Shown below is a copy of one of the spreadsheets developed to make
preliminary estimates of amplifier performance using Microsoft Excel. It's useful for checking noise, gain distribution, compression and the like. Output power prediction and output transformer characteristics can be estimated by another spreadsheet in the package. The spreadsheets are related and data is
interchanged automatically between them as the design develops.
Spreadsheets are as ubiquitous as computers although they may not be used for purposes more sophisticated than balancing the family checkbook. A
spreadsheet can be used to perform some of the functions provided by conventional "programming languages" (the closest one to spreadsheet language being "Basic").
Spreadsheets are simple to program, provide some sophisticated mathematical functions and can even be used in an optimization mode by employing internal
"Visual Basic" capabilities that provide branching, looping, for-next and other techniques commonly associated with programming.
The spreadsheets generated for use with this book are specific to the manual calculations described in the appropriate chapters with a few enhancements.
Various "what if?" scenarios can be quickly evaluated with these tools with little likelihood of computational error (once the spreadsheet is free of bugs).
The amplifier configuration shown in the spreadsheet below includes no "effects"
such as reverb or tremolo, it's just an amplifier. A simple implementation can be made with as few as four tubes: one dual triode, a phase-splitter and two output power tubes (five tubes total if a tube rectifier is included in the power supply).
It's possible to obtain up to 50 watts output power from this configuration. Up to 100 watts is obtainable if the output tubes are paralleled (four tubes total) and the gain of the phase-splitter is increased.
Blue text indicates user entries and red text indicates calculations performed within the spreadsheet. Some of the entries are also used for calculations that occur on other spreadsheets, accurate information must be provided. If you use placeholders for preliminary estimates, remember to replace them with accurate values before committing to a design. It's recommended that one accumulates the tube data sheets (easy to find on the internet) and makes copies of them before starting this exercise, as well as all other parts documentation that may affect performance. The documentation may be useful in the event problems are encountered when the design is proofed (the "breadboard" stage).
From the data sheets bias voltages and currents may be extracted as well as typical and maximum expected gain and distortion values. These operational requirements should be added to the spreadsheet as tube types are selected.
Gain values entered in the spread sheet need to be obtainable - tube data sheets
will provide guidance on this subject. Incorrect (overly optimistic) gain entries will ripple throughout the amplifier chain and provide false results for many of the calculated performance parameters. It's best to use conservative (safe) values when developing a new design. Voltage gain values of around 40 for example, are suggested for the preamplifier/post-amplifier stages.
At this stage of personal design development, the spreadsheet that may be most useful is the "block diagram" shown above. This sheet is useful for estimating required stage gain and evaluating compression, for example. After we've developed the skills to determine how to do these things manually, we'll make more frequent use of these tools, speeding up the design process.
Several other spreadsheets will be available for similar purposes, there is one specific to the design of the output stage of the amplifier. Another spread sheet is useful for designing the power supply and filter circuits. As noted previously, the spreadsheets were created with Microsoft "Excel", therefore any spreadsheet program used must have the ability to read "Excel" files.
Use of the tools presumes basic background knowledge of the design process. If one is willing to work through all of the chapters on design, then the
spreadsheets can be extremely useful and time-efficient. I don't recommend using them in a vacuum, however; a thorough understanding of how the results are obtained will result in confidence in the "answers".
We will work through a practical design example in chapter 22.0, indicating how spreadsheets can be employed to design the architecture, and many of the details of a medium power amplifier, in a matter of a few hours.
For now, in order to boot up our design exercise, we need to determine, as a minimum, the gain distribution throughout the stages that make up the amplifier.
We need certain vital pieces of information such as input voltage level and output voltage level + output impedance in order to get things moving. If one hasn't a clear idea of what to do with this information at this point, it's a good idea to review chapter 5.0 and prepare a set of specifications. Once one is familiar with the design process and (especially) manipulating the spreadsheets, the
procedure will become straightforward and simple.
Risking boring and repetitious suggestions that may cause lack of interest, I strongly recommend that the remainder of this text (excluding appendices and certain chapters that aren't directly related to design) be read and hopefully understood before starting a design, even as an exercise. As we've noted, all design is iterative but commencing a design without sufficient information to make informed decisions can be frustrating due to the large amount of repetitive work necessary. Frustration can stimulate a negative mental feedback process that results in poor decisions and poor implementation.