Nakamichi PA-5 and PA-7 Design Help Requested

[Apogee]

Greetings all,

I have recently discovered the Stasis amps that Nakamichi produced under license from Nelson. I have acquired several with the intent of re-capping them and making them the best they can be.

To that end, when reviewing the schematics between the 5 and the 7, there is a 10uf 35v Cerafine in the circuit on the 7's that has been left out of the 5. It appears the front ends are very similar minus the offset adjustment that is included on the PA-7. I have circled the subject cap in red.

What is the purpose of this cap, and can I eliminate it in the 7's?

I've attached both schematics for reference.

Would there be any problem with replacing the 4.7K offset trimmer in the 7's with a 5K unit? I ask because I'm unable to find a multi-turn 4.7 that will fit while still allowing access for adjustment. Also, any problem with replacing the other 150 ohm bias trimmer with a 200 ohm unit? I'm dealing with the same size and access issues on this one as well.

I know very little about circuit design, so I thought that I'd ask.

Any help would be much appreciated!

Thanks in advance,

Steve

PS - I apologize for the blurry scans.

PA-5 schematic on left, PA-7 on right

[Apogee]

Wow, nobody knows?

I realize that I can just remove it and see what happens, but I'd rather not take a chance blowing up a very large amp while experimenting.

Any help or insight would be much appreciated!

Thanks,

Steve

[Zen Mod]

you'll not blow it with omitting that cap from circ , but without cap , Vas output impedance is variable in frequency domain

if you leave it in place , you'll just get better sound than without

trimpot variants you mention are perfectly OK for substitution

nice trick is to measure set value of each trimpot ( after desoldering from pcb ) then setting new one on same value before soldering in pcb

[Apogee]

Thank you Zen Mod.

What I don't understand is how the smaller unit (PA-5) is able to run without it. It was left out by the factory. Wouldn't the smaller amp react the same way and sound better with it installed? I'm not sure it's purpose, that's why I asked.

I'm of the mind that the fewer caps in the signal path, the better. Hence why I'm asking.

Nice tip regarding the trimpots. Hadn't occurred to me but I'll definitely do it.

Thank you,

Steve

[Zen Mod]

if you look at schematics , you'll see difference in drivers used

maybe those not equipped with cap have less capacitance .... dunno - now I'm too lazy to look in datasheets

anyway - that cap isn't directly ( in usual sense ) in signal path - it's role is to minimize nonlinearity of VAS stage output impedance

in any case - simple tests - with and without that cap will give you enough meat for conclusion

there is no danger of instability

[JamesBrennan4]

I have both a PA-5 and a PA-7. To me, they sound the same.

Here's some info from Bob Cordell's book, "Designing Audio Power Amplifiers". Vbe Multiplier, pages 40 and 41.

"The Impedance of the Vbe multiplier rises at high frequencies... For this reason, the Vbe multiplier is often shunted by a capacitor of 0.1 to 10uF. A shunt capacitor of as little as 0.1uF eliminates the increase in impedance at high frequencies."

On my CAS-1, similar circuit, I have tried no capacitor, 0.1uF and 10uF. I did not hear any difference.

However, lower impedance may help curb potential high frequency parasitic oscillations.

[Zen Mod]

conclusion - engineers are just big kids

[Nelson Pass]

Most designers just stick them in - "they can't hurt".

[Apogee]

Originally Posted by Nelson Pass

Most designers just stick them in - "they can't hurt".

It's the "they can't hurt" part that I was concerned about. Without knowing if it was part of the original Stasis design, combined with not finding it in the other amp, is why I questioned it.

I was just trying to figure out if it was there to make the amp as stable as possible across the widest possible audience at perhaps a slight degradation of the sound vs running it a little more "risky" to get that extra margin of performance.

I would assume that Nak would have tended to lean on the ultra conservative side as they likely wanted the most reliable product possible.

As I mentioned, I don't have the design knowledge, so I didn't know how to interpret its purpose but was looking to eek out the best performance possible. Hence why I asked.

Thanks to all for the help.

Best regards,

Steve

[dggs]

I have my PA-7 for almost 20 years. Have always been wondering how easy/difficult/possible it is to convert it to balanced input. Thanks Apogee for posting the input stage schematics. Can I just disconnect C102 and feed negative input to R108(?) (2.21K) ? Any comment would be appreciated!