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two pre amps into stereo power amp?

Discussion in 'Amp Central Station' started by doghouseman, Apr 7, 2021.

  1. doghouseman

    doghouseman Tele-Meister

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    I have a stereo power amp rackmount.

    I would like to run the mono signal from one pre amp into one side of the power amp, and the mono signal of another pre amp into the other side of the power amp.

    Then I would use an AB switch to switch between the pre amps.

    Then from one output channel of stereo power amp out to a cab, and the other channel out to a different cab.

    I am assuming this is not a problem?

    I guess i should make sure that the two cabs are the same ohms?
     
  2. Wally

    Wally Telefied Ad Free Member

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    I am going to think that when in stereo mode, each side of that stereo power amp will need a load at all times. Unless you want to run both preamps simultaneously, you can probably bridge those stereo amps for a mono output with double the power. Check your manual on that possibility of bridging the outputs for mono. In that mode, you would then be switching prior to the preamps to select which preamp you want to run...or both if wanted.
    If you want the ability to run both of those preamps simultaneously, each power amp will need its own speaker load.
     
  3. doghouseman

    doghouseman Tele-Meister

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    Well, I was hoping I could do this because I could route the pre amps to different speakers.

    I dont want to switch the pre amps and run mono out of the power amp because that means I have to use the same speakers for each pre amp.

    But, yes, I was a little worried that one side of the power amp will not be seeing a load at all times.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Any tube power amp needs a load when it is powered up. What you want to do is simple since you have...or are planning to have...different speakers. Simply run each preamp to its own power amp, each with its own load, and run your ABY switch in front of the preamps.
     
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  5. 4pickupguy

    4pickupguy Doctor of Teleocity

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    If you switching between the inputs, the unused side of the power amp still sees a load it’s just not seeing an input signal. If you are switch between outputs then you always have an unloaded side of the amp.
     
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  6. doghouseman

    doghouseman Tele-Meister

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    Yes, but that requires buying another power amp ;-) I was hoping to use my stereo amp as two separate power amps.
     
  7. Wally

    Wally Telefied Ad Free Member

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    as I said, I think this is doable. You have two preamps,correct? You have a stereo power amp....think of that as two mono amps that need not be fed a signal at all times. You have two different speaker loads....one for each power amp. This is in essence establishing two complete amps. Set the rig up and think of each as a separate amp..preamp/power amp/speaker. Place your switch as you wanted prior to the preamps and play. You can run either rig alone or together.
    your speaker loads need to match the requirements on the stereo power amps.

    i don’t think I am misunderstanding things, but I have not finished my morning tea yet, either.
     
  8. Jay Jernigan

    Jay Jernigan Tele-Afflicted

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    Like 4 pickupguy said, it ain't a problem. Avoid ground loops for quiet operation, of course, and you are golden.
    A stereo power amp is essential two separate amplifiers. If you have a compatible speaker load on each side and the amp has two separate volume controls, you can balance the outputs of the two cabs, or make them different, if you prefer.
     
  9. doghouseman

    doghouseman Tele-Meister

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    Yes, I was hoping to treat it as two separate amplifiers. Each side has a load on the output (speakers). I am just switching the pre-amp input to be into one side or the other, either channel A or channel B, of the stereo power amp.

    One reason I am kicking this idea around is that I have used a head switcher in the past, but switching into the same cab (two heads, one cab). This means that you sometimes dont have a good speaker to match your preamp. In this case, I can switch my cabs at the same time I switch my pre-amps, so I can use the right cab for the right pre-amp.
     
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  10. Ed Storer

    Ed Storer Tele-Afflicted Silver Supporter

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    There's a key piece of information missing: Is your stereo power amp tube or solid state?

    I believe you said you were going to run two speakers, one for each channel. Solid state amps have no problem running with no load.
     
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  11. doghouseman

    doghouseman Tele-Meister

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    Yes, rocktron velocity 150 power amp.
     
  12. ce24

    ce24 Poster Extraordinaire

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    Switching A-B inputs, no load problem....switching A-B ouputs is a load problem.
     
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  13. W.L.Weller

    W.L.Weller Tele-Holic

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    Be sure to match the cabinet impedances if you are trying to match output levels from your 2 channels. That is to say, use a pair of 8Ω cabinets, or a pair of 16Ω cabinets, etc.

    Since that power amp is solid state, its output power is dependent on the speaker impedances. Lower impedances, more wattage. Looks like it can handle speaker loads down to 4Ω per channel. Check your manual since my advice is worth what you paid for it.
     
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  14. Wally

    Wally Telefied Ad Free Member

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    So...late but useful info. SS power amp....as noted, solid state amps can do without a load. When in doubt, manuals are the most valuable resource.
     
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  15. doghouseman

    doghouseman Tele-Meister

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    Yes, I checked the manual first, but it did not list using the stereo amp as two separate power amps, each with different speakers, and switching. I guess this is a non traditional set up.
     
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  16. Wally

    Wally Telefied Ad Free Member

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    In effect, a true stereo system is a construction of two preamps each feeding power amps each driving its own speaker load. In the audiophile world, those preamps, power amps and speakers are identical to each other.
    In the guitar world, many refer to the use of two amps as being ‘stereo’ even if the two amps——amplifiers consist of preamps, power amps and speakers—-are not identical. I am of the camp that considers a stereo amp for guitar as being more like that audiophile setup in which the two paths are identical. Example: the Gibson stereo amps that were built to amplify the stereo ES-345/355 guitars that were introduced in the late ‘50s. I have owned a Bell-logoed Gibson GA-79 in mint condition which is one of those amps. Had it been the RVT version with reverb in addition to the tremolo, I would still own it. However, I keep two Super Champ for use with my 1966 ES-345 TDC stereo Gibson....one amp for each pickup. This constitutes a stereo amplifier with the ability to establish a wide soundstage by placing the amps as far apart as I wish. A minimum of six feet is my preference. That six feet of separation is a crude rule of thumb for establishing a proper stereo soundstage in the high fidelity audiophile world.
    When using different amps for a multiple amp situation without stereo effects, I consider that to be an expanded mono amplification scheme. Ommv.
    What you are wanting to do is to take a stereo power amp and attach different preamps and speakers to each separate power amp. You then want to use the two amplifiers...preamp, power amp, and speaker load...separately as two mono amps, which is what they would be imho. With an ABY switcher, you could run them separately or together in...as I would call it...expanded mono.
    Since you are wanting different speaker loads for different Sonic results, there is no problem with loads on the power amps. You can do what you describe in post #1. What is seemingly confounding you is the fact that the power amps happen to be sitting in one construct. No big deal...they are separate power amps.
     
  17. doghouseman

    doghouseman Tele-Meister

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    Yeah, this stereo power amp is a relic of the 80s rack mount era. It might have been made in the 80s, not sure. Rocktron was big on rack stuff in the 80 and 90s. I remember that even 4x12 cabs were wired so that they could be switched into stereo mode. All this rack stuff works great with a rack mount processor, which are usually in stereo, like the old Alesis Quadraverb and Midiverb and the Lexicon rackmount stuff. I am just using the rocktron power amp because 1) it sounds great and it is 2) a lot cheaper on the used market than the newer SS power amp stuff (like the Quilter stuff) 3) it is built like a tank.

    There are a lot of preamps that are pedal sized these days, so you can put your pre-amp on your pedal board and use a rack power amp for amplification. All of this is easier to deal with than a traditional tube amp - lighter and more reliable. So that is what I have been doing these days.
     
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