Long time forum reader and first time posting. I am a long time electronics tinkerer, and recently acquired some Nakamichi equipment that was, according to the WWW, designed by the famous Nelson Pass. It was this fact of Mr. Pass's influence in these amplifiers that got my attention (since I just built two ACA 1.8 amps). I ended up with two PA-7 Stasis amplifiers, a CA-5 preamp, and a CA-5II preamp.
The first thing I noticed was that the CA-5 sounded waaaay better than the CA-5II. At least to my ears. The CA-5II may need some attention - or a new owner.
The PA-7s have me a little worried. They don't seem to have much top end sparkle. The have an extremely nice midrange though. I'm not sure if I need to make some adjustments to the DC offset or bias settings because of age, or if I should look into doing a recap at this point. I was much younger when I first heard these amplifiers. I remember being incredibly impressed with the PA-7 driving some reference level Infinity speakers. Any recommendations on what I should do with the PA-7s?
The first thing I noticed was that the CA-5 sounded waaaay better than the CA-5II. At least to my ears. The CA-5II may need some attention - or a new owner.
The PA-7s have me a little worried. They don't seem to have much top end sparkle. The have an extremely nice midrange though. I'm not sure if I need to make some adjustments to the DC offset or bias settings because of age, or if I should look into doing a recap at this point. I was much younger when I first heard these amplifiers. I remember being incredibly impressed with the PA-7 driving some reference level Infinity speakers. Any recommendations on what I should do with the PA-7s?
try searching through forum, there was a mention of few problems, especially one cap (feedback?) being substantially small
use advanced search on google, targeting diyaudio.com
use advanced search on google, targeting diyaudio.com
Loaded question - I can't believe ZM didn't ask to send to him for proper disposal, he must be losing his edge.
But he's right about that feedback cap- search around for that. i never had a PA-7 but have heard about that cap a few times. Something about reaching for better measurements at the expense of sound quality.
Additionally check/replace all the electrolytic and tantalum caps, particularly in the front end. Check bias. The amp might not be operating properly at all...
Thanks for the replies. I will have to do some additional searches on the forum. I did a several and did not turn up the particular issue with the feedback cap. In fact, I did not find much of anything at all about the PA-7. Sounds like replacing the capacitors is a good idea. What about the power supply filter caps? There are four huge electrolytics. The look fine to me, but that may not mean anything.
Thanks for the contact. I am building a parts list to order to replace all the electrolytic caps. Some will be hard to find it seems. The 1800 µF 50V pair for the PS filter is a problem with the form factor. I wonder if I can use 3300 µF instead. The others I have found in either Silmic II or Cerafine and as an alternative Nichicon KZ. I am also looking for suggestions for some kind of film type caps for the 10 and 20 µF 200V fat coupling cans.🙂
yes, you can use 3m3 instead of 1m8
film type - my fave for fat cans bypass are motor runs, but you probably don't have a space for that ...
film type - my fave for fat cans bypass are motor runs, but you probably don't have a space for that ...
If you are referring to C102/C202, I believe those are the ones mentioned above as "feedback caps". In any case, they are not coupling caps (i.e., C102/C102 are not in series in the signal path).
The difference between what Nakamichi used here and what was used on otherwise rather similar Threshold Stasis amps of the time is that Nak used 10µF films rather than much higher (220µF, I think) in the Threshold. The concern in the use of the smaller value was/is that it results in some rolloff in the deepest bass.
The best and most useful discussion I have read of this issue is in the link below (about a PA-5, but it is the same issue). There is a suggestion made in this link about how to address the putative shortcoming of the Nak implementation.
New toy, Nakamichi PA-5 amplifier, some pics and measurements | Audiokarma Home Audio Stereo Discussion Forums
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I found the posting on AK about the feedback capacitor. If I go in and recap this PA-7, can I expect the high frequency content to improve? Both of my PA-7s seem to be rather light in the higher registers.
