Why Common Ground Amps?

kn505

I picked up the 2.3tl last weekend. Since I am anxious to hear how they sound, I hooked up a pair of Musical Design (75 watts per channel in stereo) amps in mono (probably giving me 125 watts) to a Lexicon CP1+ Pre to drive the speakers. I got tiny sounds from the speakers. I had to run up stairs to get the remote for Pre so I can increase the volume. When I returned, there was smoke coming out of one of the amps. I then powered every thing off. I took the fired amp out, set the un-fried amp to stereo and hooked it back up. This amp was also bad because the music was coming out of the right speaker only.
I am poking around and learning that these SDA speakers need common ground amps. I dont know if the Musical Design amps were common ground or not, but I am wondering whether these speakers might fry my amps because the amps were probably not common ground OR there was something previously wrong with the amps (since they have not been used for more than 4 years)?
The speakers are now hooked up to a small receiver and they produce sound. However, I would like to know if the speakers were hurt in any way with my fried amps?
I just bought a Carver TFM 55X and it should be coming soon. Is there anything I should do to avoid similar problems to the new amp?

Maybe it might help the other new owners of the SDA speakers to avoiding similar problems with a SDA 101 course. I would like to know some answers to the questions:

Why SDA speakers require common ground amps?
What is common ground amp?
How one tell if the amp is common ground or not?
What happen to the speakers (or amps) if they are connected to the un-common ground amps?

engtaz

So you don't blow the speakers and the amp. SRS2's + non common ground amps = 8 mid drivers blown & 1 tweeter and 1 amp damaged. Simple math. The channel share the same ground.

heiney9

All you questions have been answered numerous times (100's) on the forum before. Do a search and you'll find all the info you need.

1) if the amp has a floating ground (not sharing the same grounding point) because of the way the dimensional drivers have to be wired to achieve the SDA principle (with the cord attached) a non-common ground amp see's a direct short. You can NOT use bridged stereo or mono amps with SDA's unless you have the special AI-1 interconnect cable. Some mono amps (NOT bridged stereo amps run in mono) can have their negative speaker terminals strapped to create a common ground. ALWAYS CHECK WITH THE AMP MANUFACTURER BEFORE DOING SO.

2) an amp that does not have a "floating" ground and where both the L and R channels have a common ground point.

3) using a VOM you can check continuity between the negative speaker terminals on the amp (unpowered) if it reads Zero or close to Zero it's common ground.

4) damage to either or both.

H9

F1nut

I dont know if the Musical Design amps were common ground or not

Moot point as you were running them bridged, which basically makes them mono blocks and therefore, non-common ground. The AI-1 solves non-common ground issues, but bridged amps, in general, are only rated to drive 8 ohm loads. Your SDA's are 6 ohm nominal and dip lower, meaning no bridged amps.

What happen to the speakers (or amps) if they are connected to the un-common ground amps?

You already found out.

Ron Temple

The Carver is common ground.

kn505

Thanks for the inputs.

The carver tfm 55x worked very well with the speakers after 30 minutes warming up. But there was some problem with the amp during the first 30 minutes. The sound was cut in and out and there were clicking noises when the sound was out. I sent the amp back to the seller and am looking for a new amp.

There is a adcom 5802 that I can get for around $700. There are some discussions about using this amp for SDA speakers. (If I get this amp, I know that I will need to connect the two negative terminals together to make it common ground.)

There is a California Audio Labs CL-2500MCA amp on ebay with a buy-it-now price for $2500 +shipping. It is 5 channels amp with 500 watts/channel. I sure can use the other 3 channels later for home theater purposes. If you have any experience with this amp and would like to share, it is greatly appreciated.

How do I know whether this amp is common ground? Can I just to connect the negative terminals from the two channels together to be in the safe side (or that is a no no for a multi amp)?

Does anyone know the fair market price for this amp?

Thanks in advance.

heiney9

heiney9 wrote: »

All you questions have been answered numerous times (100's) on the forum before. Do a search and you'll find all the info you need.

1) if the amp has a floating ground (not sharing the same grounding point) because of the way the dimensional drivers have to be wired to achieve the SDA principle (with the cord attached) a non-common ground amp see's a direct short. You can NOT use bridged stereo or mono amps with SDA's unless you have the special AI-1 interconnect cable. Some mono amps (NOT bridged stereo amps run in mono) can have their negative speaker terminals strapped to create a common ground. ALWAYS CHECK WITH THE AMP MANUFACTURER BEFORE DOING SO.

2) an amp that does not have a "floating" ground and where both the L and R channels have a common ground point.

3) using a VOM you can check continuity between the negative speaker terminals on the amp (unpowered) if it reads Zero or close to Zero it's common ground.
4) damage to either or both.

H9

I hate quoting my own stuff see #3 above. No it's NOT alright just to tie the neagative terminals together to be "safe". Only do that if the amp in non-common ground and the manufacturer or service tech states it is OK to do so.

YES, you can tie the negative terminals together on the Adcom 5802 and that would be a great choice.........and the price is very good at $700.

H9

concealer404

F1nut wrote: »

Your SDA's are 6 ohm nominal and dip lower, meaning no bridged amps.

Does this include the SRSs? Sorry for the stupid question....

Bobsama

concealer404 wrote: »

Does this include the SRSs? Sorry for the stupid question....

Yes. Any Polks with the interconnect cable NEED common ground. That means all versions of SDA-1's (1, 1A, 1B, 1C), SDA-2's (2, 2A, 2B), CRS's (CRS, CRS+), and SRS's (SRS, SRS 2, 1.2, 1.2TL, 2.3, 2.3TL, 3.1TL). If they have the SDA effect, they NEED common ground (or a work-around, as found around here).

You've already seen what happens because of the interconnect cable. If you are unsure of common ground or not, do NOT use the interconnect cable.

RCFGuitars

heiney9 wrote: »

No it's NOT alright just to tie the neagative terminals together to be "safe". Only do that if the amp in non-common ground and the manufacturer or service tech states it is OK to do so.

I found it interesting that the manufacturer tech who emailed me regarding my Rotel RB1090 confirmed that the amp is common ground, but that the neg. terminals could be straped together and "that may even be a good idea".

heiney9

RCFGuitars wrote: »

I found it interesting that the manufacturer tech who emailed me regarding my Rotel RB1090 confirmed that the amp is common ground, but that the neg. terminals could be straped together and "that may even be a good idea".

If it's a common ground amp by design what advantage would strapping the negative terminals have? When I spoke earlier I guess my point was that not all non-common ground amps can have the negative terminals strapped.

H9

Anyways this subject has been posted about hundreds of times over the past 8 years so there's plenty of info to go around.