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Using 'Lightspeed' passive LDR attenuator w ZKIT1 (Read 6476 times)
Evo
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Using 'Lightspeed' passive LDR attenuator w ZKIT1
05/10/10 at 14:07:04
 
I recently saw a 'Lightspeed' passive LDR attenuator which looked pretty tasty as a pre-amp. The blurp says that it uses a optocoupler device that is a sealed unit which consists of a high performance LED that shines on a light dependent resistor (LDR). This means that it is a passive preamplifier with 'no contact points in the form of a "wiper" as in the case of a potentiometer or "switches" as may be with a discrete stepped attenuator'.

All sounds pretty good, but I guess that it usually works with poweramps (otherwise in intregrated amps the volume control mechanism there would undo all the passive LDR attentuator good work, right?). Also it works best apparently with amps with an input impedance of more than 50K. So....

1) What do people think would be the best way to convert the ZKIT1 into a poweramp? I was thinking that I could remove the pot and put a resistor along each signal path where the pot was with a value a fair bit greater than 1k. Is that correct? Also given the way I have soldered my pot and made my case getting at the pot and removing it might be quite difficult without desoldering a lot of connections (transformers, RCAs, speaker jacks) to remove the board. Is there an feasible alternative that doesn't involve removing the pot?
2) Does anyone know what the input impedance of the ZKIT1 would be? How do you measure input impedance?
3) Also if anyone has heard the 'Lightspeed' or another passive LDR attenuator, any thoughts?
Cheers,
Matt
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chris_m
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #1 - 05/10/10 at 14:54:36
 
I haven't come across that but will have to go check it out.  Am I right to assume that a resistance values is changed by a change in the intensity of the LED?   Avoiding contacts is nice but I wonder what the potential for thermal noise, etc. at the front end of the gain path might like.  It would be interesting to hear from someone using this and what they think.
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Doorman
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #2 - 05/10/10 at 20:27:54
 
I've recently modded my TS GG pre to incorporate optocouplers as the attenuating device.
I can't praise this mod enough. removing the attenuator from the circuit has astonishing results.
Don
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dank
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #3 - 05/10/10 at 22:27:19
 
Oh, I get it...finally.  You are running the audio thru the optically sensitive resistor and controlling its resistance by how much current you send thru the LED (probably with a pot) right?

I was having a heck of a time trying to figure out how you could send the audio thru the optical coupler and make it work properly.

What device are you using, and is there a spec sheet on-line?

Dan
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chris_m
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #4 - 05/11/10 at 02:27:23
 
Dan, that's the same conclusion I came to.  Here's a DIY link with a full description:

http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/

Chris
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #5 - 05/12/10 at 14:16:47
 
""Oh, I get it...finally.  You are running the audio thru the optically sensitive resistor and controlling its resistance by how much current you send thru the LED (probably with a pot) right?" - yep and the pot's not in the signal path, so attenuation without contact points. Blood clever idea I reckon.
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #6 - 05/14/10 at 15:31:14
 
I have been looking at info re: digital attenuation (via computer software) vs LDR attenuation and theoretically digital attenuation should work better. Given that I only use computer as source, it makes sense that I should first try bypassing the pot in my zkit1 and use the computer's attenuation to adjust volume. Am I right in thinking that the resistance of the pot varies between 100R and infinite, so I will to replace the pot with a resistor greater than 100R? What value would people suggest? I don't want the amp to clip.

Thanks

Matt
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dank
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #7 - 05/14/10 at 16:29:19
 
Matt

Leave the pot in.  Set it to full volume and control the system volume from your PC.  As long as the hum isn't too loud, you should be all set.  If you simply have to replace the pot, you would need two sets of 2 resistors (one set per channel) in series (R1 + R2 = 100k) where the wiper is the common R1/R2 connection, R1 goes to the input, and R2 goes to ground.  The pot at full volume would be R1 = 0 (just a wire), R2 = 100k.  The pot at minimum volume would be R1 = 100k, R2 = 0 (this has the input to the amp grounded - minimum volume.  Remember the pot is audio taper (to match our ears) so 1/2 volume will NOT be R1=R2=50k.  If you find a volume setting other than full that you want, you will have to measure with a VOM.  Also remember, there is nothing magic about the 100k value...you wouldn't want to go higher, but I'm using 50k in mine as 100k seemed to be quite sensitive to noise.

Dan
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #8 - 05/14/10 at 22:55:39
 
Thanks Dan.
Sorry about my rudimentary electronics knowledge. I didn't really know how pots worked so from what you are saying it acts like a variable voltage divider. Because of the logistical nature of sound if I find a volume I like then I will have to measure Vin and Vout and calculate the resistances: Vout=(R2/R1+R2)*Vin. Am I right?

The reason I wanted to remove the pot was as per information given about the LDR attentuator which suggested that it is a pot's wiper that adds distortion to the signal. The LDR reportedly removes the distortion by removing an contacts in the attenuation process. That was my undertsanding anyway. Given that 1) I have heard only great things about how the LDR attentuators improve the sound and 2) that digital attentuation is reportedly the most accurate, I figure that I might be able to get a simliar effect to having a LDR attentuator by removing the pot from the signal path in the zkit1 and using digitial attenuation to change volume. This mod would be much easier to do than try to get hold of a matched pair of optocouplers and build the LDR attenuator (particularly as I couldn't just add a module within the amp enclosure as the heat would likely decalibrate the attenuator, so would have to build a separate enclosure). I figure that if I leave the pot in the signal path this distortion will still exist even if is turned up to full volume (and at full volume the hum does get a bit annoying).
What do you think? Is my rationale sound?

