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Old 25th June 2008, 01:20 AM   #7121
mlloyd1 is online now mlloyd1  United States
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John Curl's Blowtorch preamplifier
lindsay:
can you post a pic? jp isn't a suffix that describes any idss ratings (unless things have changed recently)

are you sure there is not a "GR" somewhere on the front?


mlloyd1

Quote:
Originally posted by LHMAudio
... and they appear to have a JP suffix...
 
Old 25th June 2008, 12:16 PM   #7122
jan.didden is offline jan.didden  Europe
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RF sensitivity:

Some people actually use a bjt-input opamp as an AM receiver:

http://www.i-trixx.com/uk/index.php?...d=080625_13801

(It looks as if the LM386 is only used to amplify RF, but in the text they admit that it also does its part of the demodulation).

Jan Didden
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Old 25th June 2008, 01:35 PM   #7123
ken.berg is offline ken.berg  United States
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Default MCM JFETs

Quote:
Originally posted by LHMAudio
I ordered the Toshiba JFETS 2SK170 and 2SJ74 from MCM. I received a partial shipment (the rest are due this Friday) and they appear to have a JP suffix. Earlier posts have suggested there are different quality parts and given the low price at MCM I assume these are not the high quality parts. I have three questions:

1. Is the JP in fact the suffix or a country abbreviation, and
2. If the JP is the suffix, what does that mean?
3. If the JP is not the suffix, how to ID the suffix?

Thanks.

Lindsay

Lindsay -
The FETs I received from Newark (MCM) last week are all sufffix BL (6-12mA IDss) as printed on the devices themselves.

The antistat bags were simply labeled 2SJ74 JP (I assume for Japan as country of origin?)
 
Old 25th June 2008, 02:27 PM   #7124
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default RFI

Here are some RFI simulations on the fron-end of a couple different amps. Notice that the output stage is replaced by an ideal buffer. Also notice that these amps have different tail currents. As it appeared that a higher tails current results in less AM demodulation, one should take this into account.
The following amps have been tested:

1. Bob's EC-amps, two versions, one with BJTs at the input, the other with JFETs. I had had to introduce some minor modifications to suppress instabilities: C3 increased from 150 to 220pF, R14 decreased from 100 to 47 Ohm and the VAS loaded with 10pF and 100 Ohm in series. The JFET version, however, still gave some unrealistic THD figures. After replacing the NPD5564 by a 2SK389, the problems were gone. Tail current: 4mA

2. D. Self's blameless amp. Tail current: 6mA

3. The PGP amp. Tail current: 2mA

4. The PCP amp (improved version of PMP). Sum of IPS collector currents: 2.6mA (this amp has no tail, instead a CFB IP stage)

RFI signal: Fc=10MHz, A=0.1Vpk, Fm=20kHz and m=0.7, applied to the non-inverting input. Input filters disabled of course.
The following AM demodulation products (20kHz) were observed.

Bob-JFET:. 0.722mV
Bob-BJT:.. 0.311mV
Self:........ 0.170mV
PGP:........ 5.10mV (0.85mV @4mA tail current)
PCP:........ 1.1mV

The reader may draw his own conclusions from these figures, that is, as long as he don't think JFETs are always better.

Next, I've also looked at the effect on THD20 (in ppm) of an un-modulated carrier of 10MHz and 0.1Vpk.
The AF signal is 1Vpk and 20kHz.
..............no carrier... with carrier
Bob-JFET:. 0.233 ....... 0.260
Bob-BJT:.. 0.409 ........ 0.445
Self:........ 2.9 ........... 3.2
PGP:........ 0.039 ....... 0.024
PCP:........ 0.018 ....... 0.031

As one can see, the THD figures are hardly compromised by the HF carrier, but remember, the amplitude is "only" 0.1V. At 1V we get a different picture. I've only tested the PCP amp equipped with true (although non-existing) complementary trannies.

