Hum issues on transformer output preamp

[Richard]

Hi Andy, yes we found, I think, the Alps motorised pot isn't suitable for shunt pot mod as it has the channel grounds on one tag and we need them separate for SPM.

If I'm following your diy pre is essentially Pre3. If so I used mine with great satisfaction (no hum!!) using the shunt pod mod to adjust gain to a lower level.

Mine was built as standard but using a 47K series into a 20K pot to lower max gain from the standard +15.5dB to 5dB. Using the 20K pot keeps input resistance in a usual range but I know some used 100K into the 50K pot without a problem.

There are other choices and more info in the FAQs here,
http://www.world-designs.co.uk/forum...ead.php?t=6885
and in the last part here,
http://www.world-designs.co.uk/forum...ead.php?t=5795
log in to see the pics

[bob orbell]

Richard, both motorised and manual pots are the same, both have 6 pins connected only to each track and each wiper. Bob

[Richard]

Thanks Bob, yes the 27mm Alps blues look similar but we definitely had someone back in the shunt pot thread days who had a common-channel earth problem with a motorised pot and couldn't use it. It may have been a different motorised Alps series or different make. Easy enough to check though by using a meter across the 2 channels' pins to see if any are common.

That said, my post really was to point Andy (if he hasn't tried it) to a way to lower effective gain without changing the circuit, and make the vol control more usable.

To give an idea of the difference in attenuation using a shunt pot,

a standard 50K log pot gives approx attenuation at these positions,
Min, then 1/4 -30dB 1/2 -16dB 3/4 -9dB Max 0dB

that 50K pot wired as a shunt pot using a 100K series resistor gives,
Min, then 1/4 -36dB 1/2 -24dB 3/4 -17dB Max -10dB

[bikerhifinut]

Originally Posted by bob orbell

Richard, both motorised and manual pots are the same, both have 6 pins connected only to each track and each wiper. Bob

The RK271 does Bob and you are correct.

However the RK168 which is the current production motorised ALPS pot has its tags arranged inline with a common ground tag. These are the ones I referred to.
There is a way round the shunt mod with the RK168 which involves using the 4 channel version which has 2 pairs of tracks each pair having a separate ground. They are very common on places like eBay and for very good prices. Caveat emptor as they say as most vendors are in China. The few that have landed here have all seemed good quality and no worse on the channel matching than the old RK116 motorised pot I have here. Which isn't as good as the 27mm, quelle surprise!


Its still possible to get the RK271 "blue" for now but they are discontinued in motorised form by ALPS, and the manual version is now only manufactured in 100k values.

RS will do you a RK271 motorised pot for £20, and Matthew supplies them also at a fair price. I am minded to lay in a small stash as they will eventually become rocking horse poo.


I'm not keen on the shunt mod but I'm not getting into an argument about it, I think its a fudge to fix a problem that doesn't exist, that's my opinion and nothing more.


Ideally, in the long term I would love to have some sort of resistance pad attenuator that is remote controllable and that way it should be possible to get fine control of volume where it counts at lower to moderate levels and also it removes in one go the issue of channel matching as 1% resistors would give near perfect matching.

Andy.

[bikerhifinut]

It's also feasible to attenuate the incoming signal by simply using a series resistor on the "hot" input tag of a standard pot. The log law will be altered but whether its noticeable I can't say. The effect on noise on the signal will be the same as if a shunt pot was used, near as dammit. I don't subscribe to the notion that the carbon track on a shunted pot is somehow not in the signal circuit, It has to be to provide a return to earth.

[Richard]

Thanks for the pot info and glad you got there in the end Andy

[Richard]

Hi Andy, Bob, all,

Looking at those vol pot attenuation figures we can see that most amplifier set-ups will be "throwing away" a large amount of input signal at usual listening levels; around -50dB for 1/10 rotation, -35dB for 1/4 and -20dB for 1/2 etc. Depending on speaker and power amp sensitivity some systems will throw away 10dB more than others and we never worry about it. My own is usually between 1/4 and half way, almost never above, and I don't find any quality difference between those levels.

It may seem counter-intuative but I don't think this is necessarily a bad thing. Just because an amplifier is capable of full gain doesn't mean it will work better at full gain. Many simpler circuits produce less distortion when run at lower output and doing so may be putting them in their sweet spot.

If we want to try a different route to "lose" some of the excess gain and perhaps improve distortion levels we could try using it in a feedback loop. 10dB global might work. Being more adventurous the pot could be tried in a local feedback loop around the first stage, a la Quad 34. No idea if it would work and I don't have a Pre3 to try but it might be fun for someone; the pot could be the sole volume control or used as a separate "gain control" with the existing pot being used as volume control, food for thought anyway.