Nakamichi PA5 mk1 update

[Lars Nielsen]

Hi all,

I have just got my hands on one of these nice old power amps.. I am considering to make some updates. The schematic is attached, so i will refer to reference names. My goal is to remove all unecessery caps..

1. What will happen if C102 is shorted? The lower bandwidth limit will be removed from 7hz to DC. That would be nice for the overall performance, but will the output still be stable arround 0V on the output? There is no DC offset adjustment in the PA5, but I see that the PA7 have this, but the circuits are the same..?

2. Remove C101 and R101. No need for bandwidht limits in my world.. The R102 might be changed to 10k for better S/N reduction.

Please submit your comments and inputs..

Regards,
Lars

[jam]

1) DC offset will be a problem.

2) Expose yourself to RFproblems.

A fet differential might go someway to solving above issues.

Jam

[goody75]

I'm not 100% on this but I think the DC offset adjustment is on the PA7 MK2. The PA7 MK1 has an output ac coupling cap.

[goody75]

I'm not 100% on this but I think the DC offset adjustment is on the PA7 MK2. The PA7 MK1 has an output ac coupling cap.

[Lars Nielsen]

Originally Posted by jam

1) DC offset will be a problem.

2) Expose yourself to RFproblems.

A fet differential might go someway to solving above issues.

Jam

I am thinking out of the box now..

What about mounting a 500k trimmer, across R114, R115 and the middle to R109. I can then adjust the DC offset to zero mV at this point. Then short the C102 and re-adjust according to the output.

Alternative i could replace C102 with a new better one, maybe a little bigger too.. What about M-cap, any other suggestions?

[Lars Nielsen]

Originally Posted by goody75

I'm not 100% on this but I think the DC offset adjustment is on the PA7 MK2. The PA7 MK1 has an output ac coupling cap.

There is no output capaciter in these designs.. But I am wondering why the PA7 has offset adjustment, and the PA5 does not, the designs are the same...

[Lars Nielsen]

Now my Nakamichi PA5 is updated..

I have shorted the C102, so the amp is now DC coupled. There came about 200mV DC offset at the output, so i installed a DC offfset trim pot, the PCB is already prepared for this.

I have also removed the input filter, for better bandwidth response.

The sound is now at a another level, the amp is still not perfect, but now it is higih end!

Regards Lars