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Author Topic: 4 ohms or 8 ohms  (Read 6415 times)

Ken McDonald

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4 ohms or 8 ohms
« on: September 23, 2011, 04:51:35 pm »

I'm a newbie so hi to everyone, I'm in the process of buying some PA speakers and this point is bothering me. I know that the current from the output stage of an amplifier required for a 4 ohm speaker is greater than that of an 8 ohm speaker. I know that the output therefore would be louder given all other conditions being the same.

If there is no discernable difference in quality between 4 and 8 ohms and the benefit seems to be more volume with absolutely no trade off. Given that this particular amplifier is designed to run with both 4 and 8 ohms impedance then why does anyone ever buy 8 ohm speakers for their PA?

Why do they make so many of them?

Any help much appreciated thank you
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David Parker

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Re: 4 ohms or 8 ohms
« Reply #1 on: September 23, 2011, 06:05:52 pm »

I'm a newbie so hi to everyone, I'm in the process of buying some PA speakers and this point is bothering me. I know that the current from the output stage of an amplifier required for a 4 ohm speaker is greater than that of an 8 ohm speaker. I know that the output therefore would be louder given all other conditions being the same.

If there is no discernable difference in quality between 4 and 8 ohms and the benefit seems to be more volume with absolutely no trade off. Given that this particular amplifier is designed to run with both 4 and 8 ohms impedance then why does anyone ever buy 8 ohm speakers for their PA?

Why do they make so many of them?

Any help much appreciated thank you

8 ohms is pretty much a standard. If you buy two up front for one on each side, then you want to add some, two more gives you a 4 ohm load on each side, which most amps will handle.
« Last Edit: September 23, 2011, 09:29:40 pm by Mac Kerr »
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Ken McDonald

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Re: 4 ohms or 8 ohms
« Reply #2 on: September 23, 2011, 06:21:48 pm »

8 ohms is pretty much a standard. If you buy two up front for one on each side, then you want to add some, two more gives you a 4 ohm load on each side, which most amps will handle.

Why do they make so many of them?



Thank you David, the thing is in a band situation you would normally just have  bass bins and the top speakers either side of the stage. So having extra speakers connected in parallel to create a 4 ohm impedance per channel would be the exception more than the rule. Given a band situation I still have no idea why they all don't use 4 ohm speakers.
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Tim McCulloch

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Re: 4 ohms or 8 ohms
« Reply #3 on: September 23, 2011, 09:42:36 pm »

Thank you David, the thing is in a band situation you would normally just have  bass bins and the top speakers either side of the stage. So having extra speakers connected in parallel to create a 4 ohm impedance per channel would be the exception more than the rule. Given a band situation I still have no idea why they all don't use 4 ohm speakers.
Voltage drives speakers.  What you think of as power is the current delivery capability of the amplifier's power supply.  While related they are not the same thing.
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Randall Hyde

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Re: 4 ohms or 8 ohms
« Reply #4 on: September 24, 2011, 01:09:25 am »

I'm a newbie so hi to everyone, I'm in the process of buying some PA speakers and this point is bothering me. I know that the current from the output stage of an amplifier required for a 4 ohm speaker is greater than that of an 8 ohm speaker. I know that the output therefore would be louder given all other conditions being the same.
All other things being the same, you'd probably expect 3dB more gain. Alas, all things are not generally the same. For example, you can usually expect more distortion at 4 ohms from a given amplifier and speaker cables (damping) than at 8 ohms.

Quote
If there is no discernable difference in quality between 4 and 8 ohms and the benefit seems to be more volume with absolutely no trade off. Given that this particular amplifier is designed to run with both 4 and 8 ohms impedance then why does anyone ever buy 8 ohm speakers for their PA?

Why do they make so many of them?

Any help much appreciated thank you
First of all, many PA speakers are 4 ohms. I have JBL SRX 725 cabinets, SRX 728s cabinets, Mpro 225 cabs, and Mpro 418 cabs that are all 4 ohms.

