[DESIGN] Original(?) mike-amp design

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I think it´s a good idea for a separate designing forum.

So, things like "why my ssl isn´t working???" should be kept in the clone forum, that would also work as a troubleshooting forum and the real electronics ideas would be in the designing forum, you know... That´s beneficial for everyone, I bet...
 
I think a design forum is a great idea. I doubt I'll be posting anything, but I'll be reading everything and trying very hard to follow.

:thumb: :thumb:
 
i like the idea of a seperate design forum as well. i'm not sure i could contribute anything, but like Seth said i would always be hanging out and trying to learn as much as i could from the great minds here.
 
Svart,

I think Charlie put it very well. The idea isn't to "separate the men from the boys" or any other such elitist thing, but rather to provide a place where these less popular but fascinating topics can be hashed out, at their naturally more-leisurely pace, without disappearing beneath pages of "will this meter work in my SSL clone?" posts. Bear in mind, since it would be a design and theory forum, I would expect frequent visits by newbie and intermediate DIYers grappling with aspects of electronic theory.

In very simplified terms, while the character of the present forum is mostly "how to build", this new forum would be "how it works." There'll be many instances where it won't be clear in which forum a topic should be posted. No problem, since most of us will continue to read and contribute to all the forums, anyway. But at least the clutter will be alleviated somewhat.

Earlier, Kev also made an excellent point, that some top-notch designer types might be attracted here if there were a forum specifically for these topics. We're very lucky to have a guy like PRR, someone very knowledgeable who also happens to have the patience to wade through pages of threads. But who knows what other heavyweights might show themselves if the discussions of interest to them were easier to find, and not subject to such a rapid turnover?
 
> won't output loading have an effect on the feedback? Would you require a further buffer stage to isolate the effects of varying output loads?

A good question, one I've often wondered. In fact the answer seems to be "no". There is no difference between separate DC-load and feedback resistors, and one resistor doing double duty. Output impedance is low either way, and the double-duty scheme is a little more efficient (and a nickel cheaper).

> classic "ring of three" topology

An example of combined DC-load and feedback. It also happens to have low output impedance before feedback, but that is not essential.

BTW, how "classic" is that? I never saw it before the mid-1970s, when AR started using it as phono preamp. I discovered it about the same time, but it seems odd that I don't see it in any older gear.

> but an input transformer might be nice.

There are 10^69 ways to skin cats. And "another" way might also be "nice". This is my amp design, and I gave good (IMHO) reasons for exploring a transformerless idea. I also sometimes have good reason to favor transformer-input; especially in my concert hall which gets many volts of ground-crap between the stage and the booth. But this one is transformerless. (At least at the input: there is a variation with output transformer, which may give lower noise, or may melt mikes...)

> this thread seems to have met the usual fate

Partly my fault. I should have known better than to start a long project just before school started. Last week was quiet; this week is a madhouse. Half the professors need new PCs and the other half have virus infestations so bad that net-ops is giving me nasty-grams to clean up my subnet.

> Clone Lab - Design Lab

I'd be comfortable with just "Lab" and "Design". But that is just me talking. I don't get into a "lab" much any more; just think about things.

> if you post an empty message in a topic, you'll get an email telling you when there is a new post in that particular topic

ummm... why not use the "Watch this topic for replies" feature at the bottom of the topic, instead of posting blanks or drivel?
 
Hey guys,

I have always found the "how things work" stuff the most interesting... and though I don't yet have anything important to add (still a newbie), I'm all for encouraging more of it. I'm one of those people who doesn't fell entirely comfortable doing something till I feel I understand how it works (at least vaguely) and the Lab has been a great educational resource, and is a constant source of inspiration!! :thumb:

JustinS
 
I like the transistor approach because you can test the individual components and match them etc.
I often wonder if these guys who use 10 opamps in their designs get any sleep at night, knowning that their part could change at any time without warning, or go out of production altogether.
It has happened here befor, I know that. Major headache. Uprevs that no longer match the old product, changing specs, broken dies that force them to use anothe maker and relabel, you never know whats inside them chips!
cj
 
[quote author="PRR"]ummm... why not use the "Watch this topic for replies" feature at the bottom of the topic, instead of posting blanks or drivel?[/quote] All too often, I browse this forum without being logged-in, so I have missed this before now...I now notice this handy feature! Thanks PRR! Maybe you spend too much time here? Just kidding :green:

[quote author="CJ"]I often wonder if these guys who use 10 opamps in their designs...[/quote] A good reason to use something verry standard in designs...I doubt there is much drift in the good ol' TL072 as long as you buy from TI. OP275 are probably pretty much a known quantity. But as far as this goes, you can test the opamps too! You're just testing a couple hundred transistors at one time...see, its faster! :razz: i have experienced the same sort of trouble with discontinued transistors, CJ. There's no sure bet in any of this, really.

