Multiple Small Subs - Geddes Approach

[markus76]

Originally posted by gedlee
Correct, there is no data either way, so the best we have is logic and analysis and neither favors you claims.

"You seem to be digging in your heals and grabbing at straws just to "be right". "

Originally posted by gedlee
Your last sentence is not one that I agree with, which is perhaps, the issue. I see no reason to assume that the signals need to be identical. In fact, I have said many times that, in theory, the signals to each sub should be decorrelated, meaning NOT identical.

No signal, no modal problems—I can understand that
In order to decorrelate a signal there needs to be a signal in the first place. For example, if there's a low frequency event in the left channel only, then your setup won't have that signal in the center channel and the right channel. But your optimization is based on the assumption that there IS that signal in the center and right channel.

Now I don't only sound like a broken record, I feel like one. Next time I'll just link one of my last posts randomly.

Best, Markus

[markus76]

Originally posted by gedlee
This isn't always possible and not always effective. Cutting the ground lug is not such a big deal, because the equipment is still grounded, just not through that lug. Its grounded through the signal wire shield. If a short to ground occurs it will conduct the current through the signal wires and then to ground, but this should still blow the fuse before any damage is done.

Earl, it's grossly negligent to recommend cutting the ground lug. It's there to protect you if a device fails. Sooner or later every device fails in one way or another. I wouldn't want to be responsible for someone getting killed by one of my "recommendations".

Markus

[gedlee]

Originally posted by markus76

No signal, no modal problems—I can understand that
In order to decorrelate a signal there needs to be a signal in the first place.

No need to be sarcastic.

Originally posted by markus76


For example, if there's a low frequency event in the left channel only, then your setup won't have that signal in the center channel and the right channel.

As it should be.

Originally posted by markus76

But your optimization is based on the assumption that there IS that signal in the center and right channel.

Best, Markus

No system could do what you are expecting, not the JBL, not anything. I make the assumption that the LFs are mono or nearly mono, which, as I said, everyone assumes since it's correct virtually all of the time. You CAN assume anything that you like about the signal (left only, right only, etc.), but you have to assume something, and no matter what you assume it is always possible that the assumption will be wrong. Assuming what I do is not an error, to assume anything else would be.

[markus76]

Originally posted by gedlee
No system could do what you are expecting, not the JBL, not anything.

Answer:

Apply a highpass by running all speakers as "small" and optimize the AVR's sub out. This will deal with ANY source signal and any cheap AVR will work.

Originally posted by gedlee
I make the assumption that the LFs are mono or nearly mono, which, as I said, everyone assumes since it's correct virtually all of the time. You CAN assume anything that you like about the signal (left only, right only, etc.), but you have to assume something, and no matter what you assume it is always possible that the assumption will be wrong. Assuming what I do is not an error, to assume anything else would be.

So your assumption is right because you assume that everyone else assumes the same, ergo my assumption is wrong. Uhm, I can live with that

Have a great day, Markus

[gedlee]

Originally posted by markus76


Earl, it's grossly negligent to recommend cutting the ground lug. It's there to protect you if a device fails. Sooner or later every device fails in one way or another. I wouldn't want to be responsible for someone getting killed by one of my "recommendations".

Markus

Marcus

Grounding for safety in audio equipment is not necessary and its often not even done. My AVR is not grounded, perfectly legal and reasonable. Grounds are mostly for power equipment (drills, table saws, etc. that take lots of current and have a tendancy to short to the case which is often metal. Audio equipment is very low amperage and the internal circuits are always issolated from the mains by a transformer. The only failure in equipment like this that could be a danger is a short to the case BEFORE the transformer, but this is VERY unlikely. As such its really overkill to use a power line ground. Albeit the equipment should be grounded somewhere just for noise and hum rejection, but this need be and only should be done at one place.

