Zach's 700B Thread

[Gepetto]

A note about pre-driver transistors on the PL amps.

Oscillation. All the drivers AND pre-drivers were changed to ON Semi MJ15024 devices by the owner. Random wall to wall oscillation was occurring on the lower half of the right channel signal. As you might expect, the upper half was rock solid but not the bottom half. Reverting to the original faster RCA410 pre-drivers fixed this issue. As a sidebar, the entire right channel had the wrong size transistor mounting screws installed, 24 screws of a larger size putting undue stress on the B and E leads going into the socket. 3 of the imitation sil pads were perforated and one of the MJ15024s had a nasty burr around one of the mounting holes.

Things to check for Zach, these are things you run into quite often.

By the way, the screws used to hold the TO-3 transistors in should have a thread major diameter of 0.136-0.140", not the 0.156" I was measuring on the defective screws.

 

[Zach C.]

Zach
The shielded input wires are tied to themselves and NOTHING else. They are stiff enough to be self supporting. Separate these from AC and DC power wiring. The amp has a Power Supply Rejection Ratio that is very good and it will reject noise coming in on the power supply rails. However it will not reject noise coming in through the input wires as the amps purpose is to amplify these.

The photo was provided in the case you show to illustrate how to wire the 2 AC thermal cutout switches from the top edge of the amp rather than the original single wire AC run right through the center of the amp chassis.
View attachment 7790

I'm with you now. I thought you meant that the bias transistor leads were tied off some manner, since these are prone to breakage.

Couple other items:

Melted C14R??

Douche move on my part- I was trying to get a couple of solder joints prettied up on the front of the replacement WO board and melted the casing. oops. That board is still waiting for the transplant patient to stabilize before it goes in.

DO NOT use MJ15024 or MJ21196 for the predrivers. Unless they are confirmed BAD, use the SJ2741 or RCA410 predrivers that are stock with the amp. Use of MJ15024 in the predriver location on the lower half of the amp output can lead to spurious oscillation (you can hear it too). A local feedback loop is formed between Q7, Q12 and Q14, 16, 18, 20, 22 on the lower half of the output bridge. If any of the SJ2741s is confirmed bad, get a replacement, they are available.

Good to know. Mine seem to be good- I'll stick with those for now.

Thanks!

Zach

 

[Gepetto]

I'm with you now. I thought you meant that the bias transistor leads were tied off some manner, since these are prone to breakage.
Thanks!

Zach

I did mean that Zach
I posted another close up of how to do that.

 

[Zach C.]

I did mean that Zach
I posted another close up of how to do that.

So, both. gotcha!

thanks

Zach

 

[laatsch55]

The 400 and 700 take different power supplies. The 400 takes DC from the rails. The 700 takes AC from the secondary side of the transformer BEFORE rectification. The ground for that 3 hole connector block can go to the star grpound or the middle of the 3 hole terminal srip on the chassis backwall. To emphasize-----the speaker out grounds go to the star ground ONLY, the input RCA's are grounded to



[h=2]Joe's Improved Grounding Scheme[/h]

The following is a copy of Joe's post on the subject.

Here is what I did to achieve this.
1. Removed the white 22AWG chassis ground wire that runs from the copper bus pad between the input RCA jacks to the White Oak PL14_20 PCB, pin 5R.
2. Removed the 16AWG bus wire jumper that Phase normally installs between this same copper bus pad and the right channel speaker ground binding post.
3. I left the 2 signal grounds in the signal twisted pairs that originate at this copper bus pad between the input RCA jacks that run to the White Oak PL14_20 pin 2R and 2L respectively.
4. I added an 18AWG tinned solid copper bus wire jumper between the right and left channel ground (black) speaker binding posts.
5. I added a #10 ring lug with 2 white 20AWG wired into the ring lug on one end and into the White Oak PL14_20 PCB pin 5R and 5L respectively. The #10 ring lug was attached under one of the large bulk capacitor ground bus bar screws that connect the 2 bulk caps together.
6. I changed R2L from 56 ohms to 2.7 ohms.

This makes the large ground bus bar between the 2 bulk caps into the Single Point Ground for the entire amplifier. This wiring scheme eliminates the common mode noise from the large speaker ground from entering and affecting the low level input and low level ground reference on the White Oak PL14_20 PCB (caused by the bus wire jumper that Phase Linear installed between the right speaker negative binding post and the copper pad at the input RCA jacks)

Hope this is clear what I did. I will try to document in a diagram later on​

 

[Zach C.]

The 400 and 700 take different power supplies...

Thanks for the clarification, and the quote. I think I follow that. I'll pick up a resistor, too.

Zach

 

[laatsch55]

Also Zach, let us know if your getting too much information, we have a tendency to do that here. The info may seem conflicting at times so point those out so we can eliminate those, sometimes thjere are two ways to do the same thing that works. I have asked for help on other forums in the past and have been inundated with good intentioned posts but it seemed overwhelming at times, looking back over your thread it looks like it's happening here.

When in doubt follow Joe's advice, he IS THE authority and the rest of us defer to him in PL matters.
BUT the rest of us will have a trick or two up our sleeve occasionally.....

 

[Zach C.]

Also Zach, let us know if your getting too much information, we have a tendency to do that here. The info may seem conflicting at times so point those out so we can eliminate those, sometimes thjere are two ways to do the same thing that works. I have asked for help on other forums in the past and have been inundated with good intentioned posts but it seemed overwhelming at times, looking back over your thread it looks like it's happening here.

