100K Volume Control for Tube Pre
I've determined that my diff balanced 12AT7-based Atmasphere MP-1 phono section has an output impedance in the range of 5-10K. So to use an LDR volume control, I need either raise input impedance of the LDR series/shunt to above 100K, or modify the phono stage. I like Panelhead/George's idea of building a variable voltage regulator to tinker with input impedance. Does anybody know how far down under 5V control voltage one would have to go to push impedance of the series/shunt elements into the 100K-200K range? At that level will distortion products of the resistive elements become problematic?
I've determined that my diff balanced 12AT7-based Atmasphere MP-1 phono section has an output impedance in the range of 5-10K. So to use an LDR volume control, I need either raise input impedance of the LDR series/shunt to above 100K, or modify the phono stage. I like Panelhead/George's idea of building a variable voltage regulator to tinker with input impedance. Does anybody know how far down under 5V control voltage one would have to go to push impedance of the series/shunt elements into the 100K-200K range? At that level will distortion products of the resistive elements become problematic?
My idea
Dave,
My testing has all been done with two stereo controls to allow dual mono volume controls. But from what I have seen a pair of dual 250K linear pots will get you in the ballpark. Then diddle the input voltage to dial in exactly the value you want.
What I have seen is about a 15 -20% drop in the middle. So a 100K control at the extremes might go 80 -85K in the middle. But most likely a 250K pots will come in higher the 100K.
I have not done any testing yet with the variable voltage. But I hoped the values I want will be achieved with maybe +/- 0.5 volt.
Changing the voltage much more will require recalculating the value of the 100 ohm series resistors to keep from running to much juice through the LED's. Mine are 200 ohm because there is only one LED instead of two.
The taper is a little off using linear pots. It would be close to linear taper if you only used one LDR shunt and a fixed series resistor, ala Melos. But by using a variable shunt and series the taper is second order. And since the diodes are not linear, the this make the taper a little closer to log. But it still comes up faster than a log pot.
I am going to add a LDR control to my active line in the next two weeks. And keep my LDR passive for comparison. I am sold on LDR's for attenuation. Need to see if I am sold on passives.
And, if you are on the fence, blow 400.00 and buy a real Lightspeed from Georgehifi. His are most likely much better sounding than mine is. I am just learning how to use these. He has it down.
George
Dave,
My testing has all been done with two stereo controls to allow dual mono volume controls. But from what I have seen a pair of dual 250K linear pots will get you in the ballpark. Then diddle the input voltage to dial in exactly the value you want.
What I have seen is about a 15 -20% drop in the middle. So a 100K control at the extremes might go 80 -85K in the middle. But most likely a 250K pots will come in higher the 100K.
I have not done any testing yet with the variable voltage. But I hoped the values I want will be achieved with maybe +/- 0.5 volt.
Changing the voltage much more will require recalculating the value of the 100 ohm series resistors to keep from running to much juice through the LED's. Mine are 200 ohm because there is only one LED instead of two.
The taper is a little off using linear pots. It would be close to linear taper if you only used one LDR shunt and a fixed series resistor, ala Melos. But by using a variable shunt and series the taper is second order. And since the diodes are not linear, the this make the taper a little closer to log. But it still comes up faster than a log pot.
I am going to add a LDR control to my active line in the next two weeks. And keep my LDR passive for comparison. I am sold on LDR's for attenuation. Need to see if I am sold on passives.
And, if you are on the fence, blow 400.00 and buy a real Lightspeed from Georgehifi. His are most likely much better sounding than mine is. I am just learning how to use these. He has it down.
George
100K Volume Control for Tube Pre
George,
I liked your idea of lowering voltage output at the control regulator, because that would let me use just one (possibly motorized) dual pot to control volume, and adjust channel balance with two stock 10K trim pots that vary plate voltage to a LS tube. I would think that after sufficiently reducing the regulator's output, then either a 100K or 250K dual pot could be used to obtain the desired high impedance. So far I have just a 100K dual log pot. Can you suggest a US source for cheap 250K or higher dual pots?
George,
I liked your idea of lowering voltage output at the control regulator, because that would let me use just one (possibly motorized) dual pot to control volume, and adjust channel balance with two stock 10K trim pots that vary plate voltage to a LS tube. I would think that after sufficiently reducing the regulator's output, then either a 100K or 250K dual pot could be used to obtain the desired high impedance. So far I have just a 100K dual log pot. Can you suggest a US source for cheap 250K or higher dual pots?
No luck
Hi George,
Connected the schematic as shown using a 100k volume pot, but the it appeared not to work.
Some symptoms:
1. I'm using a 5.39V source, which when the volume pot was at either extreme, dropped to 5.36V. I sort of expected that it would drop to 0V.
2. I did not connect the ground of the LDR's to anything in the circuit
3. The 1k pots were set at 400 ohms.
Any idea why this did not work?
Thanks
Ryan
Hi George,
Connected the schematic as shown using a 100k volume pot, but the it appeared not to work.
Some symptoms:
1. I'm using a 5.39V source, which when the volume pot was at either extreme, dropped to 5.36V. I sort of expected that it would drop to 0V.
