This is a great idea. I've been pondering - has anyone trying driving the leds with a PWM source? I'm asking since i'm now in the process of building a digitally controlled (PIC) preamp and have messed with pwm drivers for leds before, and i think you could create a "digital pot" using PWM that would requiere but two output pins...
mikelm said:
Mike, the output of your cd player, did you measure this also with your DMM from output to ground? If so, WRONG, it is the SERIES resistance from the output back to the opamp amp output and this can only be measured with the cd player on and mute circuit off and NOT with a DMM, or if you have a circuit diagram of the output post it up and I can tell you what the output impedance of your player will be.
Cheers George
George,
My CD player DAC o/p is buffered with an op-amp. This op-amp has feedback so the the o/p imp is effectively zero.
This op amp is connected directly to the o/p phono sockets with a 20 ohm wirewound non-inductive resistor. This is, more or less, how I always do my CD player o/p's.
The regular transistor operated mute function relies on cheap series resistors of a few hundred ohms which niether of us like.
Please believe me, my CD player o/p impedance is about 20 ohms.
mike
My CD player DAC o/p is buffered with an op-amp. This op-amp has feedback so the the o/p imp is effectively zero.
This op amp is connected directly to the o/p phono sockets with a 20 ohm wirewound non-inductive resistor. This is, more or less, how I always do my CD player o/p's.
The regular transistor operated mute function relies on cheap series resistors of a few hundred ohms which niether of us like.
Please believe me, my CD player o/p impedance is about 20 ohms.
mike
Input, not output.
Mike,
When using a pot for voltage divider, the output impedance always varies.
What stays contant, (maybe not) is the input impedance. Try measuring for open input to ground. Then rotate the pot.
The input impedance is what loads sources and sets the LPF. The Lightspeed is speced at 7K. I would think it varies about 10 -20% as the pot is rotated.
George
Mike,
When using a pot for voltage divider, the output impedance always varies.
What stays contant, (maybe not) is the input impedance. Try measuring for open input to ground. Then rotate the pot.
The input impedance is what loads sources and sets the LPF. The Lightspeed is speced at 7K. I would think it varies about 10 -20% as the pot is rotated.
George
The following data might interest anyone thinking about making a volume control for an active pre using LDRs. To match up LDRs I used a 5.2V source & a 12-position rotary switch with fixed resistors. Out of 40 factory-sorted LDRs I found four quads matched within 5%-- enough for a balanced standalone passive and a balanced volume control. I'm thinking that to get the LDRs even more closely matched for the balanced appication, I'll put a 10K or 20K PCB trimpot in series with each LED, and use these trimpots to converge the resistances of the LDRs at the spot on the volume control that produces average listening levels. Turns out my tube integrated phono/LS wants a 250K attenuator. I'm not optimistic about the LDR's low power rating and distortion numbers at higher signal voltages but I'll give it a go.
Typical resistance for NSL32SR2S "D" sorted LDRs on 5.2V power:
LED R = Photocell R
5.6K = 195R
26.6K = 623R
81K = 1.9K
148K = 3.9K
219K = 6.4K
470K = 19.2K
690K = 35.7K
1M = 70K
1.18M = 87K
1.5M = 130K
1.76M = 180K
Typical resistance for NSL32SR2S "D" sorted LDRs on 5.2V power:
LED R = Photocell R
5.6K = 195R
26.6K = 623R
81K = 1.9K
148K = 3.9K
219K = 6.4K
470K = 19.2K
690K = 35.7K
1M = 70K
1.18M = 87K
1.5M = 130K
1.76M = 180K
David Garretson said:
Please see http://www.diyaudio.com/forums/showthread.php?s=&threadid=80194&perpage=10&pagenumber=52. Post #515.
Thanks for the data.
Re: Input, not output.
This what I did and I got 46k 38k 48k for right & 66k 39k 44k for left
In each series of three figures the first is with volume at minimum, second is the lowest measured value and the third is full volume.
So your 20% guess is in the right ball park.
mike
Panelhead said:
This what I did and I got 46k 38k 48k for right & 66k 39k 44k for left
In each series of three figures the first is with volume at minimum, second is the lowest measured value and the third is full volume.
So your 20% guess is in the right ball park.
mike
Constant input impedance...
I have suggested a method using 10 position switch in lightspeed tone control. With this method u can calibrate the resistance to the exact value, but it works like a stepped volume control.
here
Other, not so simple method is to use another matched pair of vactrols and get an error signal to change the LED drive.
Gajanan Phadte
I have suggested a method using 10 position switch in lightspeed tone control. With this method u can calibrate the resistance to the exact value, but it works like a stepped volume control.
here
Other, not so simple method is to use another matched pair of vactrols and get an error signal to change the LED drive.
Gajanan Phadte
Dual mono controls
Nordic,
That is why I use dual controls. Not all sources have good channel balance to begin with. I have a bunch of OJC records that have very poor level matching. Suspect they were mono mixes, butchered to stereo.
Plus when you build everything, being able to listen to each channel alone is vital for troubleshooting.
Georgehifi uses a source and scope to factory calibrate the two chanels of his Lightspeed controls. This is great fort people who do not want to fiddle with two volume pots.
All my favorite commercial preamps owned in the past, Superphon Rev 2G, AI Modulus 2D, and Croft Epoch all used mono controls and no balance. So I have never build a diy unit with a stereo control.
