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In Reply to: Re: Too low vol setting bad for dynamics ? posted by zanash on July 24, 2005 at 11:51:18:
>low cost vol pot have all sorts of none linear faults at low levels...
How do you know Trackside's Arcam Delta 60 amp has a cheap vol pot?
Arcam uses good quality components, I own some equipement of them.That said, you're right, and furthermore since they are log pots. The only relevant non-linearity is a tolerance in the curve angle vs pedestal resistance. Such a non-linearity would give balance error between channels, but, once set, cannot degrade the signal. Or I'm missing something (it interests me, since, as a secondary job, I teach electronic components and sensors technology in a MS level engineering school in Marseilles)?>Also badly designed input and output stages can be overwhelmed...
Same question as above, how do you know Arcam badly designed input and output stages in their Delta 60 amp?> I belive
I don't. Arcam, as most manufacturers, and all the serious ones, uses the standard consumer input level. The gain is set in order to give the expected steady output power level on a reference impedance (8 ohms) with the standard consumer input level + a headroom. So, as long as the load impedance is not lower than specified (often 3 ohms), and the input level is not higher than the standard level + its headroom, you just cannot overwhelm any design that comply with its specifications.> Attenuation will help here to
Attenuation will help before the preamp, between the CD reader and the PA, that's were the level mismatch occurs. I agree with the attenuator builder in this matter>Many to notch preamps have selectable input sensetivites to help overcome this problem.
All of them (at least in high-end) should. It's the best way to manage the two standards, balanced vs unbalanced (you bet I'm a "balanced" afficionado!) and their very different levels.
Follow Ups:
It was a genral statment...the arcams are of an ok build quality, not top notch, tend to use standard components and the designs are conservative but well laid out. I would happily own an arcam, have happily owned an A60, cd73, Black box......That said a little tweeking and some good quality components certainly raise the performance from ok to top notch. Your right about degrading the signal, but a problem pot will effect the percived end sound to a greater extent than is often realised. I know this from work done on several mass market integrateds I've modded. Once the cheap pots are swapped out for say a Bournes conductive plastic of the same value the unit realy start to sing.
> That said a little tweeking and some good quality components certainly raise the > performance from ok to top notch
Sure. Sad you cannot tweak the layout, that's were most improvements can be made (parallel layout of current loops, guarding high impedance tracks, separate layout of all grounds, decoupling between source IC or discrete transistors power lines, and FB return of following stage, highest possible components density - with thermal behavior sims obviously- etc etc.) You soon go to the point where a 2-layer board is just not enough, and you 'll use a four-layer board for good planes above the tracks and components that need it.> Once the cheap pots are swapped out for say a Bournes conductive plastic
> of the same value the unit realy start to sing.
A common problem with cheap pots is that the wiper, rubbing onto the conductive track, tears off small particles and dust from the conductive material. There are two contradictory constrains for the pot designer; applying a light rubbing pressure (to avoid eating the track too fast), but dust will accumulate under the wiper, increase the wiper resistance, and give bad sound (because the dust resistance is non-linear, as for most percolating systems). Or appying an high pressure, but you eat out the track sooner, and the pot won't last long. For treble and bass controls mainstream designer use high wiper pressure pots (most are linear BTW), as they guess the pots will be used sparcely, for the volume pot, they use low pressure pots.
Or, the best solutions are:
-using a hard track material, and apply a high pressure. That is the Cermet pots case . Cermet is a mix (up to the nanoscale) of metallic oxydes -ceramics- and a conductor (tungsten, molybdenum or allied silver). People don't like them in audio circles, I don't know why, all my pots are now cermet, and I'm happy with them
-using a supple, "lissom" material, that will grow hollow under the wiper pressure then get back to its initial shape. So, no tearing off dust particles. That's the conductive plastic pots you were talking about.
> I know this from work done on several mass market integrateds I've modded. Once the
> cheap pots are swapped out for say a Bournes conductive plastic of the same value the
> unit realy start to sing.
;-)
Yes ....I agree, by my tweeking philosphy is that the designer had a plan [hopefully] but the money men decided what components he could use. So by almost reverse engineering the kit you can improve the performance and raise it to the level the designer wanted. If you change the topology your changing the design.
Yes, but some designers are more objectivist and others are more subjectivist. So for objectivist designers the component choice might not be down to the money men. Maybe Quad would be a good example of an objectivist and Naim a subjectivist.
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