Audio Impedance 101

[bnesound]

Hello group. I was wondering if one of you experts could set me on the forward path to impedance nirvana over a subject I could never wrap my head around.

I have an old solid state console with a program output of 600 Ohms transformer balanced. The original power amps on the console also have 600 Ohms transformer balanced inputs. I have since modified the wiring to feed a pair of Mackie powered monitors with 20k Ohms balanced inputs and everything seems kosher. Now what I want to do is put the original amps back in but also keep the Mackies working simultaneously. What is the best way to split the output out of the console without a drop in output level? Should I use a 600 Ohm splitter, like a passive, unpowered RDL STD-600 divider/combiner? I have also been told something like an active, powered RDL SR-DA3 would work, 20k Ohms in, 150 Ohms out would also work. I want to maintain the lowest noise and best frequency response.

Thanks for any advice in simple English would be greatly appreciated.

 

[TomT]

If the Mackie amp is truly 20 K balanced, you really don't have to do anything. Just parallel the inputs with the original amplifier.

At 20K you are bridging the 600 ohm load, that is, you are only extracting a small sample of the audio signal, not doing a "1 for 1" transfer like was done with the original monitor amp at 600 ohms. So there should be almost no change whether the Mackie is connected or not.

Where you may have a problem is if the Mackie's inputs are actually unbalanced (e.g. the typical RCA jack input); you may find some hum introduced. Also, even if the Mackie's inputs are "active balanced" (that is, into an IC) you may also get some hum. A couple of electrolytic capacitors (oh, maybe 47 uf) might cure this.

Another problem might be matching the levels between the two amps because they undoubtedly have a different amount of gain. Of course, if both amps have input level pots, may not be a problem.

Unlikely that anything would blow up--just leave the Mackie connected at the monitor feed & hook the original power back up & see what happens.

 

[bnesound]

Thanks Tom, I will try this. This might solve the feed to the amps but I did not mention that I may also need another set of outputs to feed some outboard gear. When I have done this in the past as a parallel 600 to 600 I have experienced some loss of outlet level. I guess I should have mentioned that before. So now that I need a third balanced output would I still be better off with a splitter or DA? Oh, where would I put that cap on the TRS? Thanks again

 

[TomT]

Driving several feeds from the monitor feed would best be done with a DA. Then you can control the level fed to each amplifier. Of course, the downside is that your monitor level pot on the console still determines how loud or soft your feed is to every device. Depends on what your goal is--this would fine, for example, if you wanted a simple way to feed separate amplifiers for studio speakers, control room speakers & a lobby speaker pair (for example). On the other hand, if you needed to have both a monitor feed to an amp in another room (which had it's own input level control) & a feed to a recording device, then you should feed the DA from the program line outputs. That way the level would not go up and down as you adjust your monitor feed in the control room.

Broadcast Tools makes a DA that will provide three separated isolated feeds from one stereo feed:

http://broadcasttools.com/view_product.php?pid=112

You would set it up to be fed +4 dbu (typical level in older consoles for an outboard monitor would be around + 4 to +8). The input is bridging (as I discussed before), should only have very minor change to the output level of whichever feed you attach it to. Once installed, you just leave it in line. Since the outputs are isolated from each other, even if the Mackie input is really unbalanced or is quasi-balanced; you shouldn't have any problem hooking it up directly.

Now all you wold need is a tiny screw driver to adjust the microscopic pots on the DA.

 

[bnesound]

Hi Tom,

You mentioned a 47uf cap might get rid of a hum on my Mackie Powered Monitors. Where would you insert the cap on a balanced input?

Thanks


Brian

 

[TomT]

First thing to try since the monitor feed is balanced and supposedly the Mackie input is balanced is to connect a wire from any kind of ground bus on the console to the ground post or chassis ground of the Mackie. This may cure any hum problem. If you have it, use shielded two wire cable and connect the shield to the #1 pin on the XLR connector going into the Mackie. Then connect the shield to the ground bus of the console.

If this doesn't cure it, try an axial cap--(lead each end) and connect it on either side of the line; hang them on the output terminals of the console monitor feed.:

E.G.

transformer output 1----- +cap- ----amp input 1

Transformer output 2---- -cap+ ---- amp input 2

(where 1 and 2 out of the transformer are your 2 output terminals and amp input 1 is the positive--usually the tip of a 1/4 inch plug or pin 2 of an XLR; while input 2 on the Mackie would either be the ring on a stereo 1/4 inch plug or pin#3 on an XLR).


May work, may have no effect.

Another thing to try is a small value--say 1 uf--cap to ground across both sides of the line... + to the audio wires and (-) to a good ground point.

Voltage value of the caps is of little importance--25 volts would be a good value.