Some simulation results investigating TPC/TMC/‘pure Cherry’ and other obfuscating stuff. This started out as an exercise to see if LTspice can simulate ‘real life’ stuff from when commercial PA design was important to me.

I hope yus gurus will use it as a base point for some of the ‘compensation’ discussions in the threads on Bob & Doug’s books.

I’m also hoping Bob & Doug will be encouraged to do more ‘real life’ work on ‘pure Cherry’ rather than declare it unworkable.

Starting point is Douglas Self’s fig 6.16 from his 4th ed. cos that’s close to what I was playing with in 1990. I’ve used Bob Cordell’s BJT models and some FET models from Scott Wurcer. These give the worst results of any models I can find so are likely to be close to real life. I’ve used mjl21194/3 cos they are the worst of Cordell’s OP devices and closest to Self’s mj802/4502 and the Sanyo device I used. This gave Self'’s results for THD with frequency … and also what I remember from 20+ yrs ago, which was encouraging.

Remember, all dis theory en simulation is useful … not cos the mathematical rigour or how it supports some pet theory .. but ONLY if it has some connection to ‘real life’ and gives us insights into the behaviour of amps and how to improve / design them.

Self & Cordell’s books are useful … not cos they expound many theories … but cos …

I

Some tweaking of stability done with added ‘zeros’ with judicious use of C2/7/9 and .TRANS. Check if C4 across the ‘bias spreader’ is appropriate and if something across R15 is needed.

BTW, this is all stuff you check & tweak in ‘real life’ … eg if you change devices in production

The Open Loop responses are done with a

First off is

Distortion at

I hope yus gurus will use it as a base point for some of the ‘compensation’ discussions in the threads on Bob & Doug’s books.

I’m also hoping Bob & Doug will be encouraged to do more ‘real life’ work on ‘pure Cherry’ rather than declare it unworkable.

Starting point is Douglas Self’s fig 6.16 from his 4th ed. cos that’s close to what I was playing with in 1990. I’ve used Bob Cordell’s BJT models and some FET models from Scott Wurcer. These give the worst results of any models I can find so are likely to be close to real life. I’ve used mjl21194/3 cos they are the worst of Cordell’s OP devices and closest to Self’s mj802/4502 and the Sanyo device I used. This gave Self'’s results for THD with frequency … and also what I remember from 20+ yrs ago, which was encouraging.

Remember, all dis theory en simulation is useful … not cos the mathematical rigour or how it supports some pet theory .. but ONLY if it has some connection to ‘real life’ and gives us insights into the behaviour of amps and how to improve / design them.

Self & Cordell’s books are useful … not cos they expound many theories … but cos …

*for their pet topologies & ideas*, they show ‘real life’ results from their experience and relate them to the simulations and calculations. eg asking Self to pontificate on the virtues of CFAs is non-productive. He doesn’t see the virtues … and more importantly, hasn’t done much ‘real life’ work on them.I

**check virtual (and real) stability**with a large variety of loads**including pure capacitance from 33p to 100n**as described in http://www.diyaudio.com/forums/analog-line-level/218373-discrete-opamp-open-design-231.html#post3266142Some tweaking of stability done with added ‘zeros’ with judicious use of C2/7/9 and .TRANS. Check if C4 across the ‘bias spreader’ is appropriate and if something across R15 is needed.

BTW, this is all stuff you check & tweak in ‘real life’ … eg if you change devices in production

The Open Loop responses are done with a

**Tian probe**(LTspice4\examples\Educational\LoopGain2.asc) at the –ve feedback point. Connect R5 to Ground for this. .AC closed loop responses (not shown) to check for excessive peaking. Connect V2 to R5 for this. For these and plain .TRANS, you need to get rid of the ..- connection between InN and the base of Q3
- U1 the THD meter

*Of course, a Tian probe measures***Loop Gain at that point**rather than Open Loop. My apologies to da pedants.First off is

**Two Pole Compensation (TPC) around the VAS**. I’ve sorta used Cordell’s method by splitting Cdom and taking a tweak resistor R23 to earth. C7=4p7 from the stability tests.Distortion at

**20V into 8R**up to 10th harm. using**Eugene Dvoskin’s Total Harmonic Analyzer**from the LTspice Yahoo Group including stuff from jcx. The distortion residual shows 2 cycles of 20kHz with the fundamental superimposed at 0.01% to give some idea of scale. Ignore the lower freq waveforms. Takes a long time.#### Attachments

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