As you can imagine, cranking more power out of the same amp stresses the amp a bit more (i.e., produces more heat, which is bad). As a general rule, for example, I generally run my amps at about twice the minimum impedance they are rated for; so if an amp will handle a 4-ohm load, I usually run it with 8 ohms.  If it can handle a 2-ohm load, I usually limit it to 4 ohms. I've never had a problem with amplifiers running this configuration. OTOH, I have had some low-end amps (Crown XLS 602) overheat and shut off when running them at 4 ohms (their minimum impedance).  Some might think I'm foolish to waste the power, but again, I'd point out that I've never had a problem with amps except when running them at their minimum impedance.

Now as to why you would buy 8 ohm cabinets rather than 4 ohms, well, you can always stick *two* 8-ohm cabs on an amp that will handle 4 ohms (assuming you're wiring them in parallel, which is the standard way to do it). This lets you hook up twice as many speakers (or use half the number of amps), all other things being equal. When you learn about power compression, you'll find out that it's better to run two speakers at half power than one speaker at full power. Once the voice coils heat up (which they will if you start shoveling full power into them), their efficiency decreases dramatically.

If loudest volume is your concern, I'd be far more interested in the sensitivity of the speakers than their impedance. If one speaker has a 96 dB SPL (1w/1m) rating and another has a 99 dB SPL (1w/1m) rating, then it will take *twice* the power for the first speaker to reach the same volume level as the second (i.e., 2w/1m).

Of course, ultimately you probably care a lot more about how the speaker sounds. But generally, highly sensitive (efficient) speakers are better quality than less sensitive speakers (in the pro audio realm).
Cheers,
Randy Hyde
« Last Edit: September 24, 2011, 01:11:56 am by Randall Hyde »
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Ken McDonald

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Re: 4 ohms or 8 ohms
« Reply #5 on: September 24, 2011, 05:32:08 am »

All other things being the same, you'd probably expect 3dB more gain. Alas, all things are not generally the same. For example, you can usually expect more distortion at 4 ohms from a given amplifier and speaker cables (damping) than at 8 ohms.

Thank you for your responses everyone.

Please don't think I am being obtuse here or deliberately awkward, as I am genuinely trying to get to the bottom of this particular area.

I would have thought that the impedance of the cables would be the same irrespective of speaker impedance. Obviously the cable would be need to be capable of delivering the extra current required by a 4 ohm speaker. But if you had a heavy duty cable to cover maximum current than that should be OK.
The distortion discrimination thing between 4 and 8 ohms is one of my problems. You see I don't know anyone who can tell me what impedance a speaker is by listening to it. No one I know can listen to a band and, without physically checking to see what speakers they are using, and then say "Oh Yes they are using 8 ohm speakers.



As you can imagine, cranking more power out of the same amp stresses the amp a bit more (i.e., produces more heat, which is bad). As a general rule, for example, I generally run my amps at about twice the minimum impedance they are rated for; so if an amp will handle a 4-ohm load, I usually run it with 8 ohms.  If it can handle a 2-ohm load, I usually limit it to 4 ohms. I've never had a problem with amplifiers running this configuration. OTOH, I have had some low-end amps (Crown XLS 602) overheat and shut off when running them at 4 ohms (their minimum impedance).  Some might think I'm foolish to waste the power, but again, I'd point out that I've never had a problem with amps except when running them at their minimum impedance.

I was suggesting using an amplifier running within it's design spec. These class D amps apparently don't get very hot ( I don't know) but I can see your point with regards to running your amps so they don't work so hard and therefore live longer due to that. So if your amp works with a 2 ohm load then using 4 ohm speakers  would still be louder than an 8 ohm speaker. But even so they wouldn't design 8 ohm speakers just so you can run a 4 ohm rated amp that it doesn't have to work hard.

