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John Curl's Blowtorch preamplifier
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Old 25th June 2008, 11:28 PM   #7141
john curl is offline john curl  United States
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Welcome back, Nelson. It is true that slight mismatches like that can be still used effectively.
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Old 26th June 2008, 01:20 AM   #7142
Nelson Pass is offline Nelson Pass  United States
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John Curl's Blowtorch preamplifier
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Old 26th June 2008, 01:23 AM   #7143
john curl is offline john curl  United States
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Jam told me you were gone.
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Old 26th June 2008, 01:42 AM   #7144
GK is offline GK  Australia
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Default Re: RFI

Quote:
Originally posted by Edmond Stuart
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.

Hi Edmond.

Instead of 10MHz, you might want to retry this experiment with RF carrier signals approaching the unity loop gain frequency of each amplifier.

This is where both the error signal is large and the input stage itself may still have enough gain to send plenty of the RF carrier onto the VAS. This is where marginal designs will fall flat on their face even with lower level RF signals.

Cheers,
Glen
 
Old 26th June 2008, 06:49 AM   #7145
PMA is offline PMA  Europe
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I appreciate Edmond's hard try to simulate RFI influence, but IMHO there is one of the biggest difference between real life and virtual life. I have quite a lot of time domain measurements and FFT analyzer measurements, I can post something to give Edmond a better hint.

Measured at the OUTPUT of power amp, whole audio chain, single-shot trigger. Repetition frequency is pretty variable, quite low, in audio band.
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Old 26th June 2008, 09:24 AM   #7146
Sigurd Ruschkow is offline Sigurd Ruschkow  Sweden
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Guys,
there are many other Internet stores than MCM that have the 2SJ74 and 2SK170! Even BL grdes. Both here in Europe and in the USA.




Sigurd


Quote:
Originally posted by john curl
Don't return them, but you you might have to try to buy some 2SK170 GR to best match. In any case, you can use what you have. I will explain later, if necessary. Still, it might be that Toshiba is 'cherry picking' parts for Chas Hansen, and we may not be able to get 2SJ74BL through normal channels, easily.
 
Old 26th June 2008, 09:51 AM   #7147
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default RFI

Quote:
Originally posted by PMA
I appreciate Edmond's hard try to simulate RFI influence, but IMHO there is one of the biggest difference between real life and virtual life. I have quite a lot of time domain measurements and FFT analyzer measurements, I can post something to give Edmond a better hint.

Measured at the OUTPUT of power amp, whole audio chain, single-shot trigger. Repetition frequency is pretty variable, quite low, in audio band.
Hi Pavel,

I was already waiting for your input. Thanks.
So you recommend using a pulse, but what height and width?

Although I agree with you that one should measure the whole audio chain, I have to limit my simulations to various front-ends, as this thread is about the BT and variations on pre-amplifiers, NOT power amplifiers. So, in order to stay OT, I must exclude RFI effects on power output stages.

In the meanwhile, if you don't mind, I'll complete my sims using a 1MHz RFI source (on request of Glen).

Cheers,
Edmond.
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Old 26th June 2008, 10:33 AM   #7148
homemodder is offline homemodder  Portugal
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Thanks Edmond, very informative although i was surprised at the cfb results at first but the latter results say it all.
 
Old 26th June 2008, 10:38 AM   #7149
GK is offline GK  Australia
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Quote:
Originally posted by PMA
I appreciate Edmond's hard try to simulate RFI influence, but IMHO there is one of the biggest difference between real life and virtual life.......

Bzzzttt.

Here is a real life result. The AM waveform shown below is what get across my speakers when a ~3 meter bare wire antenna and a tank circuit consisting of a fixed capacitor in parallel with a slug tuned inductor is connected to the input of my preamp and tuned to the frequency of the 50kW AM broadcast station less than 10 kilometers away.

All discrete BJT circuitry and no audible demodulation from the speakers.

But then again, WTF would I know about RFI immunity.
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Old 26th June 2008, 11:42 AM   #7150
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default Re: Re: RFI

Quote:
Originally posted by G.Kleinschmidt
Hi Edmond.

Instead of 10MHz, you might want to retry this experiment with RF carrier signals approaching the unity loop gain frequency of each amplifier.
[snip]
Cheers,
Glen
Hi Glen,

I've repeated the sims with 1MHz. I think a good average between a fT of Bob's EC amp (1.9MHz), blameless (550kHz) and PGP/PCP (~800kHz)
With 0.1V RFI:

................. AM: ....THD20 from ... to (in ppm)
Bob-JFET:. 2.8uV ......... 0.233 ..... 1.28
Bob-BJT:.. 1.5uV .......... no effect
Self:........ 135uV ......... 0.135 ..... 3.07
PGP:......... 61uV .......... no effect (@2mA tail current)
PGP:.......... 9uV ........... no effect (@4mA tail current)
PCP:.......... 9uV ........... no effect

With 1V RFI:

................. AM: ....THD20 from ... to (in ppm)
Bob-JFET:. 0.5mV ......... 0.233 ..... 18.2
Bob-BJT:. 0.16mV ......... no effect
Self:....... 85.3mV ......... 0.135 ..... 93.3
PGP:........ 250mV ......... 0.039 ..... 8.9 (@2mA tail current)
PGP:....... 10.6mV ......... 0.039 ..... 8.8 (@4mA tail current)
PCP:......... 78mV .......... 0.019 ..... 0.025

Clearly, Bob's front-end with BJTs in the input stage is the winner, that is, according to my simulations, for what it is worth...
And the PGP/PCP amp? Well, they obviously need good input filtering and shielding. Apparently, ultra low distortion amps have a price.

Cheers,
Edmond.
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