SS amp theory

[sam9]

To a point. It is really essential that you compare SPICE results to cicuits that you actually construct.

There are degrees of skill and understanding people attain with SPICE. With time and experience the ability to simulate circuits can increase to the the level of prediction becomes very, very good. This can be a satisfying journey all on it's own for some. For other who see SPICE as a tool to get them to their final goal - operating hardware - it may not be worthwhile to use it for anything beyond a first approximation.

 

[Greg Erskine]

Build an amp with mirror image input and VAS that uses current mirrors in the LTP's. There are three or four examples in the book. Come back and tell us your experiences. By "build" I mean physically construct, not just a SPCE model.

I think I know what you are getting at. While I haven't build one of Randy's designs, I have built somewhat similar cct with current mirrors and ccs on the input and VAS sections (which are in favour in most "texts" not just Randy's). On the other hand, I have built an AKSA, which contradicts most of the books I have read, that soundly trounces my other "more correct" amps. My AKSA experiences would prevent me "Build(ing) an amp with mirror image input and VAS that uses current mirrors in the LTP's" because I now have preconceived idea of what I think is right. This may end up not being 100% correct but I am very comfortable with my current position.

Both Slone's and Self's books dismiss the more basic circuits out of hand and don't even discuss them. From my perspective this is very unfortunate. But I still would not discount these books because they were valuable to my understanding of amplifier design and an important part in coming to my conclusions. It is an interesting fact we all have access to the same information, we all build lots of amps but we end up with many different final results.

I would guess there are many happy owners of Slone amps out there and it would be incorrect for me to not accept that for lots of people super symmetrical designs with current mirrors, active ccs and extensive protection circuits is the right solution.

 

[sam9]

I'll be a little more explicit. An input section with mirrored LTPs, each having a current mirror can't normally drive the VAS. I learned this gem the hard way. It's not a question of whether it sounds good -- the circuit doesn't work -- no sound comes out -- nada! It can be made to work, I think, but only use of a bunch clamping diodes. If you can find a schematic of a Stochino high slew rate amp, I think that's an example. There are probably other means but Slone didn't discuss them. The designs appear to work in a SPICE simulators but in in real life. Like I said there is a thread here about it.

I think he must have been under pressure to complete the book and published certain designs without actually constructing them. As far as I know other than those three or four all the others in his book work as claimed. I've built two of them and many have built the Opti-Mas - if with the exception of those in question all his other amp designs seem pretty good.

I've had a couple of e-mail cvorrespondances and he seems a pretty decent sort. Others who have met or spoken with him concur as far as I know.


Going on to Self -- my guess is that his lack of interest in symetrical IS&VAS configurations comes from the difficulty of incorporating current mirrors. You may have noticed he's very keen on current mirrors. So if he to choose between current mirrors or symetrical small signal stages and choose the current mirror. AQ reasonable choice one way or the other, I think.

 

[Greg Erskine]

Thanks sam9. no misunderstanding now.

 

[tlf9999]

I'll be a little more explicit. An input section with mirrored LTPs, each having a current mirror can't normally drive the VAS. I learned this gem the hard way. It's not a question of whether it sounds good -- the circuit doesn't work -- no sound comes out -- nada! It can be made to work, I think, but only use of a bunch clamping diodes.

to me, it is a combination of the right devices and Re's in the current mirrors.

You will notice that the current through the mirrors are undefined - you will not be able to calculate it. and if you run the same circute through different simulators you will get vastly different values for the current.

To me, that is totally unacceptable.

 

[Brion55]

Well, I'm glad everyone is in agreement here about what book(s) to read.

 

[sam9]

You will notice that the current through the mirrors are undefined - you will not be able to calculate it. and if you run the same circute through different simulators you will get vastly different values for the current.

To me, that is totally unacceptable.

Not being one of those who has felt they had the time (or brains, or ambition) to delve deeply in to the innards of SPICE, your comment is also a caution about SPICE. Given undefined current, two different simulators return two different values (at leaast one of which must be wrong) but neither returns an error message. A more sophisticated user than myself might notice something amiss right away, but until my level of improves I'll continue to be a little skeptical of circuits on paper.

Meanwhile, for someone who has not even soldered before, the first few chapters of Slones "Electricity & ELectronis" is very helpful. Especially since it gives the reader step by step instructions to actually construct something (a regulated DC benchtop PS) useful.

While I'm still grumpy about the time I spent before realizing something was wrong about the CMs, I recognize I don't know the inside story of how it got published.

As an aside about "paper circuits": I recently tried building an appearently good idea that was published in EDN. Maybe it is valid or maybe not, but in any caseit's not as simple and straight forward as it first appeared. I'm not writing it off yet, but if it does pan out it may be despite some key information that was not provided to the reader.