I connected a CA-5 up to the PA-7 this afternoon. As soon as the CA-5 relay clicked in to connect the outputs, there was a very loud pop followed by a hum. When I switched off the CA-5, the loud pop repeated. So, I disconnected it and put a multimeter on it and looked for DC. On startup and shutdown, I get over 1 volt of DC for just a quick second and then it receeds to about 8 mA. What on earth could this possibly be? Bad output transistors?
8mA , typo?
you meant 8mV?
can you remember - did you had longer period before, before relay clicks in ?
often happens that electrolytic in delay circuit goes south, so delay time is shorted
logic sez that - if anything is wrong, speaker protection should not allow relay to kick in, so say that your speaker protection still think everything is good
so, if 8mV is what you see at output and relay really is ON, transistors are most likely OK, and hum you have is of different reason
same in both channels?
you meant 8mV?
can you remember - did you had longer period before, before relay clicks in ?
often happens that electrolytic in delay circuit goes south, so delay time is shorted
logic sez that - if anything is wrong, speaker protection should not allow relay to kick in, so say that your speaker protection still think everything is good
so, if 8mV is what you see at output and relay really is ON, transistors are most likely OK, and hum you have is of different reason
same in both channels?
Zen,
I am talking about the preamp relay and yes I meant 8 mV. Apologies. I was upset and my typing was not great. Actually, I connected the CA-5 preamp to the meter again. Before power switch is on, it measures 0V (grounded). When the power switch is engaged, it still measures 0V until the relay clicks. Just as the relay clicks, it measures over 2 volts at the RCA outputs for a split second, then the voltage varies +/- 0.04 Volts until I turn off the power. When the power is turned off, another huge DC voltage spike up to around 2 volts then it goes to 0V when the relay disengages. Strange - and speaker killer level.
The PA-7 faithfully delivered the spike to my Dynaudio Contours. Not good...
I am talking about the preamp relay and yes I meant 8 mV. Apologies. I was upset and my typing was not great. Actually, I connected the CA-5 preamp to the meter again. Before power switch is on, it measures 0V (grounded). When the power switch is engaged, it still measures 0V until the relay clicks. Just as the relay clicks, it measures over 2 volts at the RCA outputs for a split second, then the voltage varies +/- 0.04 Volts until I turn off the power. When the power is turned off, another huge DC voltage spike up to around 2 volts then it goes to 0V when the relay disengages. Strange - and speaker killer level.
The PA-7 faithfully delivered the spike to my Dynaudio Contours. Not good...
sorry, I was thinking that power amp is in question
anyway, overhaul time
do you have schematic?
if you check with DVM on outputs, is it same on both channels?
anyway, overhaul time
do you have schematic?
if you check with DVM on outputs, is it same on both channels?
It is good practice -- really a rule for many of us -- that the amp is the last thing on, and the first thing off.
This will insure that any power-up or power-down transients, whether due to a fault or not, will not go through the amp and to the speakers at full volume.
This will insure that any power-up or power-down transients, whether due to a fault or not, will not go through the amp and to the speakers at full volume.
Same on both channels. Yes, I have the service manual. I downloaded it from HiFiEngine.com.
I have that rule too - except I don't know where my mind went this time. I normally try used vintage stull on a cheap setup that I don't care about. I don't know why I thought I could get away with it this time. I wonder if it would have been less of an impact if I had used the two ACA amps instead.😱
As you work through the CA-5 to see if it has an issue, if you need a comparison, I have a CA-5 that I can take measurements on. I have it on all the time (a very good preventative to inadvertently turning on the amp first), but can cycle it if need be. Good luck in getting it sorted.
RocketScientist - can you post a picture of your preamp, inside
as far I can see , there are two iterations of CA5, one being much simpler
interesting, that one certainly unable to throw DC on output, being AC coupled
as far I can see , there are two iterations of CA5, one being much simpler
interesting, that one certainly unable to throw DC on output, being AC coupled
The one I have is the simpler version, the CA-5. It has 10 µF film coupling capacitors (with DC spikes - go figure). I will try to post some photos later today. There appears to be evidence of some arcing on the output jack daughter board.
The more complex version, CA-5II, is the one that has opamp servos and tone controls.
The more complex version, CA-5II, is the one that has opamp servos and tone controls.