[Barry_NJ]

I'll be curious to read what you think...

Me too! I'd really like to hear a solid critique/evaluation of the CDA vs. SDS boards. If there really isn't a big difference in the sound of the two I'd likely go with the less expensive option. IF however the SDS is superior, I'll spend the extra $95.

[jtwrace]

In my system and to my ears the SDS sounds better.  Yes, cleaner with more micro detail. 

What I mean't was about the resistor instead of the pots.  I'm still  over this one.   

[Nick77]

In my system and to my ears the SDS sounds better.  Yes, cleaner with more micro detail. 

What I mean't was about the resistor instead of the pots.  I'm still  over this one.   

I felt the SDS board yielded more control and better detail also.

[wushuliu]

As a side note, I'm betting for SDS resistor mod, using the naked Vishays would pay off huge - especially since you only need 2... 

[wushuliu]

Many thanks Neil. The resistor mod is a must. Plain and simple. Before what I heard wasn't $95 better than the CDA. Using 3.3k PRPs, the gain roughly in the same place I had before, was a game changer. No one will be disappointed. I will most certainly be getting the naked Vishays for this...

[Nick77]

Wush so what your saying is the gain pots are holding back the amp and switching to a fixed gain using a resistor improves SQ? I have my gain set to about 1 oclock, what should i try? I guess it would be trial and error with cheap resistors to determine exact resistance for my setup?

Would the same be true switching to a better gain pot? Could one say that the cheap plastic pots degrade the sound?

[Barry_NJ]

No trial and no error, just measure the resistance you've dialed in with the pot and get resistors of that value.

[Nick77]

No trial and no error, just measure the resistance you've dialed in with the pot and get resistors of that value.

Okay thanks, how does one do that?

[poseidonsvoice]

Nick and all,

Let's learn to walk before we run!

Anatomy of a potentiometer



1 = ground.
2 = wiper or output.
3 = input.

Here is a picture of a 10K linear potentiometer. Notice where he put the leads, i.e. one lead on the input and the other on the output/wiper:


The minimum resistance of the pot is obviously 0 ohms or a direct short.




Here the pot is centered, so the resistance is about 5K.



Here the resistance is about 10K, or maximum attenuation.

Get it?

This post probably belongs in the "Lab" as well.

Required reading right here.

Anand.

[Nick77]

Anand, thank you! Very informitive, i guess then another test would be to try a better pot, i like having the flexablility of adjustment.

Are the plastic one's 10K?

[poseidonsvoice]

Anand, thank you! Very informitive, i guess then another test would be to try a better pot, i like having the flexablility of adjustment.

Are the plastic one's 10K?

One of mine measures 10K and the other measures 9.95K so plenty close to 10K.  I just set them where it approximates to about 26dB of gain, which is ~ 5K.

Anybody who has a good oscilloscope, please corroborate, it would be nice to know what resistance settings correspond to various levels of gain, either in dB or "X".

Anand.

[jtwrace]

Anybody who has a good oscilloscope, please corroborate, it would be nice to know what resistance settings correspond to various levels of gain, either in dB or "X".

Anand.

I'll loan you my http://www.easysync-ltd.com/product/520/ds1m12.html  It sits more than it gets used.   

[nwboater]

Can anyone explain from a technical perspective why two resistors in a voltage divider configuration with resistances being the same as the original pot would cause the sound to be different? I am baffled!

Thanks,
Rod

[jtwrace]

Can anyone explain from a technical perspective why two resistors in a voltage divider configuration with resistances being the same as the original pot would cause the sound to be different? I am baffled!

Thanks,
Rod

That makes two of us!

[dwk]

Well, whenever you start discussion 'sound quality' there is a lot of speculation and subjective opinion involved, but.....

First, the fixed resistor is replacing not only the resistive element of the pot, but also a long wire that connects the pot to the board.  Since this resistive element is in the feedback loop, it is rather sensitive to contamination from emi/rfi etc. Eliminating the wire is likely to be a large part of the improvement.

Of course, I think the original mod was simply to stick the resistor at the end of the same long wire and that was also reported to be an improvement, so it's possible that the quality of the resistive element is also a factor. I don't think it's too much of a stretch to think that a conductive plastic pot and the associated mechanical contact has a different noise/distortion signature than a fixed resistor. Whether this is enough to be an audible factor even in a feedback loop is a valid question, but it's not uncommon for people to report significant benefit when switching from a cheap pot to a stepped attenuator in a preamp - in fact it seems to almost be 'accepted wisdom' that stepped attenuators using fixed resistors are much better.

So, it's probably a collection of factors
- eliminating the long wire leads
- eliminating the mechanical contact in the pot
- replacing the conductive plastic element with a fixed resistor.

As always, YMMV.

[wushuliu]

Well, whenever you start discussion 'sound quality' there is a lot of speculation and subjective opinion involved, but.....

First, the fixed resistor is replacing not only the resistive element of the pot, but also a long wire that connects the pot to the board.  Since this resistive element is in the feedback loop, it is rather sensitive to contamination from emi/rfi etc. Eliminating the wire is likely to be a large part of the improvement.

Of course, I think the original mod was simply to stick the resistor at the end of the same long wire and that was also reported to be an improvement, so it's possible that the quality of the resistive element is also a factor. I don't think it's too much of a stretch to think that a conductive plastic pot and the associated mechanical contact has a different noise/distortion signature than a fixed resistor. Whether this is enough to be an audible factor even in a feedback loop is a valid question, but it's not uncommon for people to report significant benefit when switching from a cheap pot to a stepped attenuator in a preamp - in fact it seems to almost be 'accepted wisdom' that stepped attenuators using fixed resistors are much better.

So, it's probably a collection of factors
- eliminating the long wire leads
- eliminating the mechanical contact in the pot
- replacing the conductive plastic element with a fixed resistor.

As always, YMMV.

Unless I'm mistaken this is the appeal of the Lightspeed attenuator/passive pre, the assumption being that all pots degrade the signal to some degree.

Regardless, there's a big enough step in sound quality that anyone could hear.

[wushuliu]

Nick and all,

Let's learn to walk before we run!

Anand.

Thanks for posting that Anand! 

[walkern]

In my VERY limited experience, different types of resistors sound different as well.  So there may be something to the notion that one type of resistor is a better 'match' to the sound of the amp than another, or than a conductive plastic attenuator.  Certainly high quality resistors sound different (usually better) than lesser quality resistors as well... so I'm thinking that there are more factors to consider in addition to the ones noted by DWK.

In any case, it was pretty obvious to me that using resistors in place of the pots clearly improved the quality of the sound of the SDS amp I own.  And I haven't even tried the really nice high quality types of resistors yet.  Thanks to everyone who was willing to try out this tweak and report on their findings.

Neil

[jtwrace]

In any case, it was pretty obvious to me that using resistors in place of the pots clearly improved the quality of the sound of the SDS amp I own. 
Neil

That's all that matters.  Thanks for your effort.  When I get my second SDS built I will try it.  In the meantime, I look forward to others comments on this possible tweak.

[nwboater]

Thanks all for your explanations. I'm still a little baffled, but do accept what you are hearing. It's a bit of a disappointment though as I'm using a 'receiver-less' system. I'm going from the soundcard in my HTPC straight to the power amps. Having individual amp gain controls is a plus. So if I get one, or more, of these amps I'll have to set the gains on the pots at the right place for system balance, then change to fixed resistors.

Rod