So guys,
I was knocked out by a cold for a few days but here‘s what I was able to work out after that:
I‘ve read through „Handbook For Sound Engineers“ (Glen Ballou) and „Small Signal Audio Design“ (Douglas Self) very carefully and have the feeling I got a much better grasp on what this topic is about.
The Project
I am designing a monitoring controller based on Eurocards in a subrack. The cards (8-input card for example) are supposed to be quite universal, so I can use them in different projects in the future. My university is actually interested in a controller for their 3D audio dome, based on this project. Douglas Self talks about worst case output impedances of 2,5k (passive volume knob). This controller will only be interconnected with professional studio gear, so I‘m not quite sure which range of impedacnes to plan for at the moment.
Requirements
fairly flat frequency response and low noise
universal and stable
Inputs
I want to use this schematic now, explained in detail by Douglas self.
(Fig. 14.12)
In the following calculations, I have always added the typical capacitor deviation in the direction that makes matters worse.
With values of R7/R8 = 100R and C1/C2 = 100p (+ 10%), expecting a worst-case output impedance of 2,5k , I get a RF filter rolloff at 556 kHz.
--> Should I assume lower output impedances in the professional environment to be able to have a lower RF-Filter rolloff? I looked up some of our devices: They were all in the 50 – 100 Ohms range. I will probably never connect something like a passive volume controller.
With C3/C4 values of 100µ, the DC-blocking rolloff is at very low 0,02 Hz. What I learned from Small Signal Audio though is: The reactance of the caps is supposed to be a very small fraction of the total input impedance, so that tolerances don‘t affect symmetry and therefore CMRR. 100µ (-20% = 80µ) gives me an Xc of 1k @ 2Hz. Thats 1% of 100k (R5/R6).
--> I picked 2Hz pretty randomly. Down to which frequency should this ratio be so low?
--> What about polarized caps? Self always explicitly mentions non-polarized caps but in commercial products I mostly find normal polarized ones. I have to say I find NP‘s a bit bulky and expensive. What difference does it really make?
--> Self states that early in a system (Though not by the DC blocking caps) the low-end frequency response should be defined. Should that also be done in a simple monitoring controller, i.e. should I add an RC highpass filter after this input circuit?
The book ommitts various parts of this schematic because they were discussed in erlier chapters. I added the feedback caps for HF stability for example. 100p (+10%) over 2,2k gives me a cutoff at 723 kHz.
--> Can I put feedback caps across both R2 and R4, or will that affect symmetry? Isn‘t the cold signal filtered twice, but the hot signal only through the second stage? I‘m not sure about that.
Outputs
This is an exact schematic from Small Signal Audio: (Despite the polarized caps)
(Fig. 15.3 c)
I looked into the datasheets of a few professional products and the lowest input impedance I found was 3,6k (Studer Vista). If I‘m aiming at a DC-blocking cutoff frequency of 2 Hz (As Self accepts in one unbalanced example), round the input impedance to 3k, I get a value of 26µ for C2. It has a reactance of 3k @ 2 Hz / 300 R @ 20 Hz. That‘s about the same as the input impedance for very low signals, so I suppose it shoud be 10 times lower.
--> At this point I‘m not completely sure at which output cap reactance to aim. Which are the lowest frequencies to concider when calculating the reactance?
--> Do I need output DC-blocking at all with my design that only consists of [input]-[routing / attenuation]-[output] ? How much DC can this circuit build up? The blocking cap would probably also protect following equipment from a latch-up state in my circuit, right?
--> When comparing this balanced output to the equivalent circuit from http://sound.whsites.net/project87.htm I notice that it has a 10k parallel and 1k series resistor on the input. Is this needed when feeding it from a low impedance source inside my system?
Please be forgiving, as I am just learning all of this stuff and hope I didn‘t make any major mistakes. If I did, correct me. I already learned a lot from verifying all of the calculations I read in Small Signal Audio myself. I hope you can clarify these questions that remained.
Best regards,
Adrian
