Nakamichi PA-5 upgrades
I'm looking for some input on upgrading a Nakamichi PA-5 power amplifier.
This subject was discussed a few times before; however, there is no thread with the complete list describing PA-5 mods in enough details and discussion on them. I tried to make one and would like to get the community opinion on it. Few mods like recap are simple; however, some others were just mentioned in different threads without specific as to how they were made. That’s where help is really needed.
So here is the list:
-Replace electrolytic capacitors in L and R amp PCBs with Nichicon Audio grade KW or KZ and limiter PCB with Nichicon PW;
-Replace trimmers with multi-turn Bourns;
-Replace two 47000 mkF/80V filtering electrolytes with new 47-52 K mkF 80 V or 100V. Here I don’t know where to source replacements; Mouser and Digikey have a few but quite pricey; eBay has some less expensive but could it be trusted?
-Replace a Toshiba 15G4B41 diode bridge with HEXFRDs or with something faster and low-noise. What replacement(s) could be recommended?
- Short the C102/C202 10 mkF/200V electrolytic capacitor at the input to make the PA-5 DC coupled, remove C101/201 and R101/201, and install a 500 K DC offset pot across R114, R115 and the middle to R109 as suggested by Lars Nielsen here: http://www.diyaudio.com/forums/const...k1-update.html
Alternatively, upgrade the C102/202 to 470 mkF to improve low frequency roll off from ~50hZ to ~2 Hz, as discussed here in the post#4: http://www.diyaudio.com/forums/pass-labs/163741-improve-sound-nakamichi-pa-5-a.html
-Do modifications mentioned by the member Stephensank in the recent thread here: http://www.diyaudio.com/forums/digit...ge-switch.html
Unfortunately, he was not very specific about these mods: “A Nak PA-5(w/LF corner freq 10uf film enlarged by addition of at least 150uf/50v Nichicon lytic to remove subsonic BW limit) would be a huge improvement” and “first PA-5 & PA-7 had the subsonic cutoff & oversensitive protection mute circuits (easy 2 resistor changes to cure that bit)”.
- Add cascode modifications to PA-5. Here I am completely lost. What are they and would they improve sound?
-Replace internal wiring. Jon Soderberg offers this for PA-5/PA-7 but he did not provide any specifics. Most likely he uses heavy gauge wiring. Anyone tried this?
-Replace input RCA jacks and output speaker terminals. For me, both seems to be of sufficient quality.
Any other ideas? Let’s post them here as well as discussions.
I have a service manual with PA-5 schematics but can’t attach it here since it’s too big. Hifiendgine has it or I can e-mail it to you if needed.
Thank you for your help.
If it's older than 15 years replace the power supply caps with ones
comparable to the originals. Panasonic or Nichicon is plenty good enough.
Arrange to get the amplifier some serious ventilation and then raise the
bias until the heat sinks run at 50 deg C after an hour of operation.
Serious ventilation is what you think it is - a fan is not out of the question.
Many thanks for your advice, Mr. Pass. I planned power supply caps change but didn't think about setting bias this way.
Actually, your bias guidelines mean that the best working temperature of the PA-5 amplifier is 50 C. Mine NEVER was that hot meaning it is (seriously) underbiased. Nakamichi in the PA-5 SM suggests turn the amp on, wait 20 minutes and then adjust bias to 40 mV.
Why Nakamichi did this?
Did most of the mods listed above: changed small caps and large filtering caps, replaced with multiturn Bournes bias pots and installed DC offset ones, changed diode bridge to IXYS and implemented mods suggested by Stephensank (thank you Stephen for sending me the detailed info). The amp works, delivering 150W in 8 Ohm loads before clipping.
However, I got a couple of potential issues:
First, bias in one the channels didn't go lower than 59-60 mV while in the other one it could be freely adjusted in the 30-80 mV range. Before upgrade bias in both channels was easily set up to 40mV as per service manual.
This doesn't bother me much since I wanted to increase the bias anyways as per Mr. Pass advice. Now the temperature of fins is 41-42 C after 2 hours of idling with bias at 59 mV in both channels; I'll increase the bias to one that gives ca 50 C. However, why does the bias in ONLY one channel can't go lower than 59 mV? Could it be a serious problem? Both bias pots are new.
Second, The DC offset jumps to ~58 VOLTS at the moment of powering up the PA-5 and stays there for 3-4 seconds until relay click. After this, the bias is approx quickly revert to ~1 mV, set up this way with newly installed DC offset pots. Could this huge voltage jump at power up damage the speakers?
do you have schm?
Yes but the file is very big to post here. Got it from HiFi engine. Will try to scan and make PDF of it tomorrow.
I'm seeing just PA-7 there
anyway - try to connect 8R/1W (or even smaller power) resistor across amp's output pre-relay , then measure voltage peak during powering up
it's most probably just miniscule current peak , normal for that amp
in fact , pretty normal for any amp , prior to connecting actual load on output
Here is the schematics, attached:
It shows Stephen Sank mods: added 150 mkF/50 V electrolyte accross C101/201; changing R155/255 to 220 Ohm and R157/257 to 2.2 kOhm, and adding a 5 kOhm pot for DC offset accross R114/R115 as in PA-7.
did you tried what I said ?
Yes, just tried this.
With 8R/200W (didn't have smaller W load :D) connected across amp's output pre-relay the voltage spike was 1.8 V max if measured pre-relay.
If the load was connected to speaker terminals, the voltage at them was 2 mV or so before relay clicks and increased to 57 mV R. Ch/43 mV L. Ch. AFTER relay was engaged. Then it quickly went down.
Thank you for clarificatoin of this issue.
Time to put this babe into the system...
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