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Old 25th May 2008, 09:56 PM   #1326
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I'll start posting tomorrow.
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Old 26th May 2008, 01:48 AM   #1327
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So...

I obtained 8 samples of the NSL32SR3 and 8 of the NSL32SR2S
and measured the resistance versus the current through the diode
at room temperature.

The data for the NSL32SR3:
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Old 26th May 2008, 01:49 AM   #1328
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The data for the NSL32SR2S:
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Old 26th May 2008, 01:50 AM   #1329
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Here's a graph of the NSL32SR3:
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Old 26th May 2008, 01:56 AM   #1330
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Here's a close-up:
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Old 26th May 2008, 02:03 AM   #1331
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Here the graphic for NSL32SR2S:
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Old 26th May 2008, 02:04 AM   #1332
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And here's the closeup:
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Old 26th May 2008, 02:22 AM   #1333
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Here's a distortion curve for the NSL32SR3:
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Old 26th May 2008, 02:26 AM   #1334
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And here's a distortion curve for the NSL32SR2S.

These curves were created with a 600 ohm source impedance,
and the current through the diode was set to produce attenuation
of -0.1 dB, -0.5 dB, -1 dB, -2 dB, -3 dB, -6 dB, and -10 dB
which corresponds to

50 Kohm, 10k, 5k, 2.3k, 1.5k, 600, and 277 ohms.
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Old 26th May 2008, 02:30 AM   #1335
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And last post of the evening, here's a scope shot of the waveform
and distortion, showing the 3rd harmonic content.

This shows that the element's resistance decreases in response
to waveform voltage. Used as a shunt to ground, this is a
compressive characteristic. Used in series, it has an expansive
characteristic.
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Old 26th May 2008, 06:31 PM   #1336
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Default R2 vs R3

I built my first unit from R3S units. It worked fine, even tried it for a while with linear pots to take advantage of the non-linear response. This was entirely because I ordered the wrong parts from Allied.
The next couple were all built using R2. And then swapped to log pots for voltage control. The only difference I could find was more better control at high attenuation.
The R3 grade, and linear pots ramped up quickly. Normal listening was a little hair trigger. Highest attenuation was not enough sometimes with Lowthers. Swapping to more normal efficiency speakers helped with this a bunch.
Sonically, the R2 and R3 sounded identical to me. There appears to be some difference is distortion, but my speakers most likely swap this.

George
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Old 26th May 2008, 08:00 PM   #1337
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I chose the NSL32SR3 for its better matching, linearity, and range,
and created attached circuit.

The values of R1 and R2 were chosen to make the summation of
the resistors of LDR1 and LDR2 equal to 10K ohm, which then sets
the minimum input impedance of the circuit with the rail voltages
you have (in this case the commonly found +/-15V).

Then you attach the potentiometer with the values shown with the
bottom end going to the -15V rail and the top to a variable regulator.
The nominal value of this regulator is 7.5V, but it needs to be
adjustable so that you get the range you want.

The range that this was adjusted for was -1 dB at the top and
-40 dB at the bottom. More range is possible, but at the expense
of the taper characteristic.

The buffer section consists of matched 2SK170 and 2SJ74, matched
for Idss. If you do not have matched parts and DC offset is an
issue, then consider using an output coupling cap.

The bandwidth of the active buffer is extremely wide, and for that
reason 1K ohm input and output resistors are appropriate.

The distortion of the active buffer is about .003% on a clear day
at 1 V, which is less than you will be experiencing with the LDR
portion of the circuit.

Of course you can tart this up as you like - I consider this the
minimal circuit.
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Old 26th May 2008, 08:01 PM   #1338
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Here is the attenuation curve I got, showing dB of attenuation
versus the clock position of the volume control.
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Old 26th May 2008, 08:05 PM   #1339
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Lastly, here are the distortion figures I got with a constant 1 volt
input to the attenuator. More input voltage naturally results in
greater distortion, except at the -6 dB value, where the 3rd harmonic
distortion of the two LDRs cancels. From -1 to -6 dB, the distortion
is compressive, and from -6 to -40 dB the distortion is expansive.

