John Curl's Blowtorch preamplifier part II

[Curly Woods]

Originally Posted by stinius

John

It's ML JC2

No it is a ML JC-2

[Steve Dunlap]

Now, in between, what have I learned? Well, talk is cheap, and quality control can make or break you.
The 'low bid' can well be the 'high bid' once the headaches start.

I actually said "fair bid" not low bid. I will stop being cheap now.

[Steve Eddy]

Originally Posted by john curl

One company, strongly recommended to us, was located in California, and they did audio enclosures for other audio manufacturers, already.

Neal Feay perchance?

se

[PMA]

Neal Feay is known even here as a producer of high quality enclosures.

[john curl]

No comment.

[Steve Eddy]

Originally Posted by john curl

No comment.

se

[john curl]

Everyone, I would like to point out that different people like to put their time and energies into different things. This is important to note.
For example, some people find it a wonderful challenge to see how cheaply they can get something built. I, once, had to make a phase locked tape capstan servo for essentially the same price as the precision AC motor that it replaced. In this case, every resistor counted. I still wince at some of my tradeoffs, but it worked and went into production, for years.
Others, are in competition with a bunch of metal 'artistes' who just love to make metal masterpieces of beauty and impressiveness. In this case, at least 1/2 of ones time, money and energy has to be devoted to external and internal appearances of the product. Look at a high price sports car or sedan. It is the same in audio.
Well, when criticism comes up, often it is one camp against another, slinging the comments.
In my case, while I don't like to bother with metal fabrication, I have grown used to quality examples, and it is hard to go back. Does this mean that an equivalent sounding preamp, for example could not be made in a pie tin, or cake box? No, but it would be difficult to sell one, made that way.

[simon7000]

Housekeeping.

SY

Thanks for the kind words, are you sure you are into the spirit of this thread!


ikoflexer,

I think you asked why unbalanced power supplies were an advantage. Since most of the traffic here really runs on misunderstanding, I will start at the basics.

Music when produced is a complex 4 dimensional modulation of air pressure. We reduce this to two undulating voltages (Bell's term) for most of the purposes discussed here. As we are not immortal (I apologize in advance if I am wrong about this) there are only frequency components above 0 cycles per second. This means that the voltage goes above and below zero. The rate at which it does that is the slew rate. Louder passages have higher voltages and slew more for the same time period between zero crossings.

A typical music signal spends most of it's time around the zero crossings in terms of voltage level. I am not concerned here with the large level limitations, that is another issue.

In a Class B output stage one device is used to pull the signal voltage up and another pulls it down. (John uses current output in the Blowtorch as mentioned elsewhere). There is a dead zone around zero when neither device is on. Since much of the music is around zero crossings this distortion is easier to hear that many other kinds. Note that timing errors from either digital or analog time delay causes also cause problems.

In a class AB output both upper and lower devices are on at the zero crossing. This gives double the gain for the signal which is not as bad, but just past the zero point the gain shifts back to normal, again increasing nasty things.

In a class A circuit much of this is avoided, but there still are gain modulation issues, due to other factors.

If we offset the power supply voltages then the hand off between output devices does not occur as close to the zero crossings, so it actually happens less often and is more masked by the desired signal.

In the Blowtorch I have gathered that there are two class A devices opposing each other at each stage. If they were perfectly match the total distortion would be less, but it would occur at the zero crossings. Since the N's and P's are not perfectly matched the total distortion is greater but does not happen where the problem is most easily heard.

I expect you can now begin to understand why tight tracking of the power supplies is not always a good idea. Also if you carry the thought a bit more you can see some of the advantages of a current output.

The biggest advantage of current out has more to do with other factors, but that is also a different issue.

[ikoflexer]

Thank you!

Originally Posted by simon7000

I expect you can now begin to understand why tight tracking of the power supplies is not always a good idea. Also if you carry the thought a bit more you can see some of the advantages of a current output.

I just don't understand what you mean by tight tracking of the power supplies.

The biggest advantage of current out has more to do with other factors, but that is also a different issue.

I am curious what the biggest advantage of current out is.

[Bob Cordell]

Originally Posted by simon7000

Housekeeping.


In a class AB output both upper and lower devices are on at the zero crossing. This gives double the gain for the signal which is not as bad, but just past the zero point the gain shifts back to normal, again increasing nasty things.


If we offset the power supply voltages then the hand off between output devices does not occur as close to the zero crossings, so it actually happens less often and is more masked by the desired signal.

I expect you can now begin to understand why tight tracking of the power supplies is not always a good idea. Also if you carry the thought a bit more you can see some of the advantages of a current output.

The biggest advantage of current out has more to do with other factors, but that is also a different issue.

Simon, not to be picky, but in an optimally-biased class AB stage the gain (actually total gm) is the same at crossover as it is far away from crossover. Unfortuantely it wiggles around crossover. This is basically what Barney Oliver pointed out long ago. What you pointed out above is the so-called gm-doubling situation that can occur when a class AB stage is strongly biased above the optimal bias situation.

Because junction temperatures move around a lot in an output stage, Vbe moves around quite a bit. The optimal bias is usually said to occur when about 26 mV is present at idle across each output emitter resistor. Now consider that Vbe changes at 2.2 mV/C. Even a 10C change in temperature (which will not be instantaneously be compensated) will introduce a change of 22 mV into Vbe of the output transistor. For this reason it is difficult to stay near optimum bias in the face of real-world program signal swings. For that reason, many designers prefer to err on the side of higher bias if they are willing to take care of the slightly higher idle temperature. The slight increase in distortion under mild gm-doubling conditions is usually considered to be far less sonically degrading that the crossover distortion that results when an output stage is under-biased.

Separately, the offset you mention is not an unreasonable thing to do, and people have been doing it with op amps in audio applications for years. Doug Self's Crossover Displacement (XD) output stage is not much more than applying this principle to an output stage.

However, achieving such offsets by mis-tracking of the power supplies is not the way to go. I'm not necessarily saying that precise tracking of the positive and negative rails is essential, but in any properly designed circuit if they happen to track perfectly that should never be a bad thing.

Cheers,
Bob