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Old 7th June 2009, 04:19 AM   #101
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Yes, no final application would be complete without such a bypass. I only speak of the issue because my initial power-up configuration had no bypass to begin with. I wanted to see how it did without any bypass to start.
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Old 7th June 2009, 04:32 AM   #102
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I have no objections whatsoever to the publications of you many observations, and I certainly find them interesting.

But not everyone here is as experienced as you and me, and they might jump to the wrong conclusions without understanding fully what lies behind your observations. I merely wish to clarify those.

Look forward to further results from you, as already mentioned. And I would be interested to see the effect of load resistance (e.g. 1k, 10k & 100k). Open circuit output connected to a probe only probably behaves like a pure capacitive load ??


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Old 7th June 2009, 04:36 AM   #103
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Yes, I understand. It is good to be clear here and I enjoy collaboration with others on these fun topics.

I certainly don't intend to be misleading and if you find that I have, please let me know.
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Old 7th June 2009, 09:10 AM   #104
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Okay, I have prepared more data for all to use.

I've got the B1 on a regulated +/-5V supply, the results are as follows:

Quiescent current: 14.9mA

14mV offset (input grounded)

Frequency response is within 2dB from 100kHz to 10MHz (measured with HP 654A sig. gen and Tek 2247A scope)

Frequency response graph from 1Hz to 100kHz will follow in an attached image along with graph comparing distortion of HP signal source against amplifier output.

Gain at 1kHz:

Loaded by Fluke 179 DMM: 0.990
Loaded by Fluke 179 DMM and 1000Ω resistor: 0.967
Loaded by Fluke 179 DMM and 470Ω resistor : 0.943
Loaded by Fluke 179 DMM and 100Ω resistor : 0.790

Output Impedance at 1kHz: 25.2Ω (Something weird about that, as if transconductance is very low or something's wrong. Maybe my Iq is too low and I have JFETs at the low end of the spec.) (Or I did the math wrong three times).

I did very thorough testing to determine the source of the aforementioned oscillation. The oscillation turned out to be within the B1 and not my function generator. I did some tests to see what it took to make it oscillate:

Load: 1.5m RG56 Coaxial cable, unterminated.
Input: 470Ω resistor to ground and Tek P6109 10x probe with 2247A scope.

Power supply: Regulated +/-5V bypassed each side to ground with 1.5F poly caps within 2cm of B1 board.

Results:

With B1 direct to supply no oscillation was detected but there was peaking at 10MHz when driven with HP 654A oscillator.

With B1 V+ coupled through 1mH inductance to supply oscillation occurred at 864kHz at 1.18Vp-p.

With B1 V+ coupled through 50H inductance to supply oscillation occurred at 6.06MHz at 6.44Vp-p.

To reduce susceptibility to oscillation 100Ω resistance was added in series with output to decouple the reactive load; results follow:

With B1 v+ coupled through 1mH inductance to supply oscillation occurred at 864kHz at 50mVp-p

With B1 V+ coupled through 50H inductance to supply oscillation did not occur.

The above data suggests that this circuit should be stable in most audio applications provided the power supply is adequately bypassed and the output decoupled or kept within certain tolerances of load reactance (no big long runs of coax).

Everybody please comment here; I hope others do some tests to see what happens for them; make sure I'm not fudging anything up seriously.

I might attempt a noise measurement with the analyzer if I have time tonight but who knows if I'll get to it.
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Old 7th June 2009, 09:21 AM   #105
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Well,

a) +/- 5V supply is for me very low. I never tested anything below +/-9V; most of the time +/- 12V. If I am not wrong, the capacitances of the JFET increase quite a bit at such low voltages. Worse is that they get very non-linear and vary with Vds significantly (when you supply a voltage swing, e.g.); this might explain also the oscillation you observed.

b) 14mV offset is large. You matching does not seems to be very good. I regularly get below 2mV with little effort. I suggest you measure Idss at the same voltage (Vds) as you later use as rail voltage in the circuit.

The rest looks more or less OK. The output impedance I expect to be around 25 ohm. 1/Yfs for each JFET is about 30R, so 2 in parallel gives 15R; plus 10R degeneration (?) -> 25R.

See also :

http://www.diyaudio.com/forums/showt...52#post1774452

Again your low rail voltages might lower the Yfs further at Iq.


Patrick
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Old 7th June 2009, 09:22 AM   #106
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Here are the scans of the spectrum analyzer output;

1st, top is input, bottom is output.
Attached Images
File Type: gif b1-vrs-pre-amp-distort.gif (33.3 KB, 799 views)
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Old 7th June 2009, 09:23 AM   #107
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FR (mdB vertical scale, ie 0.4 dB full scale)
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File Type: gif fr-in-mdb.gif (42.0 KB, 770 views)
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Old 7th June 2009, 09:34 AM   #108
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I cannot draw any conclusions from the spectrum analyser outputs. The output has about 20dB higher noise floor than the input (coming from power supply?), but the 2nd harmonics from both the input and the output are about the same amount higher than the noise floor (about 20dB for both).

The FR is essentially flat, as I think 0.01dB is within measurement accuracy ?? Also the dip at LF is due to the coupling cap you are using ? Did you measure the input before or after the coupling cap ?

See also

http://www.diyaudio.com/forums/showt...55#post1774455


Patrick
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Old 7th June 2009, 09:49 AM   #109
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As far as I know, the distortion is compared to the fundamental frequency of stimulus and not the noise floor when dealing with an amplifier.

Correct me if I'm wrong; and I will research this to be sure.

It seems to me as if the amplifier simply has a higher noise floor than the signal source, and also some distortion. Would others please comment?

I tested the circuit with +/-9V supply and noted worse offset and more proneness to oscillation.

I will measure distortion later when I have more time.
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Old 7th June 2009, 09:51 AM   #110
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I must add that during the frequency response measurement in the image show; I did not use capacitor coupling anywhere in the circuit. This was direct coupled entirely.
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