Hello, looking for some advice concerning a stepped attenuator I just purchased. Forgive me if this in the wrong forum but the knowledge here seems second to none so i thought id try. Anyway, I wanted to build one myself but not yet confident in my soldering/overall expertise. I was previously using a fixed series/variable shunt device with a [FONT=Arial,Helvetica,Geneva,Swiss,SunSans-Regular]Blue Alps Potentiometer[/FONT] with caddock resistors. It was a nice improvement over my active 5.1 preamp with better low level detail and clarity etc. However, upgraditis hit me and i then purchased the axiom (walker edition) from luminous audio as i heard good things about it. It previously used a TKD control but the company recently changed to a custom built 24 stepped attenuator and also has the caddock resistors with vampire rca's. I am using it with a sony 9000es cd player and Kinergetics research Chiro amp that has an input impedence of 25k ohms. The speakers are custom 2 way monitor size speakers with a 90db efficiency. .
After 20 hours of use, I can say the vocal clarity and lifelike presence is a big improvement. However, the bass and background instruments are dark/dull and distant sounding and the overall balance/ musicality just has less impact. The company does offer to change the resistors to different values to get better volume. But the overall volume seems ok on most recordings with the control at about 75%. I didnt have this issue with the Blue Alps potentiometer I was previously using. Question, is this known to happen with the stepped attenuators or does this sound like there is a mismatch or impedence issue with my unit. Any help is greatly appreciated.
Thanks
After 20 hours of use, I can say the vocal clarity and lifelike presence is a big improvement. However, the bass and background instruments are dark/dull and distant sounding and the overall balance/ musicality just has less impact. The company does offer to change the resistors to different values to get better volume. But the overall volume seems ok on most recordings with the control at about 75%. I didnt have this issue with the Blue Alps potentiometer I was previously using. Question, is this known to happen with the stepped attenuators or does this sound like there is a mismatch or impedence issue with my unit. Any help is greatly appreciated.
Thanks
First, you need to define your goal. Do you want the most-accurate reproduction, even if it doesn't "sound" as good, to you? I would hope so.
Next, you need to determine which setup is more accurate, by measuring, not listening (unless maybe your listening has been measured and is known to be accurate
.
Finally, choose the one that measures the flattest, i.e. "spectrum out" best-matches "spectrum in".
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Regarding your question about the possibility of something like an "impedance mismatch" issue: There are far too many variables for the amount of data that you have provided, for anyone here to really know. But I can imagine that the impedance of the attenuator, combined with some capacitance or inductance, either in a connected circuit or stray, could theoretically form a high-pass or low-pass filter. As with most similar types of speculation, even though that could happen, there is no way to know if the MAGNITUDE of the effect might be significant, or negligible, without measurements.
At any rate, the possibility of impedance issues creating undesired filtering effects is probably one of the main good reasons for using active buffer amplifiers, before and/or after a passive attenuation system.
Even if you were one of the zealots who thinks that it's bad to put more opamps in the sacred signal path, you could still use a couple of buffers just as a test, to see what the attenuator COULD (and should) sound like, under the proper conditions. Otherwise, without extensive measurements, there would be NO way to be sure WHICH component is causing any effect that you hear, or why.
Next, you need to determine which setup is more accurate, by measuring, not listening (unless maybe your listening has been measured and is known to be accurate
.Finally, choose the one that measures the flattest, i.e. "spectrum out" best-matches "spectrum in".
----
Regarding your question about the possibility of something like an "impedance mismatch" issue: There are far too many variables for the amount of data that you have provided, for anyone here to really know. But I can imagine that the impedance of the attenuator, combined with some capacitance or inductance, either in a connected circuit or stray, could theoretically form a high-pass or low-pass filter. As with most similar types of speculation, even though that could happen, there is no way to know if the MAGNITUDE of the effect might be significant, or negligible, without measurements.
At any rate, the possibility of impedance issues creating undesired filtering effects is probably one of the main good reasons for using active buffer amplifiers, before and/or after a passive attenuation system.
Even if you were one of the zealots who thinks that it's bad to put more opamps in the sacred signal path, you could still use a couple of buffers just as a test, to see what the attenuator COULD (and should) sound like, under the proper conditions. Otherwise, without extensive measurements, there would be NO way to be sure WHICH component is causing any effect that you hear, or why.
A picture would say a lot. It sure is bad to put more opamps (with opamps in general "less is more") in the signal path as gootee posted but it would be a good test to isolate the "problem". edit: I think gootee is right about using buffers with these specific fixed series/shunt attenuators. They are lower in resistance in many cases compared with normal attenuators.
So it would help a lot if you mention the resistance of the attenuator. If that one is for instance a 10 kOhm unit you probably have found the problem.
You could use a buffer before the stepped attenuator to check if the Sony is the cause. You will lighten the load on the Sony then. If your problem still exists please remove the buffer and use it after the stepped attenuator and check what happens.Maybe it is the amp that influences the sound character in some way.
I reread you post some times and : a fixed series/variable shunt device will behave as a low impedance/resistance at low volume levels. My guess is that the Sony is overloaded but to really determine that you will have to give more information. My experiments with these attenuators were stopped long ago as I found out they need far more thoughts on the source and amp side. More than I wished to care for.
So it would help a lot if you mention the resistance of the attenuator. If that one is for instance a 10 kOhm unit you probably have found the problem.
You could use a buffer before the stepped attenuator to check if the Sony is the cause. You will lighten the load on the Sony then. If your problem still exists please remove the buffer and use it after the stepped attenuator and check what happens.Maybe it is the amp that influences the sound character in some way.
I reread you post some times and : a fixed series/variable shunt device will behave as a low impedance/resistance at low volume levels. My guess is that the Sony is overloaded but to really determine that you will have to give more information. My experiments with these attenuators were stopped long ago as I found out they need far more thoughts on the source and amp side. More than I wished to care for.
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You can measure the input resistance of the stepped attenuator on any selected input (arbitrary) and then worse case output impedance is 1/4 of that value and occurs at -6dB attenuation.
If the total resistance of the attenuator is more than 5 - 10K you definitely need a buffer. Cable capacitance will be a major issue starting with a 10K attenuator and depends on cable length. Amplifier input impedance should be at least 10X higher than the worst case output impedance to minimize interaction with the taper of the attenuator.
A buffer will fix all issues regardless of attenuator impedance.
If the total resistance of the attenuator is more than 5 - 10K you definitely need a buffer. Cable capacitance will be a major issue starting with a 10K attenuator and depends on cable length. Amplifier input impedance should be at least 10X higher than the worst case output impedance to minimize interaction with the taper of the attenuator.
A buffer will fix all issues regardless of attenuator impedance.
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