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Old 12th October 2005, 02:30 PM   #1
Heiney9 is offline Heiney9  United States
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Question Adcom...common ground amp??

This forum is always so helpful, I have a question. I have a friend who just bought a pair of Polk Audio SDA-SRS 2.3tl's. These are the speakers that have a cable running between them that cancel out interaural cross talk. So a signal from the left speaker is sent to the right and vice versa. With the cable provided the power amp connected can only be a common ground amp. He is going to use an ADCOM GFA-5800 to power them. He is being told that it is a NON-COMMON Ground amp and that he will damage the speakers.

Maybe someone can explain how the Adcom 5800 series is Non-common ground, becasue I thought only true mono-blocks or possible dual-mono (in 1 chassis) were the only Non-common ground designs. What makes the 5800 a non-common ground amplifier?

Thanks

Brock
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Old 13th October 2005, 03:45 AM   #2
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Hi Heiney9,

A stereo amplifier can easily have a non-common ground by having seperate power supplies for the two amplifier channels, so everything LEFT is referenced to the ground for the LEFT channel which appears at the outside ring of the input RCA and the black speaker post. The RIGHT channel will reference to it's power supply and appear at it's in/out terminals likewise. The two MAY share a common transformer but with duplicate secondary windings.

Cheers,
Greg
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Old 13th October 2005, 01:37 PM   #3
clem_o is offline clem_o  Philippines
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But, a source would probably have a common ground, so once such an amp is connected to, say a preamp, Left and Right would still wind up 'commoned' anyway, wouldn't it?

Maybe the Adcom 5800 uses bridged outputs?! I'm really not familiar with the model...

Cheers!
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Old 13th October 2005, 02:04 PM   #4
anatech is offline anatech  Canada
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Hi Heiney9,
The GFA-5800 is not bridged. Each channel is referenced to ground with a 100R resistor. Measure between the two negative speaker terminals and make sure you read about 200 ohms. If not, fix this.

To use these speakers you will need to short the two negative terminals together. Then enjoy the music. You may have some additional back ground noise by defeating the resistive connection between channels but it shouldn't be serious.

-Chris
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Old 13th October 2005, 08:16 PM   #5
K-amps is offline K-amps  United States
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Chris,

I think his issue is more the output ground , not input ground (which as you correctly point out is 100R). Question is, can he tie the 2 grounds of the speaker posts?

I know I have run loads of either ground (channel a and channel b while the hot side in both cases was channel a) on a GFA-5802 (300wpc also single Toroid) with no issues but that was testing. Not sure I can state he will be fine long term. If I were to take a guess, I'd say he will be ok because the SDA's are connected together via 2 huge coils with DCR or 2 ohms each. So in series, they will present a 4 ohm load to tie the grounds which in my opinion will have little or no effect of the GFA's pseudo ground (i.e. 2 secondaries one for each channel). I think he can use the 5800 with the 2.3tl.

What do you think?
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Old 13th October 2005, 10:34 PM   #6
anatech is offline anatech  Canada
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Hi Arif,
No, in this amp, the channel grounds are referenced independently to chassis ground through a 100 ohm resistor each. If the amp is run into the Polk's without shorting the two ground terminals, Brock's friend may burn out the two 100 ohm resistors.

There is but one transformer with independent windings supplying power to each channel. Each amp is treated as if it's a mono block. If you look at the schematic, it shows the main ground for each channel connected to chassis ground through this 100 ohm resistor. Neat idea eh (Canadian)?

I didn't even have to look for the input ground. The inputs are referenced to the channel ground first.

-Chris
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Old 13th October 2005, 11:40 PM   #7
K-amps is offline K-amps  United States
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Would tying the ground output posts still route the grounding via the 100R to chassis or would they then be irrelevant?

I guess I am asking because he will not be shorting (bypassing) each 100 R will he?
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Old 13th October 2005, 11:57 PM   #8
anatech is offline anatech  Canada
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Hi Arif,
No. The chassis will then be 50 ohms away from the circuits instead of 100 ohms. Each amplifier will now have a short ground to ground to the other. This is what we want for these speakers. Better use something greater than 22 GA here.
There is no normal reason for the output currents to flow into the chassis ground, they are returned to the power supplies.

-Chris
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Old 14th October 2005, 12:15 AM   #9
K-amps is offline K-amps  United States
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Gotcha!
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Old 19th October 2005, 02:33 PM   #10
Heiney9 is offline Heiney9  United States
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Thanks very much for the responses. For some reason I'm not getting e-mail notifications when someone replies to a thread I'm subscribed to .

I'm pretty sure I understand now how an amp with secondary windings can actually not have a common ground.

Hi Chris we've corresponded before about a Nak SR-3A problem, as always you are knowledgeable and helpful. Also, K-amps you've been helpful in the past as well. I'm not a tech and though I do understand some electronics theory, I don't know my around the insides of units. It's also been along time since I could read and interpret a schematic.

I have another question I need some input/opinion. It's about Adcom which I know you 2 are familiar with. I've owned a GFA-545 since new in about 86-87. I use it pretty much daily and it's now getiing to be almost 20 years old. It seems in the past the unit ran warm, I could always feel a little heat (nothing excessive) coming from the unit, which is normal. Lately I've noticed that the amp is a bit lean in the sound it puts out and doesn't seem to have the oomph it once had. This is very slight, but since I've owned the sytem for 18 years I can notice subtle differences pretty quickly. The unit doesn't seem to put out any heat at all anymore. I can reach under the chassis and feel the heat sinks and they are almost ice cold. I'm thinking maybe a bias issue?

Is it typical to take these in at this age and have it cheked to make sure everything is in spec? Is it possible it needs to be refreshed and will it make it sound better? This may all be my imagination, but something just seems off with my system and I find it odd that the amp once ran warm and now seems to run very cold.

Any opinions or advice or stories about what could cause this would be appreciated. I heistate to take it to the local repair shop because the guy there has no manners and is not willing to even discuss what could be wrong and possible fixes. You basically drop it off tell him what's wrong and he tells you what it costs and he'll be in touch. I'm sure he can do the work properly, but then again since he's not very social it's hard to get a good read on him.

TIA

Brock
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