Any questions?
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Old 26th May 2008, 08:12 PM   #1340
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I forgot to mention that because I have not followed the entire
thread, it is possible that I have duplicated something without
knowing it. If that's the case then

Excuuuuuuuse Meeeeee!

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Old 26th May 2008, 08:16 PM   #1341
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> Any questions?

Yes please.

I wonder if you would kindly measure the distortion using a fixed series resistor (say a Dale 10k) and only one LDR as shunt. From 0-40dB would be great.


Thanks in advance,
Patrick
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Old 26th May 2008, 08:17 PM   #1342
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> I forgot to mention that because I have not followed the entire
>thread, it is possible that I have duplicated something without
> knowing it. If that's the case then
> Excuuuuuuuse Meeeeee!

No no. This is great.

At last someone publish some independent data based on actual measurements, rather than just posting figures from the manufacturer.


Thanks for that,
Patrick
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Old 26th May 2008, 08:20 PM   #1343
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Quote:
Originally posted by EUVL
I wonder if you would kindly measure the distortion using a fixed series resistor (say a Dale 10k) and only one LDR as shunt. From 0-40dB would be great.
I have shown the figures for 600 ohm resistance, but when I have
a chance I'll take a look.
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Old 26th May 2008, 08:20 PM   #1344
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One more question :

Can you perhaps describe the sonic difference between expansive and compressive distortions ?


Patrick
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Old 26th May 2008, 08:24 PM   #1345
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> I have shown the figures for 600 ohm resistance, but when I have a chance I'll take a look.

I meant replacing LDR1 in your schematics with a 10k fixed R.

There are too many claims in the thread that 2xLDRs (one series and one shunt) is better (has less distortion) than one fixed R and one LDR. It would be great to see some measurement data from an independent source.


Patrick
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Old 26th May 2008, 08:48 PM   #1346
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Here you go. Keep in mind that the figures are a little misleading
if you don't remember that with larger attenuation, the probability
that you will be seeing the higher output voltages is quite low.

Also, ignore the noise at low levels on the -1 dB curve - the impedances
are high, and the circuit unshielded.

Also Also, it is not feasible to get more than about 35 dB attenuation
with this circuit.
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Old 26th May 2008, 09:02 PM   #1347
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I have been holding out building one hoping NP will contribute somehow...and now here he is...with a high input Z buffer too. I'll patiently wait some more for a 'maximum' circuit.
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Old 26th May 2008, 09:06 PM   #1348
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Thanks. Great Info.

Patrick
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Old 26th May 2008, 09:13 PM   #1349
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Quote:
Originally posted by EUVL
Can you perhaps describe the sonic difference between expansive and compressive distortions ?
With pleasure. The distortion is 3rd harmonic, and when you look
at the distortion wave juxtaposed against the original sine wave,
you will see that the peaks and troughs are lining up with the peaks
of the signal peaks.

When you look the positive signal peak and see that at the same
time the distortion is also enjoying a positive peak, and similarly
when the negative signal is lined up with a negative peak on the
distortion, that means that the signal is distorted to make the
peaks larger - this would be expansive.

Seen more commonly, when the signal peaks line up with distortion
peaks of the opposite phase, this means that the distortion is
making the peaks of the original signal slightly smaller. This is
compressive.

Here is an example of this on a scope trace. In this example the
distortion is compressive. If you inverted the phase of the distortion
waveform it would be expansive.
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Old 26th May 2008, 09:17 PM   #1350
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Very well explained.

But I wonder how the human ear interprets that -- expansive = brighter sound ??

Also a question on your distortion graphs. Is the X-axis the voltage before the source resistor or is it the voltage across the LDR ?


Patrick
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