Amplifier Sensitivity, Decibels, and You!

Discussion in 'Solid State' started by 240 Volts, Jan 30, 2006.

  1. Markw

    Markw Super Member

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    Most likely

    Sounds like you're thinking about trying to feed a line level signal into a phono preamp. Bad idea on many levels. :no:

    Well, if you exclude tuners and tape decks, you might be right.
     
  2. zenith2134

    zenith2134 Addicted Member

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    Actually this is not what I'm attempting at all, since all of my 70s stuff lists tape in's and Aux at 150mV, while all the built-in phono stages are rated at 2.5mV. Big difference there. I still highly doubt that tape decks and tuners were anywhere near 2 volts until the late 80s
     
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  3. Markw

    Markw Super Member

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    Well, since you failed to state wether that was a minimum or maximum (which you DID imply) I think you should take a little of that heat yourself.

    Really? How about an example from the 60's?

    http://home.indy.net/~gregdunn/dynaco/components/FM3/index.html

    There's advantages to having been around when these things were new.
     
  4. zenith2134

    zenith2134 Addicted Member

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    Guess there are. I stand corrected, sir.

    Still, no one really answered my question...
     
  5. Markw

    Markw Super Member

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    And, what was your question?
     
  6. zenith2134

    zenith2134 Addicted Member

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    My question is - Wouldn't distortion rise when you are exceeding the maximum input voltage of the input?? I would assume that the preamp diodes handling the input path would be driven past their safe operating parameters a bit.
     
  7. Markw

    Markw Super Member

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    Yes, but you need to define what that "maximum" voltage is. A sensitivity rating is not a maximum.

    Using that 2.5 mv rating for a phono input, that's not a maximum rating, that's a voltage at which a certain level can be reached but it can accept a much higher input than that. Re-read your specs and you'll see a maximum for the phono input. If you exceed that maximum rating then you will most certainy get distortion.

    Likewise, for those line level inputs, odds are that 150 mv figure is to attain a certain output level. If it goes higher than that, you simply turn the gain control down, so I guess you can say that it's basically unlimited.

    Line level stages have the volume control on the amp to limit their input level.

    The phono stage has no gain control on it's input and it can easily be overdriven. The volume control on the amp comes after the phono preamp stage. That's why line-level signals fed into there can sound perfectly horrible (not to mention the RIAA curve)

    Many source units allow adjustment of their output so the gain control on the amp stays pretty constant. Virtually all tuners and tape decks did back in the day.
     
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  8. zenith2134

    zenith2134 Addicted Member

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    Thanks for clearing this up. I understand much better now
     
  9. 240 Volts

    240 Volts Strange, yet oddly normal

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    zenith2134, I thought the maximum allowable input level would be easy to find from the amplifier's specifications - but I was wrong!!
    The specifications I have seen only give a maximum figure for the phono input, there is nothing at all for the line level inputs (tuner/tape/aux).

    I've had a quick look at a few schematics to see how the line level inputs are typically handled.
    Pioneer SX-737 Receiver and SA-9500 Amplifier : Signal goes through source selector switches, then to the balance control, then to the volume control.
    Pioneer SX-850 Receiver : Signal goes through source selector switches, then to the balance control, then to a buffer amplifier operating from the 51volt supply rail, then to the volume control.

    If these units are typical of most vintage gear then there is nothing at all to worry about when connecting the 2volt output of a CD player into the line level inputs of a vintage amplifier - simply exercise some restraint with the volume control and all will be well. :thmbsp:

    - Richard B.
     
  10. zenith2134

    zenith2134 Addicted Member

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    Oh okay, now I see. So just heed the warnings about setting gain and all is cool. Nice.

    Guess I was thinking of more modern and cheaper gear where if you input too hot, the pre clips like crazy... But I digress: I gave away all of my equipment that does that
     
  11. RaymondLeggs

    RaymondLeggs Super Member

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    I like loud music!
     
