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Old 24th May 2015, 06:22 PM   #1
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Default Phase Linear 700b refurb and full complementary conversion

I'm working on a PL700b for a friend. As the title says, its completely original and currently working fine. My friend wants it gone through completely as well as having it converted from the original quasi complementary output stage to a full complementary output. Before getting started I've done a lot of research on this amp. I've found quite a bit of info on what to do in the way of upgrading it, but the conversion info is spotty.

My big concern is that I perform the quasi to full comp conversion properly. I've redrawn the original schematic for the PL20 control pcb to show the planned changes for the conversion. I'm hoping there is someone familiar with these amps that can tell me if I'm on the right track or if I've missed something.

I've attached the original and modified drawings. Any info would be appreciated.
Attached Images
File Type: jpg PL700b_PL20_quasi.jpg (286.5 KB, 456 views)
File Type: jpg PL700b_PL20_Full_Comp_Conversion.jpg (651.3 KB, 451 views)
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Old 24th May 2015, 06:38 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by Chamberman View Post
I'm working on a PL700b for a friend. My big concern is that I perform the quasi to full comp conversion properly.
You should do the usual cap replacements etc. and have him use the amp for a while before even considering such a drastic circuit change.
You could get into big trouble going beyond this.

Last edited by rayma; 24th May 2015 at 06:41 PM.
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Old 24th May 2015, 06:53 PM   #3
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Originally Posted by rayma View Post
You should do the usual cap replacements etc. and have him use the amp for a while before even considering such a drastic circuit change.
You could get into big trouble going beyond this.
I started working on the control pcb already. C6 was toast in both channels and his low freq response was down about -9dB at 20 Hz in the L channel and about -6dB in the R. I planned to do most of the control pcb work, install the wattsabundant output protection board while and then use the amp as a quasi for a bit to see how it does.

I'm also installing a new set of main caps and a new bridge rectifier.

So the full comp conversion was going to be the last stage after I had completed basically all other work and verified there was no oscillation or any other problems present.

The thing is that this amp has the XPL910 outputs in one channel and Fairchild 909's in the other. My friend plans to use this to power his Acoustats so it will be working on a 4 ohm load. I don't expect it to be that reliable without some more modern output transistors in there.
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Old 24th May 2015, 07:01 PM   #4
rayma is online now rayma  United States
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Originally Posted by Chamberman View Post
So the full comp conversion was going to be the last stage after I had completed basically all other work and
verified there was no oscillation or any other problems present.
Good approach. The comp conversion could change the stability enough that you might have real problems.
Just make sure that he understands this, and is willing to pay what it will cost for your time and the parts.
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Old 24th May 2015, 09:05 PM   #5
wg_ski is offline wg_ski  United States
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Replace C11 with a 100 volt low leakage type. Any leakage at at all and you'll get wierdness that can take days or weeks to diagnose because the symptoms won't make any sense.

Or return wire #13 on the board to the output (where it normally connects on most amps) instead of the base of the output transistor bank. This can increase crossover distortion in the normal quasi-comp under-biased class B mode. Converting to full comp mitigates this distortion and allows you to run pretty much any bias in the outputs without oscillations. The full comp conversion actually improves stability margin if you use the recommended 4-7 MHz drivers and outputs. Anything faster than the predrivers (15MHz) will cause oscillations, but they aren't as rugged anyway. Heat sinking is still a limiting factor on quiescent bias, but you can crack them open a bit (a few mA per device) in full comp mode and it improves things a lot over running the outputs in cutoff.
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Old 24th May 2015, 09:25 PM   #6
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wg_ski, that's good info thanks.

I am going to replace Q7/Q10 with some new 2n3440/2n5415 pairs that I have had in my parts drawer for about 15 years. I also plan to add base stopper resistors on them as well as the output transistors.
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Old 24th May 2015, 11:02 PM   #7
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Quote:
I don't expect it to be that reliable without some more modern output transistors in there.
Hummmm .... Mixing old with new = trouble. These old classic amps were
designed and compensated with the old low ft devices as a major factor.

Like the old leach amp . I just started from scratch .... instead of trying to
"update" it.

PS - the old outputs are most likely MORE reliable (with this circuit).

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Old 24th May 2015, 11:51 PM   #8
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Take a look at the Phoenix Audio web site.A good deal of info is available there.Increasing C6 to 470uf and installing back to back diodes on the back of C6 as well as matching the input diff pair will improve stability.RCA 410 drivers with MJ21196 outputs seems to be a common improvement.I tried the full comp output stage and heard little or no improvement but it is your choice.Cheers.
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Old 25th May 2015, 12:30 AM   #9
sregor is offline sregor  United States
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There is always white oak boards. Explore White Oak Audio (if you get in a bind
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Old 25th May 2015, 03:42 AM   #10
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Originally Posted by ostripper View Post
Hummmm .... Mixing old with new = trouble. These old classic amps were
designed and compensated with the old low ft devices as a major factor.
The last official output transistors that the factory recommended in their service bulletins were MJ15024's. Those have the same 4mhz fT as the MJ21196, its just the MJ21196's have a much better SOA. Luckily I will not be breaking new ground in this regard as many others have used the MJ21195/96 in their PL amps with great success.

I welcome the comments though, thanks!
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