Speaker Output Grounding Questions

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
Good stuff, Andrew. I wonder if we tend to confuse 'grounding out' with 'linking to a reference voltage"...

I think you nailed it there. Always pithy! Every forum needs a @King Fan.

Here’s a sketch to illustrate my point. The top two images are 3 batteries of differing voltages wired in series. You can see that changing the ground point of the circuit does not change the voltage differential between any two of the “taps”. The absolute voltage (measured to ground) of each tap changes, but if you wired a light bulb across each of the 3 pairs of taps, they would all light up the same no matter where you ground the circuit. Ground it anywhere you want and end up with the same result.

Finally, extend the same logic to 3 inductors wired in series. They each provide an AC voltage instead of a DC voltage. Since they are all in phase with each other, we can add and subtract them like DC. No matter where we ground the circuit, the relative voltage between any pair of taps will be the same. Grounding the secondary (anywhere, as long as it is only in one place) ensures the AC average voltage will be 0VDC and not some random floating voltage.

1642561482410.jpeg
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
I tried using all 3 taps on that transformer exactly as you describe and I ended up with a potential difference between the speaker jack body and the amp chassis. Not huge, but enough to give a definite tingle if touched it with bare hands.
I ended up removing the 4 ohm and tying the wire now common to 8 and 16 to ground. It made 8 and 16 useable but I was disappointed I couldn’t use all 3 taps.
I’ve since read threads on here by other users puzzling over how to use all 3 to no avail.

So make sure I am interpreting your experience correctly. You had the secondary floating—no connection to chassis at any point of the secondary—and the speaker jack (isolated) picked up enough voltage from somewhere to deliver a continuous shock into your bare fingers?

That’s enough empirical evidence for me. Ground the secondary.
 

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,291
Age
62
Location
Finland
Using isolated loudspeaker jacks it is possible to easily get as many impedances. For example H125_ six output secondary, mathematically there comes 15 separate outputs. Isolated output jacks also allows to use different impedance and different power rating loudspeakers same time choosing suitaple outputs, following winding ratio/impedance AND power calculating.

This is example for more common three output OT which has six different options but continuing that it builds up...

NFB is another feature where loadspeaker load obviously does not effet much so it can be taken where is is most convenient.
 

Attachments

  • PHOTO-2022-01-19-13-47-25.jpg
    PHOTO-2022-01-19-13-47-25.jpg
    161.7 KB · Views: 8
Last edited:

Bitsleftover

Tele-Holic
Joined
Oct 17, 2016
Posts
681
Location
The North of England
So make sure I am interpreting your experience correctly. You had the secondary floating—no connection to chassis at any point of the secondary—and the speaker jack (isolated) picked up enough voltage from somewhere to deliver a continuous shock into your bare fingers?

That’s enough empirical evidence for me. Ground the secondary.
Yes. Exactly that. Cliff jacks. My speaker cable had a metal jack plug and when I touched it and the chassis at the same time I got a zap. Not huge. But enough to command some colourful language.
I am, as always, open to the possibility I wired it completely wrong. I asked the question on here but I can’t for the life of me find the conversation. I dropped the 4 ohm option and wired 8 & 16 ohm common wire to the grounded lugs of the now grounded switchcraft jacks.
 

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
609
Location
Orlando
Was this determined experimentally or just theoretically?

I just don't see how you lose signal across any of those pairs just by connecting a single tap (any one of them) to ground. You are just tethering the whole secondary to 0VDC instead of letting it float wherever it wants to be. The appropriate AC signals still appear across all the taps. Floating to me means no reference to ground in the entire circuit. If you connect any one of those taps to ground, they all become tethered to 0VDC. Right?

it took me a few days, but i think i get it now.
so to be clear, you saying tie tap 1 to ground, which ties the entire 2ndary to ground (0 volts) and that'll do it? from then on, it doesnt matter what taps i actually use to get the load and impedance i need.
thats easy.

i need to revisit a few of my amps...
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,001
Location
Salt Lake City
I still don’t see the harm in using tap (1) to ground the secondary. Important to use isolated speaker jacks though.
Hey, @andrewRneumann , you’ve done a greqt job with these helpful analyses, and you know a lot more EE than I do. Maybe expand a minute on why this OT needs isolated speaker jacks? I know ground returns through chassis are bad in theory, but my experience has been like Mr. Keen's, that it doesn’t make an audible difference in some cases. Is there a safety or other reason here? Something to do with switching impedance?

(Keen citation near the end of this page…)

http://www.geofex.com/article_folders/stargnd/stargnd.htm
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
Hey, @andrewRneumann , you’ve done a greqt job with these helpful analyses, and you know a lot more EE than I do. Maybe expand a minute on why this OT needs isolated speaker jacks? I know ground returns through chassis are bad in theory, but my experience has been like Mr. Keen's, that it doesn’t make an audible difference in some cases. Is there a safety or other reason here? Something to do with switching impedance?

(Keen citation near the end of this page…)

http://www.geofex.com/article_folders/stargnd/stargnd.htm

Non-isolated jacks will ground the sleeve portion of the secondary taps to the chassis. If you have multiple jacks you will end up with multiple short circuits through the chassis.

I illustrated using tap “1” as the chassis ground. You could use any tap, as long as it’s just one (for the same reason as using isolated jacks—multiple grounds will be short circuits through the chassis). If you wanted to use NFB, the tap you ground does matter. Say the NFB is based on the voltage across the 8-Ohm secondary, then ground tap “2” and and take the NFB from tap “5”. Leave tap “1” open in this case. All the speaker jacks will still work as advertised.

