Adding an attenuator switch

[jpkuhn]

I plan to purchase a kit with the CH01 chassis and 4 N72 preamps. I really would like to add a 20dB attenuator switch to the preamps. Do you think it would be possible to add one. I know how to implement it with one double inverter switch and 3 resistors. The resistors can be soldered directly on the switch. My questions are, is there a point on the PCB where I can break the input tracks, between the transformer and the 48V feed to insert such a switch and is there a risk of bad issues due to the extra wiring ? Thanks.

[tpryan]

I really would like to add a 20dB attenuator switch to the preamps.

There's already a 20dB pad built into the selector switch, albeit after the input transformer. The minimum gain overall is 15dB, and the input transformer is capable of handling quite high levels without saturation. Do you really think you need it?

Do you think it would be possible to add one.

Of course.

My questions are, is there a point on the PCB where I can break the input tracks, between the transformer and the 48V feed to insert such a switch?

The best way to do this is to wire between the PCB and the input transformer. There's no need to cut tracks, since you need to wire short jumpers from the transformer to the PCB anyway.

And is there a risk of bad issues due to the extra wiring?

More wiring increases the opportunity for noise pickup, but you should be OK if you take reasonable care.

-Tim

[jpkuhn]

Thanks for answer.
Do you mean that with, let's say a D112 inside a kick drum, using the lowest gain there is no risk of overloading the preamp ? I currently use several pres (Buzz, Sebatron, SPL) and I often engage the pad on kicks, snares and OHs. It is true that my preamps do not have an output level pot like SCA's. What do you think ?

[tpryan]

Well, let's do a little math. Using your D112 as an example, let's assume you're recording a monster drummer who can hit the drum so hard that the SPL at the mic is 140dB. AKG says the efficiency of the D112 is 1.8mV/Pa of pressure. We want to know how many millivolts the mic produces at 140dB SPL. Solving this formula for P:

dB SPL = 20 Log (P / 0.00002)

we get 200Pa of pressure. Multiplying 200Pa times 1.8mV/Pa gives us a 360mV output. Converting to dBu, since that's what we do in audio:

dBm = 20 Log (.360/.775)

gives us about -6.7dBu at the input, which is a pretty hot mic signal.

Even so, the 10468 input transformer is capable of +9dBm @ 1% distortion @ 20Hz, so we've got 15.7dB of headroom at the transformer, even with a 140dB source! Since the the transformer is usually wired for a 1:2 step-up, the signal out of the transformer will be up 6dB, to -0.7dBu.

With the gain control set to minimum, the pad will subtract 20dB, giving -20.7dB at the input to the first amp stage. With the output control fully CW, the amp adds back 29dB, for a level at the output of +8.3dBu.

Since the N72 clips at about +27dBu, you'll have not quite 19dB of headroom in this scenario, at least as far as the N72 is concerned. The rest of your signal chain should be happy with a +8dBu signal, but with hotter signals you may have problems. In such instances you may need to pad the output of the N72, not the input, but it depends on what you're plugging into.

[drunton]

Tim, I love the rigorous math. As a EE knowing all the formulas is great. It's the application of stringing them together that is the hard part to learn.

[jpkuhn]

Thanks for the maths, it does'nt harm from time to time. I will try the preamps without a pad.

[screws]

I use my N72 on kick all the time. No distortion, and my favorite studio drummer hits the kick like godzilla coming down the street.