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    How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

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    calauver1

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    Join date : 2017-01-16

    How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by calauver1 on Mon Jul 24, 2017 8:17 am

    Hello, I am wanting to use a multi amp/speaker switch so I can switch between my VTA ST-120 and my Marantz 2325. I understand that the tube amp should not be switched on without a speaker load and you might say, just turn it off before you switch, however I want to have insurance in case I or my wife forgets. The switch I am looking at is the Beresford TC-7220 Audio crossover http://www.homehifi.co.uk/S/tc-7220.htm and it does not provide a load for the off speakers. Is there a way to modify the ST-120 so it will not be damaged if it is accidentally switched on without a load and if so, will it have any sonic effect? I apologize if this subject is out there, but I couldn't find anything. Thanks
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    Peter W.

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

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by Peter W. on Mon Jul 24, 2017 10:42 am

    Wow! A good question and well asked!

    OK, many after-market speaker selector switches have an "all off" function, so the issue is real.
    With the item in question, were you to select "Amp B", you would be disconnecting "Amp A" altogether. So this is a double whammy based on that switch.

    I can make one simple suggestion, which would be to install a headphone outlet box permanently connected to the amp outlets and leave a headphone connected at all times. Pretty basic:

    Phono Jack in a box. Amp --> 100-ohm @5-watt resistor --> jack for each channel. Common to common.

    Or build your own switch using a 4P/DT switch with no 'center-off'. Given you want a simple idiot-proof switch, the one linked would do nicely without any fancy bells, whistles or the associated costs.

    http://www.mouser.com/ProductDetail/CK-Components/7401SYZQE/?qs=sGAEpiMZZMsqIr59i2oRcnavdJzZ%252bjCJ3ijMyWenAVQ%3d

    Go to the last description on the data sheet. 5A @24 V should be more than enough for your intentions.

    Best of luck with it.
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    peterh

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    Location : gothenburg, sweden

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by peterh on Mon Jul 24, 2017 3:05 pm

    calauver1 wrote:Hello, I am wanting to use a multi amp/speaker switch so I can switch between my VTA ST-120 and my Marantz 2325. I understand that the tube amp should not be switched on without a speaker load and you might say, just turn it off before you switch, however I want to have insurance in case I or my wife forgets. The switch I am looking at is the Beresford TC-7220 Audio crossover http://www.homehifi.co.uk/S/tc-7220.htm and it does not provide a load for the off speakers. Is there a way to modify the ST-120 so it will not be damaged if it is accidentally switched on without a load and if so, will it have any sonic effect? I apologize if this subject is out there, but I couldn't find anything. Thanks
    Install a 100 ohm 5w resistor permanently connected to the 16ohm winding, install inside the amp.
    This resistor (( or simular) :
    http://www.arcolresistors.com/resistors/hs15-aluminium-housed-resistor/
    can be screwed in the chassies to permit good cooling. 10w ( or even 5w) should do just fine.
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    calauver1

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    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by calauver1 on Mon Jul 24, 2017 8:20 pm

    I think my amp only has 8ohm and 4ohm taps and I am using the 8ohm. Can I connect the resistor to the 4ohm ?
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    peterh

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

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by peterh on Tue Jul 25, 2017 12:06 am

    calauver1 wrote:I think my amp only has 8ohm and 4ohm taps and I am using the 8ohm. Can I connect the resistor to the 4ohm ?
    Your transformer has a 16 ohm tap curled up inside ( it's the one connected to the feedback
    network).
    The issue of what you use is irrelevant as the resistor is to be connected inside and permanent,
    thus it won't interfere with what you are using. And it's a point that it is permanently
    soldered as one do not want it to ever be disconnected.
    It's the open output that might be dangerous for the amp.

