Hi,
A friend of mine built an attenuator based on LDR design.
I noticed that L+R channels are imbalanced.
when the pot knob is turned up, it starts from R channel +5dBFS louder then L channel (minimum),
but when knob turning up, the imbalance gets smaller and smaller, until around -20dbFS (around middle) it's almost perfect, then Left channel (when turning up) became louder and louder - up to -5dBFs, and it's almost perfect when in full 'open' position.
I was told it is just a problem with LDR and it possibly can't be resolved, but I remember reading some audio purist using LDR att. and I don't think they wouldn't notice a 5dB difference between channels, so I presume it's possibly to have it ideal (0.1dB - I can live with, 0.2dB - would be maximum)
Could you please help me - what could cause this sort of problem ?
How to resolve it ?
Is it simply swapping a pair of LDRs or sth else ?
I really appreciate your help, here - thank you in advance
If LDR design is not perfect and it's very difficult to find pair of LDR with 0.1dB difference - could you please advise me, what type of passive att. would be good ? - in a 'human - 99%' price range
A friend of mine built an attenuator based on LDR design.
I noticed that L+R channels are imbalanced.
when the pot knob is turned up, it starts from R channel +5dBFS louder then L channel (minimum),
but when knob turning up, the imbalance gets smaller and smaller, until around -20dbFS (around middle) it's almost perfect, then Left channel (when turning up) became louder and louder - up to -5dBFs, and it's almost perfect when in full 'open' position.
I was told it is just a problem with LDR and it possibly can't be resolved, but I remember reading some audio purist using LDR att. and I don't think they wouldn't notice a 5dB difference between channels, so I presume it's possibly to have it ideal (0.1dB - I can live with, 0.2dB - would be maximum)
Could you please help me - what could cause this sort of problem ?
How to resolve it ?
Is it simply swapping a pair of LDRs or sth else ?
I really appreciate your help, here - thank you in advance
If LDR design is not perfect and it's very difficult to find pair of LDR with 0.1dB difference - could you please advise me, what type of passive att. would be good ? - in a 'human - 99%' price range
Now let me say first of all I have never built a Lightspeed, but when I first read about it my first thought was that matching L and R channels and getting them to track accurately and maintain that accuracy long term might be somewhat difficult.
Personally, I would go for something like an ALPS motorised pot every time.
The ALPS is the classic quality audiophile pot that has been used for decades, for example
ALPS|RK27111250K|POTENTIOMETER, 50K | Farnell United Kingdom
ALPS|RK27111250K|POTENTIOMETER, 50K | Farnell United Kingdom
Thank you for your input, so do you think ALPS is 'close' with sound quality to ldr ? if yes, then maybe I'll drop the idea with ldr....
I dunno I am not constructor myself, my knowledge in the area is minimal.....
I dunno I am not constructor myself, my knowledge in the area is minimal.....
I can't really comment on the LDR type control for sound quality as I haven't built one.
The ALPS has always been the "Gold Standard" for quality attenuators and I and countless others (manufacturers and diy) have never had a problem.
The LDR "Lightspeed" is an intriguing concept but the wide tolerance of the LDR's to me implies close channel matching will be difficult without a lot of individual tweaking and compensating.
The ALPS has always been the "Gold Standard" for quality attenuators and I and countless others (manufacturers and diy) have never had a problem.
The LDR "Lightspeed" is an intriguing concept but the wide tolerance of the LDR's to me implies close channel matching will be difficult without a lot of individual tweaking and compensating.
I am bending towards ALPS, or anything with best sound quality possible, but also without L/R imbalance.
thanks for your input, do you think ALPS is as best as it gets ?
or it is a chance to invest a bit more in pot and have better sound ?
what about goldpoint or this :
HiEnd attenuator 48 steps silver passive preamp Caddock | eBay
thanks in advance for help
The Lightspeed does not normally suffer from severe channel imbalance. If you are experiencing it, you have either made an error when building the device or have mismatched components.
Why not post on the main Lightspeed thread as you can get advice from those who have built the thing, know the circuit well and don't need to imply its qualities or limitations?
