The way I match mine is at three/five levels, coresponding to 9am, 12 noon, and 3pm on the volume control, then they are very quickly soldered in situ (this is important as too much heat will upset the matching), then a final tweak using the 1k trimer which is always used on the side with the most gain.
After I'm happy with that all four LDR's are then incased (potted) with a super hard high temp wax (racing ski base wax) then rechecked after it has cooled, the wax I believe adds to the thermal stablity of all 4 LDR's and to keep prying Chinese copy cats from looking, they can remove the wax but will undoubtly kill a couple of LDR's doing so.
As far as the talk about distortion, what is there is so minimal at the cd levels we are using, it's not even worth talking about, it's second hamonic in nature and so small it's insignifcant, and they say 2nd harmonic is pleasing to the ear anyway, that's why tubes are loved so much.
Cheers George
After I'm happy with that all four LDR's are then incased (potted) with a super hard high temp wax (racing ski base wax) then rechecked after it has cooled, the wax I believe adds to the thermal stablity of all 4 LDR's and to keep prying Chinese copy cats from looking, they can remove the wax but will undoubtly kill a couple of LDR's doing so.
As far as the talk about distortion, what is there is so minimal at the cd levels we are using, it's not even worth talking about, it's second hamonic in nature and so small it's insignifcant, and they say 2nd harmonic is pleasing to the ear anyway, that's why tubes are loved so much.
Cheers George
Andrea,
We bought 30 pcs NSL32SR3 (NOT the SR2 recommended by George) from RS at 1.64 Euro per piece. You will need at least 25 if you want 4 with real good match (say 0.5dB) as they really varies A LOT.
Or order selected ones, e.g. SR2S. If you add 30% import tax and VAT, then RS is not that expensive anymore.
Patrick
We bought 30 pcs NSL32SR3 (NOT the SR2 recommended by George) from RS at 1.64 Euro per piece. You will need at least 25 if you want 4 with real good match (say 0.5dB) as they really varies A LOT.
Or order selected ones, e.g. SR2S. If you add 30% import tax and VAT, then RS is not that expensive anymore.
Patrick
EUVL said:
Hello,
the SR3 are cheaper at Farnell too, but less matchable (as also George says). But if someone is already ordering at allied from the US and is willing to buy some on my behalf there are no taxes.. I am just trying to be cheap (as usual
)@RS UK they cost 1.21£ but there is a 10£ charge for orders under 100£ (extended range item)
@RS Italy cost is 3,04€ (no surcharge since it is already in the price
)Cheers
Andrea
LDR volume control in tube pre
Thinking again about the LDR to replace the stepped series attenuator in my Atma-Sphere MP-1 tube pre. I have a regulated 6V DC source available inside the chassis that powers the tube filaments. Would the LDR LEDs tolerate a 6V power source without requiring further downregulation to 5V? Would any adjustments to George's circuit be necessary?
Thinking again about the LDR to replace the stepped series attenuator in my Atma-Sphere MP-1 tube pre. I have a regulated 6V DC source available inside the chassis that powers the tube filaments. Would the LDR LEDs tolerate a 6V power source without requiring further downregulation to 5V? Would any adjustments to George's circuit be necessary?
David Garretson said:
Just to be sure I would build another simple +5vdc regulated supply and use the low dropout +5v reg from National semiconductors, just to keep things super smooth.
And have you any idea what the ouput impedence of your of your circutron phono stage is? I know it used to feed into the 100k volume control but now it will see 7k odd, if it is in the order of 1k I would definatly give it a go, especially if you can up the impedence of the next stage to over 200k which should be no problem if it is tube also.
Cheers George
100K Volume Control for Tube
GeorgeHifi,
I suppose output Z of the LDR series/shunt would be the value of the shunt LDR R in parallel with the grid R of the next tube LS, which in the MP-1 is 1M. Perhaps this is benign? But I don't know output Z of the upstream 12AT7 phono stage or how it will interact with input Z of the LDR. It would probably be optimal to have input Z of the LDR control as close as possible to the stock 100K series attenuator.
GeorgeHifi,
I suppose output Z of the LDR series/shunt would be the value of the shunt LDR R in parallel with the grid R of the next tube LS, which in the MP-1 is 1M. Perhaps this is benign? But I don't know output Z of the upstream 12AT7 phono stage or how it will interact with input Z of the LDR. It would probably be optimal to have input Z of the LDR control as close as possible to the stock 100K series attenuator.
Re: 100K Volume Control for Tube
If the Z out of the phono stage is unknown then just build it as I've shown and make your next stage 200k. It won't hurt anything and you have a better than even chance all will be fine as far as a good impedence match goes.
You will know if the impedence match is bad if your gain is reduced alot.
Cheers George
David Garretson said:
If the Z out of the phono stage is unknown then just build it as I've shown and make your next stage 200k. It won't hurt anything and you have a better than even chance all will be fine as far as a good impedence match goes.
