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Old 20th February 2008, 11:34 AM   #1161
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bottom line, to go under about 47k with the volume control pot, I would have to enlarge the capacitor at the output of the stage before the pot.

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JG
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Old 20th February 2008, 11:48 AM   #1162
jkeny is offline jkeny  Ireland
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Were comparative tests made between voltage & current fed LDRs or have I missed something?
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Old 20th February 2008, 12:04 PM   #1163
AndrewT is offline AndrewT  Scotland
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Hi G,
would the sound deteriorate if two 470nF caps were fitted in parallel?
Would you notice the difference between F-3db changing from 7Hz to 15Hz?
To the first Q. I suggest you may not hear any deterioration.
To the second Q. I think you will hear F-3db going up to 15Hz. I would recommend going down below 2Hz if your equipment can cope with a widerband signal.
To achieve 2Hz the cap would need to be ~1.5uF for Zin=47k
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Old 20th February 2008, 12:20 PM   #1164
mikelm is offline mikelm  England
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I moved over to current drive.

It was not a direct comparison though because I implemented current drive with with series resistor / shunt LDR.

The shunt resistor is a 22K vishay bulk foil.

I am very happy with the sound it is incredibly clear & clean.

I bought some other resistors to check out the difference but I cannot motivate myself to try them because the sound is already excellent.

For me, well implemented current drive is clearly better than well implemented voltage drive.
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Old 20th February 2008, 12:41 PM   #1165
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Hi AndrewT

I do not care much about under 20-30Hz, but the experience shows a 470nF cap instead of a 100nF cap gives better bass, even with a 100k pot after it. Another fact, the AN M7 preamp is not good under 47k (the 3 tube riaa).
We can discuss about impedances in tube chains, but I'm afraid it is too off topic.
Thanks for your help, but I will not purchase new caps for my DAC and RIAA for sure.

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JG
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Old 20th February 2008, 05:20 PM   #1166
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Quote:
Originally posted by mikelm
I moved over to current drive.

It was not a direct comparison though because I implemented current drive with with series resistor / shunt LDR.

The shunt resistor is a 22K vishay bulk foil.

I am very happy with the sound it is incredibly clear & clean.

I bought some other resistors to check out the difference but I cannot motivate myself to try them because the sound is already excellent.

For me, well implemented current drive is clearly better than well implemented voltage drive.
Are you refering to the CCS or something different? If it's something different, I'd be interested in the details.
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Old 20th February 2008, 06:55 PM   #1167
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by Giordano
I'm happy that your concept is nearly the same, however I do not understand the fit function.
In our design for each channel we have 2 series LED/LDR per input (of which fortunately only one needs to be calibrated, the other is operated simply as a "switch" of sort, with just a trimmer to equalize the fixed "on" resistances) plus one common shunt.

We plan to make a modular design with an independent inputs+attenuator board (with its own uC) for each channel plus one or more uC board(s) for UI, display, remote, etc. (the automatic calibration system may be "split" between the UI controller board and the "channel" ones and/or require yet another subsystem on its own... not all the details have been addressed yet).

That is, e.g. for a "typical" stereo preamplifier with 4 inputs you'd have 10 LDR to calibrate. A larger system with 6 channels and 5 inputs each would require 36 calibrated LED/LDRs!

The problem is: the I/R charactiristic of the LED/LDR is neither linear nor equal between different samples... matching "by hand" such a large number of LDR is simply out of question.

If you want to have the possibility to "dial in" whatever attenuator you'd like using a significant fraction of the enormous dynamic range of the LED/LDRs without having to "re-calibrate" everything each and every time, you need to know in advance which currents to set in order to get whatever resistance value you'd need.

The easier way to do this would be to build a full "calibration table" with the LED drive current value required to get each resistance value that may be required, and store it on a eeprom. And of course this should be done for each LED/LDR in the system.

To get enough precision and dynamic range, you'd need at least a couple of 16bit words for each value pair, and you'd need to calibrate and store A LOT of them for each LDR!

This has quite some drawbacks. Most notably:[list][*]it would require a lot of memory... way much more than what would be available on any uC (unless resorting to an external memory chip, that is).

[*] endless time required for the calibration procedure: LDRs are slow and very heat sensitive... the calibration procedure should be something like this:

  1. define some desired Rset value
  2. ramp up/down the LED current while measuring the LDR resistance until it approach the desired Rset value
  3. turn off LED and wait to let the device cool down
  4. turn on the LED at the last set current and check resistance
  5. if |R - Rset| < desired_precision we're done, else loop (back to step 2)
  6. turn off LED and wait to let the device cool down
  7. loop from step 1 for all required/desired resistance values... [/list=1]
A valid alternative (suggested by Andy) that allows to save a LOT of memory (and not only that) is the use of a "fit" function. You find a simple function that "fits" with the I/R characteristic of the LDRs within a given precision and code it into the firmware. Then, instead of building and storing many large tables, for each LDR you only have to find and store just the few parameters of the fit function...

at "run time", when you need some resistance value, instead of looking it up among the ones available in a large calibration table you can simply calculate the current which is required to get it.

This way you can actually "dial in" (virtually build) whatever attenuator you'd like on the fly, directly from the unit UI, without ever reprogramming the system...

imagine e.g. using different attenuation curves and/or different I/O impedances for different input channels to get the best match with the different sources...

...did I say that your imagination is the only limit to the flexibility and versatility of such a system?!
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Old 20th February 2008, 11:47 PM   #1168
mikelm is offline mikelm  England
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Quote:
Originally posted by Paul Ebert
Are you refering to the CCS or something different?

It was CCS - very close to Peter's design
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Old 21st February 2008, 12:25 AM   #1169
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Hi in reference to the post # 1160

http://www.diyaudio.com/forums/showt...06#post1433006

a second hand Lightspeed Attenuator for sale, you all can stop ringing poor Richard he sold it to the first caller this morning.

Cheers George
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Old 21st February 2008, 08:29 AM   #1170
jkeny is offline jkeny  Ireland
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Well, if the series resistor/shunt LDR is as good (in CCS form anyway!) would it not simplify matters to go with this configuration initially for the auto-calibrate model? Apologies if this has been teased out before!

In the completely opposite direction to the simplification above - has anybody considered this for a differential vol control - posiible with auto-calibration!
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