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Thread: grounding load box w/BNC jack for 'scope?

  1. #1
    Senior Member SoulFetish's Avatar
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    grounding load box w/BNC jack for 'scope?

    I just finished building a new bench and I'm building a new load box rated for 200W(min) at 4Ω, 8Ω, & 16Ω. I'm going to install a BNC jack to the enclosure for easy access to the signal into the load. I want to double check the best way to handle the ground (in ref to earth).
    Some builders/manufacturers ground the sleeve of the output jack to a ground reference somewhere in the amplifier. Others prefer to isolate the output signal to eliminate any ground loops.
    So, my question in regards to the outer conductor in a 'scope probe is this: is there any risk in attaching the probe ground to the sleeve in an isolated signal? How would you handle grounding the enclosure of the load box so that it can safely take both isolated and non-isolated input signals?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  2. #2
    Supporting Member The Dude's Avatar
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    Do you always plug the gear your testing into an isolation transformer or isolated variac? (You should, but many don't.)
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  3. #3
    Senior Member SoulFetish's Avatar
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    Here is how I was thinking of doing it:
    load-box.jpg
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by The Dude View Post
    Do you always plug the gear your testing into an isolation transformer or isolated variac? (You should, but many don't.)
    Here is an article which makes a good case for not using an isolated transformer in many instances:
    Isolation Transformers
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  5. #5
    Senior Member Enzo's Avatar
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    Problems may arise on solid state amps wher the speaker return is not to ground. If your amp is earthed, and your scope as well, then grounding the BNC tot
    Education is what you're left with after you have forgotten what you have learned.

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    Think about how the box needs to be set up with a bridged amp. You don't want either the load or the scope grounding the amp's output.
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    Old Timer J M Fahey's Avatar
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    You know I love lateral thinking , if if I canīt solve a problem (happens all the time), I try to solve a simpler one instead
    You know, 90% of results with 20% the problem/investment/effort.

    In my load box, resistors go to an isolated jack ... so far same as you, so I can load standard/bridged/ungrounded amps , which is the main point.

    The BNC "hot" pin can go, through a switch, to input jack Hot or "ground" (sleeve actually).

    I add 2 posts for multimeter, one to jack tip, one to sleeve, I use spring speaker type red/black ones, so they hold multimeter probes there to measure VAC across load no matter what.

    BNC ground goes to a crocodile clip so I can clip it straight to amp chassis so no problem even if amp is bridged or speaker is grounded through a small (but important) resistor.

    But ... but ... thatīs not what "they" suggest !!!! "they" talk about expensive scope differential preamps, floating ground, the works !!!!!

    Really????? letīs see what problems does my setup bring.

    1) plain grounded output (think Twin Reverb or tube Marshall): no problem at all .

    2) mixed feedback guitar amp: you are measuring (with scope) slightly higher voltage (typically around 1V) than actually going to speaker, because you are including the drop across current sensing resistor ... so 20VAC is probably 19VAC .
    No big deal, scope is used *mainly* to check for distortion, clipping, oscillation, etc.
    You want exact value for precise power measurement? ... thatīs what the multimeter terminals are for .
    No multimeter?
    You want to be precise?
    Flip the switch, read voltage across grounding resistor, substract from main voltage measurement.

    3) bridged amplifier.
    Again no big deal, itīs made of two out of phase **exact same** amplifiers.
    You check one for anything you want (yes, from hot to ground) , flip the switch back and forth as many times as you want to check the other.
    You will miss NO problems at all (clipping/distortion/oscillation/hum/buzz/spikes/noise/etc.)

    But .... but .... I see only half the actual voltage sent to the load!!! How can I calculate, say, full power?

    Think again
    Maybe itīs twice what you see on screen?

    Besides, meter is floating and across the full load.

    Whenever I get a computer back Iīll post a schematic.
    nevetslab and SoulFetish like this.
    Juan Manuel Fahey

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    I have my load so I can split it for working on stereo or dual-mono power amps where I want to simultaneously load both channels. I bring out my 4 ohm load resistors to isolated combo binding/banana posts. I can get patchable 1x16, 1x8, 1x4, 1x2 or 2x8, 2x4, 2x2 ohm loads.
    J M Fahey and nevetslab like this.

