Can't hear a difference in SVT4 "bridge cable"

packhowitzer · #1 07-29-2011, 10:51 AM

I have owned a US made Ampeg SVT4 pro for 10 or 11 years now. I bought it the first year that they had the current styling. It was sold to me "new" off the floor of a now out of business big box retailer. They were able to scrounge up the box and manual, but that's it. I bought a couple of speakon equipped speaker cables to go with it a few weeks later and went on my merry way.
I set the amp to bridge mode via the switch on the back of the amp. I hooked up the cables as instructed in the manual. HOWEVER, I never knew that the end of the cable going in to the amp had to be wired +1 Hot, +2 Return. I just used the cables as they were out of the package. I never got the proprietary Ampeg cable as I bought the amp off of the floor.
As far as I can tell, the amp has never suffered any ill effects from my oversight. It's been pushed pretty hard from time to time as I haven't always had PA support at every gig.
I recently saw a thread where someone mentioned a 'special Ampeg bridge cable' and I thought "uh-oh" Through the last few years I became aware that GK used a special set up on their speakon connectors to get their amps to biamp properly. I never had any idea Ampeg did this. I see now that there is a mark on the back of the amp stating the wiring method for bridge mode, i just didn't know this was anything other than standard info.
SOoo...I grabbed a cable, opened the end, and rewired it as listed: +1,+2 (instead of the standard +1,-1). I marked the end that was now "special" and plugged it in. Ready to finally unlock the true potential of this head. BTW, has been used with many different cab configs, but it is currently running through 2 mesa boogie powerhouse 4x10s. I powered her up and started to play and....
honestly I couldn't tell much difference if any. I hooked it up the new "correct" way and played: sounded good. I hooked it up the old "wrong" way and played: sounded the same. I unswitched the mono mode and ran each cab in dual mono 8ohm per side: sounded fine (if not quite as loud).

My questions are does the power from bridging only really reveal itself if the amp is really cranked (i was probably at a healthy 11o'clock gain and 2o'clock master)? Have I been doing damage to the amp with my incorrect cable usage? It doesn't seem like it to me. This amp is quite a workhorse. In fact the only issue ever has been a bad effects loop jack which was an easy fix. Ampeg doesn't seem to make nearly as much noise about their "special cable" requirements as GK does. That's why I never noticed the need until recently. I read my manuals and try to follow instruction for the care and operation of my gear.

Anybody got any thoughts? Comments? Jimmy-advice?

Thanks, everybody!

BassmanPaul · #2 07-29-2011, 11:10 AM

If you are using a standard +1 -1 speakon cable you are only using one channel of the amp. GK uses the standard +1 -1 pair for the woofer and the +2 -2 pair for high range. Thus a four core cabe is required. Your cable should be wired +1 >>> +1 and +2 >>> -1. +1 and +2 are the amp end only and must be clearly marked as such. It won't work the other way around.

RickenBoogie · #3 07-29-2011, 11:17 AM

And, as far as the volume issue is concerned, it may be that the cabs are already at their limit to how far they'll go. When a speaker cab reaches it's limit, any "extra" power it receives is converted to heat, (voice coil), not sound. Just a thought.

packhowitzer · #4 07-29-2011, 11:29 AM

Thanks Paul. So essentially, I was only using one channel of this amp anytime I ran it in bridge mode. And I think I still have some work to do to get the cable up to snuff. I don't fully understand your wiring example (me being green to cables, not your explanation).

RickenBoogie, you bring up a valid point. I understand that I can only get so much sound out of a cab. I don't really need them to be any louder than they are now. i just want to get the most out of my gear, and I want the amp to run in an efficient manner, etc... I waited a long time to get my gear and saved my pennies, so I just want to get as much out of it as possible.

billfitzmaurice · #5 07-29-2011, 11:36 AM

Quote:

Originally Posted by RickenBoogie

And, as far as the volume issue is concerned, it may be that the cabs are already at their limit to how far they'll go. When a speaker cab reaches it's limit, any "extra" power it receives is converted to heat, (voice coil), not sound. Just a thought.

+1. The problem with bridging is that so few understand what it actually does. It's not about watts, its about volts. Bridging doubles the voltage swing, and if your speakers can't make use of double the voltage all you're accomplishing is to greatly increase the likelihood of blown drivers. That's why I recommend that if you have to ask any questions about how to bridge you probably shouldn't be considering doing so to begin with.

packhowitzer · #6 07-29-2011, 01:04 PM

Quote:

Originally Posted by billfitzmaurice

+1. The problem with bridging is that so few understand what it actually does. It's not about watts, its about volts. Bridging doubles the voltage swing, and if your speakers can't make use of double the voltage all you're accomplishing is to greatly increase the likelihood of blown drivers. That's why I recommend that if you have to ask any questions about how to bridge you probably shouldn't be considering doing so to begin with.