2. I did not connect the ground of the LDR's to anything in the circuit
3. The 1k pots were set at 400 ohms.
Any idea why this did not work?
Thanks
Ryan
Re: No luck
Hi Dr H, you need to gound the - of the 4x LED's to ground and also the ground or - of the 5vdc/reg power supply and one side each of the shunt LDR's x2. I've quickly redone the circuit of it so it is a bit easier to follow, and marked the chassis/rca earth points, also make sure that each section of the dual pot is wired and works opposite to each other.
Dr.H said:
Hi Dr H, you need to gound the - of the 4x LED's to ground and also the ground or - of the 5vdc/reg power supply and one side each of the shunt LDR's x2. I've quickly redone the circuit of it so it is a bit easier to follow, and marked the chassis/rca earth points, also make sure that each section of the dual pot is wired and works opposite to each other.
Attachments
Re: 100K Volume Control for Tube Pre
Dave,
I built a balanced control. Never used it and recently pulled the matched LDR's out.
THis how I rigged it. Two series elements, one to + and one to -. Then two shunt LDR's, one end tied to the respective series LDR, the other to ground. Input on one end of series LDR, output taken off where the series and shunt LDR's are tied together.
This gets you one channel. Matching up 8 exactly will be tough. I went for 2 sets of 4. Used one set for series and the other for shunts.
Since you are running all these off of one pot, a 100K make be fine to get you to 100 -200K. You may even get by with a 50K.
Good Luck!
David Garretson said:
Dave,
I built a balanced control. Never used it and recently pulled the matched LDR's out.
THis how I rigged it. Two series elements, one to + and one to -. Then two shunt LDR's, one end tied to the respective series LDR, the other to ground. Input on one end of series LDR, output taken off where the series and shunt LDR's are tied together.
This gets you one channel. Matching up 8 exactly will be tough. I went for 2 sets of 4. Used one set for series and the other for shunts.
Since you are running all these off of one pot, a 100K make be fine to get you to 100 -200K. You may even get by with a 50K.
Good Luck!
100K Volume Control for Tube Pre
Panelhead/George,
Yes I understand. I was just thinking that a way to increase impedance might be to wire the photoresistive portion of the optocouler as you suggest (with one series/shunt pair per signal phase), and "dim" the control elements by wiring just the LED portion in series or a combination of series & parallel for the four LEDs sharing a ramp on the control pot.
Panelhead/George,
Yes I understand. I was just thinking that a way to increase impedance might be to wire the photoresistive portion of the optocouler as you suggest (with one series/shunt pair per signal phase), and "dim" the control elements by wiring just the LED portion in series or a combination of series & parallel for the four LEDs sharing a ramp on the control pot.
No luck
Still no luck. Even with the ground connected as shown.
Two questions:
1. When using 1mA current for matching LDR's, what sort of resistances do the LDR's give? In the megaohm range? I was expecting lower numbers.
2. The volume control is used to change the voltages to the LDR's. When the voltage from one gang on the pot reads 5V, what should the other read? 0V? In my case, I'm using 5.39V input and the lowest voltage I get is 5.36V, not zero.
Thanks for any input.
Ryan
Still no luck. Even with the ground connected as shown.
Two questions:
1. When using 1mA current for matching LDR's, what sort of resistances do the LDR's give? In the megaohm range? I was expecting lower numbers.
2. The volume control is used to change the voltages to the LDR's. When the voltage from one gang on the pot reads 5V, what should the other read? 0V? In my case, I'm using 5.39V input and the lowest voltage I get is 5.36V, not zero.
Thanks for any input.
Ryan
Re: No luck
Sounds to me like you have connected your pot's wrong or that they lack connection to ground. Try to measure the pots without the rest of the circuit
Edit: Sorry - just looked at the circuit again. Since you have very low difference from 0 ohm and 200k in series, then the path is surely broken somewhere. If not in the resistors, the wires, or the PCB then it must be the LEDs.
Check the resistors, PCB etc with an ohm-meter.
Sounds to me like you have connected your pot's wrong or that they lack connection to ground. Try to measure the pots without the rest of the circuit
Edit: Sorry - just looked at the circuit again. Since you have very low difference from 0 ohm and 200k in series, then the path is surely broken somewhere. If not in the resistors, the wires, or the PCB then it must be the LEDs.
Check the resistors, PCB etc with an ohm-meter.
Matching LDRs
Interesting idea from Audio Asylum poster on how ensure close matching of LDRs in the passive:
http://www.AudioAsylum.com/forums/tubediy/messages/116896.html
Posted by Istanbul Wasconstantinople
Use a pair of LDRs and an opamp.
------------------------------
One LDR is for the volume control and the other is a reference. You feed a control voltage to the op-amp which measures the voltage drop across the reference LDR and adjusts the lamp voltage to both LDRs accordingly.
Provided the LDRs are in the same environment, this will compensate for temperature drift and nonlinear light response of the LDR. This should suffice even if the LDRs aren't perfectly matched.
Note, LDRs are not as linear as regular resistors. The impact on sound quality is unknown.
