Audio Electronics Tutorial

[tpryan]

I've just started working on some "About the Circuit" pages for each of the preamps, and I'm wondering what's particularly confusing or mysterious about them from a novice's point of view. What topics should be stressed? Thanks for your suggestions.

-Tim

[Bastiaan]

For me that would be the basic idea of what a transistor, capacitor etc is actually doing in the circuit, and what the general idea is behind it.

I have some basic understanding of stuff, but usually regard a transistor as a black round thing that can amplify stuff, a capacitor as something to smooth out AC and act as a buffer, and a resistor as something creating resistance/friction.

Hope this helps you.

[Bastiaan]

Maybe explain what all the seperate "units" are doing. I noticed on the drawings its all divided into power suppply, first stage etc etc.....


Provide a schematic of black boxes, and explain a bit on what that black box is dpoing

[tele_player]

I think the basics of electonics, DC theory, AC theory, resistance, capacitance, inductance, Ohm's Law, semiconductors, etc are better served by tech school textbooks.

On the specifics of these designs, rationale behind the setup of the gain switch (internal gain staging), negative feedback (A12 trim pot) power supplies/local regulation, transformer balancing, DC coupling and servos, might be useful to some. Maybe a note on headroom and the need (or not) for meters and overload LEDs.

[Randyman...]

The block diagram would be cool to see where the mic's signal is flowing, where the PS's power is going, and where the gain stages are (A novice like me can't make sensible conclusions from the schematic alone).

Going into the basics of the N-P-N (or whatever) of a Transistor, or at least knowing what the E, C, and B do would also be helpful. I think most people brave enough to attempt a DIY of this level of monetary investment are already vaguely familiar with resistors and capacitors, and possibly transformers (that is about where I am at - I get lost on Transistors). Explaining the OpAmp would also help clear up some fogginess.

I do agree that the bare basics should probably be taken care of by the builder (hot-link to other "Basics" web-pages perhaps?), unless you just really want to include it.

More info on troubleshooting would also help (like CHECK THOSE Q6 NUTS ), but there are so many scenarios, phone/email support seems to be ideal here (it was for me). But knowing more about the circuit basics will help anyone troubleshoot more accurately on their own.

[drunton]

Tim, this is a cool idea.

How about details like setting the values for the mic input pad. Why you staged the gain in certain ways. Why a 1:10 transformer is appropriate for a 2520 but not for a 990 opamp which wants a 1:2 and how these three subjects interact for optimal design.

Plus, how you use IV curves to set bias and then still be able to control the gain of the transistor stages. The 1272 is pretty complex in its feedback and design. Even understanding 1 stage design gets quite complicated when trying to follow how multiple stages play together.

[PRR]

Not to do Tim's work, but just to gauge what level of explanation you-all want....

> Why a 1:10 transformer is appropriate for a 2520 but not for a 990 opamp which wants a 1:2 and how these three subjects interact for optimal design.

It isn't ratio but impedance. The numbers you give are customary because most mikes are about 150Ω. If we liked 10Ω or 2,000Ω microphones, we would use different ratios.

(However, a very long length of twisted shielded cable works best at about 100Ω-300Ω, even with large changes in cable dimensions. And there are situations where mikes are very far from their amplifiers. So the 150Ω impedance range is pretty well forced upon us.)

Remembering (from a prior yet unwritten chapter) that impedance scales as square of transformer ratio: with 1:10 the amplifier sees 150Ω*10*10 or 15,000Ω; with 1:2 the amplifier sees 150Ω*2*2 or 600Ω.

The noise of a transistor (or anything) can be considered in two parts: a noise voltage and a noise current. The noise voltage adds to the signal voltage. The noise current flows through the source impedance and creates a voltage that adds to the signal voltage.

Hmmm.... we have a voltage and a current, sounds like a resistance. In fact you can see it that way too.

As you increase the transistor current, noise current increases (logically) and noise voltage decreases (for reasons we can defer to another chapter). So the noise resistance of a transistor varies with the bias current.

The 2520 uses a fairly low bias current, perhaps around 0.1mA. The 990 uses a fairly high bias current, I think around 2mA. The optimum source impedance for the 2520 is much higher than the optimum source impedance for a 990.

But wait, there's more. Before transistors there were vacuum tubes, which have negligible noise current. In classic low-noise tube design, you use the highest possible step-up. (For reasons covered in the transformer chapter) 10K or 20K is about as high as you can go for good wide-range audio response. So when transistor amps appeared, the world was full of good 1:10 transformers, and that's what designers aimed for. System impedances tended to be around 10K.

But many designers started exploring the high current possible with transistors. Working at low impedances and quite high current and power, a transistor can equal or exceed the signal to noise performance of a tube. Deane Jensen's 990 (among others) targetted very low source and feedback and mixing resistors. System impedances tended to 600Ω or less.

And as in life, a happy medium is better than going to extremes. Both the 2520 and the 990 approaches can work fabulously well, but so can 2K-3K impedances.

> how you use IV curves to set bias and then still be able to control the gain of the transistor stages.

The gain of a transistor amp is NOT set by bias current. Well, it has an effect.... you can show this by setting up a 1-transistor amp with a fixed load resistor and an adjustable current. Gain is proportional to current. BUT the input overload point is only 10mV-20mV, and the distortion near that point is about 26%! You can't make hi-fi amps that way. Moreover, in a real amp the bias current will change from part to part, and with temperature, so the gain is never the same unit-to-unit or day-to-day. BJT transistors always use feedback to set the gain.

[tpryan]

PRR, I'm gonna have to put you on the payroll if you keep it up!

[drunton]

PRR - that's a great summary. It glues well with one that I happened across on rec.audio.pro. I understand your explanation very well. The original RAP discussion thread had examples including using data from Jensen's "high performance op-amps" summary appnote.

My point is that, other than finding this by perusing usenet or SCA's message board, how can someone really learn this.

Regarding feedback, gain and bias - I understand what you are saying. My example is this. Let's say I want to modify the 1272 "first stage" preamp to give it much less gain. The problem is that the internal feedback resistors affect the bias also. So, how do I go about doing this. If it were a textbook 3 BJT circuit with transistor decoupling between stages, it would be easy, but Neve didn't use textbook examples.

[chikkenguy]

neg. feedback, the whys, hows, etc for general application and also pertaining to the kits would be interesting to me.

load isolators also.

something on overload leds would be excellent, i think.

[SoundBase]

Just a belated response to tim's orignal post.

As for myself, I've done some reading and am moderately comforatble with most single stage circuits, both tube and ss. When I look at the schematic for the N72, I get amazed aty how all the stages are coupled together (some dc and some not) and all off the varous feedback networks involved in stabilty and gain control. Consequantly for me some questions would be:
1) how do you design stages scuh that you can dc couple them ?
2) How do all the feedback networks work to change the gain and/or maintain stabilty?
3) I understand the capacitors between the collector and base to roll off high freq gain and maintain stability, but what do the caps between the emitter and base do ?

I drop my jaw in awe to think that rupert designed all of these without spice programs and just hand calulated everything....

Just my $0.02,
Chuck Hanning