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    Bypass capacitors on long leads

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    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Bypass capacitors on long leads

    Post by sKiZo on Mon Oct 21, 2013 12:40 pm

    Did a custom case and relocated the input leads to the rear, so the input leads are pretty long. Twisted wire, and run as far away from the power as possible, and lucky me, no noise or hum.

    However ...

    Getting back inside for a couple minor changes and figured now's a good time to ask.

    Any long wire CAN act as an antenna, and even though the signal is clean, I don't imagine it would hurt to add bypass capacitors to these lines. Normally we'd be talking maybe 0.05mV maximum deviation - not a lot, but as they say, every little bit counts. Got a bunch of 0.1uf that I normally use for such applications to buffer transistor gates and such. Add one to each end of the line? Just one at the jack? Center conductor, ground, or both? Double up with both 0.1uf and 0.01uf for broad spectrum noise cancellation?


    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bypass capacitors on long leads

    Post by Bob Latino on Mon Oct 21, 2013 4:19 pm

    sKiZo,

    The VTA ST-120 amp, even in your custom configuration, should be near dead silent with your ear on the speaker grilles using speakers of average efficiency (88 to 92 dB). Push/pull amps by their very nature tend to be quieter than direct heat triode (DHT) amps. There is always some noise cancellation in the output transformers of push/pull tube amps. On the VTA ST-120 the CENTER driver tube (initial voltage amplifier for both channels) is the tube which can cause hum if the tube is noisy to begin with. Sometimes a 12AU7/12BH7 type tube which has been tested for low noise can be used in the center position.

    Personally I don't think that your idea of adding bypass capacitors will lower the (near inaudible) noise level of your amp at all. You are welcome to try though? They key things in these amp that lower hum/noise levels are ...

    1. Parts placement on the chassis
    2. Having the plates of the power and output transformers at right angles to each other
    3. Twisting ALL wires that will carry AC voltages for any appreciable distance
    4. Using selected low noise driver tubes

    As an aside to number 3 listed above - When you twist wires carrying AC together, both wires are always at right angles to each other and any inductive interaction between the two wires will near zero.

    Bob

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Bypass capacitors on long leads

    Post by arledgsc on Tue Oct 22, 2013 10:06 am

    Since tube amps are high impedance by nature adding 0.1uF or 0.01uF to ground in the signal chain drastically affects the audio frequencies as well.   The ST-120 input impedance is about 270K ohms and with a corner frequency of 20Khz (-3dB point) takes only 30pF of capacitance.  So the nano-farad caps will heavily filter the audio signal.  If I was to experiment I would try something like a 10-15pF mica but it would be best not to rely on them at all.

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Bypass capacitors on long leads

    Post by sKiZo on Tue Oct 22, 2013 11:23 am

    OK then ... something I hadn't thought of ... I'll hold those caps for my next solid state project.

    I can say they do make a difference in gated circuits ... here's a Carver H9AV I did a while back, with bypass added in strategic locations ...



    If noise were an issue (which it's not - I can barely hear a bit of hiss out of the speakers with my ear right at the tweets at pretty robust volume when it's idling), I'd still think it'd be worth a try. Easy enough to snip them out if it's a problem.

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Bypass capacitors on long leads

    Post by arledgsc on Tue Oct 22, 2013 12:08 pm

    Oh, I see.  Power supply decoupling is another story.  It seems the Carver mod adds some addtional caps across the electrolytics.  The theory is that different caps have differening capacitive reactance at higher frequencies.   Electrolytics tend to become inductive at higher frequencies.  That is the purpose of the 1.0uF ESL bypass cap in the ST-120 power supply to provide high frequencies an easier path to ground.  I hope Carver didn't use electrolytics for stage couplers.  If so I see why someone would bypass these electrolytic stage couplers which is common practice in speaker crossovers.

    I only get a hint of white noise and a very tiny amount 60Hz hum holding my ear to the speaker.  The white noise seems to be just electron noise.  Remarkably quiet for a tube amp.  I use a passive attentuator and it doesn't matter if volume is maxed or at minimum.  I sit 6 ft. from the speakers and can hear nothing until the music plays.  If you have access to an oscilloscope it will certainly show any interference on the inputs that is otherwise inaudible but could be mixing with the audio signal to create undesirable noises.


    Last edited by arledgsc on Wed Oct 23, 2013 9:26 am; edited 2 times in total

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Bypass capacitors on long leads

    Post by sKiZo on Tue Oct 22, 2013 1:01 pm

    Yup ... the Carver did have some issues with AC noise in the signal path. They also cut a couple corners when it came to bypassing a couple of the ICs, and I don't suppose I helped any by replacing some of those with current tech Burr Browns. Even considered running 0.1 AND .01 caps parallel wherever I added a bypass to catch a broader noise band, but that was just a passing thought. Best I can say is the mods made for some significant improvements in the imaging and overall SQ.

    Only hum I get is a bit of mechanical vibration from the PT. Once again, not so you'd notice unless you're right on top of it. Nothing going thru to the speakers - with all the wire re-routes and mods, I figure I got dumb lucky there. I did use the rubber washers between the iron and chassis, but may have gotten the holes a bit tight, so there could be some thread contact. Long as I'm in there, I'll add rubber washers below the board, and maybe add a dab of black silicone to help isolate the mounting screws. May help, may not ... no biggie if it doesn't.

    Even with that, the ST120 is quieter than my McIntosh MC2205, but that's like ... 35 years old? And the Big Mac ain't no slouch when it comes to "black" either, so either way, I'm a happy camper.

    Or will be once I figure out the meter thing ... I'll be bangin' away on that later today and keep my fingers crossed.

    Bob Latino
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    Posts : 2378
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    Re: Bypass capacitors on long leads

    Post by Bob Latino on Tue Oct 22, 2013 1:19 pm

    sKiZo wrote:
    Only hum I get is a bit of mechanical vibration from the PT. Once again, not so you'd notice unless you're right on top of it. Nothing going thru to the speakers - with all the wire re-routes and mods, I figure I got dumb lucky there. I did use the rubber washers between the iron and chassis, but may have gotten the holes a bit tight, so there could be some thread contact. Long as I'm in there, I'll add rubber washers below the board, and maybe add a dab of black silicone to help isolate the mounting screws. May help, may not ... no biggie if it doesn't.
    If you get a slight amount of mechanical vibration from the power transformer > Before you add rubber washers try either loosening or tightening the four neoprene isolation washers that came with your kit. Nearly all power transformers have a slight amount of mechanical vibration. As long as this vibration is not audible from your listening position, I would ignore it.

    Bob

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Bypass capacitors on long leads

    Post by sKiZo on Tue Oct 22, 2013 7:30 pm

    I know what I need ... audiophile grade transformer screws!

    Went to the hardware for this and that today, so I picked up some crowned faucet washers ... long as I got em, shouldn't hertz to throw them on ...

    And I'll never have to worry about the PT leaking ... clown 

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