[wrxr7]

I'm hoping that there is a simple answer to this simple question.

I would like to change the input loading of my Bugle from 47k to 62k to better facilitate a Shure M97xE cartridge.  I have read conflicting information on how to accomplish this.

The Bugle manual states that "There are two unused resistor locations on the board, R16 and R34, where you can solder in whatever load resistance you need."  Does this mean that to achieve an input load of 62k ohms that I simply need to solder in two 62K ohm resistors to locations R16 and R34 without changing anything else?

I have also read, and the schematic seems to show, that R16 and R34 are in parallel with the current 47k resistors.  Does this mean that the values of R16 and R34 must be combined with the current 47k ohms to make 62k?  If so, what value resistors need to be added to result in a 62k ohm input load?

Thanks for taking the time to address this hopefully simple question!

[Brinkman]

With a resistor in place, adding resistance in parallel will actually lower the sum resistance. Here's the formula:
Resistance (total) = 1/[(1/R1)+(1/R2)+(etc.)], where R1 and R2 are the values of the two respective resistors.

Since you are only dealing with two resistors, you could use the product-over-sum formula: R(total) = (R1*R2)/(R1+R2)

If the two resistors were equal values, the R(total) would simply be half that value, or R/2.

You might scratch your head and say to yourself, 'wouldn't it just be easier to replace the 47K with a 62K resistor than to deal with all that math?'

And you would be right. 

[jcg0322]

The Bugle manual states that "There are two unused resistor locations on the board, R16 and R34, where you can solder in whatever load resistance you need." 

I am not sure why Jim states this in the manual. Even with 100M ohm resistors intalled at these locations, the input resistance can never be higher than 47k ohms.

We might be missing something here Brinkman, and your math is correct, but I would like Mr. Hagerman to chime in and clarify this technical point.

Thanks,
Jack

[Brinkman]

The full quote actually is, "For moving coil cartridges, the loading resistance is often specified to be much less than 47k.  There are two unused resistor locations on the board, R16 and R34, where you can solder in whatever load resistance you need." (emphasis added).

The way I'm reading it, Jim forgot to put the word "lower" between "whatever" and "load" in the quote above. Of course, this is only an assumption.

[wrxr7]

So, is it safe to assume that to facilitate variable input loading, one could replace the 47K ohm resistors (R17 and R35) and wire a 100k ohm dual gang poteniometer in their place? 

Is it this simple or am I overlooking something?

[Brinkman]

Is it this simple or am I overlooking something?

Personally, I'd opt for a 100K stepped attenuator which would yield better adjustment control and channel balance. But then again, I'm just repeating what I've read rather than experienced first hand. Either way, once you found a loading level you liked (by ear), you'd have to hook your ohmmeter up to it to measure your loading if you were ever curious.

If you look at the Piccolo threads here and the manual at hagtech.com, you can get a better idea of the preferred approach to variable loading. To summarize, the implementation is a dual gang selector switch wired to resistors of various values.

By the way, the Piccolo can be powered with a Bugle power supply. Those who use it could simply keep their phono preamp loading at 100K because the Piccolo has already provided the loading. And gain (if using moving coil carts).

[jcg0322]

wrxr7,

Technically you could put a stereo type pot to replace those resistors. The reason nobody does this is because even the high quality pots ($15 - $100 range) will introduce some unwanted noise.

Brinkman's idea is good, but you could take it a step further if you wish. Once you find the correct loading using some type of pot, measure the resistance value they are set at and replace the pots with fixed value high quality resistors.

Just my 3 cents......