[quote author="Consul"]"The Brain Trust"[/quote] This gets my vote!!

Sorry PRR, we have hijacked your thread!
Peace!
Charlie
 
> Sorry PRR, we have hijacked your thread!

Ahem...

I started with a single-sided amp, shown on left. On right is two of these amps in push-pull, with a single feedback network:
janus-3.gif


Note how elegantly the feedback comes together. One objection to the single-ended form is that DC flows through the resistor we would use to set gain, so gain changes upset DC conditions. In push-pull, it doesn't. (You ask "Where's the DC?"... wait.) By changing the "20 ohm" resistor, the gain can be varied over a wide range, even down to Unity Gain which is sometimes handy. The "20 ohm" resistor (which you may remember adds to the noise of the 200 ohm mike) can go to 100 ohms (very little increase of noise) at gain of 26dB, whereas other mike-amps with higher impedance gain networks tend to have higher noise at gain of 40dB or 30dB. And as shown, it amplifies differential signals but not common mode signals.

Let's rough-out some operating parameters.

The first stage emitter current, for good low noise in microphone impedance, will be in the range of 1mA to 10mA. 3mA seems to be effective with available devices. The exact value will change when we double-up to make it differential. Also we may find a need to run some other current level. But pencil-in "3mA?".

The output stage has to carry enough current to feed the load and the feedback. In this case the feedback load is small compared to some loads we may face. To be faultless in a "Pro" studio, we are expected to be able to deliver voltages like +18dBm or +24dBm into 600 ohms, around 10V RMS and around 24mA peak if you actually have a 600 ohm load. The output stage bias current has to be at least this high. In fact in a resistance-coupled stage it has to be much higher. But for the moment pencil-in "24mA?".

But what currents? Here we have no DC path at all! Ooops!

A resistor or two to ground is logical, but interacts with the gain-set network and limits minimum gain and sets a significant common-mode gain. So let's try a couple of current sources. I've drawn them as transistors, and penciled-in the rough-guess operating currents:
janus-4.gif


What is the maximum gain possible? What is the open-loop gain (the "20 ohm" resistor set to zero)?

Remember that the emitter impedance of a silicon BJT is about 30/I, where I is current in milliAmps. And the base impedance is about Beta times higher. So the first stage, with 3mA current, has about 10 ohms emitter impedance. The second stage's emitter at 24mA will be around 1.3 ohms. Assume this transistor's Beta is 100 (just for rough-guess). The collector resistor on the first stage will probably be 0.6V/3mA or about 200 ohms. The base impedance of the second stage is about 1.3*100 or 130 ohms. 200||130 is about 70 ohms. First-stage voltage gain is about 70/6= 5. Second stage gain (unloaded) is the 1K load resistor over the 1.3 ohm emitter impedance or 1000/1.3= 770. Total gain is then 5*770= 3,800. This is just over 71 dB, and there is nothing to really stabilize the gain against variation except the feedback. And I have ignored collector impedance, which will reduce the 2nd stage gain from 770 to more like 500. We probably should not ask for 60dB of closed-loop gain. So it is close to a "good" design, but needs a little more.

Go back to the question about output current and load. What is the best output configuration for a resistance-coupled amplifier? There are several answers. It really depends how wasteful you can afford to be, and which way. Remember that a BJT can pull-down to nearly zero volts, but in a resistance-coupled amp the load only pulls-up as much as that DC load resistance can pull. In general you bias a transistor so its collector-emitter voltage is a little more than the peak AC voltage you need, and then wrestle with the resistor. If you have a power supply voltage much-much larger than the peak output voltage, you use a resistor much larger than the load resistance and set the transistor idle current to a little more than the peak AC signal. If you have a low supply voltage, you may have to idle at much more than peak AC output current so the resistor has enough "slack" to yank the load around when the transistor current falls. Here we seem to be in the intermediate case: we don't have a lot of excess supply voltage, and we don't want a lot of supply current. A rule of thumb to get you in the ballpark is: use a DC load resistor equal to the AC load, bias the transistor to sit at 1/3rd the supply voltage and twice the peak AC load current.