1642609742066.jpeg
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,001
Location
Salt Lake City
Non-isolated jacks will ground the sleeve portion of the secondary taps to the chassis. If you have multiple jacks you will end up with multiple short circuits through the chassis.
Thanks! Excellent. I've never even wired up a multi-tap OT, so I’m an infant here instead of an amp toddler elsewhere. I only know the NFB thing from reading Rob. Is the short circuit thing different if you use just one jack and an impedance switch? Ba ba goo goo… :)
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
Maybe this is the simplest way to solve the problem. Only the 4-Ohm speaker jack would need isolation if you did it like this. Tap “2” is grounded through the 8-Ohm and 16-Ohm jacks.

NFB illustrated.

1642610702924.jpeg
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
Thanks! Excellent. I've never even wired up a multi-tap OT, so I’m an infant here instead of an amp toddler elsewhere. I only know the NFB thing from reading Rob. Is the short circuit thing different if you use just one jack and an impedance switch? Ba ba goo goo… :)

An impedance switch or switches (a la @SerpentRuss ) would not require an isolated jack so long as, and I will say it again, only one tap on the secondary is grounded at a time. The NFB would be problematic in this case as it’s voltage will change as the ground tap changes when you adjust the impedance.

If you wanted to select impedance with a switch and have constant NFB regardless of switch position, that can be done. Isolate the jack, ground tap “1”, and adjust the NFB resistor value up to take into account the higher voltage it will see.

(Needs a diagram, but out of time.)

EDIT: After further consideration, I realized you don’t need to utilize tap “1” or make any NFB resistor adjustments if you wire a DP3T switch like this. The jack must be isolated though.

1642634394486.jpeg
 
Last edited:

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
609
Location
Orlando
i can take any one of those taps to ground, and they ALL share the same reference, no matter which taps i use. so Tap 5, which is readily available, to ground, (at the Screen filter cap, according to Merlin), and keep the jacks floating off the chassis. if im using NFB, then ground the same Jack im getting my signal from.
easy peasy
thanks @andrewRneumann
 

Esquier

Tele-Meister
Joined
Feb 14, 2020
Posts
116
Age
65
Location
Vernon. Canada
I bought a new SuperChamp back in the '80's and it had the speaker leads sprouting out of a white grommet. I wondered how come it didn't seem to need to be grounded. There is NFB. I've since changed the OT and added a shorting Switchcraft jack.
 

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,001
Location
Salt Lake City
If you wanted to select impedance with a switch and have constant NFB regardless of switch position, that can be done. Isolate the jack, ground tap “1”, and adjust the NFB resistor value up to take into account the higher voltage it will see.

(Needs a diagram, but out of time.)

EDIT: After further consideration, I realized you don’t need to utilize tap “1” or make any NFB resistor adjustments if you wire a DP3T switch like this. The jack must be isolated though.
I suspect I'm just being dense, but if you grounded tap 1, would it be different from Rob's 5E3 NFB with an impedance switch? If so, is that cuz the 5E3 OT there has a dedicated 'GND/COM' wire at one end of the secondary winding?

1642655203968.jpeg
 

chas.wahl

Tele-Meister
Joined
Dec 8, 2012
Posts
350
Location
NYC
I think that the difference between the Hammond 125E (subject of this thread) and an OT like their 1760E (Tweed Deluxe-appropriate "normal" multi-tap) is that the latter has one output wire (black) common to all three different output impedances you might want to use. The 125E doesn't have that simplicity. Unfortunately, the typical 4, 8 and 16 ohm combinations don't all share a common wire.

With any multi-tap-secondary OT, if NFB causes squeal or other weird noise (because it's positive feedback), then I reckon this has to be solved from the primary side, not the secondary.
 
Last edited:

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,001
Location
Salt Lake City
Aha, I suspected that little black wire was what I was missing. Thanks. And it’s not like isolating a speaker jack is a huge deal; in fact, from grounding theory, it’s best practice. I gotta study up more on windings and taps; as I say, I’ve been an 8-ohm monogamist.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
I bought a new SuperChamp back in the '80's and it had the speaker leads sprouting out of a white grommet. I wondered how come it didn't seem to need to be grounded. There is NFB. I've since changed the OT and added a shorting Switchcraft jack.

Possible the OT housing itself served as the ground.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
Doing some more thinking on this subject.

What I don’t like about utilizing isolated jack(s) and having the ground on different tap than the sleeve of the jack, is that it means signal voltage will be present on the sleeve of the output jack and everything else it touches. In a 10W amp, this is a pretty low voltage, but it’s worth keeping in mind. It also means it’s best to ground the tap that is closest to the taps in use. That would mean grounding tap “1” is probably not a good idea unless you are referencing an impedance from tap “1”.

My personal route is to never have any voltage, high or low, where someone can touch it. I like everything touchable grounded at 0V.
 

Jewellworks

Tele-Holic
Joined
Mar 2, 2020
Posts
609
Location
Orlando
i havent dont it yet, but ill probably ground tap 5, which is on the 4 and 8 ohm jacks. theyre the 2 i will most likely use the most. i have a 16ohm speaker, but it lives in another combo amp, and it takes a bit of fiddling around to plug into it. i only added the 16 0hm jack because i had the option. and with me, more is more.
 

andrewRneumann

Tele-Afflicted
Joined
Mar 22, 2020
Posts
1,587
Location
Cincinnati, OH, USA
i havent dont it yet, but ill probably ground tap 5, which is on the 4 and 8 ohm jacks. theyre the 2 i will most likely use the most. i have a 16ohm speaker, but it lives in another combo amp, and it takes a bit of fiddling around to plug into it. i only added the 16 0hm jack because i had the option. and with me, more is more.

Ok, then tap 5 gets grounded and wired to the sleeve of the 4/8 ohm jack(s). The sleeve of the 16 ohm (tap 6) will still have signal on it even when nothing is plugged into it.
 




Top