As a side issue, not all digital attenuation is accurate. If you do try out XXHighEnd like I suggested in the DAC-1 thread, hover your mouse cursor of the volume controls to pulll up and information box on how to use them because they aren't obviously .

Thanks again for your help Dan,

Matt
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dank
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #9 - 05/15/10 at 13:59:45
 
Matt

You would need to remove the load on the wiper before doing what you perpose [Vout=(R2/R1+R2)*Vin where R1+R2=100k] or you will be dealing with R2 in parallel with (1k+input Z of input tube grid) instead of just R2.  I would suggest just being careful not to move the pot shaft as you remove it from the circuit and then just measure it with an ohm meter.

If you do this, let us know if you think any sonic differences occur.

Dan

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chris_m
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #10 - 05/15/10 at 14:43:06
 
I think for the voltages produced by a battery operated VOM the impedance of the grid would look infinite and not really affect the accuracy of this measurment for practical purposes.  Alternatively the tube could be removed and then make the measurement.
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #11 - 05/16/10 at 02:37:06
 
Dan,

I am not sure I fully understand what I need to do. This what I am thinking so far. Bear it mind that I have had no teaching in electronics and only started 6 months ago to do anything with circuits so my knowledge is less than rudimentary

My plan was not to physically remove the pot from the circuit board, but to 'remove' the pot from the circuit by removing jumpers J4 and J5, which connects the signal trace to the pot, and then connecting R1 and R2 in series with a Vout running from between the two resistor to the holes where the pot output trace goes (by the 2.7k and1.5k resistor) and connect R2 to ground.

My understanding si that the equation Vout=(R2/R1+R2)*Vin where R1+R2=100k would work for an open circuit but that the load on the pot will effectively close the circuit with 100k plus input z, parallel resistance, which would make the output voltage lower than that calculated by Vout=(R2/R1+R2)*Vin where R1+R2=100k. So I need to work out what the load is so that I can work out the correct values of R1 and R2 to obtain the same output voltage as produced by the pot at the volume I desire. Am i right so far?

Dan, you think that I should remove the pot and measure the load. Where do I need to make the measurement? Is there a way to make the measurement without removing the pot as I don't want to do this if I can avoid it, due to how I have constructed my amp?

Chris M, I am not sure what your comments imply I should do?

Thaks for your help on this, sorry I'm a bit slow. I will certainly report back on any effects on sound if I can figure out what I need to do Wink

Cheers,

Matt
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chris_m
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #12 - 05/16/10 at 13:50:52
 
Matt,
My comment was in regard to my opinion you do not need to remove  the pot in order to measure the the equivalent R1 & R2 resistances for your purposes.

But I like Dan's suggestion to turn the pot at maximum and leave it there.  Why not leave in the pot for now?  That way you could actually observe how the lightspeed controls the volume by itself and whether you need to insert a fixed resistor divider in place of the pot at all.  

After some listening and playing around with volume settings and sources you will either find the optimum pot setting that you'd want to replicate with a fixed set of resistor or you may find you don't need to replicate the pot at all.  

Chris
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dank
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #13 - 05/17/10 at 13:58:02
 
Chris

You are absolutely right in that the pot resistance can be measured in (the powered down) circuit without any error due to the parallel path thru the grid.  For some reason, I was thinking that voltages were being measured (since we were talking Vout = R1/(R1+R2) Vin ) and the grid resistance of an operating tube would have an effect on the voltages.  But for an unpowered tube, the grid resistance should be infinite.

Dan
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Evo
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Re: Using 'Lightspeed' passive LDR attenuator w ZKIT1
Reply #14 - 05/17/10 at 23:34:28
 
Thanks Chris and Dan.

Glad to hear that I can measure the pot resistance without having to remove it. So to make it absolutely clear to me to work out the fix resistor values I turn the amp off and measure 1) input to wiper, and 2) wiper to ground of the pot and replace with resistors right?

In terms of turning the pot to maximum, I assume that the rationale is that the wiper is no longer in contact with resistance band of the pot so won't influence the sound. Is that right?

My concern with that is 1) that the hum returns quite noticeably when the volume is turned up to the final 1/8 of a turn. So if I compared  pot attentuation to 'lightspeed' attenuation the pot would have the advantage because it could reduce amplier hum, whereas the 'lightspeed' could not moderate  this directly - so any gains of the 'lightspeed' might be difficult to detect (I hate that hum). 2) My other issue is that I haven't got a 'lightspeed' attenuator yet so will have to get parts and build. Other than the cost there is also quite a delay in getting matching optocouplers and if the fixed resistors work as well, for me the attentuator will become redundant.

On the converse, its going to be difficult to do A-B-A comparisons with fixed resistors and I am not game for using alligator clips in a high voltage circuit!

Overall, I think that I am going to replace the pot with fixed resistor because as long as the resistors are good quality, I can't see that it will make the sound worse. I will try to listen out for any differences and let you guys know. Any recommendations re: type of resistors to use?

Thanks for your help,

Matt
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