In this case each side of the amp sees half of the 600 ohm load, so those "1K" resistors "should be" 300 ohms. For other reasons, we will lose 10 or 12 volts off the bottom, so we have 36V supply. If the transistors get 1/3rd, the resistors get 2/3rd. 2/3rd of 36V is 24V. 24V/300ohms is 80mA per side, 160mA per complete amplifier, which times 48V is over 7 Watts total dissipation! I told you it was power-hungry!

The maximum efficiency of a resistance-coupled power amplifier is about 8%. This amp more like 5%. A transformer-coupled amp makes 50% on paper and 40%-20% in practice. Class B is 78%, and a typical audio op-amp chip runs around 50% when maxed-out. If you must meet a specific dBm output level spec, a resistance-coupled output needs about 10 times the power supply of a chip. Worse, since the resistance coupled amp eats full power all the time, while a class AB chip idles stone-cold, and a console is almost never asked to deliver full-power full-load sines on all outputs at once. The resistance coupled amp is such a pig for power that you almost never see it done above a couple milliwatts output in commercial products. But scroungers like us may turn-up power parts at surplus prices. And there is an element of "Mine is bigger (and hotter) than yours!". So what the heck, eat power.

Still, with those values, it can make nearly 24V peak in 600 ohms, which is nearly +27dBm. We don't need that much.

Also: the closed-loop voltage gain is a function of these load resistors. If we can get a little more voltage gain, we would like to set the feedback for 60dB or more closed-loop gain. With two 300 ohm resistors, the middle resistor (gain-set) has to be 0.6 ohms. This is a suspiciously low value... can we really get an accurate 0.6 ohms?

Another problem: on paper, all the DC can balance. In fact, the two input devices are never going to be exactly matched. Using the monolithic matched-pairs, there will be several milliVolts of offset across the gain-set resistor, partly Vbe mismatch but mostly base bias resistor and Beta mismatch. That might be tolerable in a fixed-gain amp, but one sweet feature is the simple way we can change gain from around 1,000 to about 1. But this really needs a switch (there is no pot with over 60dB range and under 1 ohm minimum usable setting), which will want to interrupt that milliVolt offset and make milliVolt pops, which when amplified by gain=1,000 will be awful loud.

So we need a DC blocking cap in series with the gain-set resistance, which needs to be very low. And since it has an unknown DC polarity (+ or -) across it, it needs to be bipolar. What is the biggest nonpolar cap I can conveniently buy? DigiKey lists a Panasonic 6,800uFd nonpolar, at a tolerable price. When faced with a 1 ohm resistor, this gives -3dB at 26Hz, -1dB at 52Hz. That's at maximum gain; bass response improves as gain is reduced. And since high gain tend to bring up room rumble, I'm not sure we really want 2Hz bass response at maximum gain, though it is useful at lower gains.

So all things considered, I went for two 500 ohm DC load resistors and a 1 ohm gain-set resistor for 60dB gain. And then fudged again to the standard value of 470 ohms, which is only 0.54dB "off". We can't fret about half-dBs, because with open-loop gain of only 71dB the "60dB" gain-set resistor will have to be well under 1 ohm.

Oh, why bias the input bases at 10 or 12 volts? They can't sit at ground: the emitters will be 0.6V negative of the bases, plus we need a few volts across the current sources. But the bases and emitters also must follow the input voltage, both differential and common mode. Since the gain can go near unity, the input differential voltage may be several volts. And in some situations the common mode voltage is several volts. Though we hope it is much less, this type of clipping on "signal" that is rejected further up the system will give mysterious splats and buzzts. We really need to bias the inputs up enough to cover any likely signal and common-mode voltage, plus our Vbe and current-source biases.

And we will need input and output coupling capacitors and DC bias/leak resistors.

