The essence of an attenuator box of this sort is that the attenuations are known and repeatable, both over time and over channels. Switches rather than infinitely variable controls are preferred for this.Also, how much more complex would it be to have used a variable, stepped gain knob, or rotary switch, instead of a toggle switch?
Having a circuit board would eliminate any short-circuit risk. Heat shrink is a good idea but the U shape of the resistors placement make it hard to well isolate it from the input connector.. I would need to put small shrink cable on every resistor seperatly.Nice work! I was grooving to the music in the background. Some circuits seem more complicated on paper than in actuality to my untrained eyes.
What do you think a circuit board would improve, and could heat shrink tubing be used instead of double sided tape? Also, how much more complex would it be to have used a variable, stepped gain knob, or rotary switch, instead of a toggle switch?
Looking forward to the sequel!
The table of resistor values I gave Marco were for a U-pad balanced line-level attenuator. It's made up of three resistors: two identical values in series with the positive and negative input feeds respectively and a shunt resistor across the output. It's not difficult to calculate the ratio of shunt to series resistance needed for a given attenuation if you assume the source impedance is zero and the destination input impedance is infinite. Note that the circuit is symmetrical through the centre of the shunt resistor, with this centre point being at zero volts (a virtual ground). You simply calculate the values for either the upper section or the lower section, giving the series resistor and half the shunt resistor.
If the ratio of series to half the shunt is K, then the formula for an attenuation of dB is K = Alog(dB/20) -1, where the antilog is base 10. In practical terms, round the logs with care, e.g. take log(2) as 0.3 rather than 0.301023
As an example, for a 12dB pad, K = Alog(12/20) - 1 = Alog(0.6) - 1 ~= 4 - 1 = 3, so the series resistor is 3 times half the shunt, or 1.5 times the shunt value.
However, there are other factors to consider and adjustments to be made.
1. The reference resistance of (say) the shunt: e.g. Ohms, KOhms or MOhms?
2. Resistor noise if it's for low-level signals
3. Correcting for non-zero source impedance
4. Correcting for non-infinite load impedance
5. Mapping the corrected calculated values on to the standard set of available (preferred) values in the E24 or E48 ranges
6. Checking the power dissipation for high-level signals, including the change of resistance with temperature (from heating effects)
7. Checking the resistance-voltage characteristics of the chosen type of resistor (often overlooked, but usually unimportant for leaded parts)
8. Repeating all of the above when your supplier is out of stock of the values you have chosen
Ahh, the life of a design engineer...
I thought about this, I will certainly try it. If I finally get more mojo out of it, I'll do one more video about it...If you want to find out what your ISA output clipping sounds like, you could connect those attenuators in series to get enough dB reduction so that the FF800 line input does not overload. Because of impedances, you won't get exact multiples of 10dB reduction, but that doesn't matter in this case.