When you learn about power compression, you'll find out that it's better to run two speakers at half power than one speaker at full power. Once the voice coils heat up (which they will if you start shoveling full power into them), their efficiency decreases dramatically.

Given what you've just said then using a 4 ohm speaker with an amplifier at half volume would be better than using an 8ohm speaker with the same amp at full volume, to maintain the same level of power, is that right? So would that be another reason for buying a 4 ohm instead of an 8 ohm.


If loudest volume is your concern, I'd be far more interested in the sensitivity of the speakers than their impedance. If one speaker has a 96 dB SPL (1w/1m) rating and another has a 99 dB SPL (1w/1m) rating, then it will take *twice* the power for the first speaker to reach the same volume level as the second (i.e., 2w/1m).

Of course, ultimately you probably care a lot more about how the speaker sounds. But generally, highly sensitive (efficient) speakers are better quality than less sensitive speakers (in the pro audio realm).

The sensitivity is an area that baffles me too to be perfectly honest. I have been looking at some celestion SR1 speakers with spl 99dB rated at 4 ohms and then I noticed an 8 ohm SR1 also with a spl 99dB I would have thought the sensitivity ie being the amount of volume would have been greater on the 4 ohm speaker.

I need to get a handle on sensitivity because as far as I can tell dB is logarithmic multiplier but what is the starting figure that it is multiplying from? If you knew you started off with  a signal that had an amplitude of lets say 1 v peak to peak then you could use your dB figure to calculate it's final amplitude coming from an amplifier. The sensitivity of a speaker is spl measured in decibels too but 99dB is obviously a lot quieter than 120dB so given the same amp delivering the same settings to identical impedance speakers but one being 99dB and the other being 120dB would the 120 dB speaker be a lot louder? So if that is the case then a high sensitivy rating throughout the range of speakers no matter what impedance or power rating will always be better. So What is the professional desired spl figure?

As I said I'm not trying to be funny or awkward these are the points that if we were in a pub I would be asking as and when they were being mentioned. It just seems by reading it all back it looks like I'm trying to be smart. I'm not.


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Randall Hyde

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Re: 4 ohms or 8 ohms
« Reply #6 on: September 24, 2011, 07:47:30 am »



I would have thought that the impedance of the cables would be the same irrespective of speaker impedance. Obviously the cable would be need to be capable of delivering the extra current required by a 4 ohm speaker. But if you had a heavy duty cable to cover maximum current than that should be OK.
Try looking up "damping factor" in the articles here on PSW.
As the impedance drops, the resistance of the cable (tiny though it might be) begins to matter quite a bit.
Generally it's manageable at 4 ohms assuming relatively short cable lengths (50' or less) and 12 Ga or better (e.g., 10ga) speaker cable. If you're running 100' speaker cables, I'd be careful about running 4 ohms. If you're down to 2 ohms, I'd worry about anything more than a couple of feet.


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The distortion discrimination thing between 4 and 8 ohms is one of my problems. You see I don't know anyone who can tell me what impedance a speaker is by listening to it. No one I know can listen to a band and, without physically checking to see what speakers they are using, and then say "Oh Yes they are using 8 ohm speakers.
More often than not, if you see two LF drivers, it's 4 ohms. If you see one LF driver, it's likely to be 8 ohms.  Not a perfect rule, but works more often than not (e.g., my Mpro 418 subs are 4 ohms, but only have a single 18" driver; the other 4 ohm speakers I own all have two 15" or 18" LF drivers)


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I was suggesting using an amplifier running within it's design spec. These class D amps apparently don't get very hot ( I don't know) but I can see your point with regards to running your amps so they don't work so hard and therefore live longer due to that. So if your amp works with a 2 ohm load then using 4 ohm speakers  would still be louder than an 8 ohm speaker. But even so they wouldn't design 8 ohm speakers just so you can run a 4 ohm rated amp that it doesn't have to work hard.
8 ohms is a convention that goes *way* back. As I pointed out, the advantage of 8 ohm speakers is that you can stick two of them in parallel on the same amp channel (or four of them if you can go down to 2 ohms). If the amp can handle it (in my case, if it can handle 1 ohm or 2 ohm impedances), then by all means go for it. That's less weight and less space in your rack.

