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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    VTA M-125's and 16 ohm speakers

    Audiosoldier1979
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    Audiosoldier1979


    Posts : 4
    Join date : 2021-02-20
    • Post n°1

    VTA M-125's and 16 ohm speakers Empty VTA M-125's and 16 ohm speakers

    Post by Audiosoldier1979 Fri Nov 17, 2023 5:14 pm

    Hi  I could possibly be acquiring a pair of rogers ls3/5a 15ohm speakers and was wondering if there's anything specific I need to do to the internal wiring to allow everything to work properly. I bought these monoblocks second hand and have some of the original build instructions but there's nothing mentioned about which taps are which nor the ability to pair with 16 ohm speakers. I would greatly appreciate some guidance.
    Thanks
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

    VTA M-125's and 16 ohm speakers Empty Re: VTA M-125's and 16 ohm speakers

    Post by Bob Latino Fri Nov 17, 2023 8:52 pm

    Audiosoldier1979 wrote:Hi  I could possibly be acquiring a pair of rogers ls3/5a 15ohm speakers and was wondering if there's anything specific I need to do to the internal wiring to allow everything to work properly. I bought these monoblocks second hand and have some of the original build instructions but there's nothing mentioned about which taps are which nor the ability to pair with 16 ohm speakers. I would greatly appreciate some guidance.
    Thanks

    The 16 ohm tap on the output transformer is the YELLOW colored wire. Normally the 16 ohm YELLOW wire on the M-125's goes to the feedback eyelet on the driver board. To convert one of the RED speaker outputs to be a 16 ohm tap just ... (BROWN wire = 4 ohms, ORANGE wire = 8 ohms, YELLOW wire = 16 ohms)

    1. Remove the wire that is now on the RED speaker tap you want to make 16 ohm. Tape over the end of this wire (BROWN or ORANGE) so that it can't short out to the chassis.

    2. Unsolder the YELLOW wire from the driver board and connect it to the RED speaker tap that now has no wire attached.

    3. Run a piece of hookup wire from the speaker tap you just connected the YELLOW wire to (solder this connection) and run the other end of the wire to the eyelet on the driver board that the yellow wire use to be. (solder this connection)

    You have now converted that red speaker tap for 15/16 ohm speakers.

    Bob

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    Audiosoldier1979
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    Audiosoldier1979


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    • Post n°3

    VTA M-125's and 16 ohm speakers Empty Re: VTA M-125's and 16 ohm speakers

    Post by Audiosoldier1979 Sat Nov 18, 2023 12:05 pm

    Thanks again Bob. I may get tye opportunity to audition the ls35a in my system before buying. Is it OK to temporarily use the 8 ohm tap to audition them without any harm to the amps or speakers?
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°4

    VTA M-125's and 16 ohm speakers Empty Re: VTA M-125's and 16 ohm speakers

    Post by Bob Latino Sat Nov 18, 2023 12:38 pm

    Audiosoldier1979 wrote:Thanks again Bob. I may get tye opportunity to audition the ls35a in my system before buying. Is it OK to temporarily use the 8 ohm tap to audition them without any harm to the amps or speakers?

    You can plug the 15 ohm speakers into the 8 ohm tap and it will work but it is not an ideal situation. The amp will have to work harder to get volume. You will have to turn up the volume a little higher using a 15/16 ohm speaker on the 8 ohm tap than you normally would with an 8 ohm speaker of approximately the same efficiency.

    Bob

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    ELBill34
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    ELBill34


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    • Post n°5

    VTA M-125's and 16 ohm speakers Empty Re: VTA M-125's and 16 ohm speakers

    Post by ELBill34 Fri Dec 01, 2023 10:34 am

    You can make the suggested wiring change for 16 Ohm tap but you should at least give a listen on the 8 Ohm tap.
    Published speaker impedance is a nominal value and varies quite a bit as frequency changes often dipping below the nominal value.
    My Celestion DL10 speakers are rated 8 Ohm but doing an impedance sweep I find they dip just under 6 Ohm. After trying 8 and 4 Ohm taps I found I prefer them on the 4 Ohm

    Bill
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    Seamus
    Seamus
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    • Post n°6

    VTA M-125's and 16 ohm speakers Empty Re: VTA M-125's and 16 ohm speakers

    Post by Seamus Fri Dec 01, 2023 11:36 am

    Audiosoldier1979 wrote:Hi  I could possibly be acquiring a pair of rogers ls3/5a 15ohm speakers and was wondering if there's anything specific I need to do to the internal wiring to allow everything to work properly. I bought these monoblocks second hand and have some of the original build instructions but there's nothing mentioned about which taps are which nor the ability to pair with 16 ohm speakers. I would greatly appreciate some guidance.
    Thanks

    Whose and what vintage LS3/5a?

    Their impedance curves are a real roller coaster and not the best match to a transformer coupled amp.
    IIRC, as low as 7 or 8 and as high as 35 or 40.

    You may prefer the sound on the 8Ω tap.

    NEVER tape over anything in a tube amp. Do it properly with TWO layers of heat shrink.
    Tape has a nasty habit of letting go in a hot environment.
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