SDA's and common ground amps

audioluvr

So I just had a visit from a well regarded audio electronics technician (sold him one of my Carver amps) who was checking out my SDA's and B&K monoblocks. He was curious about my Dreadnought and what that was all about. I tried to explain it to him best I could but he started into talking about electronics crap and immediately lost me but essentially said that my monoblocks are common ground. Not sure if he said because they are hooked up to a common ground pre or because they are all hooked to the same ground via my home circuit.
What he did say could cause issues is if an amp was an inverting amp where one channel is out of phase with the other. After he left I tested this with my ohm meter by unhooking my speaker cables from the back of my speakers and putting my ohm meter on the two grounds. Sure as ship, they are common ground. Thoughts?

Schurkey

Common-ground, "partial common ground", and non-common ground are all possibilities depending on the design of the amplifier pair (two amplifiers for stereo, whether they're in the same or separate cabinets.)

Common-ground amplifiers have essentially "zero" resistance between the negative terminals of the two channels. Some folks claim that using the Dreadnought* improves the sonics even with common-ground amplifiers.

Partial-common-ground amplifiers are common-ground amplifiers that (typically) have a resistor between each negative speaker terminal and chassis ground. From one negative terminal to the other, there's two resistors in series. The value of the resistors varies with the design of the amplifier(s). My amps use ten-ohm resistors, for twenty ohms total between the negative terminals. Twenty ohms is enough to make SDA 1Bs unlistenable. A dreadnought could be used if the SDA speakers are compatible, but for example, the 1Bs are not compatible with the Dreadnought. A jumper wire plugged into the amplifier's negative speaker terminals bypasses the resistors, making the amplifier common-ground. Depending on the amplifier, a jumper wire might even make a pair of monoblock amps "common ground"; this is true for the Adcom GFA-565 monoblocks, but...

...it will NOT be true for any amplifier pair where one channel is inverted (some Carver amps, for example) or for any amplifier that is bridged/balanced (the speaker is driven using two "hot" terminals and no ground terminal.) The only options when using these non-common-ground amplifiers are to unplug the SDA interconnect cable, or to use a Dreadnought. A jumper wire will likely let the smoke out of the amplifier(s) or the speakers, or both.




*Awww, crap. I can never remember if it's spelled "Dreadnaught" or "Dreadnought".

audioluvr wrote: »

...B&K monoblocks... ...I tested this with my ohm meter by unhooking my speaker cables from the back of my speakers and putting my ohm meter on the two grounds. Sure as ship, they are common ground. Thoughts?

So the big question I have for you is HOW did you measure the resistance? If you have two monoblock amplifiers, and they don't have a jumper wire between the negative terminals--and the cases aren't touching--I have NO idea how you can get zero resistance between the amplifiers.

You need to have the + lead connected to one amplifier's negative terminal, and the - lead connected to the other amplifier's negative terminal. Is that how you tested your monoblocks?

I can believe that you could have CONTINUITY between the channels, but anything over one ohm is too damn much. HOW MUCH resistance did you measure?

audioluvr

I measured 2.4 ohm's between the negative terminals on my cables (didn't check between amp terminals). My big question is if someone has actually A/B'd the difference between the Dreadnought and jumper/IC. I could do this but not sure my ears and system are good enough to make the determination.

F1nut

You have to test at the binding posts with the speaker cables disconnected.

audioluvr

Thanks Jesse. I got 2.4 that way too. Must mean my cables are good...

delkal

2.4 ohms might be little high. With a cheap DMM you should be reading about 0.6 ohms (usually the lowest reading you can get shorting the DMM leads). Hold the two leads together on your DMM. Remember that number. The reading you get on the negatives should be within a few tenths.

For a two channel amp both negatives are connected internally. There is should be no resistance. Not sure what the low resistance you measured means or if it will cause you problems but I would definitely do more research.

Schurkey

audioluvr wrote: »

I measured 2.4 ohm's between the negative terminals on my cables (didn't check between amp terminals).

The cables would be some small part of the 2.4 ohms.

I'm still mystified how individual amplifiers can have "only" 2.4 ohms when the apparent ground path goes all the way back to, and including, the amplifier power cords and the wall outlet box.

IF (big IF) those amps are not a bridged/balanced design, you could jumper the two negative terminals with a length of speaker wire--or most any copper wire of 16-gauge or larger--and re-test.

audioluvr wrote: »

My big question is if someone has actually A/B'd the difference between the Dreadnought and jumper/IC. I could do this but not sure my ears and system are good enough to make the determination.

Darqueknight did this, but I don't remember the details. The post was from a long time ago. He preferred the Dreadnought/Dreadnaught.

audioluvr

Not sure what this means but when I short out my leads it reads 3.0. Yes, it's a super cheapo one.

delkal

audioluvr wrote: »

Not sure what this means but when I short out my leads it reads 3.0. Yes, it's a super cheapo one.