When in doubt follow Joe's advice, he IS THE authority and the rest of us defer to him in PL matters.
BUT the rest of us will have a trick or two up our sleeve occasionally.....

I think I'm OK from a TMI standpoint, and I have only seen one place that anything seems directly contradictory in this thread.

The situation with the output and driver T03 recommendations has me a bit thrown, but as you said, I figured I should defer to whatever Joe recommends.

I have three different recommendations here, though, so maybe some clarification would help. If I've got this right it is as follows:
Joe- leave the T03 setup I have unless something is broken
laatch- RCA410 drivers might be better due to higher gain
Don- 2sd555 are potentially prone to oscillation due to high bandwidth, and in at least one case, he was able to solve this only by switching to MJ15024 for both driver and output.

I could have that wrong, though.

The other place I see maybe minor disagreement is with the improved ground schemes between Joe and mlucitt, thought this seems to be a minor difference in application to achieve the same goal. Even these may be the same, just in a different form. I haven't really looked at both while looking at the amp.

Was there another apparent conflict you saw?

Thanks,

Zach

 

[laatsch55]

No, you've hilighted the one that was concernng me. All of us seed to have favorite driver/output combinations. We've tried a lot of different combos and the one YOU DON"T want is the 2SD555's as drivers and MJ21196's or MJ15024's as outputs, those combos WILL OSCILLATE. The 2741's are fine, I have quite a selection of most drivers and outputs, let me know if ya eed some. You CANNOT run the same transistors for the driver and output, nor can you mix types in each channel. The 2SD555 is a great output, don't be bashful using it. The drawback is they are no longer in production and if you blow one the other 9 in that channel become extranneous, unless ya let me know, I have a few.

 

[laatsch55]

The 555's will not oscillate with a 2741 or 410 as a driver. There may be another source for an oscillation, but those combos themselves will not be it. A half shorted mica insulator, a bad emitter resistor, etc, the list goes on. Although not that hard to isolate some oscillations, some others can be hairpullers.

 

[Zach C.]

No, you've hilighted the one that was concernng me. All of us seed to have favorite driver/output combinations. We've tried a lot of different combos and the one YOU DON"T want is the 2SD555's as drivers and MJ21196's or MJ15024's as outputs, those combos WILL OSCILLATE. The 2741's are fine, I have quite a selection of most drivers and outputs, let me know if ya eed some. You CANNOT run the same transistors for the driver and output, nor can you mix types in each channel. The 2SD555 is a great output, don't be bashful using it. The drawback is they are no longer in production and if you blow one the other 9 in that channel become extranneous, unless ya let me know, I have a few.

RIght on. AFAIK, all of my devices are fine, but thanks for the offer. With any luck, I won't have to take you up on it!

But this post does bring up a what appears to bedirect contradiction. From my other post Quoting Don over on AK:
"There's nothing special about the drivers. They're just lower rated versions of the outputs. At the time these amps were in productions I suppose they were cheaper then the outputs and worked perfectly fine. The cost of MJ15024's is so reasonable these days it makes sense to use them as drivers which operates them well within their tollerance."

This seems to say that he used/ uses the same for all. I think?

Zach

 

[laatsch55]

It sounds like it but I believe he was referring to using 15024's to drive 21196's. If you use a faster (from the Ft, or gain bandwidth product perspective) driver than your outputs things can get unstable.

 

[Gepetto]

As said earlier, there is a critical local feedback path set up between Q7 (TO-5), Q12 (Predriver MJ2741 or RCA410) and the even numbered final output drivers Q14, 16 etc. through D13 and R34. Examine the circuit and you will understand what I mean. This local feedback loop makes the entire assembly (Q7, Q12, Q14, etc) act as a PNP emitter follower instead of a PNP/NPN combination.


Q7 is the widest bandwidth, Q12 next widest and Q14 etc as the lowest (slowest). It is important to keep it that way so there is only one dominant pole (MJ15024) in this local feedback path.

 

[Gepetto]

BTW, it is not as critical on the upper half comprised Q10, Q11, Q13, etc as there is no local feedback loop on this half. That triad is just a straight emitter follower.

 

[laatsch55]

Why was it done like that?

 

[mlucitt]

R34 or R35?

 

[Gepetto]

Hi Lee
Need to know what you mean a little more before answering?

Do you mean why did they use the quasi complementary structure on the bottom half?

Guessing what you want to know, PL used all NPN devices because back in the day this amp was created, NPN transistors of this class were all that existed. PNP power transistors were rarer and pricier(still are today). NPN's are easier to fab in silicon than PNPs and are lower cost. PNPs are easier to fab in Germanium (and lower cost) than NPNs.

Did I get at the heart of your question?

 

[Gepetto]

R34 or R35 what Mark?

 

[Gepetto]

Hi Mark
There are 2 local feedback paths for this transistor triad.


R41/C18/D13/R35


AND

R34

Sorry I should have been more specific.

 

[mlucitt]

Joe,
Thanks, I was looking at the original PL700B schematic and R35 seemed like the logical/primary path for local FB. The path through the current limiting circuit and R34 looked like a secondary source to keep the bias of the Q7, phase inverter low until it starts to conduct by virtue of the signal from Q5, the voltage gain amplifier. Is that right?

Mark