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Given what you've just said then using a 4 ohm speaker with an amplifier at half volume would be better than using an 8ohm speaker with the same amp at full volume, to maintain the same level of power, is that right? So would that be another reason for buying a 4 ohm instead of an 8 ohm.
I certainly wouldn't make that claim. What I would claim is that using two 8-ohm speaker cabinets in parallel is arguably better than running that same amount of power into a 4-ohm cabinet because you're splitting the power across two voice coils. HOWEVER, if your 4-ohm cabinet is something like a JBL SRX 725 (dual 15" LF drivers) then you're already splitting the power across two voice coils, so replacing the 725 by a pair of SRX 715 cabinets (single 15" LF driver) won't really buy you much.


Quote
The sensitivity is an area that baffles me too to be perfectly honest. I have been looking at some celestion SR1 speakers with spl 99dB rated at 4 ohms and then I noticed an 8 ohm SR1 also with a spl 99dB I would have thought the sensitivity ie being the amount of volume would have been greater on the 4 ohm speaker.
Not really, that's a function of speaker design. Speaker sensitivities vary all over the place. I can't tell you why those SR1 speakers would differ.

Quote
I need to get a handle on sensitivity because as far as I can tell dB is logarithmic multiplier but what is the starting figure that it is multiplying from? If you knew you started off with  a signal that had an amplitude of lets say 1 v peak to peak then you could use your dB figure to calculate it's final amplitude coming from an amplifier. The sensitivity of a speaker is spl measured in decibels too but 99dB is obviously a lot quieter than 120dB so given the same amp delivering the same settings to identical impedance speakers but one being 99dB and the other being 120dB would the 120 dB speaker be a lot louder? So if that is the case then a high sensitivy rating throughout the range of speakers no matter what impedance or power rating will always be better. So What is the professional desired spl figure?
20 dB difference is about four times louder. So, yeah, 120 dB would be a lot louder than 99 dB. 99 dB is a good rock show volume; 120 dB would be painful (and causing hearing damage). Good reason not to sit 1 meter from the speakers!

I run mostly portable setups. In that realm, maximum SPL per pound (and related, maximum sensitivity per pound) is important to me. Efficient (sensitive) speakers are important so that you don't have to carry around dozens of huge amps (not to mention, trying to find power for those huge amps -- I already have to take a 60 KVA genie with me and I'm getting to the point where that might not be enough).

Personally, I get scared as the sensitivity drops to 96 dB or below (e.g., the JBL SRX 712m monitor wedges have a 96 dB rating). I once was looking at some JBL VLA932 cabinets until I noticed they had a 90 dB sensitivity rating. That means that the SRX 725 cabs I have (99 dB SPL @ 1w/1m) produce almost twice the volume at the same power level; another way of looking at it is that it takes *ten times* the power to a VLA932 to achieve the same volume as it does to an SRX 725. When I can put the cash together, I'd like to get some Danley SH46 cabs (106 dB SPL @1w/1m); those puppies require less than 1/4 the power to achieve the same volume as the SRX 725 cabs (albeit with a slightly smaller coverage pattern).

Quote
As I said I'm not trying to be funny or awkward these are the points that if we were in a pub I would be asking as and when they were being mentioned. It just seems by reading it all back it looks like I'm trying to be smart. I'm not.
Don't be silly. We all were there at one point. Soon, you'll be answering these questions for someone else.
Cheers,
Randy Hyde
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Ken McDonald

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Re: 4 ohms or 8 ohms
« Reply #7 on: September 24, 2011, 08:35:20 am »

Try looking up "damping factor" in the articles here on PSW.
As the impedance drops, the resistance of the cable (tiny though it might be) begins to matter quite a bit.
Generally it's manageable at 4 ohms assuming relatively short cable lengths (50' or less) and 12 Ga or better (e.g., 10ga) speaker cable. If you're running 100' speaker cables, I'd be careful about running 4 ohms. If you're down to 2 ohms, I'd worry about anything more than a couple of feet.