Conceptually we arrive at this:
janus-5.gif


In my next chapter I will try to show that the gain and distortion of this design is not that great, and can be improved fairly simply. I've also omitted details of stabilization against RF oscillation and DC bias.
 
that is wonderful stuff!
I am going to go through the bjt chapter in The Art of Electronics v soon and this just makes me want to gobble it up! :green:
PRR, without wanting to make your head big, I want to jump in first and say how incredibly helpful these sorts of posts/ projects are for those of us who like to know "Why does it do that?"!!!!
Keep it coming :grin:

chef

p.s. dammit Jakob, you beat me to it! :wink:
 
Sort of like waiting for the next chapter of a Flash Gordon serial at the cinema for an old fart like me. I want to add my thanks. Your approach when writing posts make the subject matter easy to understand and informative for both novice and experienced practicioners. I fall somewhere in the middle of that experience level, but I certainly enjoy reading your posts.

There are several people on this forum who take time and effort to share their knowledge and experience with the group, and whose approach to the subject at hand is immediately enlightening. Like a "Eurica moment". You are, of course, one of those, if not chief among them. Now, I we are all able to learn enough, we may be more able to engage you in decent dialogue.
 
IIRC emmiter R is 25/I. not that it matter that much for the design.

Chef did you get your book on transistors yet?

PRR your circuit looks something like some of the PASS amps at passlabs DIY section.
 
This kind of thread is really great and deserves its own place.

Can´t wait to read more!

cheers!
Fabio
 
Regarding the "ring of three"...

[quote author="PRR"]BTW, how "classic" is that? I never saw it before the mid-1970s, when AR started using it as phono preamp. I discovered it about the same time, but it seems odd that I don't see it in any older gear. [/quote]

My reference cites it as a wide-bandwidth amplifier used in instrumentation in physics labs. I'm sorry I can't provide some specific examples.

The Pultec MB-1 preamp is conceptually similar, although there is one R-C coupling between stages 1 and 2:
http://www.geocities.com/rafafreddy/DIY/pulteq-mb1/schem.jpg

Confusingly, the appellation "ring of three" is also commonly applied to a filter circuit topology known as Tow-Thomas.

Someone brought up a question about intrinsic emitter resistance. It's the reciprocal of gain and can be found as 25 mV / IC (A), or 25ohms / IC (in mA).
 
PRR,

This is a great thread, I have been looking for something in mic-pre to inspire me for a while and I hope we all will help PRR to reach a conclusion by NOT making changes to HIS concept.
I have two points/questions,
(a) How do you see this from a consrtuction viewpoint, as a "gain block" which could made up on small boards and then maybe potted like the many discrete op-amps that there are.
(b) would the usual input add ons like pads , rf filters , phantom supply etc have any affect on the noise level in the sense that the direct relationship between the mic impedance and self noise and the preamp would be changed.
Hope I am not putting the cart before the horse here! I am looking for very low noise and distortion that I could use as a preamp for ribbons
Steve
 
[quote author="PRR"]>Conceptually we arrive at this:
janus-5.gif

[/quote]

Interesting stuff!

You're building a discrete SSM2017 (the input stage), only better! :cool:

It doesn't look like that at the first glance, but the structure is really similar.
When you look at the 2017's interior, the second stage is not built from PNPs - they use a npn pair for gain, working into a folded cascode, and _that_ apparently is then buffered before it goes into the feedback resistors - but the whole structure is really the same. (Note the the second stage in the 2017 is not the long tailed pair that it looks lik at first. As in your circuit, the emitters are connected to stiff voltage.)

Of course your circuit is more elegant - using less transistors and much higher bias currents, and the design that allows direct drive of heavy loads sets it into a different class.

Let us know how the final version will look like, and how it performs!

JH.
 
[quote author="jhaible"]
When you look at the 2017's interior, the second stage is not built from PNPs - they use a npn pair for gain, working into a folded cascode, and _that_ apparently is then buffered before it goes into the feedback resistors - but the whole structure is really the same. (Note the the second stage in the 2017 is not the long tailed pair that it looks lik at first. As in your circuit, the emitters are connected to stiff voltage.)[/quote]
Here's the 2017 data sheet

JH.
 
Ok I know with my limited knowlage all I could contribute here is "48v is cool" but I feel like I'm waiting for the next issue of tape op or something.

whats next? what connects to those little bias pionts? what transistors? I hope thier really BIG. Make them all TO3 packages... the biggest op amp ever. we could pot it an old muppets lunch box.

whooo hooooo yeaaaa diga diga diga diga diga diga diga humphhh.
 

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