PreII - Volume control questions

[Rupert]

Hi All

Recently aquired the old WAD PreII kit from the new managment, which I have just built. Been using the WAD stepped attenuator until now, in a cheapo home made box, in fact I never tried a pre amp with my current system, so thought it would be worth a try. I'd convinced myself that passives were the way to go, but PreII is rapidly changing my mind.

Is there any reason why I should not replace the Alps in the PreII with my WAD stepped attenuator? I've temporary bypassed the Alps and hooked up the stepped attenuator and reckon it sounds better. The WAD attenuator is though of the shunt variety, i.e. a fixed resistor followed by selectable shunt resistors, which is of course different to the way the Alps is wired, but I don't see why it shouldn't work. It seems the shunt mod on the FAQ here turns the Alps 'L' pad arrangement into a shunt anyway.

My stepped attenuator is a 10K one, but is this the optimum value? (I'm feeding PreII with the WAD phono and driving Kit34 by the way) I've tried some different series resistors 30k and 50K and reckon the higher values sound better, and also give a better range of volume selection. I'm afraid I get a bit lost on all the impedance matching stuff, which I think is what its all about. Increasing the series resistance certainly does more than just effect the volume, it seems to affect the sound too, the tonal balance I think, lower values sounding brighter, higher values more bass?. Is this true? Anyone elses thoughts and experiences would be welcome. I worry that I am just imagining all this or thinking things are better when they just sound different.

Rupert

[Paul Barker]

I don't recall the phono II output impedance, but it is high, your problem is the coupling cap used on the output of phono II and your input impedance form a high pass filter. The resistor on theoutput of phono II (whatever that is it may be 470k or it may be 1 meg) is only there for stability if someone runs phonoII while not connected to anything, your input pot is the significant factor with the output cap of phono II and the output impednace of phonoII last valve. The valve is low current but it is used there as a cathode folower, and could drive your 10k load, though with limited power, the main issue is the low pass filter.

Enlarging the coupling cap just shifts the area of compromise.

Better to use minimum 100k log attenuator or pot. Morgan Jones once said in a little flat in London, all amps should have 470k input impedance.

[John Caswell]

Hi all,
Output impedance of Phono II is around 3.5K so should be enough for a 10K pot and as the transformer type PreII has an overall gain of around 5 you could quite safely use your stepped attenuator in shunt mode to lose a bit of gain and give you better control.
With a 10K series R you will lose 1/2 the signal with a 20K you will lose 2/3 signal. What this means is you will need to wind the volume control up. I would then adjust the series R for a sensible listening level at around 11o/c

John aka Dr John

Hi,
I remember that WAD used to recommend the 50k resistor pack should be used in PAS-II if it was to be used with the Phono-II.
I am interested in this area, as I'm using the Phono-II into a 51k Shunt attenuator into a 100k power amplifier.

[RobHolt]

Following on from what has already been said, what is the simplest and least signal path intrusive way to get more control over volume at the bottom of the pot range?

My PreII goes into a KAT6550 and feeds 96dbw speakers. Control using the Phono II is ok at low levels (though still a bit cramped) but with CD and especially an old Accuphase tuner I can barely move the pot without a huge jump in volume.

I've tried the shunt mod and didn't like it. Would perhaps using a linear pot with law faking resistors be a solution?

Cheers,

PS: I recently tried to demo my Phono II to a friend into his S&B Django magnetic pre and it wouldn't work at all - motorboating is the term I believe
The input inpedances on the Tx were high enough, well over 20k at all times but I guess the problem is the inductive load of the Tx??

Rob.

[NickG]

That doesn't supprise me, you have that big cap at the end of the cathode follower, into a inductor, so a nice LC circuit, only damped by the load of the amplifier and the output impedance of the phono stage.

[Rupert]

Hi, all

So I could try 10k, 50k or 100k, now thats hardly narrowed it down has it! I too remember reading on the WAD site that they recommended a 50k pot if using phono II, and thats what my kit came with. The circuit diagram shows 100k though, just to confuse things. I get too much volume with my existing 10k shunt attenuator although its usable, just. 20-30k gets a good listening volume at about 12oclock.

Am I right in thinking higher values cause more HF roll off?

I am assuming WAD intended their PreII and PhonoII to be used together and that some form of volume control can be stuck between them, despite whatever output caps or input impedances are present? It would just be good to know what the official value should be, or is it a suck it and see compromise. The volume control seems to be a necessary evil.

No one has said my stepped shunt attenuator is a bad thing, stick with the original Alps, so I guess its ok to use it, its seems to sound better. A potential divider type pot, like the Alps, is surely going to cause filter/roll off effects too, depending on the value?

Thanks Rupert