It means your DMM is super cheap or there is some oxidation on where the leads plug in. Do you have any deoxit?. Do a little cleaning and see if you can get it down to under 1 ohm when you short the leads. You would think it should read 0 but there is always some internal resistance (usually about 0.4-0.7 ohms) in all DMMs. The better DMMs have a zero feature but even then it can be challenging to accurately measure anything under 1 ohm.

The good news is if the negative terminals on your amp are reading close to when you just short the leads it is a common ground (or at least it is acting like one).

audioluvr

If I touch the tips of the leads together it reads 2.4 so apparently my monoblocks ARE commonly grounded. Interesting.

Schurkey

Unplug one amplifier from the wall socket, retest, and post the results.

delkal

Schurkey wrote: »

Unplug one amplifier from the wall socket, retest, and post the results.

You won't get a reading. There will not be a circuit.

Schurkey

delkal wrote: »

Schurkey wrote: »

Unplug one amplifier from the wall socket, retest, and post the results.

You won't get a reading. There will not be a circuit.

I'm expecting he'll get an "infinite" reading, which will confirm that what's providing the "common" ground is the power cords and the wall socket. I consider that to be far from ideal.

IF (big IF) he confirms that the wall socket ground terminal is what is providing "common" grounding, several inches of wire between the negative terminals will reduce the resistance, and have less "antenna" length that might pick up RFI.

F1nut

I consider that to be far from ideal.

I agree.

audioluvr

So when I unplug one of the amps I get the same reading but when I unplug the XLR I get nothing so my monoblocks are being common grounded by the pre. Next thing I should do is make a jumper that goes between the two negative terminals of the amps then bypass my dreadnought to see if I notice any difference. I may need to recruit my daughter for that though. She has much better hearing than I.

Being snowed it kinda sucks. It's either this or take up crocheting/basket weaving/etc.

PS. I may have to look for a better pre also...

Schurkey

Common ground via XLR and preamp??? Woof. I didn't see that coming.

I still think it's less than optimum.

Yes, I would slap a foot of speaker wire in there, connecting the negative terminals of the two amplifiers together. See what happens.

We're certain that the monoblocks are not bridged/balanced...right?

For the record, it's less a matter of hearing acuity, (I'm not saying hearing acuity is unimportant) and more a matter of paying attention. In other words, an educated ear/brain is more important than a gifted but ignorant ear/brain

audioluvr

Schurkey wrote: »

Common ground via XLR and preamp??? Woof. I didn't see that coming.
Me neither!

We're certain that the monoblocks are not bridged/balanced...right?

Monoblocks are true monoblocks plugged into separate house circuits. When he explained to me that they were commonly grounded via pre I just gave him a blank look and remember him saying something about "if it had two volume controls, two power supplies etc" but hey...

motorstereo

So if your mono's are common ground they should work without issue with just the cable instead of the dreadnaught?

delkal

I agree the monoblocks are acting like common ground even though they have two separate power supplies and inputs.

IIRC XLR connectors have one pin that connects all of the components grounds to the chassis. Same as hooking a wire connecting every case of every component. The case is what is connected to ground and why it was originally used in pro gear. It reduces the chances of shocks if a component does not have a ground plug.

What I still don't get is why ALL stereo amps are not common ground? The 2 channels have to be connected to the same ground somewhere in the circuit.

What am I missing?

pitdogg2

Well some see it as a short when you connect the SDA cable.

Schurkey

delkal wrote: »

What I still don't get is why ALL stereo amps are not common ground? The 2 channels have to be connected to the same ground somewhere in the circuit.

What am I missing?

The SDA circuitry requires that a portion of the left-channel electrical signal is delivered to the right channel speaker, and some of the right channel electrical energy is delivered to the left channel speaker. These electrons have to "find their way home" via the ground circuit, when that's possible the amplifier pair (stereo amplifiers) are common-ground.

1. Some amps have one channel inverted (Early Carver M1.0, all Carver M500 series, for example.) This allows more efficient utilization of power supply energy at low frequencies. It also means that the "red and black" terminals of one channel are reversed, so that the speakers aren't out-of-phase even though the amplifiers are. The result is that only the negative terminal for one channel is tied to chassis ground. (or...NEITHER negative terminal is tied to chassis ground. I need to review the schematic.)

2. Bridged/balanced amplifiers don't use a "negative" terminal that's tied to chassis ground. Each channel is, in effect, paired amplifiers with one channel inverted, and then the speakers are driven by pairs of powered terminals in reverse phase to each other. When the red terminal is at 20V +, the black terminal is at 20V -, so the speaker is driven by a 40V signal. The speaker power has no reference to "ground". Neither "black" (or "red") terminal is tied to chassis ground.

3. Some amplifiers use resistors between the negative terminals and chassis ground. I think the purpose is to reduce "noise" in the speaker-drive circuitry. They're common-ground except for the resistance deliberately inserted into the circuit. The SDA signal has to flow through those resistors unless they're bypassed. That resistance plays hell with the amp and the speakers, the SDA current could overheat the resistors. Amplifier or speaker failure could result--but until outright failure, the speakers merely sound terrible.