More often than not, if you see two LF drivers, it's 4 ohms. If you see one LF driver, it's likely to be 8 ohms.  Not a perfect rule, but works more often than not (e.g., my Mpro 418 subs are 4 ohms, but only have a single 18" driver; the other 4 ohm speakers I own all have two 15" or 18" LF drivers)

8 ohms is a convention that goes *way* back. As I pointed out, the advantage of 8 ohm speakers is that you can stick two of them in parallel on the same amp channel (or four of them if you can go down to 2 ohms). If the amp can handle it (in my case, if it can handle 1 ohm or 2 ohm impedances), then by all means go for it. That's less weight and less space in your rack.
I certainly wouldn't make that claim. What I would claim is that using two 8-ohm speaker cabinets in parallel is arguably better than running that same amount of power into a 4-ohm cabinet because you're splitting the power across two voice coils. HOWEVER, if your 4-ohm cabinet is something like a JBL SRX 725 (dual 15" LF drivers) then you're already splitting the power across two voice coils, so replacing the 725 by a pair of SRX 715 cabinets (single 15" LF driver) won't really buy you much.

Not really, that's a function of speaker design. Speaker sensitivities vary all over the place. I can't tell you why those SR1 speakers would differ.
20 dB difference is about four times louder. So, yeah, 120 dB would be a lot louder than 99 dB. 99 dB is a good rock show volume; 120 dB would be painful (and causing hearing damage). Good reason not to sit 1 meter from the speakers!

I run mostly portable setups. In that realm, maximum SPL per pound (and related, maximum sensitivity per pound) is important to me. Efficient (sensitive) speakers are important so that you don't have to carry around dozens of huge amps (not to mention, trying to find power for those huge amps -- I already have to take a 60 KVA genie with me and I'm getting to the point where that might not be enough).

Personally, I get scared as the sensitivity drops to 96 dB or below (e.g., the JBL SRX 712m monitor wedges have a 96 dB rating). I once was looking at some JBL VLA932 cabinets until I noticed they had a 90 dB sensitivity rating. That means that the SRX 725 cabs I have (99 dB SPL @ 1w/1m) produce almost twice the volume at the same power level; another way of looking at it is that it takes *ten times* the power to a VLA932 to achieve the same volume as it does to an SRX 725. When I can put the cash together, I'd like to get some Danley SH46 cabs (106 dB SPL @1w/1m); those puppies require less than 1/4 the power to achieve the same volume as the SRX 725 cabs (albeit with a slightly smaller coverage pattern).
Don't be silly. We all were there at one point. Soon, you'll be answering these questions for someone else.
Cheers,
Randy Hyde

Thanks very much Randy, your advice is very much appreciated.

Here is a link to the amp we've bought for which we are still looking for speakers.

http://www.gear4music.com/PA-DJ-and-Lighting/Behringer-PMP4000-Europower-Mixer/DE2

I think we will need something over spl 100dB. As you mention 120 would cause damage but it will save us from having to turn the amp right up and hopefully the sound level will be hovering around the 100 dB level.

We've got an option of buying a pair of celestion SR1 8 ohm speakers with a controller and a single bass bin. I'm not sure about just having one bass bin, is that normal these days, I know it is with TV surround sound systems. Although these speakers are all used.

I don't know if we should look for a better speakers with higher sensitivity and I'm not sure about the bass bins.

