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scampo77 21st July 2012 05:57 AM

possible to make amps strapable?
this is more of an exercise in thought than anything.

i have 2x JBL BP 1200.1. if i were to change all the power supply and all the outputs with the same transistors, is it theoretically or practically possible to make a jbl bp2400.1 ?

Perry Babin 21st July 2012 07:08 AM

When amps are strapped, each amp is used to drive one of the two speaker terminals. The output of one amp is inverted. Since the output of the BP1200.1 is already in that configuration, you couldn't strap them. The best option would be to use one to drive each coil of a DVC woofer.

scampo77 21st July 2012 12:43 PM

so does that mean with actual strapable amps you are restricted to two of them?

are most/all amps designed this way with one terminal inverted?

Perry Babin 21st July 2012 03:49 PM

You're limited to two amplifiers per voice coil.

All bridgeable amps have an inverted channel. Some non-bridgeable amps (already internally bridged) have two active outputs (one normal and one inverted). Some mono amps have only one active output. If they're designed to be used with another amplifier to drive a single voice coil, they will typically have a switch to make their output inverted.

1moreamp 21st July 2012 04:25 PM

There is BRIDGING and there is STRAPPING or STACKING which are you referring to?

Perry described bridging mode perfectly.

Strapping or stacking is were both speaker terminals of two separate amps are connected together into a load, plus to plus and ground to ground into the load. Certain Rockford class D amps were strap-able and a few other brands as I recall. Strapping or Stacking amps is where two or MORE amps are combined into a load. The Rockford's had sync cables that had to be connected as I recall for the strap/stack to work properly.

Strapping is similar to how DC power supplies get attached together into load configurations so smaller power supplies can be added together to gain a single combined power level. HP Harrison power supplies were strap-able if you used common length and size connection cables between the power supplies to the load, and the sense return circuitry were inter-connected so they tracked one another properly. Cables had to match each other so the load would be shared properly as the connection cables acted like resistors in the connection to the load for equalized current sharing reasons.

Perry Babin 21st July 2012 11:51 PM

I've never seen an amp that had it's speaker terminals connected directly to the same speaker terminals of another amp. I'm not saying that they don't exist but I've never seen one.

The problem with doing this with most amps is that any difference in the output (DC offset or audio levels) will cause excessive current to flow as the feedback circuit tries to compensate.

I've never had a problem directly connecting the output of DC power supplies. They're different from amplifiers because they are single-ended (they can only drive the voltage in one direction -- towards their rated output voltage). If the output voltage of one supply is a bit higher than the others, the one with the higher voltage takes the entire load until the voltage drops to the level of the other supplies. Using wire as current-sharing resistors (like the emitter resistors in an amp) does help spread the load. On the bench, I rarely have matched supplies so I use the voltage adjustment for the various supplies to equal out the load. The Harrison supplies are a bit more advanced than most 12v supplies.

1moreamp 22nd July 2012 05:58 AM

Hi Perry, yeah the old HP lab grade power supplies are bit on the high end side. I used to use them on my bench but the 115 pound weight made them a tough one to handle. I also rebuilt them for several companies in the valley that used them on everything from plating to Ion Milling. I owned five at one time myself just for bench testing and such, but each one on a 115 main would pull a full 15 amps each so each supply had to have its own AC circuit and breaker and that was for 50 amps DC each. Not very efficient old linear design. But rock hard tough and built to take almost anything you could throw at them, and Strap-able/Stack-able into loads as long as there were properly designed cables that connected them together and they were sync'ed together properly.

" Cables had to match each other so the load would be shared properly as the connection cables acted like resistors in the connection to the load for equalized current sharing reasons."

The amps don't get connected together directly at the speaker terminals of the amp, they do get attached together thru cables of exact length and size to the speaker load at the speaker, so the cable acts to allow current sharing between the output stages. There were a few car amps made to do it, but the strap mode has faded out of sight mostly due to more powerful designs being available so no need to strap amps one on top another. It requires a bit of engineering and careful balancing of output voltages and power supplies must be sync'ed together for to make it work reliably but it can be done.
Simple bridging modes like inverted channel bridging and such were always the easiest way to do things, but even those are connecting two sets of outputs together into a load, and they should also have the same output voltages only one channels output is inverted 180 degrees out of phase with the other so the voltage drop across the load doubles. At least on a blackboard it does...
This is why I asked weather or not he meant Bridging or Strapping/Stacking. They are two different approach's to the a similar end. I think you will agree they are very different ways of achieving similar goals of adding power into a load. AC or DC its all possible my friend....:)

AKHeathen 23rd July 2012 12:09 AM

take a look at the old school directed 600/1100d's, they are setup just like the jbl bp1200.1's. there are different methods that you can use, either factory provided, or external. the big factor, is that the output section has to be able to handle running the voltage 2x normal. and, you need to connect unused speaker terminals with heavy wire to complete the 2 separate boards into one output circuit. the rating is much like bridging a 2channel. if the amps are 1ohm stable, then they would be 2ohm stable strapped for the 2400. if you are looking to modify them, then you would have to check over all the components to find any possible issues. another option, would be to modify the power supply controllers to run less rail voltage, then you would change the ratings to half what they are per ohm load in each amp, additionally making the single amp stable to half the original load. then, when strapped, the amps would not exceed the original voltage. it is likely, however, that all that is not too necessary, and there may just be a cap, or a few caps, that would need to be upgraded. much better if you have master/slave and phase control. i have a maxx-link laying around in a box that can sync ans strap a large array of amps. plan on using it for a/b amps, and other setups.

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