Nakamichi PA-5 and PA-7 Design Help Requested

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
 

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

:cool:

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
 
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I see Apogee is exploring the whole Stasis thing, too, only in a far more intelligent and informed manner than I am. I would hope to learn as much as I can about these amps, since I, too, have recently developed a Stasis jones, only know zip about circuitry and hands on design, upgrades, etc., but since you chappy's are on it here, I'll just hang out and lurk, ask dumb questions, hopefully get to the point where I can even understand what you are talking about! Also interested in just how much of this is Pass 9000 and how much Nakamichi. I cannot afford Threshold to do any a/b listening tests. I've been pulsing Nelson San in emails and I must say he's been quite patient but I think this is the better path to Wisdom and Understanding, and it will spare the gent my crazy missives.

BP
 
Having worked on a few of both units. Here are my thoughts: The small electolytics on the PCBs are important to change. Some control the protection circuit and can fail. The trimmers get replaced with multi-turn devices. The main caps are hard to find for reasonable cost, but new brackets could be fitted. The relay can fail. Some of the parts like 2SC3333 and 2SA1320 are getting hard to find. I was able to source some from an obsolete part warehouse. Once the bias is set higher like Nelson indicates the sound is quite nice and somewhat compares to an S500 The cap of interest dictates the roll off freq. It can range from the 10uf found here to 470uF found on a older stasis circuit. Other designs use 220uF or 330uF What sounds best? Sometimes even, the design is de-tuned to not sound as good as the bigger flagship units.