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    Perfect time for this thread. Just ordered some thick film, 100w resistors from Newark.com for cheap, in order to build a new load box. Pulling up a chair...
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

  10. #10
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    I have my load so I can split it for working on stereo or dual-mono power amps where I want to simultaneously load both channels. I bring out my 4 ohm load resistors to isolated combo binding/banana posts. I can get patchable 1x16, 1x8, 1x4, 1x2 or 2x8, 2x4, 2x2 ohm loads.
    This is the setup we use at work.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  11. #11
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    Think about how the box needs to be set up with a bridged amp. You don't want either the load or the scope grounding the amp's output.
    Mick, could you explain what you mean by the box being set up with a bridged amp in this sense? do you mean using two probes in differential mode? If so, how would you configure tapping off signal points in reference to the isolated resistive load?
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  12. #12
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    You know I love lateral thinking , if if I canīt solve a problem (happens all the time), I try to solve a simpler one instead
    You know, 90% of results with 20% the problem/investment/effort.

    In my load box, resistors go to an isolated jack ... so far same as you, so I can load standard/bridged/ungrounded amps , which is the main point.

    The BNC "hot" pin can go, through a switch, to input jack Hot or "ground" (sleeve actually).

    I add 2 posts for multimeter, one to jack tip, one to sleeve, I use spring speaker type red/black ones, so they hold multimeter probes there to measure VAC across load no matter what.

    BNC ground goes to a crocodile clip so I can clip it straight to amp chassis so no problem even if amp is bridged or speaker is grounded through a small (but important) resistor.

    But ... but ... thatīs not what "they" suggest !!!! "they" talk about expensive scope differential preamps, floating ground, the works !!!!!

    Really????? letīs see what problems does my setup bring.

    1) plain grounded output (think Twin Reverb or tube Marshall): no problem at all .

    2) mixed feedback guitar amp: you are measuring (with scope) slightly higher voltage (typically around 1V) than actually going to speaker, because you are including the drop across current sensing resistor ... so 20VAC is probably 19VAC .
    No big deal, scope is used *mainly* to check for distortion, clipping, oscillation, etc.
    You want exact value for precise power measurement? ... thatīs what the multimeter terminals are for .
    No multimeter?
    You want to be precise?
    Flip the switch, read voltage across grounding resistor, substract from main voltage measurement.

    3) bridged amplifier.
    Again no big deal, itīs made of two out of phase **exact same** amplifiers.
    You check one for anything you want (yes, from hot to ground) , flip the switch back and forth as many times as you want to check the other.
    You will miss NO problems at all (clipping/distortion/oscillation/hum/buzz/spikes/noise/etc.)

    But .... but .... I see only half the actual voltage sent to the load!!! How can I calculate, say, full power?

    Think again
    Maybe itīs twice what you see on screen?

    Besides, meter is floating and across the full load.

    Whenever I get a computer back Iīll post a schematic.
    I see what you're doing here Fahey, and I LIKE it! It's a similar setup to what I had in mind, albeit, you thought to accommodate most commonly found audio amplifier outputs (home stereo, plus solid state and tube instrument amps). That was good thinking, and would be improvements to my initial idea. Wouldn't take but a few modifications to my original schematic. Also, I like the idea of running the sleeve of the BNC to an alligator lead. It's more consistent, and could minimize careless mistakes.
    J M Fahey likes this.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  13. #13
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    Quote Originally Posted by SoulFetish View Post
    Mick, could you explain what you mean by the box being set up with a bridged amp in this sense? do you mean using two probes in differential mode? If so, how would you configure tapping off signal points in reference to the isolated resistive load?
    Just that you need to ensure that in bridged mode one side of the load isn't connected to earth if you connect across the full bridged output. This includes the ground side of your BNC scope connection. Equally, your load resistors have to be isolated from earth. I've had plenty of success grounding the scope to the chassis. Having the BNC ground connected to a croc clip (as Juan suggests) rather than being hard-wired forces you to make a decision where to clip it. As you mention differential measurement I'm assuming you already have the techniques nailed.
    nevetslab and SoulFetish like this.

