From stereo to mono

[dpaul_gtr]

Silly question here: 

I'm making a stereo to mono cable for a friend (he wants to plug his ipod into one of those small desktop guitar mini-amps) and I've found this schematic on the interweb 

I think I understand why those two 470R resistors are there, but what's the deal with the 20K ?

[PRR]

There could be DC leaking from the iPod.

DC could upset whatever you are going into.

20K will bleed-off some DC and the audio loading is insignificant.

The 470r resistors should match much better than 10% for 1dB unbalance. 5% is wise. 2% is now cheap enough. 1% seems overkill to me. I'd probably grab five of ANYthing 220r to 1K, use the meter to find two within 3% of each other.

There is NO need for the 20K to be 1%. Anything 10K to 100K is fine. Looks like this author has a drawerful of 1% parts.

99% of the time, you could omit the 20K. Old sound guys learn to defend against the 1% bad-days, especially if it only costs $0.12.

[dpaul_gtr]

Thank you for the info! 
I've tried it without the 20K and it worked, but I'll solder one inside the 1/4 jack casing just in case. Funny thing, when I asked for the 470 ohms resistors at the local electronics shop, they only had 1%'s, usually it's the other way around.

[newperson]

what and where would you add something to block the possible DC?

[dpaul_gtr]

what and where would you add something to block the possible DC?

As PRR said, the 20k resistor doesn't block dc, it shunts it to the ground. I've thought briefly about using coupling capacitors to block the DC component (if there was any), but I wasn't keen on triyng to squeeze capacitors inside a 1/8 inch jack casing. I genuinely hope that what I just said makes sense

[PRR]

> block the possible DC?

There shouldn't be DC on the output.

But sometimes there's a little leakage.

Like a water heater. They shouldn't drip. But sometimes they do. So I put a tray under the valve.

That's not like the huge bucket sometimes found under an upstairs water heater to handle a LEAK before it ruins the floor. If a pedal downright LEAKS, you fix it. But if it just dribbles a few electrons, a medium-value resistor gives it some place to go instead of backing-up.