Best DI circuit from speaker?

[12AX7]

Right now i have a 8.2k off the speaker + to the DI jack +, then a 1k from the DI jack ground to ground. Is that a good circuit or might i be better off trying something different? I'm asking because i see a lot of variance between different amps that use a speaker derived DI, some even using caps to shape it. The amp is a marshall 50w build.

[sluckey]

then a 100R from the DI jack ground to ground.

Are you sure? Seems to me that a resistor from ground to ground would do nothing. I would think that 100Ω would connect from DI jack + to ground.

If your circuit works well for a DI input I'd say just use it. All you should need is a simple voltage divider.

[12AX7]

Sorry, i meant the hot to ground

[terminalgs]

like this:



factors to consider:  (1) the speaker's Vrms, (2) the output Vrms you want,  (3) the output impedance you want (R2).  R1 is determined as function of these three things.  This is just like determining the value of the "NFB" resistor, because, this is, in effect the same thing.  For the same reasons there isn't a generic NFB resistor value, there isn't an generic R1 value for a speaker side "line out".

input voltage:
50W at 16ohm, 8ohm, or 4ohm all produce a different Vrms on the output speaker side.  What is yours?   If your amp is designed for 50W max, but you intend always set all knobs to "6", then you can use the "intended" voltage in this figuring if you like (I'm more of a  design for "flat-out use" kinda guy).


output voltage:
your desired output voltage is determined by what you are plugging into, and what the sensitivity of the device happens to be.  If you want to plug into another amplifier's input, maybe 0.2-0.5Vrms.  If you want to plug into a mixing board,  or some device with "Pro Audio" input,  aim for 1.2Vrms (+4dBu).   mixing consoles have preamps and attenuators, so if you are low or high, they can make it up if you are off.  so you have wiggle room.


output impedance:
While output impedance can be set high (like the 1K you have) but there is no reason not to set it to 100-200ohm so it can drive anything.  You have plenty of current on your speaker voltage, so set R2 as low as possible.

[12AX7]

Thats one thing i was wondering. The 1k is 100R in some schematics. So i'll lower that. Thanks.

[sluckey]

Thats one thing i was wondering. The 1k is 100R in some schematics. So i'll lower that. Thanks.

I thought you were using a 100Ω? Oops. Never mind. I see you edited your original post.

[Colas LeGrippa]

all my line out devices use 1k5 and 150R. You can replace de 1k5 resistor for a 5k pot for more versatility. With these values, you don't interfere with the impedance of the speaker load.
1k5 and 150R in series equal 1.65 kohm.  A 8 ohm speaker in parallel with a total resistance of 1.65kohm gives 8 ohm. ( + star dust ).  If you'd use let's say two resistors of  200ohm and 20ohm,  the total load would then drop to 7.72 ohm .

Colas 

[12AX7]

Thanks. I'll try those values.