Volume pot shunt mod

[thomas27]

Hi everyone,

My crack kit is on its way and I've been thinking of ordering a new volume pot (mainly for channel balance at lower levels, otherwise Alphas are great). Goldpoints seem nice but they're very expensive to order from the UK.

Then I came across this 'shunt mod'
http://www.world-designs.co.uk/forum/showthread.php?t=5795

Has anyone tried it? It would probably not completely solve the channel imbalance at low levels but I could fiddle with resistor values - eg 47k series resistor with 10k pot to reduce the gain by 15dB as suggested in the article - and play with vol pot turned up a little more and get out of the pot imbalance zone. Would these values work with the Crack?

An alternative would be a VALab stepped attenuator but I quite like the idea of reducing the gain and the cost-effectiveness of the shunt mod.

I would love to get your thoughts on this.

Thanks a lot
Thomas

 

[Grainger49]

This shunt mod will give you a low input impedance at low volume settings.  When near zero it will draw a lot of current, possibly damaging the source.

Instead try putting a 249k (or close) in series with the wires coming from the input RCA Jacks.  This will bring the low level listening up out of the non linear portion of the ALPHA pot.  You will still be able to make your ears bleed before running out of power.

 

[thomas27]

Thanks Grainger, that's super helpful. That's exactly the simple / low-cost solution I was after.

 

[Grainger49]

I stole it from PB.  But it makes sense.

 

[mcandmar]

I would use a stepped attenuator over a pot any day, i.e. the something like the valab.  You could of course combine that with a resistor in series as Grainger talks about to reduce the overall signal.

 

[Paul Birkeland]

If you have low level channel imbalance, this is a good way to handle it: http://www.goldpt.com/mods.html

It's best to build stock first, then explore these options if necessary.

 

[thomas27]

Thanks a lot PB

Is there any difference in sound between higher value resistors in series with input (250-470k) and a potential divider before the pot with lower value resistors (as per goldpoint mods - e.g. for 100k pot 63k/30k or 85k/15k)?

Also do these methods affect the input impedance for the source?

Thanks

 

[Paul Birkeland]

Yeah, if you use too much resistance before the pot, you can pick up noise.

It's not a bad idea to keep the input impedance at 100K, which would mean also using resistors across the pot.  This will keep the performance of your Crack predictable.

 

[thomas27]

Thanks PB - here's my take to maintain 100k input impedance using common resistor values (please let me know if I am wrong!)

 

[Paul Birkeland]

Yeah, that will keep you in the ballpark.