I used to do this stuff all the time years ago but everything seems to have changed now. and I haven't played in a band for many years. So basically I'm an old codger trying to be a rock star again (not that I was before of course) and it is hard trying to make sure we get the right gear in the first place.
Thanks again Randy for your help.
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Nick Hickman

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Re: 4 ohms or 8 ohms
« Reply #8 on: September 24, 2011, 09:29:24 am »

Hi Ken,

I know that the current from the output stage of an amplifier required for a 4 ohm speaker is greater than that of an 8 ohm speaker. I know that the output therefore would be louder given all other conditions being the same.

Ah, but all other conditions won't be the same: how have you gone from an 8 ohm to a 4 ohm driver?  If you just cut the voice-coil in half, the net force produced is unchanged, so cone acceleration and SPL are also unchanged.  There are always trade-offs.

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The distortion discrimination thing between 4 and 8 ohms is one of my problems.

A power amplifier really amplifies voltage, but it has limits to both the maximum output voltage and maximum current it can supply.  If you hang a high-Z load across a typical amplifier, there's no issue with a current limit, but it'll clip when it hits its voltage rails.  If you hang a low-Z load across it, it'll run out of ability to source current long before you reach the voltage rails.  Either way, it clips (distorts).

Many power amps are designed to be pretty happy with a 4 ohm load, have a hard time and compromised performance with a 2 ohm load, and have an easy life with an 8 ohm load.  8 ohm loudspeakers are convenient because you can hang one or two of them per channel.  In an emergency (like losing a channel), you can double things up (down to a 2 ohm load).  (If the amplifier output is bridged, double those impedances.)

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I need to get a handle on sensitivity because as far as I can tell dB is logarithmic multiplier but what is the starting figure that it is multiplying from?

For sensitivity, the decibel quantity has an "SPL" qualifier.  That means "sound pressure level", a measurement of sound pressure with a reference level of 20 microPascals (RMS, unless specified otherwise).

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The sensitivity of a speaker is spl measured in decibels too but 99dB is obviously a lot quieter than 120dB so given the same amp delivering the same settings to identical impedance speakers but one being 99dB and the other being 120dB would the 120 dB speaker be a lot louder?

Of course.  If you do the sums, you find that a 100% efficient omnidirectional radiator away from any boundaries would have a sensitivity of 109dB SPL (1W/1m).  Real measurements of low-frequency drivers are usually half-space rather than full-space, and you achieve higher sensitivities by making the source more directional.

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So What is the professional desired spl figure?

You mean sensitivity for a driver?  For a woofer in half-space, 99dB SPL (1W/1m) is good (i.e. about 5% efficient).  For an HF driver, it depends on the horn (or tube) on which it's measured (more directional = higher sensitivity).  A typical compression driver on a 90x40 degree horn might be 110dB SPL (1W/1m).

Nick
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Randall Hyde

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Re: 4 ohms or 8 ohms
« Reply #9 on: September 24, 2011, 09:18:40 pm »


I think we will need something over spl 100dB. As you mention 120 would cause damage but it will save us from having to turn the amp right up and hopefully the sound level will be hovering around the 100 dB level.
Well, keep in mind that if you want to have 100 dB SPL, being able to do 120 dB will give you 20 dB headroom -- a decent amount. Remember, the higher the headroom (difference between maximum SPL and average SPL that you run), the clearer the sound is going to be. Headroom rules!


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I used to do this stuff all the time years ago but everything seems to have changed now. and I haven't played in a band for many years. So basically I'm an old codger trying to be a rock star again (not that I was before of course) and it is hard trying to make sure we get the right gear in the first place.
Thanks again Randy for your help.
Been there, done that.
I would recommend that you go through all the 400+ (probably 500+ by now) articles here on PSW. You can skip most of the press releases, but there is a gold mine of information to learn here on PSW. I sure learned quite a bit. Just be aware that some of the information found in the articles is, well, wrong. Don't be afraid to ask about something if it doesn't make sense.
cheers,
Randy Hyde
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