RMI-FC100, a single stage audio power amplifier - Page 127 - diyAudio
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Old 11th December 2012, 07:51 PM   #1261
DRZ1 is offline DRZ1  Canada
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' noisy corner ' i like that Roender

Read you while listening Dire Straits on my FC-100.......
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Old 12th December 2012, 06:55 AM   #1262
AndrewT is offline AndrewT  Scotland
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It's the variation on voltage that is more of a problem when current is very low. Hard to predict final regulated voltage.
I'm not disputing the Zener action.
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Old 12th December 2012, 07:14 AM   #1263
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Shall I buy 43V zeners to have 42-43VDC on outputs?
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Old 12th December 2012, 07:16 AM   #1264
roender is offline roender  Romania
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Quote:
Originally Posted by AndrewT View Post
It's the variation on voltage that is more of a problem when current is very low. Hard to predict final regulated voltage.
I'm not disputing the Zener action.
For 36V zener there is no significant voltage variation between 0.2 to 10mA Ir if this diode is thermally compensated by a series direct conducting junction.
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Old 18th December 2012, 01:05 PM   #1265
zdr is offline zdr  Belgium
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Quote:
Originally Posted by roender View Post
For 36V zener there is no significant voltage variation between 0.2 to 10mA Ir if this diode is thermally compensated by a series direct conducting junction.
Mihai, do you have any recommendations for DC filtering of mains AC that would not impact the sound quality of FC100?
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Old 18th December 2012, 02:31 PM   #1266
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by roender View Post
Too bad that ONsemi f..ked up some production lots of NJLs. I'm convinced that they put by mistake other type of diode inside which requires other compensation current in order to correct track the transistor Vbe over temperature.
I do not recommend to bypass all the diodes string with the pot. Just use what Andrew recommend, one resistor over the diode with way out Vf than the others.
Seems odd that they would mistake the diode type, but I also have experienced some problems. In my instance, it seems more a case of variable behavior.

One of the two amps I built would suddenly bias high, then lower. I chased the problem down to one diode that was apparently unstable. It would measure slightly different than the others with the diode setting on the multi meter, but would show about 50% higher voltage drop under load. I had originally bypassed another diode, and when I opened that connection that one read over 1V!! on the MM. However, when I ran current through that one, it dropped back to the normal reading, and was normal with the FC current load.

The take home lesson: Measure all your diodes with the expected operating current (about 2mA). Measure each one a couple of times. I would then mount them on the heat sink and short one with a clip lead. Adjust bias, then check bias with each one shorted before you button things up. If you do not shut the amp off when you move the clip lead, you should short the next one in line before you remove the clip on the prior one - or your bias will be way too high.

Sheldon
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Old 19th December 2012, 09:06 AM   #1267
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Sheldon, I have bought 175 of each NJL0281DG and NJL0302DG for the German FC-100 group-buy from VERICAL some weeks ago.
A DIY-friend of mine matched them and also measured the Vf of the integrated TT-diodes at a current of approx. 2.5mA. (12V / 4K7).
He did all the measurements twice.

The Vf of the NJL0281 - TT-diodes is in the range of 509 - 518mV; the Vf of the NJL0302 - TT - diodes is in the of 519 - 529mV.

Some of the Germans have already built the FC-100.
Their builds work with 5 active TT-diodes, and I have not heard about any adjustment / TEMPCO problems.

I therefore think that if you buy the NJLs from the current production, you won't have any problems with the internal TT-diodes.

Best regards - Rudi
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Old 19th December 2012, 03:32 PM   #1268
Sheldon is offline Sheldon  United States
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Originally Posted by Rudi_Ratlos View Post
I therefore think that if you buy the NJLs from the current production, you won't have any problems with the internal TT-diodes.
Thanks Rudi. My comments were more directed to any users who might have Thermaltraks already on hand. I think I have solved my issues by just checking each diode under actual operating conditions, and selecting the best one to bypass. Had I known that there might be some variability, I'd have checked more carefully before I installed them. Both my amps (four channels) are fine with 5 diodes in the string.

Sheldon
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Old 30th July 2013, 03:18 PM   #1269
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It's been quite a while since I became interested in this project and bought the boards and other parts in the first group buy. Sourcing parts was particularly frustrating; but finally, after many interuptions it is working as the mid drivers in my Linkwitz LX521 dipole speaker system.

I have been of the opinion that perceived differences between half decent power amplifiers would be miniscule: particularly where my 70 year old, and none-too-golden ears are concerned. How wrong I was. Particularly in the spatial domain these power amps have improved the system as much as going from the Orion speakers to the LX521's. It becomes aparent that linearity is hugely important in uncluttering the Acoustic Scene. Presumably intermod' products generate a lot of spatial "fuzz". As others have remarked, the amps seem to have total control of the drivers from low frequencies to the prickliest/sharpest transients. One of the first things I noticed was that I became very aware of the inner voices/parts such as the violas in string ensembles; indicating that some masking sounds were absent compared to lesser amplifiers. On well recorded material the AS is more simple and believable than anything I have heard before.

I almost had the amps going in December last year, but stopped work to build the LX521's. Unfortunately when selecting output devices for the second channel and finding the best place to put them on the heatsink I had some moments of madness that did not help progress. While removing and installing the board for the nth time suddenly one of the PNP thermal traks is hogging all the current. Assuming that it was faulty or that my numbering system was defective I wrongly concluded that it would have to be unsoldered. In the event I cut the leads off and removed the remaining pieces one at a time. Lo and behold a similar thing happened on the NPN side of the amplifier and this device was removed intact with a little bit of damage to the pad at the top of one via/thru hole. I now realise that I had forgotten to finally tighten the screws through the OP devices into the heatsink Loose screws =bad thermal bond=high Iq

As I have the slower thermal tracks I applied a 1UF film capacitor between input ground and the chassis. The first attempt was unsucessful at preventing oscillation (C across input connectors) but moving the caps directly between the input pin and the nearby back panel via shot leads did the trick.

Mihai, I can understand why you rank this design as the best you have heard. Congratulations, excellent work.

Keith
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Old 31st July 2013, 10:06 AM   #1270
hahfran is offline hahfran  Germany
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I happened to read this thread only yesterday. This design seems perfect to drive fast rising broadband speakers. But alas component woes by the number. Thermal track BJTs unavailable and the sK170 only with the GR spec. If I could get some thermal tracks via ebay , could I get along with just 1 pair in the power stage? ( Then a Vbe multiplier is necessary of course)
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