  14. #14
    g1
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    You probably have a reason you want to use BNC.
    I'm with Mick in preferring dual bananas. The stackability and quick reverse function makes them very versatile. And they are isolated by default.
    nevetslab likes this.
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  15. #15
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    Just that you need to ensure that in bridged mode one side of the load isn't connected to earth if you connect across the full bridged output. This includes the ground side of your BNC scope connection. Equally, your load resistors have to be isolated from earth.
    Oh. Yeah, I'm with you. I don't know why I thought you were talking about Isolating the probe through an active devise or something.

    Quote Originally Posted by Mick Bailey View Post
    As you mention differential measurement I'm assuming you already have the techniques nailed.
    Is that actually a thing?? I was just throwing out tech jargon so I could sound like one of the guys
    g1 likes this.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  16. #16
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by g1 View Post
    You probably have a reason you want to use BNC.
    I'm with Mick in preferring dual bananas. The stackability and quick reverse function makes them very versatile. And they are isolated by default.
    I was planning to put in some bananas for the multimeter. I'm not in love with using a BNC other than a convenient supply of terminated RG58. But you make a solid case for the ease of switching back and forth.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  17. #17
    g1
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    You can put dual banana at the other end of the RG58 and BNC on the scope end.
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  18. #18
    Senior Member Enzo's Avatar
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    Aside from regular scope probes, I had a few BNC to BNC cables for my scope, and I had BNC female to dual banana male adaptors. For that matter I have BNC male to dual banana female binding posts.

    70188731.jpg

    3020bone.jpg
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  19. #19
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by Enzo View Post
    Aside from regular scope probes, I had a few BNC to BNC cables for my scope, and I had BNC female to dual banana male adaptors. For that matter I have BNC male to dual banana female binding posts.
    Wouldn't ya' know, in a moment of "I have too much shit around here", I tossed (or gave away) a couple of pamona BNC to banana jack adapters during a shop purge. I haven't been burned for the most part doing that in the past, but, perhaps I should have made some room for those and held on to a couple.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  20. #20
    Senior Member Enzo's Avatar
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    You know, I would toss those project of the future amp carcasses or whatever to make space, but tools and adaptors... never.

    I probably should clean house now, I still have adaptors made for pinball machine CPU boards and dollar bill changers.
    Education is what you're left with after you have forgotten what you have learned.

  21. #21
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by SoulFetish View Post
    Here is an article which makes a good case for not using an isolated transformer in many instances:
    Isolation Transformers
    FWIW: I find nothing in that article that convinces me to change my opinion. Quite the opposite. Of course you should check for AC-chassis leakage before you send something out the door, but that's not a reason for abandoning the isolation transformer. You can check for leakage with a meter, or find out with your fingers. I prefer to be safe. Of course, it still requires some common sense. Just because you're isolated doesn't mean you can't get a jolt.
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  22. #22
    Senior Member SoulFetish's Avatar
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    What do you guys think about this configuration for a load box..

    • 3 switchable taps for 4Ω, 8Ω, & 16Ω
    • 300W max dissipation for each load impedance
    • using 3 resistors total (ish)
    Here is the wiring diagram:
    4-8-16ohm_300w_wiring_schem.jpg
    The challenge is the simplest, most efficient way to switch in and out of this. Any thoughts?

    edit: I made a mistake, the 8Ω tap will not be able to dissipate 300W. If my math is correct, it will be able to dissipate ≈150W. right?
    Last edited by SoulFetish; 10-24-2017 at 04:06 AM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  23. #23
    g1
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    Quote Originally Posted by SoulFetish View Post
    edit: I made a mistake, the 8Ω tap will not be able to dissipate 300W. If my math is correct, it will be able to dissipate ≈150W. right?
    That's what I got too. 25W per 50 ohm leg and 50W per 25 ohm leg in the 8.33 ohm configuration.
    And your 4.16 ohm drawing is missing one of the connections along the bottom.
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    Senior Member SoulFetish's Avatar
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    As much as I like the idea of using 3 physical resistors to build it, I really don't like loosing half power handling for one load impedance. I'm probably not going to need 300W all that often, but I want to build something once and build it right. So I decided to change it, and simplify the switching. Plus, I bought nice aluminum enclosure for short money and the big 100W wouldn't fit without physically modifying their mounting brackets. I'm using this configuration. 4,8, and 16Ω loads all just a bit under 300W max. Here is a layout to scale:

