Any Phase Linear Experts out there?

Przem, if you can go with the MJ21195,96 pairs, much more robust , plus they are a dedicated audio output much quieter than the GP 15024,25 pairs.
As Don has said bias is critical when driving difficult loads. 3 ohms is not a deal breaker, but you will have to force cool it. Two things that will affect the heat, bias AND Hi freq oscillations. The oscillations are the silent killer. If you don't have a scope it's extremely hard to detect, you may feel them in the chassis or not, depends. What does your Zoebel network look like??
On the bias transistor, try a 2N5088 in place of the BC. The 5088 has better resolution and range and seems to react a lot faster to heatsink temp changes.
 
The electrolytic on the top of the board, is that C6 ? If so it's usually 100uf at 16 volt. If you replace that with a 470uf at 25 volts the low end response will be greatly improved.
 
Replacing the sockets with a better quality one doesn't hurt either. Thge 10 amp rated black Augut's are excellent subs.


Przem, did you ever read my Phoenix thread on the "snap on" fans?? Good info there.
 
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Two things that will affect the heat, bias AND Hi freq oscillations. The oscillations are the silent killer. If you don't have a scope it's extremely hard to detect, you may feel them in the chassis or not, depends. (1) What does your Zoebel network look like?? (2)
On the bias transistor, try a 2N5088 in place of the BC. The 5088 has better resolution and range and seems to react a lot faster to heatsink temp changes. (3)
Ad.1 Yes, I do have it. What is 'your way' to test them?
Ad.2 It's OK, voltage +/- 15VDC, resistors not specifically hot...
Ad.3 How do you install the 2N5088 on the chassis? BD139 case is easy in mounting...
 
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#3---take an old 3403, punch out the old trannie, spread the P-Strap a wee bit, slip in the 5088 and a dab of epoxy on the top and let dry. Be sure to take a 4-40 machine screw and nut and clamp the P-strap closed while the epoxy cures.


Ad.2 It's OK, voltage +/- 15VDC, resistors not specifically hot... what??

Run a 1khz signal , get a good ,sharp sinewave displayed so you can see any irregularities and walk the freq up in 1KHZ steps up to 80K if you can. If it's oscillating you will be able to see it. Some oscillations are also level dependent. Start a 100mv input and work up to max input (around 1.75 volts).
 
Zoebel network has been modified to ensure it stays connected to the output stage before the speakers' protection module is plugged in. Resistors are installed directly on the chassis, as for PL700 PRO models. That's why I could not go for Don's protection PCB - not enough space...
 

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Przem, if those are wirewounds they are extremely bad for stability. I know, Claire Bros used em but I think they were non-inductive. If yours do not specifically say "non-inductive" assume they are. It also fits the problem profile as things get worse as the power comes up. Take 2 -5 watt 10 ohm metal oxide or metal film in parrallel and see if your heat issues go away. Inductive resistors in the PL Zoebels leads to oscillations...
 
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A temporary way to disable the bias is to solder a jumper from the base of q101 to q105. I've done this before when I had reason to believe the bias pot was intermittent or the transistor was flaky.

Another potetntial problem is the possibility of a broken wire from the driver board. The way they are all laced together, a broken wire at the board may still make connection until there is vibration or possibly even heat expansion.
 
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Another potetntial problem is the possibility of a broken wire from the driver board. The way they are all laced together, a broken wire at the board may still make connection until there is vibration or possibly even heat expansion.
I came across exactly the similar issue both in case of bias transistor and PCB wire. And same amplifier! Difficult to spot...
Good to take a note of your advice. Thank you Don.
 
The left channel of my 700B has some blown outputs. They are FPL909s. What is a good modern day equivalent to replace these with?
 
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