FWIW, my experience with both the Lightspeed and The Lighter Note is that they knock spots off switched attenuators when it comes to preserving detail. Though I suspect that even I would notice a 5 dB swing, I haven't experienced balance issues.
I did it already,
I do not imply it's quality or/and limitations, I am just asking, I need device with up to 0.2 dB imbalance
LDR l+r imbalance - YouTube
if you watch this vid, you'll notice it's not about constant difference L/R channel,
but this is inconstant, permanently changing...
I don't know myself which way to go....I'd like to utilize ldr design, as it seems for me almost ideal for audio purist,
I am seeking answer, could anyone who owns ldr design can check if their box is accurate ?
thanks for input
You could add a balance control, but unless you match optocoupler pairs by comparing resistance through their entire operating range, you will have to adjust balance every time you change the volume.
In your video it looks as if you do have two fairly matched pairs. the two on the left could be used for series resistance, the two on the right for shunt, and add balance control for critical listening. But if the video is showing four channels (not individual optocoupler attenuation) then you have an exponentially more difficult task. You don't really explain what your video is showing.
In your video it looks as if you do have two fairly matched pairs. the two on the left could be used for series resistance, the two on the right for shunt, and add balance control for critical listening. But if the video is showing four channels (not individual optocoupler attenuation) then you have an exponentially more difficult task. You don't really explain what your video is showing.
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For a pot yes, about as good as is realistically obtainable... and it is good.
A switched attenuator has the potential for absolute channel matching but has other drawbacks such as limited resolution and the possiblity of the physically large assembly picking up noise etc.
Another thought on the LDR type attenuators, and maybe this is in the threads but have they been tested for channel balance from DC to say 100khz. Any small difference in capacitance of parts between channels could give an image shift with frequency. I have no idea how close the tolererance is of such parameters on the individual LDR's
I would think that the solution to the LDR imbalance would be to hand select a pair of LDR's for closest tracking characteristic side to side. A lot of work, but doable.
Doc
Doc
I gave up and bought Khozmo shaunt attenuator, 48 steps 0.5dB
I did test it, it's super balanced from -60dB up to -1.5dB
the difference is sometimes up to 0.1dB in L or R channel - I can live with it, also it's very important I thought as with LDR (as others mentioned possibility) there was a a freq shift and now it's perfect
I did test it, it's super balanced from -60dB up to -1.5dB
the difference is sometimes up to 0.1dB in L or R channel - I can live with it, also it's very important I thought as with LDR (as others mentioned possibility) there was a a freq shift and now it's perfect
A recent post in the Lightspeed thread suggesting microcontroller control brought some thoughts to mind regarding this.
A microcontroller can provide a D/A output that drives each LED with enough resolution to give any desired resistance with excellent accuracy, precision and repeatability. These can be achieved with calibration by the microcontroller. The LDR value can be measured with a series resistor to a DC value and an A/D converter, so the microcontroller can determine the value of LED current needed for any desired resistance value of each LDR. Once calibrated (started by a pushbutton, and an LED indicator coming on to tell that it's over), a rotary encoder can be used to control the volume through the microcontroller. There's no matching needed, as the microcontroller will give each LED whatever current is needed to make the corresponding LDR the right resistance.
unfortunately my knowledge about EE is minimal, so I can't comment, but if you are right - well it could be something, as so far LDR is unusable for audio engineers like myself when precision tools are crucial
Make it easy on yourself if your just building a couple of units, get one of these and use your DMM to measure a handfull of the LED/LDR's, measure the resitances from 1mA to 20mA, it costs about $6.00 at your local electronics shop.
Cheers George
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Thanks George. But actually I wasn't the one with the LDR matching problem. The thread was started by redman.
Myself, I'd go about it using my light intensity meter (candle power) to establish set intensity points on one particular source LED. Your LED tester does this okay, but very coursely. Next feed LDR through current limiter resistor but off a constant voltage reference. LM10 being the cheapest precision reference. Next take each LDR and plot current through it at each intensity step using DVM. Plot results and compare. The whole thing could be computerized just to make doing lots of LDR's more reasonable. Key points of interest are full off resistance, full on resistance and linearity of resistance at each given point of intensity. The results are fully repeatable and accurate.
Doc
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