    I didn't want to get unbranded Chinese resistors off ebay just to save money without any access to information about the part. I was able to get these most of these 50W wirewound from Newark for under $2 a piece and didn't pay more than $3 for any of them.
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  25. #25
    Senior Member SoulFetish's Avatar
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    Here is quick build update, incorporating the helpful advice I received:
    (plus one quick question)







    This should give me close to 300W per tap. I built this to be switchable for 4, 8, and 16Ω, however, I've worked on a few old 'Supers since I designed it and ordered the parts for it. So, here is my question:
    Do you guys often have need for 2Ω loads? (or often enough?) If so, I was thinking the easiest solution would be to switch all three into a parallel configuration - ie. 1(1/4+1/8+1/16) = 2.2857. That's pretty close. Anyone see any issue with that?
    I wanted to leave the extra room to have the flexibility to install a speakon and/or banana input jack later on if I need to. The only change I would need to make is replacing the current switch with a 4P4T(minimum) rotary switch.
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  26. #26
    g1
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    Close enough. Aside from big PA type power amps, you won't have much need aside from Supers.
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  27. #27
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    My main load bank is configured as (8) 4 ohm/500W Dale Power resistors (pair of 2 ohm 1% Non-inductive 250W resistors in series, four channels per large fan-cooled heat sinks, terminated in isolated dual banana jacks, all spaced on 3/4" centers, so I can configure my loads as 1 ohm/2kWz, 2 ohm/1kW, 4 ohm 2kW or 4 ohm 500W, 16 ohm/2kW per channel, Or 2 ohm/4kW using all 8 loads. And, other variations as req'd. I also have a smaller load panel that's 4 ch of 8 ohm/200W that's portable.

    test-bench-2.jpg test-bench-1.jpg load-bank-1.jpg load-bank-4.jpg

    My scope normally is connected thru my Amber 3501a Audio Analyzer's Monitor BNC's, with the input to the analyzer via differential input. Generator output can be grounded or transformer isolated via HP 353A Transformer/Step Attenuator Patch Panel. I use a variety of signal sources...Sine, Sine Random, Pink Noise, Burst pink noise, 1/3 oct Pink Noise, Swept Sine, Square Wave, two-level tone burst Sine, etc.

    If I need to go probing, I can do it with X10 scope probe, usually directly grounded, but not always, depending whether I have the Analyzer's input attached to the gear being measured. I have differential plug-ins for my Tektronix 7633 scope when needed. If I"m using a spectrum analyzer, I have two that are balanced....one via xfmr, the other via differential inputs. Also have a good bridging input transformer for my B & K analyzers/measuring amps.

    My load banks are normally directly connected to the outputs if using dummy load, or may be test speaker. I normally monitor the output thru the Diff input of the Amber 3501a Audio Analyzer. I also have a Valhalla 2101 set up with a patch panel that can be used to directly monitor the power output of amps under load, be it dummy load or speaker load. The instrument is a wide bandwidth true RMS meter that also measures DCV.

    My main metered variac is connected to a 5kW Topaz Isolation transformer. If I have to go probing into an amp's switching power supply, I have a vintage Sony/Tektronix 326 battery powered scope so I can be totally isolated.
    Last edited by nevetslab; 12-06-2017 at 08:59 PM.
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  28. #28
    Senior Member nevetslab's Avatar
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    One load that I've never built is a dummy reactive load that closely resembles that of a speaker in both a sealed box as well as in a port-tuned box. Does anyone out there have details on building one? My shop space is down the hall from three of our rehearsal studios, one of which I share the walls with. When I'm power-testing an amp that has issues driving a speaker without having drive problems , the SPL levels too often become an issue for the clients. Resistive loads don't get you into the real-world problems that speakers present from the back-EMF they send back to the amplifier. Current limiting circuits (in solid state amps), if not well thought out and designed well can cause radical behavior. One solution is having an isolation chamber to place the speaker system into, but few of us have those or the space to put it.
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