Bill that's a very valid point. I try not to go too far above my paygrade. But I want to learn, I'm pretty sure I can be taught, and I know I can follow clear instructions. I am with you about few people understanding it fully. I have a sort of utilitarian understanding of resistance and power as it relates to audio equipment. I know not to mismatch my cabs when it comes to pairing them with my amps. The ampeg manual has a nice diagram instructing the user how to connect the cabs in several different types of operating modes. Why then, must it still be an ordeal for the average joe like me to get the most from the amp? I know some manufacturers include features that will hardly ever get used. In fact I can't see me ever using my svt4's bi-amp feature with cabs I've got. But the mono bridge feature seems common enough (and fundamentally easy enough to grasp) that I thought I would try it. So should I just give up and go plant daisies? Or should I try to dig in an give it my best shot to learn this stuff? What was your experience with all this when it was new to you (if I may ask?) You seem to be one of the most knowledgeable people here. Clearly you didn't shy away from learning these things. I don't imagine I can get the kind of education and experience you have, but surely I can learn to bridge an amp with the proper guidance. Right?

Edit: Plus would ampeg make amps that do things that are going to blow the drivers of their cabs? I know I'm using mesas, but they aren't that different in specs.

packhowitzer · #7 07-29-2011, 01:13 PM

BTW this is the extent of Ampeg's coverage of the topic of bridging the SVT4 pro:

billfitzmaurice · #8 07-29-2011, 02:36 PM

Quote:

Originally Posted by packhowitzer

Plus would ampeg make amps that do things that are going to blow the drivers of their cabs?

They would and do. Time was that there was no such thing as bridging. Then one manufacturer did it and all the others followed suit. Not because it was necessarily a good idea, but for marketing concerns. No manufacturer wants to lose sales because their amp can't be bridged to give the highest possible wattage rating on the product spec sheet. Watts sell amps. Voltage swing? Not so much.
Bridging is properly used when the voltage swing of one power amp channel is insufficient to drive the speaker being used to full displacement limited output, with a few volts left over for good measure. You have no way of knowing what that figure is, because not a single cab manufacturer publishes that information. You'd have to software model your cab/driver combination to find out what that voltage limit is, and that's a skill few players have. And then you'd need to calculate what the voltage swing of the amp is, based on the rated power into the rated load impedance.

Bob Lee (QSC) · #9 07-31-2011, 03:30 AM

Quote:

Originally Posted by RickenBoogie

And, as far as the volume issue is concerned, it may be that the cabs are already at their limit to how far they'll go. When a speaker cab reaches it's limit, any "extra" power it receives is converted to heat, (voice coil), not sound. Just a thought.

Not really. 90+% of the power going into the cab turns into heat no matter what.

Hotter voice coils have higher resistance, so they draw less current from the amp at a given output voltage. So you could push the loudspeakers to a point where you increase the amp output by, say 2 dB and the acoustic output only goes up about 1 dB. That's called power compression.

packhowitzer · #10 07-31-2011, 04:04 PM

So essentially, Bill, with my svt4 and 2 mesa 4x10s (8ohms) would you suggest I run the cabs dual mono or run them in series to drop the resistance to 4 ohms, but only utilize one of the available channels of the head? Obviously from my first post, you can see that I have used this amp for a long time without truly bridging the two internal amps. It has worked adequately. But I also do have a significant number of gigs without the benefit of PA support. I will say that there were times in the past when I thought that the amp could have been louder (since evidently I wasn't TRULY bridging it). But then again another good point was raised that my cabs may be at their limits already so no amount of increase will help- only hurt. Without going into my life story, I'm not in a position to buy or sell gear right now. I have to get by with what I've got. I know I've got some decent gear- I just want to get the most out it. And I want to try to learn as much as I can about audio gear in general and I can extrapolate from your posts, that I can't rely on simply reading a user manual or trusting a manufacturer's information to educate myself.

billfitzmaurice · #11 07-31-2011, 04:10 PM

Quote:

Originally Posted by packhowitzer

So essentially, Bill, with my svt4 and 2 mesa 4x10s (8ohms) would you suggest I run the cabs dual mono or run them in series to drop the resistance to 4 ohms

Dual mono. BTW, series would be 16 ohms, parallel would be 4 ohms. There's almost never any benefit in bridging into a 4 ohm load. It's only with an 8 ohm or higher, usually higher, load where the doubled voltage swing of bridging can be useful.

packhowitzer · #12 07-31-2011, 06:24 PM

Ok Bill. Got it. I will try the dual mono option. I guess I meant parallel not series. What I was describing was svt4 channel A into cab 1 and then out of cab 1 into cab 2. So would you go so far as to say the bridge feature is a waste of parts on an amp like the svt4? Clearly you're not recommending I do it, but let's say I had a much higher level of expertise, then would it be worth it? Should all users avoid this feature on THIS AMP? I can tell you're leading your advise down a path of "an experience user wouldn't try to bridge this amp in the first place". But can you ever see a situation that might call for it? Would Ampeg be doing the bass world a service if they removed this feature (and maybe some other "dubious" features)?