[Sheldon]

Originally posted by gedlee
Grounding for safety in audio equipment is not necessary and its often not even done. My AVR is not grounded, perfectly legal and reasonable. Grounds are mostly for power equipment (drills, table saws, etc. that take lots of current and have a tendancy to short to the case which is often metal. Audio equipment is very low amperage and the internal circuits are always issolated from the mains by a transformer. The only failure in equipment like this that could be a danger is a short to the case BEFORE the transformer, but this is VERY unlikely. As such its really overkill to use a power line ground. Albeit the equipment should be grounded somewhere just for noise and hum rejection, but this need be and only should be done at one place.

I've had the same issue with the DEQX. I'm using RCA connections. There's nothing wrong with the unit, but I had to work out a "star" grounding routing, as ground loops are easy to create with multiple pieces of gear. The more you have, the more likely it will be an issue. Sometimes connecting all pieces to the same earth circuit, or even the same power strip, does not eliminate all the loops. As someone mentioned, this is less of an issue with pro gear (balanced connections), but that's not usually what we are dealing with.

Earls method of using a single earthing point will work to sort out the hum, but might increase risk, primarily because not all users are knowledgeable enough to understand if there will be a safety issue for that particular set up.

One way around the safety issue is to securely wire all the chassis together with 14 gauge wire and lift all ground lugs with an adaptor, except for the main receiver.

Another way, if you have the knowhow (this is a DIY forum, after all), is to lift the internal chassis/circuit connections with about a 10 Ohm resistor. In parallel with the resistor, add a pair of antiparallel 25A diodes (a 25A bridge will work fine too - tie +/- together, and run the resistor across the AC connections). You might want to add 3nf or so, also in parallel, for RF ground. If your RCA connectors are directly connected to the chassis you will have to lift between the chassis and earth connection. If that helps, but not completely, you can try omitting the resistor and just use the diodes and cap.

Sheldon

[Patrick Bateman]

Originally posted by randybes
I have found these to work exceedingly well for hum

http://www.zzounds.com/item--EBTHUMX

Hum is a huge issue in the car stereo world. I've used a cheap $10 isolation transformer from Radio Shack and it seems to work well.

At first I didn't like the idea, since a cheap transformer will be bandwidth limited. But if it's only used on the sub channels, it seems acceptable.

The use of a transformer on the RCA inputs to the sub would eliminate the hum, wouldn't it?

(just thinking out loud - I'm working right now and can't test it one way or the other.)

[gedlee]

John

Yes, this should work as long as there is no ground path from the receiver to the amps or DCX.

Sheldon

"and lift all ground lugs with an adaptor"

How is this any different than cutting off the ground lug?

[pjpoes]

Originally posted by Sheldon

Another way, if you have the knowhow (this is a DIY forum, after all), is to lift the internal chassis/circuit connections with about a 10 Ohm resistor. In parallel with the resistor, add a pair of antiparallel 25A diodes (a 25A bridge will work fine too - tie +/- together, and run the resistor across the AC connections). You might want to add 3nf or so, also in parallel, for RF ground. If your RCA connectors are directly connected to the chassis you will have to lift between the chassis and earth connection. If that helps, but not completely, you can try omitting the resistor and just use the diodes and cap.

Sheldon

This would be my recommendation as well. I'm in the process of doing my final testing and tweaking of an amplifier I built with a switchmode power supply, and careful grounding is proving to be very crucial. I'm forced to lift the ground on these using just this method, and finding that the RF cap is really a good idea. I think RCA connections should not be star grounded though, I do not find this to reduce hum. Grounding signal leads (both in and out) in a star configuration is good, but it's been my experience that star grounding everything made things worse.

With it being only on the subs, probably hiss will be a non-issue, and really only the hum would be the problem. This is why I was promoting active conversion from balanced to unbalanced. I would suggest doing this all the way to the subs including changing the input to a balanced input and mounting the circuit on the plate amp, so grounds can be properly isolated.

[pjpoes]

Originally posted by gedlee
John

Sheldon

"and lift all ground lugs with an adaptor"

How is this any different than cutting off the ground lug?

I have a small amount of hum in my subwoofers, which have no earth grounds on them. I've been able to reduce the hum in my system overall by lifting the ground via the diode/resistor/cap method, but had to do so at various places. I also had to make sure the RCA inputs on the plate amps werent directly tied to the chassis ground I was lifting.