Input impedance of Phase linear 400

[BigE]

I have the PL14 boards, with 39K input impedance. I want to go to 60K or more.

Is it sufficient to change the resistor across the input to the amp to change input impedance?

thanks!

[djoffe]

I found a schematic online of PL400. From the look of this one, Zin would be about 12700 Ohms. Does yours have much higher input resistors? If so, was it a mod, or just a different vintage?

OK...here's a hint, the website:
Phase Linear PL-400 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
said:

Input impedance: PL400C-47 kOhms, PL14A-10 kOhms, PL14B-39 kOhms

I'm thinking that I may not have all the details, since there's no input blocking cap, but maybe this was just an amp for the brave.

Anyway, if you change R3, the only issue will be a bit of offset voltage at the output. If this schematic is correct, the diff pair transistors each have input bias current of around 75/200000/2/150=1.25 micro-Amps. If the input were AC coupled, then the output offset due to input bias current would be about (R3-22000)*1.25e-6.

If you choose R3=100K, the speaker offset voltage would change from its present value (assumed due largely to offset voltage) by 97.5 mV. That's probably tolerable.

[djk]

Bootstrap the input impedance.

http://passdiy.com/pdf/a40.pdf

[BigE]

Mine is the 39K version.

how can I post the schematic? I have no web-site for it....

In my amp, R1 = 1k, R2 = 39K and C22 = 220pF

[BigE]

Originally Posted by djk

Bootstrap the input impedance.

http://passdiy.com/pdf/a40.pdf

I don't have the level of sophistication to modify the driver board and circuit.

Also, the text shows a bootstrap circuit of 40K, while the PL has 39K.

[djk]

http://www.dvq.com/hifi/images/phase400.pdf

Change R2 to whatever you want, adjust R8 for minimum DC off-set.

[djoffe]

Does anyone have an idea of how many rather different schematics were used for the Phase Linear 400? The one that djk just posted (thanks!) is very different than the one I found and posted...How many other variations are out there?

[BigE]

Originally Posted by djk

http://www.dvq.com/hifi/images/phase400.pdf

Change R2 to whatever you want, adjust R8 for minimum DC off-set.

My input board is the PL-14, not the one in that schematic.

The PL 14 is far different than that... it uses a second differential pair; there is no 100K pot like R8 on the board.

I'm thinking that the input resistor (R1 = 10K) that is in the signal path to the first differential pair Q1 and Q2, and the resistor that sets input impedance (R3 = 39K) form a voltage divider.

So if I want to increase the impedance of R3 by 1/2, then I should increase R1 by 1/2 as well. So with R3 at 60K means, R1 should be about 15K.

I'm assuming that 1/4 watt resistors ought to be good enough.

[BigE]

Originally Posted by djoffe

Does anyone have an idea of how many rather different schematics were used for the Phase Linear 400? The one that djk just posted (thanks!) is very different than the one I found and posted...How many other variations are out there?

There are three in the Service Manual found at www.hifiengine.com

[djoffe]

thanks for the hifiengine link...I checked the schematics there in the service manual that seems like yours...my earlier calculations should be ok...it's close enough...if you make R1 go from 10K to 15K and R3 go from 39K to 60K, everything should still work fine. As you suspected, 1/4 Watt resistors are plenty big enough.

Good luck...and be careful...