PL700B's...White Oak or Pine Box

Peter S

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#42
Trying to be 'PC'---made in China. Next question would be; Is there enough adjustment to move the pointer to the other side of the scale? Most meters are 'right side up'
 

grapplesaw

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#44
So here is the manual I bought in 1979 for my first PL, a 700. It suggests installing 8 amp fuses for "Industrial or PA use" I now see a spec for power into 4 ohms.
About Bridging, I am driving the amplifiers with a miniDSP 4 x10 HD. The DSP functions as pre amp, EQ, and Crossover. It has unbalanced RCA as well as balanced outputs with up to 8 V RMS. Could I connect two series resistors (maybe 1 k ohms each?) across the +and - balanced output, then ground the centre point to the PL input ground. The + side of the resitor string would drive one channel, the - side would drive the other.
Voila! Amplifier is bridged! Am I missing something?
that not how it works. A little more google search Peter is required
 

grapplesaw

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#45
Trying to be 'PC'---made in China. Next question would be; Is there enough adjustment to move the pointer to the other side of the scale? Most meters are 'righside up'
Again that is not how to make it work from the opposite side. The 700b meters are special built. You can custom order one from the meter manufacturers but it’s going to cost you.
 

Peter S

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#46
Stubborn as usual, I had to try the bridging experiment (sketch in entry #33). I used a 'disposable' Sony home theatre receiver rated at 130 Watts per channel. One channel driven delivered 112 Watts into an 8 ohm load. A 1k ohm pot was connected across the Hot and Cold outputs of an Alto 8 channel mixer. The wiper of the pot was connected to ground. The scope was used in dual trace mode, one channel connected to each end of the pot. The pot was adjusted to equalize the amplitude of both channels. The signals were 180 degrees out of phase. The Hot and Cold of the mixer output were connected to the L and R inputs of the receiver....
272 Watts in 8 ohms, connected between the hot terminals of L and R speaker terminals....So the next step is to try this with a Phase Linear. Looks like my original 700 is getting volunteered for the mission. Regarding fuse size and ohm's law, an expected output of 1500 Watts into 8 ohms will draw 13.67 Amps. So even increasing the rail fuses to 8 amps still seem far from sufficient?
 

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grapplesaw

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#48
Bridging a Phase Linear has some things to be considered when building. As noted below. I have address several issues in my dual opamp build. Protective circuit must be addressed as well.

Bridged amplifiers are used widely where high power is required. This is especially so
in the pro-sound arena. A simple bridged amplifier arrangement is illustrated here


1619587611595.png


Two channels of a stereo amplifier are driven out of phase and the loudspeaker is connected across the hot outputs of the two amplifiers. A bridged power amplifier can theoretically produce 4 times the power into a given load compared to its nonbridged counterpart (one channel of the stereo amplifier) using the same rail voltages. This is because the voltage across the loudspeaker is doubled and power goes as the square of voltage. Under these conditions, each of the two amplifiers “sees” an effective load resistance equal to half that of the loudspeaker impedance. For this reason, the bridged amplifier may produce somewhat less than 4 times the power.

Bridged amplifiers have not always enjoyed a reputation for the highest-quality sound. This is partly because they are seeing half the impedance of the loudspeaker and the distortion of power amplifiers is virtually always higher when driving lower impedance loads. Moreover, the peak output current requirements are doubled, and some amplifiers may not be up to the task. Indeed, their protection circuits may be activating. Finally, bridged amplifiers are often abused. Each channel may not be rated to drive a 2-Ω load, but the amplifier may often be asked to drive a 4-Ω load in
bridged mode.

Bridged amplifiers provide only half the damping factor into a given load because there are essentially two amplifier output impedances in series with the load. This can also detract from sound quality
 

MarkWComer

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#49
Has anyone considered fitting an 'off shore' movement into the case of a damaged meter?
I think there’s an issue of any meter meeting a certain specification for current handling, so they’re not fully compatible. I believe the spec was “ohms per volt” that determined how far a meter would deflect compared to the amount of voltage applied or current going through. Even if you mechanically “zero” the pointer, you would get a false reading, either too high or too low.
 

mlucitt

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#51
I'm still confused why one would want a bridged PL700, WOPL'd or not? If your speakers are efficient at all, the 350-500 Watts per channel coming from an unbridged PL700 at full input power of 2 VRMS, will make you leave the room due to the extreme Sound Pressure Level (SPL). I know this from personal experience.

If you are powering a large tower array of professional sound reinforcement speakers outside, then please ignore this post. Otherwise, why?
 

mlucitt

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#53
I had a subwoofer and it added a huge dimension to the music. I used one channel of a PL400 in a tuned enclosure with a 12" driver. It was plenty powerful at half the db setting on the preamp "Sub Out" for a large room. If your sub is not loud enough, there are several reasons:
1. Your enclosure is sabotaging the frequency response of the driver (ported?).
2. Your driver is wired incorrectly, of inferior quality, or mismatched to the amplifier.
3. You are using more low frequency SPL than needed to balance the rest of the musical content (the Earthquake movie effect).
4. Your room, garage, auditorium, etc. is larger than the enclosure can support (you may need 2, 3, or more subs, each with it's own amplifier).
 

