The Dynaco Tube Audio Forum

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


    ST-120 As a Headphone Amp ??

    Share
    avatar
    arledgsc

    Posts : 491
    Join date : 2012-11-30
    Location : SF Bay CA

    ST-120 As a Headphone Amp ??

    Post by arledgsc on Sun Jan 07, 2018 1:10 pm

    I have moved recently and downsized greatly.  For audio listening I went from a highly tweaked and loved ST-120 amp/ speaker combo to using headphones with a dedicated headphone amp.  But I really miss the sparkle and bass of my ST-120 amp.  I have been looking at other tube amps dedicated for headphones but I really don't want to buy another amp as I already have a great one sitting in the closet.  Is there any way to power 32 ohm headphones with the ST-120????

    I realize the power and impedance challenges.  I only want to listen in the amp's lower power Class A mode anyway and probably did so 90% of the time with speakers.  One method I read said to connect two series resistors to give the amp's OT an 8 ohm equivalent load.  Then tap the 32-ohm headphones across the lower value of the two series resistors.  The resistor voltage divider values will depending upon the sensitivity of the headphones and how hard you want to drive the amp.   Like use 7 + 1 ohm or 6 + 2 ohm as the resistor load pair values.

    Has anyone done this and can you share any guidance?  I am jonesing for KT-120s and CV-181s!
    avatar
    peterh

    Posts : 1068
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-120 As a Headphone Amp ??

    Post by peterh on Sun Jan 07, 2018 1:51 pm

    Don't !
    If anything goes wrong you will risk 60w in your ear. That might permanently impact your hearing.

    You already have a dedicated earphone amp, if the earphones sound bad it's bad earphones
    Go hunt for better ones, and put away your st120 until you have a place to use speakers again.

    jsl1234

    Posts : 34
    Join date : 2017-08-28

    Re: ST-120 As a Headphone Amp ??

    Post by jsl1234 on Sun Jan 07, 2018 4:16 pm

    I wouldn't if for no other reason is I doubt the VTA120 is quiet enough for critical headphone listening re: I would think that background noise of that many tubes, while not that apparent when listening to speakers, would be really apparent if your headphones are even mildly resolving.
    avatar
    j beede

    Posts : 472
    Join date : 2011-02-07
    Location : California

    Re: ST-120 As a Headphone Amp ??

    Post by j beede on Sun Jan 07, 2018 9:46 pm

    Sounds like you could make good use of a pair of STAX earspeakers with SRD interface.
    avatar
    arledgsc

    Posts : 491
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 As a Headphone Amp ??

    Post by arledgsc on Mon Jan 08, 2018 9:00 am

    Wise observation about melting my ears and headphones. I may try it though but not as a permanent solution.
    avatar
    Peter W.

    Posts : 1019
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-120 As a Headphone Amp ??

    Post by Peter W. on Mon Jan 08, 2018 9:31 am

    OK - A dissenting view:

    There is NO, repeat, NO reason why you may not use the ST120 into any decent pair of headphones. As follows:  

    a) Obtain  a high-quality headphone jack NON-Shorting!! (more on this later).
    b) Obtain two 5%-or-better 100-ohm 5-watt resistors.
    c) Obtain 6' of good-quality 14-gauge speaker wire, polarized (marked, striped, otherwise).
    d) Obtain four high-quality connectors matching your amp output connectors. I prefer spade lugs. TWO different colors!
    e) Obtain a small project box to your taste.
    f) Obtain the appropriate strain-relief or grommets for the speaker wires.

    Drill the parts box for two speaker-wires in (grommet or strain-relief) and for the jack on the other side.
    Install one 3' piece of speaker wire in each grommet and knot (on the inside). Or strain-relief.
    Install the jack - solder-points visible. Make sure you have enough clearance for the leads and the restraining nut.

    Verify L/R/C on the headphone jack, and mark.
    Connect one resistor to L and one to R. Slide a  bit of insulation over exposed leads if you wish for safety.
    Connect the speaker wires (3' each) unmarked or unstriped lead to C. Should be two (2) wires connected to C, one resistor each to L & R.
    Connect one-each marked/striped lead to each resistor. Mark L & R.

    On the other end of the speaker wires, install the connectors - one color on the Hot another on the C.
    Verify this six (6) ways from Sunday. Amplifiers do not like cross-connected outputs, but the 120 shares the common.

    Connect the speaker wires to the amp, L, R & C.  Plug in the phones. Enjoy - you will need a volume-control on the amp if you are not using a pre-amp.

    It will be plenty quiet.

    This, essentially, duplicates a circuit developed first by Koss in the 1960s, and then used on about every receiver/amplifier since that ran directly from the main circuit and did not have separate headphone amps (few did).

    On the non-shorting jack - the circuit as-described will not support speaker connections. You would do those directly. However, AR, Koss later on and others developed a circuit that cut off the speakers when headphones were plugged in. These used a "Shorting" jack that lifted the outputs from the speaker-out to the headphones. One, the other, not both. This is a special case, but the jacks are readily available.

    I use one of these on the test-bench - adaptable  to any amplifier - and feed into a pair of Grados as my test headphones. Works just fine.

    All parts-and-pieces available from Mouser (amongst other sources) and the entire shebang should cost less than $15 inclusive of shipping. I used 1% resistors, by the way. Why not?
    avatar
    arledgsc

    Posts : 491
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 As a Headphone Amp ??

    Post by arledgsc on Mon Jan 08, 2018 10:28 am

    @Peter W. Thanks Peter. Are you saying a single 100 ohm resistor off the transformer tap that feeds the phones? This would work for transistor amps but the OT in a tube amp would not be seeing the correct load impedance. Am I missing something?
    avatar
    Peter W.

    Posts : 1019
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: ST-120 As a Headphone Amp ??

    Post by Peter W. on Mon Jan 08, 2018 10:42 am

    arledgsc wrote:@Peter W.  Thanks Peter.  Are you saying a single 100 ohm resistor off the transformer tap that feeds the phones?  This would work for transistor amps but the OT in a tube amp would not be seeing the correct load impedance.  Am I missing something?

    Matters not. Come off the 16-ohm tap (or highest impedance tap available). I use this commonly on my (all vintage) ST70, ST35 & Scott LK150 with no untoward results.

    You can mess with the resistor value, but for the sake of the headphones, I would not go _BELOW_ 100 ohms. Keep in mind that Koss came up with this circuit to allow for headphones in the pre-transistor era, when suppliers such as Scott, Dynaco, et.al. simply did not allow for them.
    avatar
    arledgsc

    Posts : 491
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-120 As a Headphone Amp ??

    Post by arledgsc on Mon Jan 08, 2018 10:56 am

    Interesting and thank you!
    avatar
    MarcVBelgium

    Posts : 104
    Join date : 2013-12-06
    Age : 65
    Location : Antwerp, BELGIUM

    Re: ST-120 As a Headphone Amp ??

    Post by MarcVBelgium on Tue Jan 09, 2018 8:11 am

    I use this "Nixie" on a Sennheiser HD-650 with GREAT results......

    http://nixiekits.eu/product.php?key=headphone_amp

    Sponsored content

    Re: ST-120 As a Headphone Amp ??

    Post by Sponsored content


      Current date/time is Wed Dec 12, 2018 10:58 pm