Mine is the 275 version, the circuit differs a bit from the 280 schematic shown. Cordell writes that it is difficult to keep the CFP stable,.
Hi Jack,
I've serviced many CFP type amplifiers and they are generally well behaved. I don't see them coming back except on a ten year cycle that I had my better customers on (for a checkup only).
I think that when you are designing a CFP stage, that's where some extra effort comes in. But once the design has been stabilized it should be reliable. Question. There are usually some small bypass capacitors that are critical to stability. The stacked output design is also stable as long as these capacitors are in good health. If yours are film type capacitors, they do tend to go open over time in some sets. Replace with a good ceramic capacitor when you repair yours, or dive in now and at least check them. If you don't see any bypass capacitors close to the output stage, you'd better install some.
If someone made a parts substitution for the outputs or drivers, that can create an unstable circuit. Then you have to stabilize the output stage again, and that is experimental work. A simulator will only get you so far in the physical world.
Just because a circuit is a CFP design doesn't make it inherently unstable. A service guy can though!
-Chris
I've serviced many CFP type amplifiers and they are generally well behaved. I don't see them coming back except on a ten year cycle that I had my better customers on (for a checkup only).
I think that when you are designing a CFP stage, that's where some extra effort comes in. But once the design has been stabilized it should be reliable. Question. There are usually some small bypass capacitors that are critical to stability. The stacked output design is also stable as long as these capacitors are in good health. If yours are film type capacitors, they do tend to go open over time in some sets. Replace with a good ceramic capacitor when you repair yours, or dive in now and at least check them. If you don't see any bypass capacitors close to the output stage, you'd better install some.
If someone made a parts substitution for the outputs or drivers, that can create an unstable circuit. Then you have to stabilize the output stage again, and that is experimental work. A simulator will only get you so far in the physical world.
Just because a circuit is a CFP design doesn't make it inherently unstable. A service guy can though!
-Chris
@Bob Cordell: I hope this is not entirely off topic in this context ... but somewhere on diyaudio I found some component spice files that were newer than the ones you have on your website. This spice file a.o.t. include files on the LSK489 and LSJ689 from Linear Integrated Systems.
I've tried to include these files in an LTSpice simulation, however, when using the files LTSpice gets very slow and basically halts. So I wonder if there might be an error somewhere in the files (I use them the way I normally include an external .model file) 😕 ... The spice files are:
Cheers & thanks for any help,
Jesper
I've tried to include these files in an LTSpice simulation, however, when using the files LTSpice gets very slow and basically halts. So I wonder if there might be an error somewhere in the files (I use them the way I normally include an external .model file) 😕 ... The spice files are:
Cheers & thanks for any help,
Jesper
The only difference I can see between the two files is the change from _1 to _2 in the .MODEL LS689_1 PJF and same for the 489.
Is that correct?
does LT run properly with _1 or with _2?
Is that correct?
does LT run properly with _1 or with _2?
Hi Andrew,
Thanks for considering. Just to make absolutely sure I tried the lsk489 models again and much to my surprise they both work now 😕 ... So, although the simulation I previously did with the LSK spice files worked with other spice files, apparently I somehow have made a mistake in using the LSK489 files. My apology for this .. 😱
As far as I can see there are differences in the LSJ689 files (the three first parameters) but not in the LSK489 files. Isn't that correct?
Cheers,
Jesper
Thanks for considering. Just to make absolutely sure I tried the lsk489 models again and much to my surprise they both work now 😕 ... So, although the simulation I previously did with the LSK spice files worked with other spice files, apparently I somehow have made a mistake in using the LSK489 files. My apology for this .. 😱
As far as I can see there are differences in the LSJ689 files (the three first parameters) but not in the LSK489 files. Isn't that correct?
Cheers,
Jesper
yes, I thought I had read that as the same, but you are right the 689 has changed the three parameters.
I'm very curious about the conditions that cause it to fail, driven hard, hot ambient, shorted output? How did it fail the first time?
First time failure was output open due to a faulty speaker inter-connect. Second time was a blown output transistor. Dealer repaired both at no charge - he did a bad job soldering one of the Linn banana plugs in the first instance. The output zobel seems to have been an afterthought.
Hi Jack,
Standard dealer servicing I'm afraid. The amp has't failed multiple times - the service has.
-Chris
Standard dealer servicing I'm afraid. The amp has't failed multiple times - the service has.
-Chris
Bob,
Have you seen the Marantz 250 or 1200 power amp topology? CFP output with a VAS in the output stage and sort of an AC virtual ground input to the output stage allowing the front end to run on a lower supply. Here's the 1200 service manual that has nearly the same schematic as the 250 but easier to read, in .pdf if you are interested:
http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Marantz-1200-Service-Manual-1.pdf
I'm not saying that it is good, just different and interesting.
Output stage might be interesting with an OP amp front end.
Have you seen the Marantz 250 or 1200 power amp topology? CFP output with a VAS in the output stage and sort of an AC virtual ground input to the output stage allowing the front end to run on a lower supply. Here's the 1200 service manual that has nearly the same schematic as the 250 but easier to read, in .pdf if you are interested:
http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Marantz-1200-Service-Manual-1.pdf
I'm not saying that it is good, just different and interesting.
Output stage might be interesting with an OP amp front end.
Hi Pete,
They sound good, even by today's standards.
If you look at the input side of the amplifier, you will see a buffer that was what Nakamichi's "HTA" circuit was all about. That would have been the Nak product in the 90's, mid 90's I think. I forget what HTA stood for now.
-Chris
They sound good, even by today's standards.
If you look at the input side of the amplifier, you will see a buffer that was what Nakamichi's "HTA" circuit was all about. That would have been the Nak product in the 90's, mid 90's I think. I forget what HTA stood for now.
-Chris
That is a CFP follower for high input Z, isn't it?
Bob did a similar thing with a FET CFP for a differential input IIRC.
What is odd, is with that front end the amp could be driven as inverting with all the advantages.
Hi Pete,
Yes, it was a buffer. They found better sound by driving a BJT diff pair with a lower impedance. Marantz did this is a few amplifiers, including the 500. They were most definitely ahead of their time.
-Chris
Yes, it was a buffer. They found better sound by driving a BJT diff pair with a lower impedance. Marantz did this is a few amplifiers, including the 500. They were most definitely ahead of their time.
-Chris
Hi David,
Where did you see them? I can't locate those resistors. Must be blind or something.
-Chris
Where did you see them? I can't locate those resistors. Must be blind or something.
-Chris