Peter S

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#56
Not that I have to prove my insanity, but the new plan is four PL700's. Two WOPL'd 700 series II's will drive the MF and HF line arrays. No question that 400 WPC is overkill for the plannar tweeters but there are 8 plannars at 80 Watts max each, per line array. Besides, I like the PL sound. The plan is to use two more bridged 700B's to drive the LF's. Pictured cabinets are copies of JBL cinema subs but built with 1" MDF and internal cross bracing. The cabinets weigh 171 lbs each. I used RCF drivers (97.5 db/w/m) rather than JBL (99 db) The drivers claim 1500 Watts "AES standard" and 3000 Watts peak power handling....1.5 db less efficient but approx 3 db more power handling, so with enough power, more overall output than the JBL sub. The listening area is approx 1400 sq ft with 17' ceiling.
Rather than subject my still functioning original 700 to the bridging test, I will attempt to revive one of the remaining wrecks from the latest acquisition, only to beat it to death again with some bridging tests.
So if MJ151196's have a maximum Ic of 16 Amps and there are 5 in parallel could the rail fuses be increased beyond 8 amps? I vaguely understand that the 16 collector current would push the transistor beyond it's 'SOA' and continuous operation at the maximum potential bridged output would at the very least, overheat the power transformer, but with temp controlled fans, and attention to current sharring, emitter resistors, transistor matching..
 

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mlucitt

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#57
1.5 db less efficient but approx 3 db more power handling,
Nice speakers. Note that JBL is very conservative in their power handling specifications. I have never seen an equivalent speaker take more power than a JBL.
You do realize that with sensitivity of 97.5 db/w/m, and 1500 Watts of power you are treading into dangerous territory.
For example, if 1 Watt=97.5db at 1 meter and every doubling of power is +3db of loudness, then at 1024 Watts you have 127.5db of loudness. The accepted scale of loudness rates 115db-140db as "Serious Injury". But it's cool, you will probably never reach that point because:
There are three broad categories as to why speakers can only handle a certain power level:
  1. Thermal. Power is hot! Excessive power will heat voice coils to the point where they can expand and bind up in the voice coil gap; heat can deform VC formers, soften adhesives, burn crossover resistors, all kinds of things. Speakers can tolerate some thermal abuse for short periods, but if too much heat builds up for too long, the speaker will suffer a thermal-related failure.
  2. Mechanical. Excessive power input will demand that the speaker’s drivers try to move long distances, perhaps farther than they were designed to. This can cause the driver’s voice coil to “jump the gap,” or move so far that the voice coil comes out of the gap, misaligns, and can’t return to its original position. That’s an excessive power-caused mechanical failure.
  3. Magnetic/Compression. This happens when the power input is so great that the speaker’s magnetic structure saturates and can’t convert the input signal into additional mechanical energy A speaker is a transducer—a device that converts one form of energy (electrical) into another (mechanical/acoustic)— and at this point, it stops “transducing.” At these extreme power levels, the speaker actually puts out a softer signal. This is known as compression, because the speaker compresses its output in response to a greater input signal.
https://www.audioholics.com/room-acoustics/the-decibel-db
 

Peter S

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#58
Thanks Mark, I am familiar with the compression phenomenon in regards to loud speakers and this has reinforced my motivation to build a system with extreme headroom.
"if 1 Watt=97.5db at 1 meter and every doubling of power is +3db of loudness, then at 1024 Watts you have 127.5db of loudness" This seems to be pretty much the result. My Radio Shack SPL meter has indicated levels peaking at 117 db, 4 meters from the speakers. Not quite sure how the inverse square law would translate this to the level at one meter, considering this is an indoor environment and the woofers are in 'quarter space' (on the floor, against the wall) and the line arrays have a non-spherical radiation pattern.
Agreed that these sound levels are crazy---and harmful, I never let facts get in the way of fun! So one of the 'wrecks' has come to life. This amp had the PL20B board inside the amp but disconnected, seems like bad news.
 

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Peter S

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#60
The left channel of this future 'lab rat' has a mixure of XPL909 and TP9054 outputs, The right channel is almost all 2SD555's.....But there it is; Over 430 Watts per channel!
Before attemping to subject this amp to a bridge mode test, what checks could be performed to prevent an early failure? Could current sharring (or transistor matching) be tested by measuring voltage across emitter resistors under load?
Obviously, it would be best to yank all the outputs and install a fresh set of MJ21196's but I'm hesitating to risk them!
 

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