You will know if the impedence match is bad if your gain is reduced alot.
Cheers George
Sorry leave your next stage at 1megohm forget about changing to 200k.
You will know if it is loading up your phono stage too much if the gain comes down in comparision to the line level inputs, which by the way will sound far better than they used to. If it does reduce in gain then you can try some mods to bring the phono Z down to about 1-2k. Then all will be ok.
Below is a very low (100ohm) output z for a tube output (Developed by Allen Wright SLCF).
Cheers George
You will know if it is loading up your phono stage too much if the gain comes down in comparision to the line level inputs, which by the way will sound far better than they used to. If it does reduce in gain then you can try some mods to bring the phono Z down to about 1-2k. Then all will be ok.
Below is a very low (100ohm) output z for a tube output (Developed by Allen Wright SLCF).
Cheers George
Attachments
100K Volume Control for Tube
Thanks, George. I just ordered qty 30 NSL-32SR2S with hopes of closely matching 8 for for the diff balanced attenuator. I've got a Radio Shack 100K dual pot. I suppose I'll run all 4 matched series elements on one ramp and all 4 shunt elements on the other ramp. Luckily I won't have to worry about L/R balance, as the pre has separate trim controls that vary voltage to the LS tubes allowing minor balance adjustments.
Thanks, George. I just ordered qty 30 NSL-32SR2S with hopes of closely matching 8 for for the diff balanced attenuator. I've got a Radio Shack 100K dual pot. I suppose I'll run all 4 matched series elements on one ramp and all 4 shunt elements on the other ramp. Luckily I won't have to worry about L/R balance, as the pre has separate trim controls that vary voltage to the LS tubes allowing minor balance adjustments.
David if you get 2 sets out of the 30, I will gladly help you recover some of the costs... I can get some from Farnell, via another company, but it gets too complicated and too many peole want a profit in the chain.
Question... would it be possible to use 2 in series, for greater impendance..
Question... would it be possible to use 2 in series, for greater impendance..
Brain never rests
I measured two I built. Mine is dual mono, got 8 -10K with 50K linear, 22-29K with 100K linear stereo pots.
So in a stroke of genius, it dawned on me that my LM317 could be used to dial in the impedance wanted. I plan on replacing the fixed voltage adjust resistor with a 1K pot. My series resistors are 200 hm due to only one LDR per pot track.
I will raise the voltage on the on measuring 22K - 29k, and lower it a little on the one measuring 8 -10K. I want 15- 20K on one and 10 -12K on the other.
If you are unsure, connect a VOM from input to signal ground. As you rotate the pot it will show how much the total resistance varies from full attenuation to wide open. I suspect my linear pots are better at keeping a constant impdeance, but might be wrong.
The linear pots are more hair trigger, listening level is in the first 20- 25% of rotation.
I measured two I built. Mine is dual mono, got 8 -10K with 50K linear, 22-29K with 100K linear stereo pots.
So in a stroke of genius, it dawned on me that my LM317 could be used to dial in the impedance wanted. I plan on replacing the fixed voltage adjust resistor with a 1K pot. My series resistors are 200 hm due to only one LDR per pot track.
I will raise the voltage on the on measuring 22K - 29k, and lower it a little on the one measuring 8 -10K. I want 15- 20K on one and 10 -12K on the other.
If you are unsure, connect a VOM from input to signal ground. As you rotate the pot it will show how much the total resistance varies from full attenuation to wide open. I suspect my linear pots are better at keeping a constant impdeance, but might be wrong.
The linear pots are more hair trigger, listening level is in the first 20- 25% of rotation.
Re: Quick question
Dr.H you only need to put the 1k trimer on the louder of the two chanels, then you can adjust two ways, most accurate is to calibrate with an audio generator and occiloscope between 9am and 3pm on the volume control with a 1k sine wave and match the gain of the two chanels to be equal.
Or you can do it by ear and pick a vocal track that you know has a good central image and with the volume control at your normal listening position and adjust the 1K trimer so the image is centered between the two speakers. It is possible to use this 1k trimer as a mild balance control this way.
Cheers George
Dr.H said:I have set the LDR's up as per George's implementation. How do I match the LDR's using the 1K pots? Waht do I need to meaure and how.
Thanks
Ryan
PS, I will be using my DACT stepped attenuator as the voltage adjuster (at least to start of with....)
Dr.H you only need to put the 1k trimer on the louder of the two chanels, then you can adjust two ways, most accurate is to calibrate with an audio generator and occiloscope between 9am and 3pm on the volume control with a 1k sine wave and match the gain of the two chanels to be equal.
Or you can do it by ear and pick a vocal track that you know has a good central image and with the volume control at your normal listening position and adjust the 1K trimer so the image is centered between the two speakers. It is possible to use this 1k trimer as a mild balance control this way.
Cheers George