IIRC, this idea came from Melos.
Interesting idea from Audio Asylum poster on how ensure close matching of LDRs in the passive:
http://www.AudioAsylum.com/forums/tubediy/messages/116896.html
Posted by Istanbul Wasconstantinople
Use a pair of LDRs and an opamp.
------------------------------
One LDR is for the volume control and the other is a reference. You feed a control voltage to the op-amp which measures the voltage drop across the reference LDR and adjusts the lamp voltage to both LDRs accordingly.
Provided the LDRs are in the same environment, this will compensate for temperature drift and nonlinear light response of the LDR. This should suffice even if the LDRs aren't perfectly matched.
Note, LDRs are not as linear as regular resistors. The impact on sound quality is unknown.
IIRC, this idea came from Melos.
Re: Voltages
If you are using a 5vdc reg the 100ohm limiting resistor is right, if you use 9vdc reg you will blow the led's instantly with the 100ohm limting resistor it needs to be a higher calculated resistance.
This is a formula for led limiting resistors, work on a maximum of 20mA and you'll be safe.
http://www.diyaudio.com/forums/attachment.php?s=&postid=939925&stamp=1150264203
Cheers George
Dr.H said:
If you are using a 5vdc reg the 100ohm limiting resistor is right, if you use 9vdc reg you will blow the led's instantly with the 100ohm limting resistor it needs to be a higher calculated resistance.
This is a formula for led limiting resistors, work on a maximum of 20mA and you'll be safe.
http://www.diyaudio.com/forums/attachment.php?s=&postid=939925&stamp=1150264203
Cheers George
Attachments
100K Volume Control for Tube Pre
I breadboarded GeorgeHifi's circuit using 8 LDRs as needed for a balanced volume control in my integrated tube pre/phono stage. This was just to determine impedance behavior prior to matching up the LDRs for a buildout. 4 Series LDRs share one ramp of a 100K dual log pot and 4 Shunt LDRs share the complementary ramp. A 100R current-limiting resistor is on each Series LED and 150R is on each Shunt LED-- unfortunately I didn't have enough 100R for the job.
Across the range of the pot, input Z is between 15K-20K. R input-to-output is 20K at full attenuation. This may be OK for a standalone passive, but it won't get near the 100K ladder I need to accomodate the 10K impedance of my phono section.
I determined that 5V control voltage and R of 1.35M on the LED of a single LDR drives its photoresistor to 100K. Series-wiring the LED elements of 2 LDRs drives each LDR to slightly more than double its R. So I guess to increase impedance I can either use a much larger pot, reduce control voltage, or perhaps series-wire four LED elements while retaining George's suggested wiring of the photoresistive elements.
George, at higher attenuations does the LDR become progressively nonlinear, such that even careful bench matching may not produce an accurate high-impedance balanced volume control? If this is a problem, perhaps I can either (1) design an op-amp based control schema a la Melos as per the above posts, or (2) buffer my phono section as per your previous suggestion, or oh no (3) sell my vinyl. I'd prefer #1 if I someone could help figure it out....
Regards,
Dave
I breadboarded GeorgeHifi's circuit using 8 LDRs as needed for a balanced volume control in my integrated tube pre/phono stage. This was just to determine impedance behavior prior to matching up the LDRs for a buildout. 4 Series LDRs share one ramp of a 100K dual log pot and 4 Shunt LDRs share the complementary ramp. A 100R current-limiting resistor is on each Series LED and 150R is on each Shunt LED-- unfortunately I didn't have enough 100R for the job.
Across the range of the pot, input Z is between 15K-20K. R input-to-output is 20K at full attenuation. This may be OK for a standalone passive, but it won't get near the 100K ladder I need to accomodate the 10K impedance of my phono section.
I determined that 5V control voltage and R of 1.35M on the LED of a single LDR drives its photoresistor to 100K. Series-wiring the LED elements of 2 LDRs drives each LDR to slightly more than double its R. So I guess to increase impedance I can either use a much larger pot, reduce control voltage, or perhaps series-wire four LED elements while retaining George's suggested wiring of the photoresistive elements.
George, at higher attenuations does the LDR become progressively nonlinear, such that even careful bench matching may not produce an accurate high-impedance balanced volume control? If this is a problem, perhaps I can either (1) design an op-amp based control schema a la Melos as per the above posts, or (2) buffer my phono section as per your previous suggestion, or oh no (3) sell my vinyl. I'd prefer #1 if I someone could help figure it out....
Regards,
Dave
Hi Dave, I don't know which George your asking but you are persistant if nothing else with this path your taking on, wish you luck in your quest, glad it's you and not me, these little buggers can do your head in, if you don't have enough coffee in the house.
As to your question about about the resistance curve of the 32sr2s I have never done a resistance v light intensity graph on these, but i get the impression after playing with them for so long that they have almost an anti logarithmic curve to them.
Cheers George
As to your question about about the resistance curve of the 32sr2s I have never done a resistance v light intensity graph on these, but i get the impression after playing with them for so long that they have almost an anti logarithmic curve to them.
Cheers George