Amp/Preamp Asylum

Phase Linear 300 redesign

47.24.177.99

Tiberius

A

I recently completed a redesign of the Phase Linear 300 amplifier. I designed a new driver circuit that used the existing circuit as a starting point. I kept the common base amplifiers and the op amp input circuit, I changed the compensation and bias circuits and finally changed the outputs from bipolars to lateral MOSFETs. I retained the power transformer and heat sinks and chassis from the original amplifier.

I laid out the pc boards with kiCAD and they are 4 layer boards with 2 signal layers, a power plane layer and return plane layer. The original power transformer output voltage sags a bit under full load which limits the output power to about 125 watts at 8 ohms. The circuitry is capable of over 200 watts.

Follow Ups:

RE: Phase Linear 300 redesign - Tiberius 08:56:25 10/05/24 (0)
After almost 2 months of use I'm still happy with the redesign. I did switch to a higher voltage transformer for some testing and I can get 200 watts RMS out now.

I've tried a few different op amps and I've had the best results with the older OPA627 and the somewhat newer ADA4627. I had to mount the latter on a SOIC to DIP adapter board. When the lateral MOSFETs warm up there must be a slight parameter change and I was getting a lot of overshoot with the TL071 and LM356 op amps. So much in fact that it was very audible and I could see it in the square response when I went back and measured. That does not happen with the OPA627 or the ADA4627.
RE: Phase Linear 300 redesign - Mike Mount 16:02:58 09/10/24 (2)
I have thought about this post, your effort and result, since reading / seeing it.
BRAVO MAN! I think it is beyond awesome to see (and here) legacy products
*restored* - rebuilt way beyond what was possible in the day.

THAT is truly beautiful. Great Job!!! really nice layout and end result.

I saw too many of those amps come through the front doors / in our shop,
to ever think of owning or even rebuilding one... A most right(eous) thing!
I love it.
- RE: Phase Linear 300 redesign - Tiberius 17:34:05 09/10/24 (1)
  Thanks for the kind words.
  - RE: Phase Linear 300 redesign - farfetched 05:10:46 09/11/24 (0)
    I was thinking the same thing. You know how to hobby, Tiberius.
    
    enjoy the lovely tunes. I listened to RCA's Rosa Ponselle cd last night. Mono and early electric recordings of course, but you halve the volume (it's mono!) and those recording still tell you so much about the quality of old voices. It's amiracle.
    
    .
    / optimally proportioned triangles are our friends
show us a pic when you can, my brother had a beautiful Phase Linear 700 /nt - Story 15:22:30 08/14/24 (0)
RE: Phase Linear 300 redesign - hawkmoon 03:32:55 08/07/24 (0)
Now you need to update a classic 400! Nice stuff
Nice work! Looks clean. - Ralph 14:44:04 08/06/24 (3)
Have you played it?

Have you measured anything on it yet? I'd be interested in how the distortion vs frequency curve looks.
- RE: Nice work! Looks clean. - Tiberius 15:25:34 08/06/24 (2)
  I have it hooked up currently and have been using it for a couple weeks now. I'm very happy with how it sounds. It has a lot of power reserve as you'd expect.
  
  The distortion analyzer I currently have measures down to 0.03%. The amplifier measures 0.03% at low power and at full power from 20Hz to 20 KHz, so the THD is 0.03% or less. I recently purchased an HP339 analyzer but don't have it yet. It can measure lower than 0.03% so I'm curious if it's lower than that or not.
  
  I have an RC filter on the input to roll off the response at 120KHz.
  
  I'm posting the square wave response. The flat part of the trace looks a little thick but that's the oscilloscope, it's not an oscillation.
  
  The only downside is that there is a little oscillation as it powers down. The Phase Linear 300 had a relay on the output and I'm going to use a relay on this amp as well.
  - Good to see those older Phase linears put to use. - Ralph 10:47:15 08/13/24 (1)
    I am curious though- do you get the same distortion measurement at 7KHz?
    
    It might be the amp is lower distortion than your analyzer can see...
    - RE: Good to see those older Phase linears put to use. - Tiberius 12:36:48 08/13/24 (0)
      I retested the amplifier THD with an HP339 analyzer I just purchased last week. At 132 watts into 8 ohms, I measured the THD at 0.005% at 20KHz, 0.004% at 10KHz, 0.0015% at 2KHz and 0.001% at 500 Hz. Phase Linear rated it for 0.009% at 120 watts out and 0.002% at 1KHz, so my modifications meets or exceeds the original ratings. I doubt I could hear a difference between 0.05% and 0.005% anyway, but I am glad my circuit met the original specifications. When I bought this amplifier it was not working so I had no way to do a listening test between the original and my circuit. I'm happy with how it came out.

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