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Thread: Operadio PA amp, 5000 ohm output?

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    Operadio PA amp, 5000 ohm output?

    Hello,

    I have an old Operadio 1A65. The schematic can be found here:

    http://www.theused.com/resource/resource.php?curPage=27

    I am wondering why the output impedence is so high? Is this really just a pre-amp? If so, what kind of power amp would accept this 5000ohm signal?

    Is there such a thing as a 5000ohm speaker.

    Any information is appreciated. Thanks!!!

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    Lifetime Member Enzo's Avatar
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    SOmeone might have a 5000 ohm headphone.

    This is a line amplifier, not a power amplifier. Well, a PA system mixer really. That output tube is a 6SN7, and that isn;t going to power much speaker.

    The output transformer allows you to match the output to a wide range of impedances, from 10k ohms (5000-0-5000), down to 125 ohms. You can drive a balanced line or a single ended line.

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    Thanks for the reply!

    So if it's very low output (I've noticed that myself,) is it really designed to drive a speaker directly?

    If it is designed to go into a power amp, (even a solid state one) which taps would I use, and should I set it up for single ended or balanced line?

    Ideally I would like to use this for background music, guitar, or as a vocal amp.

    Thanks!

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    Lifetime Member Enzo's Avatar
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    It is a line amp, it is not meant to drive a speaker.

    Experiment with connections to a power amp. If the amp has balanced inputs, try that, otherwise, single ended. Of single sided, start with as high an impedance as possible.

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    Thanks again!

    Any guidelines for impedance to use on the balanced input? Is there a typical range that power amps would be expecting?

    How will I know if I'm off? Poor signal quality? Excessive volume or too quiet?

    Naturally I don't want to fry anything on either end!!!

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    Noodle of Reality Steve Conner's Avatar
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    Yes, that's how you'll know if you're off: poor signal quality, excessive volume, or too quiet, or things getting fried.

    I'd start with the 125 ohm tap and work up from there.

    The output is transformer balanced, so it should be able to drive any kind of power amp input, either balanced or single-ended, without problems. It'll probably work best if you leave off the ground jumper mentioned in the instructions, unless your power amp isn't grounded.

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    Re-reading this thread now, and never having actually tested this with a power amp, I realize that it appears I was given conflicting advice.

    Likely the power amp I will use will be single ended.

    Should I use the 125ohm tap to start for connection to the power amp, or highest impedance (10kohms)?

    Note: the schematic link does not appear to be valid anymore.

    Perhaps a more concise question would be this:

    A modern solid state pre-amp, what would the single ended impedance be?

    Or rather, what impedance would a modern solid state amp expect?

    Thanks!

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    Lifetime Member Enzo's Avatar
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    I was talking about matching, and Steve was talking about signal level.

    With a multitap output, the lowest impedance tap will have the lowest voltage. SO if you start there and work up, you will be starting with the lowest signal level, then step up until it gets to where you need it.

    Other than level, I doubt the actual impedance will be your issue.

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    Thank you!

    I've been looking for a nice mono power amp only and haven't had a lot of luck.

    I think I have a chance at a newer Crest amp that I can bridge mono.

    I will try to report back with results in another few years or so, lol.

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    Senior Member km6xz's Avatar
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    Most modern power amps(anything actually) are intended for bridging input so as long as the source Z is lower than the 10-50k input Z of the amp, the amp will be happy. But the line amp was built in the days that interconnections between gear was expecting terminating inputs...600 ohms to 600 ohms for max power transfer. They were rating level in power, dbm. Now, with bridging inputs, they are dbv, voltage ratings. So, the amp will not care but the line amp will not have its best signal to noise ratio or rated frequency response if mismatching its output impedance.
    Say that the power amp has a nominal 50k input Z, select the tap on the line amp to its highest Z. Note also that the line amp was expected to supply higher voltage(power) to the line than consumer power amps want now....-10dbv but pro amps are looking for either -10dbv or +4dbv or if older, +4dbm. If a pro style amp try the highest tap on the line amp output and see how that works. I suspect you will get lots of drive, and that will be the one with the best frequency response.

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    Thank you for the additional info!

    I have recently acquired a Crest Vs900 amp, and I am ready to make some connections.

    For sure I do not want to damage anything!

    I am used to mid-fi home stereo equipment, so even working with balanced line is new to me.

    The Crest input is listed as 20k ohm balanced, 10k ohm unbalanced.

    The Operadio 1A65 has 6 taps:

    GND - 5000 - 500 - CT - 500 - 5000

    The Operadio 1A65 schematic says:

    "With a jumper between "ground" and "center"

    CT & 500 = 125 ohms single ended
    CT & 5000 = 1250 ohms single ended
    500 & 500 = 500 ohms balanced line
    5000 & 5000 = 5000 ohms balanced line

    "With a jumper between "ground" and 5000"

    5000 & 5000 = 5000 ohms single ended

    "With a jumper between "ground" and 500"

    500 & 500 = 500 ohms single ended


    The Crest Vs900 3 input terminals are:

    GND + -


    Enzo, you mentioned being able to do 10k, 5000-0-5000, but I didn't see that config on the schematic. Is 10k possible? Would that be balanced & using no jumpers?

    Connected as?

    Operadio - No jumpers
    Left 5000 tap to Crest -
    Right 5000 tap to Crest +
    Operadio (CT? GND?) to Crest GND


    If so, I could get 10k balanced, that's 50% of the amp listing 20k balanced.


    Otherwise my plan is to connect as follows:

    Operadio connection- jumper from gnd to 5000

    Left side 5000 tap to Crest - tap
    Right side 5000 tap to Crest + tap

    Ground wire from Operadio GND to Crest GND

    (the reason for the ground wire, is the Crest manual states for parallel mode, a ground on the A input channel must be present. Then the + and - is jumpered to channel B, allowing the same source signal on both channels, output to two speakers.)

    The goal being 5000 ohms single ended, 50% of the amp listed at 10k unbalanced.


    To connect to the Vs900 from the Operadio as a balanced 5000 ohm would be:

    Operadio Jumper "GND to CT"

    Left side 5000 to Crest -
    Right side 5000 to Crest +
    Ground or CT wire to Crest "GND"

    Thanks for the contributions so far.

    Does anyone have any other suggestions or comments?

    Much appreciated!

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