    If you use the 4 ohm tap i'd suggest a 50 ohm resistor.
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    bluemeanies

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    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by bluemeanies on Tue Jul 25, 2017 2:03 pm

    Okay, so let me butt in here.
    I had the same thought since I have a HT and 2channel listening. A pair of Bob Latino's mono blocks for 2channel and the Outlaw 7700 amplifier for HT. So two venues with one pair of speakers.
    I thought of the idea and drilled the holes on the boxes for the switches after that I had an experienced engineer (who is also on this forum) actually put two of these switches together for me.
    I call it the Gratrix switch box. Well made with quality toggle switches.
    Everything was purchased online...CHEAP.

    When I want to listen to 2channel I have the switch labeled accordingly.
    They were not difficult to make and they are better than anything I have seen on the internet costing less without sacrificing quality.
    I have had them for about 2years and no issues.
    I have also on more than one occasion had the switch toggled to the 7700 while turning on the m125's with zero effect on the m125's.

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    calauver1

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    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by calauver1 on Tue Jul 25, 2017 7:38 pm

    Looks good, but I still want insurance that I won't damage the tube amp. I have a habit of turning on my amp then I go over to my vinyl rack and look for what I want to listen to. Sometimes it's 10-15 minutes before I play something and if I forgot and left the switch on the other amp, it could do damage. All good info and I appreciate it.
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    evoroadster

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    Location : Honolulu, Hawaii

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by evoroadster on Thu Jul 27, 2017 1:01 am

    In my case I have 2 amps with one set of speakers. Each amp has their own set of speaker cables hooked up. The set not plugged into the speakers are plugged into a home-built dummy load using 2 each 8 ohm 100W noninductive load resistors. When I want to switch amps I swap connectors at the speakers with the set on the dummy load.
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    cci1492

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    Location : Bergen County NJ

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by cci1492 on Thu Jul 27, 2017 9:10 pm

    evoroadster wrote:In my case I have 2 amps with one set of speakers.  Each amp has their own set of speaker cables hooked up.  The set not plugged into the speakers are plugged into a home-built dummy load using 2 each 8 ohm 100W noninductive load resistors.  When I want to switch amps I swap connectors at the speakers with the set on the dummy load.

    Exactly what I've been doing. I know myself and this is the safest for me.
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    sKiZo

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    Location : Michigan USA

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by sKiZo on Fri Aug 04, 2017 7:45 am

    I use an old Niles DSP-1 two position switch. It provides a constant load to whatever amps are hooked up.

    Wondering if the 100ohm resistor mentioned earlier has any impact on sound quality or power output? seems to me that might tend to attenuate the low end a bit when the amp goes live?
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    Peter W.

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    Location : Melrose Park, PA

    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by Peter W. on Fri Aug 04, 2017 8:06 am

    sKiZo wrote:I use an old Niles DSP-1 two position switch. It provides a constant load to whatever amps are hooked up.

    Wondering if the 100ohm resistor mentioned earlier has any impact on sound quality or power output? seems to me that might tend to attenuate the low end a bit when the amp goes live?


    On sound quality, not much, but not zero.
    Yes, it does reduce the overall impedance of the system.
    I don't like internal permanent modifications for this rather basic purpose. I prefer a simple external device that is as fail-safe as is practical. I mentioned the headphone box option as many amplifiers (and pre-amps) under discussion here do not have such an option, so it serves two purposes. And, one may always unplug the headphones, removing it from the circuit when the speakers are connected.
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    sKiZo

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    Re: How can I modify ST-120 to have a load on the speaker terminals with no speakers attached?

    Post by sKiZo on Sat Aug 05, 2017 10:31 am

    I have a nice workaround here for headphones in the Maverick TubeMagic DAC that I use for myHTPC's digital/analog conversion. That's actually better known as a headphone amp and does yeoman duty in that capacity.

    Bonus, it also allows me to tailor the "flavor" of the ST120's tube set. Currently running a GE 6SN7 center and matched pair of 5814's driving KT120's, and those would be a bit "dark" for my tastes except for the Tesla 6CC42 acting as a tube buffer in the DAC. Together ... magic!

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