  12. mike5500

    mike5500 Active Member

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    got me thinking (dangerous stuff)

    well, this thread got me thinking about the whole 2V P-P and input sensitivity issue. And I was wondering..."if one were to take say, a 5532 differential op-amp and whip up a conditioner circuit to chop that 2V P-P down to about 200mV P-P, wouldn't that balance out the gain structure of the receiver/ power amp so you can really drive the outputputs to give you some headroom?"

    what do you guys think?:beer:
     
  13. xylopilot

    xylopilot Member

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  14. romanoff

    romanoff New Member

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    Vintage Panasonic Stereo

    I have a 1969 panasonic AM/FM stereo receiver amplifier w. turntable.
    Turning the volume knob on the receiver/amp creates extreme static. Is this due to age and/or dust.? What is the best way to clean/correct it? Thanks.

    Steve
    Maine
     
  15. Urizen

    Urizen Lunatic Member

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  16. Excumbrian

    Excumbrian Active Member

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    OK I understood some of it. My maths is ok (though I'm not sure what the * represents). May I ask a few simple (stupid?) questions?

    1: "with the following typical gain : phono amplifier +60dB, pre amp -20dB, power amp +30dB : The overall gain is therefore +70dB" -- why is there a reduction in voltage in the pre-amp?

    2: What should I look for in an integrated amp (on the vintage second-hand market) to be reasonably sure I can put a CD or DVD player through it safely? Is there any way of knowing what the sensitivity of the amp is if there is no manual?

    3: If there is a mismatch, what can one do about it? Is there a way of reducing the signal voltage from the modern gear?
     
  17. klm1

    klm1 New Member

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    alright, I'll try, here goes;
    1) The * indicates "times", or multiply.
    2) The preamps usually do not have a gain stage, so they only "absorb" power,
    and do not "produce" any.
    3) No, unless you have a schematic and/or can do the math, you won't know the mismatch amounts- you can, however, measure it.
    4) Yes, that is what Bogie was referring to(...a direct box...)
    However, due to the nature of the circuitry in general, some people may
    notice a "change" in the sound.

    Help?? Hope so...:thmbsp:
     
  18. 240 Volts

    240 Volts Strange, yet oddly normal

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    Location:
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    Excumbrian, sorry for the delay in responding to your questions, I'd just like to expand a little bit on the answers from klm1 above.

    Under "normal" operating conditions the output from a pre-amp will usually be lower than the input. For example, if the input signal is the typical 2volt maximum from a CD player and the output is feeding a power-amp with an input sensitivity of 1volt (for full power), then listening at anything less than maximum volume involves the signal level being reduced (maybe by -20dB to -40dB at average listening levels, depending of course on speaker efficiency and personal preference).

    Some very high quality pre-amps are purely "passive" devices, i.e. they consist only of source selection switces and a volume control. By their very nature the output must always be lower than the input.

    Usually the higher voltage from the modern gear does not cause any problem with older amplifiers. If the volume control has to be set so low that it's difficult to accurately adjust it, then it's always possible to fit in-line attenuators between the source and the amplifier.

    Hope this helps to clarify things for you, if not then don't hesitate to ask again. :yes::yes:

    - Richard B.
     
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  19. dartman_pl

    dartman_pl New Member

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    Richard B,

    First of all many thanks for this thread!
    Now, I think I understand a little this voltage issue.

    I have in my system :

    1.Technics SH-X1000 DAC with output voltage 2,5Vrms and impedance 600Ohm

    2. Yamaha preamp CX-1000 with 150mV input sensitivity and pre out 1,5V

    3. Yamaha amp with 1,62V input sensitivity and 60kOhm impedance.

    I understand that there is a perfect match between pre / amp and very big mismatch between DAC and pre / amp.
    That's the reason why I turn a volume only a little 8 o'clock
    room is overloaded of sound!
    Am I right?

    Should I simply remove active preamp and use high quality passive controller?

    Thanks,
     
  20. majoco

    majoco ZL2MC

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    Location:
    New Zealand
    Re the question about 'back in the 70's 150mV input sensitivity was the the norm, now we have 2volts' or something like that. CD's have much greater dynamic range than records or tape could ever cope with, therefore the headroom (level above 'normal' before the onset of clipping in the input stages) must be able to cope with this range. In theory there is no noise from a CD so the dynamic range could be over 90dB, as good as your modern pre-amps and probably a lot better than the room that you are listening in!
     
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