0.1V carrier: AM product = 0.37nV. THD20 rose from 4.7ppb to 5.0ppb
1.0V carrier: AM product = 1.25uV. THD20 rose from 4.7ppb to 24ppm

AM demodulation is almost nihil. Not surprising, as the circuit is 100% symmetrical. But at strong RFI levels the THD rises considerably. That's precisely what Glen has predicted.


Cheers,
Edmond.
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Old 25th June 2008, 03:45 PM   #7125
john curl is offline john curl  United States
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I would like to follow up on the noise measurements of the BF862. I originally saw a number of 'measurements' that were all over the map, so to speak. The problem with this part, besides limited voltage breakdown, is that it is not SPEC'ED for low frequency low noise. This can be a big problem as one batch of devices might be very quiet, and the next batch could very well be very noisy, as there is no 'quality control' requirement imposed by the spec sheet on the device.
It is well known that 1/f noise is an almost uncontrollable variable in quality fets, and requires a good attention to fab detail to get it right. This is what the fabricators tell me, in any case.
Independent screening of a FEW samples of the BF862 has shown it to be a pretty fair device, with perhaps twice the 1/f noise of a typical 2SK170, BUT with lower input capacitance that would make it a specific contender for condenser microphone input electronics, among other uses, just like Scott Wurcer mentioned on this thread a few weeks ago.
This does not mean that EVERY batch will be as quiet, but since two independent batches have been independently sampled with Quanteks (an obsolete technology as stated here on this thread by others) it looks OK to try them, and include them in the few devices that we have left to work with.
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Old 25th June 2008, 05:23 PM   #7126
PMA is offline PMA  Europe
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Though this is a BT thread, I would like to mention

http://stereophile.com/solidpreamps/308para/
 
Old 25th June 2008, 05:38 PM   #7127
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default RFI

(continued from post #7124)

And here are the (simulated) results with a hefty RF input of 1Vpk.

AM demodulation:
Bob-JFET:. 1.5V
Bob-BJT:.. 2.15V
Self:........ 68mV
PGP:........ 4.44V (@2mA tail current)
PGP:........ 1.77V (@4mA tail current)
PCP:........ 2.74V

THD20:
Bob-JFET:. 148ppm
Bob-BJT:.. 395ppm
Self:........ 105ppm
PGP:........ 26.4ppm (@2mA tail current)
PGP:........ 5.07ppm (@4mA tail current)
PCP:........ 900ppm

As we can see, the distortion of the PGP amp is the least affected by this large RFI, probably because of the NDFL stage that attenuates RF before it enters the VAS.

The PCP amp, on the other hand, clearly gives up. Apparently due to mismatches between complementary devices, because with true complements (see previous post), the rise of THD is limited to 24ppm.

As for Bob's EC amp, these figures clearly demonstrate that JFETs perform better with regard to distortion as well with regard to AM demodulation.

NB: These results were obtained without any filtering at the input. So I think that in real life with good shielding etc., amplifier inputs will never subjected to the terror of a 1V 10MHz interference. Therefore, the figures presented in my previous post may be more representative.

Cheers,
Edmond.
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Old 25th June 2008, 07:42 PM   #7128
scott wurcer is offline scott wurcer  United States
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Speaking of RFI I had reason to tell someone to try the lopsided Tom Holman input stage. Anyone ever try a complimentary version of that? I also found a recent (if you can believe that) very favorable review of a refurbished APT preamp, with the usual "you would have to spend X1000's of dollars today to equal its performance".
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Old 25th June 2008, 07:47 PM   #7129
LHMAudio is offline LHMAudio  United States
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Once I got the FETS under good light with a magnifying glass I could see the grade markings.

So what do these markings mean?

2SK170 BL
2SJ74 GR

Thanks,

Lindsay
 
Old 25th June 2008, 07:49 PM   #7130
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Blue, Green. See datasheets. It means different transconductance and maximum Id.
 

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