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I'm trying various log volume pots as a source line attenuator. They all seem to dull the dynamics and roll off the treble to some degree.
Would using a stepped attenuator with quality descrete resistors solve those problems? I found this outline to try.
thanks
charles
Step1 Attenuation = Infinity dB, Rx = 250000 ohms, Ry = 0 ohms, (249K - 0)
Step2 Attenuation = 69 dB, Rx = 249911 ohms, Ry = 89 ohms, (249K - 88.7R)
Step3 Attenuation = 64 dB, Rx = 249842 ohms, Ry = 158 ohms, (249K - 158R)
Step4 Attenuation = 59 dB, Rx = 249719 ohms, Ry = 281 ohms, (249K - 280R)
Step5 Attenuation = 54 dB, Rx = 249501 ohms, Ry = 499 ohms, (249K - 499R)
Step6 Attenuation = 49 dB, Rx = 249113 ohms, Ry = 887 ohms, (249K - 887R)
Step7 Attenuation = 44 dB, Rx = 248423 ohms, Ry = 1577 ohms,(249K - 1.58K)
Step8 Attenuation = 40 dB, Rx = 247500 ohms, Ry = 2500 ohms,(249K - 2.49K)
Step9 Attenuation = 36 dB, Rx = 246038 ohms, Ry = 3962 ohms,(243K - 3.92K)
Step10 Attenuation = 34 dB, Rx = 245012 ohms, Ry = 4988 ohms,243K - 4.99K)
Step11 Attenuation = 32 dB, Rx = 243720 ohms, Ry = 6280 ohms,243K - 6.34K)
Step12 Attenuation = 30 dB, Rx = 242094 ohms, Ry = 7906 ohms,243K - 7.87K)
Step13 Attenuation = 28 dB, Rx = 240047 ohms, Ry = 9953 ohms,237K - 9.76K)
Step14 Attenuation = 26 dB, Rx = 237470 ohms, Ry = 12530 ohms237K - 12.4K)
Step15 Attenuation = 24 dB, Rx = 234226 ohms, Ry = 15774 ohms232K - 15.8K)
Step16 Attenuation = 22 dB, Rx = 230142 ohms, Ry = 19858 ohms232K - 19.6K)
Step17 Attenuation = 20 dB, Rx = 225000 ohms, Ry = 25000 ohms226K - 24.9K)
Step18 Attenuation = 18 dB, Rx = 218527 ohms, Ry = 31473ohms221K - 31.6K)
Step19 Attenuation = 15 dB, Rx = 205543 ohms, Ry = 44457 ohms205K - 44.2K)
Step20 Attenuation = 12 dB, Rx = 187203 ohms, Ry = 62797 ohms187K - 63.4K)
Step21 Attenuation = 9 dB, Rx = 161297 ohms, Ry = 88703 ohms,162K - 88.7K)
Step22 Attenuation = 6 dB, Rx = 124703 ohms, Ry = 125297 ohms,124K - 124K)
Step23 Attenuation = 3 dB, Rx = 73014 ohms, Ry = 176986 ohms,73.2K - 178K)
Step24 Attenuation = 0 dB, Rx = 0 ohms, Ry = 250000 ohms (0 - 249K)
Follow Ups:
To get natural balance and soundstage one must fine tune playback volume. Two dB (or greater) steps will not cut it. Frequently one can hear that the best setting is half-way in between two steps. I use a stepped attenuator with 1 dB steps and find this to be OK, but others suggest that 1/2 dB steps are better.
The high frequency roll-off depends on the resistance selected and the load capacitance (cable plus destination). Lower resistance and/or lower capacitance means less high frequency roll-off. However, the resistors are in parallel with the load and if they are too low they may overload the drivers at the source. Passive attenuators are best used with short runs from the attenuator to the amplifier input. You can also minimize the effects by using (expensive) low capacitance cable. The sound quality issues will be the same with stepped on continuous attenuators, except that the continuous attenuators may have more opportunity for noisy contacts.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
The output impedance at the -6db step (step 22) will be the highest.
At that position the output impedance will be 62.5k ohms.
With only 100Pf of shunt capacitance (cable + first stage Miller) the -3db point of the low pass filter will be at 25kHz and the phase shift will extend down to 2.5kHz, more capacitance and it gets worse.
For a resistive passive attenuator to really work proper and not cause HF roll-off you need a 20k ohm unit and sources that are happy driving 20k ohms.
I use a AVC (autoformer volume control) which always has lower output impedance than the source.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It looks like an L-Pad with only two resistors in each channel at any given time and constant input impedance. It should sound good!
That is what I use in my preamp.
Mike
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