General question on big and old analog circuits designs

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
41
My question is about circuits like the one attached. I actually don't know what it is, and it's not my intention to know it for now. I actually refer to those big and old circuit diagrams which have thousands of opamps, transistors, and analog components in general. For a digital/microcontroller guy (like me), this might take a longer time to understand it all compared to an analog designer with a vast experience. As I said, my question is not to understand this circuit, but to know if nowadays these circuits are being significantly reduced using microcontrollers or special ICs. Let's say you're given one of these big diagrams with obsolete parts and you have to redesign it from scratch. I would like to hear people who have had the experience of redesigning an old circuit. What would your approach be? Of course, the first step would be to understand the application of the circuit.
 

Attachments

This is for a device called "Boss CE 300 Analog Chorus" from the 1980's. Don't know exactly what it does but you can get a used one on EBay here.
Since this is for audio, you couldn't replace much of it with a microprocessor but modern analog components would simplify it considerably.
 

OBW0549

Joined Mar 2, 2015
3,142
Let's say you're given one of these big diagrams with obsolete parts and you have to redesign it from scratch. I would like to hear people who have had the experience of redesigning an old circuit. What would your approach be? Of course, the first step would be to understand the application of the circuit.
Yes, that's certainly the first step, to find out as much about the application as possible.

Next, I'd try to "de-jumble" the schematic: identify the various functional blocks in the diagram, and re-draw the schematic on multiple sheets, with only one or two of the functional blocks on each sheet to make it easier to figure out. Trying to work with a schematic like the example you posted is apt to lead to difficulties, and taking the time to put it in more readable form is well worth the effort.

Beyond that, I'd say it's best to do the re-design at the highest level possible to take maximum advantage of simplifications possible with modern components. Try to re-design entire functional blocks, or groups of functional blocks, and simplify them rather than searching (probably in vain) for direct substitutes for the obsolete parts in the original design.

I've been through this sort of thing a few times, and the above is the approach I took.
 
I wouldn't attempt to reverse engineer the device, if I could generate a set of requirements for functionality then I would design a new one from the ground up.
 

BobTPH

Joined Jun 5, 2013
2,198
Since this is for audio, you couldn't replace much of it with a microprocessor but modern analog components would simplify it considerably.
Seriously? Where have you been for the last 30 years?

From Wikipedia:
Digital audio is also the name for the entire technology of sound recording and reproduction using audio signals that have been encoded in digital form. Following significant advances in digital audio technology during the 1970s, it gradually replaced analog audio technology in many areas of audio engineering and telecommunications in the 1990s and 2000s.
Bob
 

WBahn

Joined Mar 31, 2012
25,068
My question is about circuits like the one attached. I actually don't know what it is, and it's not my intention to know it for now. I actually refer to those big and old circuit diagrams which have thousands of opamps, transistors, and analog components in general. For a digital/microcontroller guy (like me), this might take a longer time to understand it all compared to an analog designer with a vast experience. As I said, my question is not to understand this circuit, but to know if nowadays these circuits are being significantly reduced using microcontrollers or special ICs. Let's say you're given one of these big diagrams with obsolete parts and you have to redesign it from scratch. I would like to hear people who have had the experience of redesigning an old circuit. What would your approach be? Of course, the first step would be to understand the application of the circuit.
As you say, the first step is to understand what the circuit is supposed to do, in detail. Ideally, either find or back out a set of performance specifications if you can. If you have to back it out, then welcome to the joys of reverse engineering. While that schematic is pretty dense, it also seems reasonably well laid out in terms of functional groups and provides quite a few clues with the various labels here and there. Then you are in a much better position to decide how much of the functionality can be implemented in, say, a digital signal processor.

Since this is clearly an audio application you can almost certainly reduce it significantly using modern digital technology. But you are also trading one headache for another in that you are still going to have to figure out, one way or another, an acceptable set of performance specifications and required functionality in order to develop your code and your user interface.
 

RPLaJeunesse

Joined Jul 29, 2018
82
One approach is the "black box" approach. Define what goes in, what gets done with that, and what comes out. Assuming you know what gets done, the choice of how depends on many factors, one of which is soft vs. hard tooling. An all-analog design like the example requires a lot of tooling, that costs money and is difficult to modify if changes are needed. Soft tooling isn't necessarily cheaper, but it is often easy to modify - good if you are attempting something new and not 100% certain of what really needs to be done. Soft tooling is often in the form of microprocessor code, or code for an FPGA or other (re)programmable device. Nowadays the circuit above is minimized by the use of a DSP core surrounded with ADC and DAC and the few needed analog input & output signal conditioning circuits. Or maybe an ARM CPU with the necessary analog support. Personally I've looked at the Orange Pi Zero for a multi-drop group intercom function, just needs the added controls & connectors.
 

AnalogKid

Joined Aug 1, 2013
8,251
It's not a bad design at all; each functional block is pretty tight in its component count and design stability (the ability of 100 assemblies to perform exactly the same with a minimum of calibration adjustments). As above, it is an audio effects processor based on a delay line called a BBD (bucket-brigade device). The delayed audio is recombined with the direct audio to make a single voice sound more full, or one voice to sound like many singing together ("chorus").

ALL of this can be done inside one digital signal processor, which is how someone aiming for the low to medium quality market would do it. Someone aiming for the golden-eared retro audio crazies might go all analog, but that BBD part is going to be hard to find. As a compromise, you could put a digital delay line in the middle of all analog input and output processing.

I've done some old circuit redesign (called technical refresh). Most of the time there is a good reason for keeping at least some of the analog stuff. Digital is different Almost anything that was a state machine can be replaced with a microcontroller, and pages of very fast random logic can fit in a single CPLD. FPGA's are all the rage, but for the vast majority of this kind of retrofit their unique characteristics are not required.

ak
 
Top