subwoofer to the monitor setup, a quad noise gate, and a high speed CD duplicator. That pretty much completes the studio for now. That mic, though, therein hangs a tale- Just for your benefit, I'm reprinting the saga of Shure SM82 from a previous thread-

Shure SM82, No shit. I found this puppy at a flea market, in the bottom of a box with some dead old Chinese strat knockoffs, with leaves and cobwebs. I bushed it off, and took a good look. Ah ha!- Shure SM82-unidirectional microphone- serial number (4 digits) It's about a foot long, with a thin cylindrical body, a heavy grill (with the required dent), XLR connector.
I immediately came to the following conclusions:
1. It's a Shure, so if it works, it has to be worth something.
2. They don't put serial numbers on a SM57, so it's not cheap.
3. It was intended for some field application- hence the heavy duty grill.
What the hell, I said, "how much?". "What'll you give me for it?"
"$10.00?"- sold. I got it home, and plugged it into my Avalon AD2022, and-nothing. Oh well, probably a dead mic, and a $10 gamble lost. But-an SM81 is a condenser. Let's try some phantom power....WRONG! Even with the gain set at minimum, it fried the left side of the Avalon, and blew up a set of Sennheiser HD280's, which I was wearing at the time. For some reason, the Rolls headphone amp survived. I dropped the cans to the floor screaming, but I was soon aware that I couldn't hear that, or much of anything else.
After I ran in circles turning everything in sight off, the little light bulb came on over my head, and the 3 hours or so it took for most of my hearing to come back gave me plenty of time to look this baby up on line under discontinued items. God love Shure, they actually have spec sheets and owners manuals for all their discontinued items on line, right back to 1926.
The SM-82 is a *line level* broadcast mic with a built in limiter and preamp, designed to operate on a mercury battery that can no longer be legally made (and there is no lithium equivalent), *or* phantom power. It is capable of transmitting its signal over 1.5 kilometers of cable with no preamp! Designed for television and sports broadcast applications, it has apparently got a little bit of a private cult following. It was a favorite of war correspondents, because you can jack it into a telephone plug with the correct adaptor.
Well, I've got a couple of pres with line level inputs, but guess what? They don't produce phantom power. So I bought a little phantom power supply, and ran it into my Joemeek twinQ (for some reason, the Avalon was in the shop). Believe it or not, Guitar Center replaced the cans. I didn't talk too much about how they got broken- LOL.
Any way, the mic sounded *GREAT*. The only problem was- the phantom power supply was a little noisy for critical recording. So one day I was feeling brave, and I said to myself, "Gee, suppose I set the input impedence on the Avalon at 50 ohms, which is basically line level?". I can tell you I was sweating when I turned on the phantom power. And it worked!
The last one rolled off the assembly line in 1987, and cost about $680 list at the time. I have no idea what the street price was, but it wasn't a cheap mic. It's exactly what an SM-7 would be if it were a condenser. It has the high end detail of a really good condenser with almost no reach. It pretty much rejects everything that isn't right in front of it. The pickup field and proximity effect are very well defined, so it's a little picky about movement. If you can sing without moving, it rocks! It is also a *great* acoustic guitar mic. It picks up almost no ambient noise, just like a dynamic. This is a little known fantastic mic, if you can get it to a line level input *with phantom power*.-Richie
 
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