Sad to say the two most expensive units I have owned had stereo controls. But neither stayed more for two years, they were a VLT Ultimate and CAT Ref. I still have the Croft and AI, the AI for 17 years. The Croft for only 12. The CAT was better than the VTL, the Croft easily bettered the CAT.
All my diy solid state line stages have ended up better sounding than the Croft. Most did not start out there, but got there after some work.
The Lightspeed is very good also. But is system dependent. In some systems it betters the actives, in others it lacks dynamics and low end in comparison.
Andyp, where are you on yours?
Nordic,
That is why I use dual controls. Not all sources have good channel balance to begin with. I have a bunch of OJC records that have very poor level matching. Suspect they were mono mixes, butchered to stereo.
Plus when you build everything, being able to listen to each channel alone is vital for troubleshooting.
Georgehifi uses a source and scope to factory calibrate the two chanels of his Lightspeed controls. This is great fort people who do not want to fiddle with two volume pots.
All my favorite commercial preamps owned in the past, Superphon Rev 2G, AI Modulus 2D, and Croft Epoch all used mono controls and no balance. So I have never build a diy unit with a stereo control.
Sad to say the two most expensive units I have owned had stereo controls. But neither stayed more for two years, they were a VLT Ultimate and CAT Ref. I still have the Croft and AI, the AI for 17 years. The Croft for only 12. The CAT was better than the VTL, the Croft easily bettered the CAT.
All my diy solid state line stages have ended up better sounding than the Croft. Most did not start out there, but got there after some work.
The Lightspeed is very good also. But is system dependent. In some systems it betters the actives, in others it lacks dynamics and low end in comparison.
Andyp, where are you on yours?
Re: Dual mono controls
in others it lacks dynamics and low end in comparison.
Andyp, where are you on yours? [/B][/QUOTE]
Obviously wasn't impedance matched properly, or gain was low on the poweramp, so the volume on the Lightspeed was past 3 o'clock?
This has been the only time I have had this happen.
Cheers George
Panelhead said:
in others it lacks dynamics and low end in comparison.
Andyp, where are you on yours? [/B][/QUOTE]
Obviously wasn't impedance matched properly, or gain was low on the poweramp, so the volume on the Lightspeed was past 3 o'clock?
This has been the only time I have had this happen.
Cheers George
Impedance
Mike,
Mine uses R3 diodes, not R2 so it measures different. But I get 22K - 29K from it with a single pair per 100K pot and 5 volts.
I have been away from home for the last weeks weeks, but plan on trying 50K switched log pots next. Hope to see 10K then.
The numbers you are getting are way to high for a passive. Are you sure this is input of circuit to signal ground? The number you listed for output impedance made sense. These input impedance look to be double of mine.
Glad you are happy with the sonics now.
I feel sorry for Georgehifi. Builds a worldclass line and sells for 400.00. And everyone wants to modify it to suit their ideas.
The pics of yours made me feel good about the products I build. And I got a real backhand compliment at the last audio get together I went to. The host said that if my equipment looked as good as Luther's, or if Luther's sounded as good as mine, we would have something.
I feel mine looks fine and I know Luther is very happy with how his sounds. Guess we both build to make ourselves happy.
George
Mike,
Mine uses R3 diodes, not R2 so it measures different. But I get 22K - 29K from it with a single pair per 100K pot and 5 volts.
I have been away from home for the last weeks weeks, but plan on trying 50K switched log pots next. Hope to see 10K then.
The numbers you are getting are way to high for a passive. Are you sure this is input of circuit to signal ground? The number you listed for output impedance made sense. These input impedance look to be double of mine.
Glad you are happy with the sonics now.
I feel sorry for Georgehifi. Builds a worldclass line and sells for 400.00. And everyone wants to modify it to suit their ideas.
The pics of yours made me feel good about the products I build. And I got a real backhand compliment at the last audio get together I went to. The host said that if my equipment looked as good as Luther's, or if Luther's sounded as good as mine, we would have something.
I feel mine looks fine and I know Luther is very happy with how his sounds. Guess we both build to make ourselves happy.
George
Re: Impedance
with a regular resistor volume control the electrical mid point o/p impedance should be approximately 1/4 of the i/p impedance ( if the driving impedance is low and o/p load impedance is very high )
i.e. 100K pot, mid point = 50K each side, these 50k's effectively in parallel = 25K
with lightspeed the LED's and LDR are both probably non-linear so given this is so, the comparison of my figures for i/p & o/p seem to make reasonable sense.
I would like to try the lower impedance devices sometime ( why do you call them diodes ? I thought LDR = light dependent resistor )
cheers
mike
Panelhead said:
with a regular resistor volume control the electrical mid point o/p impedance should be approximately 1/4 of the i/p impedance ( if the driving impedance is low and o/p load impedance is very high )
i.e. 100K pot, mid point = 50K each side, these 50k's effectively in parallel = 25K
with lightspeed the LED's and LDR are both probably non-linear so given this is so, the comparison of my figures for i/p & o/p seem to make reasonable sense.
I would like to try the lower impedance devices sometime ( why do you call them diodes ? I thought LDR = light dependent resistor )
cheers
mike
psymo said:
You don't need a buffer.
Going back to your other posts, the problem you have is, you have Pete Daniel chip power amp, it is 20k input, I have answered with a solution to this, here it is again, replace the 20k resistor with a 100k or 200k and then the Lightspeed will become the loading to ground instead of the 20k resistor. But never switch on your amp unless the Lightspeed is conected and powered up, or you will get highish dc offset from your amp.
You have not stated what CD player you have, and it's output impedance.
Cheers George