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Itegrated and power amp combo theory question.

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by cosaw, Oct 10, 2004.

  1. cosaw

    cosaw
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    Hello All,

    Will shortly be building two subs to match with my main hi-fi speakers. I'm trying to decide how best to crossover and power the subs. I'm doing this on a budget and trying to keep the cost down. I'm thinking of using the natural roll off of my main sealed speakers and matching up my subs with a low pass active crossover with seperate amp. The question is can I use my integrated amp in tandem with a power amp i.e. take a signal from my integrated amp to feed through my active crossover to my subs? Would the signal from the integrated amp (say from a tape loop) be the same as from a preamplifier? A picture should make it more clear:

    [​IMG]

    Is this acceptable/workable?

    Thanks,

    Simon
     
  2. davehk

    davehk
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    The output from a tape loop is before the volume controls - this would be a problem since when you vary the volume only the level to the integrated PA will change. Better to use pre-out sockets on the integrated, if it has them. You can do this even if the pre-outs are feeding the integrated main-ins, just get some y-adapters and cables.

    Also, tape-loop outputs are generally much lower level than pre-amp outputs.
     
  3. cosaw

    cosaw
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    Thanks for the reply davehk.

    Problem is no pre-outs whatsoever just two tape loops all other input sources and speaker outs. Is there anyway I could use the amps 1/4" headphone socket as this sees the volume control and on my amp the main speakers are not (automatically) muted when you plug in a headphone jack. Would I need to modify the headphone signal to match it to the equivalent of a preamp or is this totally unfeasible? Are there any other know workarounds?

    Thanks,

    Simon
     
  4. davehk

    davehk
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    What's the make and model of amp? - a mod may be possible
     
  5. wookie

    wookie
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    You can make a high level to low level cable with a pair of phono plugs two 25k resistors and two 1k shunt resistors.

    This is quite popular with car audio circles.

    Its basically an "L PAD" voltage divider.

    I have made loads of them over the years and run a power amp from a Mini System with a pair!

    http://www.epanorama.net/circuits/speaker_to_line.html
     
  6. cosaw

    cosaw
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    Davenk: it's a an old technics amp from the 80s, probably nothing special but it was my dads and with further additions to the system I'm trying to keep the price down.

    Wookie: Thanks for that I'll have a look, just to clarify does this system just involve the one volume control? i.e. does the volume control on the primary amplifier then control the volume to all the speakers or does it involve the use of sperarate controls?
     
  7. wookie

    wookie
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    Just the one volume control is used.

    It basically converts a high level amplifer output to a low level power amp input (without loading up the amplifer).

    Example of load presented to amp is :- A 50w amp into 4 ohms will make 25w into 8 ohms, here we are presenting a load of 26,000 ohms so the amp aint gonna struggle with that!


    Its exactly the same circuit you will find in pretty much any subwoofer with high level speaker cable inputs!
    It also has a flat response.

    HTH.
     
  8. cosaw

    cosaw
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    Wookie - I've now got something to fall back on in case I can't hook up via headphone socket.

    Thanks for your help!

    Simon
     
  9. davehk

    davehk
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    Wookie's solution is pretty good. The only possible real problem is that any crossover distortion on the integrated's output is going to appear as part of the signal on the input of the external PA and be amplified.

    With an old amp like you have, this may be noticeable. In which case, my own approach would be to convert one of the tape loops to give you pre-out/main-in setup, taking the output from a suitable point inside the amp and maybe fitting a buffer amp, depending on the cicuitry (which implies internal mods to the amp - but I can do that sort of thing. If you can provide the exact model number, I'll see if I can find the schematics and look at the feasibility).
     
  10. cosaw

    cosaw
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    Davehk:

    Funily enough I uploaded the schematics to some webspace of mine for some spanish chap I noticed trying to get hold of them months ago. They're still up there they just need putting side by side you don't need to feel obliged to look at them afterall I'd prefer to deal with the problem externally (the amp was my late fathers it has sentimental value and I don't want to tamper with it you see):

    The links:
    http://www.alastair-ward.freeserve.co.uk/SU-Z22 A.gif
    http://www.alastair-ward.freeserve.co.uk/SU-Z22 B.gif
    http://www.alastair-ward.freeserve.co.uk/SU-Z22 C.gif
    http://www.alastair-ward.freeserve.co.uk/SU-Z22 D.gif

    If this was possible could the mod be built as an external unit which took a signal from the tape out?

    Simon
     
  11. davehk

    davehk
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    Not in quite that way - you'd still have the issue of volume control. However, you could come up with a unit that took the output from the tape out, put it through an external volume control, buffered it and then provide 2 sets of stereo outs both controlled by the external volume knob. One pair feeds back to the tape-in on the integrated, the other to the external amp. Then set the integrated to tape-monitor mode and use its internal volume control to set the relative levels between the two sets of speakers. The external vol control is then used to control the overall volume.

    This would be a little more expensive, since you would need to include additional circuitry for power supply.

    Looking at the schematic. I think there may be suitable tap-off points on the collectors of Q301/302 on sheet B. However, it's not possible to tell from the schematic if a buffer amp is needed since there are no signal level indications - one would need to 'scope it.
     

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