Simplest discrete op-amps

[burdij]

I was thinking that might be a possiblity, but how do you produce a null signal with a method that itself is free of TIM distortion?

The observer affects the observed.

 

[mikep]

[quote author="burdij"]I was thinking that might be a possiblity, but how do you produce a null signal with a method that itself is free of TIM distortion?

The observer affects the observed.[/quote]

exactly. I have been wonding how to deal with this myself. perhaps the summing/nulling function should be built into your reference monitoring amp? or perhaps use a more powerful amp for the source then passively sum the signals right into a speaker/headphone? Id love to hear more ideas.

 

[JohnRoberts]

[quote author="burdij"]I believe the article cited talks about non-harmonic noise interaction within the cochlea of the ear caused by an, as of yet, unknown effect.

Leach describes an amplifier with low TIM as one with less than about 20db of gain per stage, set by local feedback loops. There are several articles authored by him and others on TIM effects. One problem with TIM is measuring it. It is a case of the measuring instrument's TIM performance. Kind of like measuring .001% harmonic distortion with a harmonic distortion analyzer (like mine) with .01% minimum resolution. And as far as I know IMD and TIM are two different things, that in fact, IMD is an in-band phenomenon of the interaction or modulation of high frequency in-band components by lower frequency in-band components.

Here is a short excerpt from Maxim App. Note 2119 regarding the two common methods for measuring IMD:

.

The SMPTE (Society of Motion Picture and Television Engineers) standard specifies a two-sinewave test signal consisting of a low-frequency, high-amplitude tone linearly combined with a high-frequency sinewave at 1/4 the amplitude of the low-frequency tone. The SMPTE specification calls for 60Hz and 7kHz as the two sinewaves. When a non-linear device is subjected to a two-tone test signal, intermodulation products appear as sidebands around the high-frequency tone. The percentage intermodulation distortion is defined as the percentage of amplitude modulation, represented by the second and third order pair of sidebands, of the high-frequency signal. Second order sidebands around the high frequency tone are spaced at a frequency equal to the low-frequency tone (FH ± FL). Third order sidebands are spaced at twice the low-frequency tone or FH ± 2FL. (FH and FL correspond to the highfrequency and low-frequency tones, respectively.2)

CCIF intermodulation distortion testing consists of using two equal-amplitude, high-frequency signals spaced closely in frequency. Signals used for test data presented include 13kHz and 14kHz. The audio analyzer measures only the amplitude of the difference tone or low-frequency product resulting from the two high-frequency test signals. Here, the percentage intermodulation distortion is defined as the percentage of amplitude modulation which the low order or difference signal (FH - FL) represents on the high-frequency signals.

burdij wrote:

2. The gain block must have a fairly high slew rate at a moderately high gain so that it can achieve a good GBW for a significant number of harmonics of the fundamental of each intermingled wave in the wave "packet".

I didnt understand this one. Could you explain ?

As far as this comment goes, it all has to do with the Fourier Series. If you don't reproduce a goodly number of the harmonics of the fundamental, including their phase relationships (since there is a complex component involved with the AC waveform), you can't possibly result in an accurate "reproduction" of the input waveform including its transient phenomenon. An important concept in FM electronic music synthesis, I believe.

This stuff isn't my idea, by the way, this has been around since well, 1927, I suppose. I was just trying to get a list of some achievable goals for the design of a simple discrete gain stage, not get mired in the psychoacoustic mumbo jumbo. Let's face it, most things can look very good when run through test equipment, but, as we have all experienced, can vary widely when subjected to the harsh criticism of the listener.[/quote]

If you don't stress the ear with out of band signal it will be less likely to distort.

Yes TIM is difficult to measure because it doesn't exist in a properly band passed audio path.

The difficulties associated with higher slew rates were well explored and understood by guys like Baxandal designing electronics for RADAR in WWII. TIM, SID, etc was rediscovered by audiophiles decades later looking for some holy grail to hang their quest for some new distortion mechanism on.

I suggest reading the Leach paper I referenced. It is brief and concise.

JR

PS: I don’t suggest that there are not examples of gear that may measure well on one test and still sound bad with real music, but what you are hearing is not what was measured.

 

[Martin B. Kantola]

[quote author="clintrubber"]And we shouldn't share these on purpose, since it'll be better that everyone has a highly personalized collection of snippets.[/quote]

Exactly, it's a very personal thing! Read somewhere that Rupert Neve listens to pink noise. Can't imagine how that works... Gotta ask him about it some day.

Since I have a recording studio, it's pretty easy to produce various test signals. As you say, there are different signals that reveal different problems. Simple recordings with lots of depth (such as church acoustics) are great to evaluate low level performance and detail. One of my other favourites is using a single female vocal, close-miked, it contains so much demanding information, especially in the upper range of the spectrum (any type of clothing or lack of it works for me...) And our hearing seems to be especially sensitive to the human voice.

The biggest problem I have is the demands on the playback system, speakers, amps and so. Takes a very good (expensive) system. Struggling to find a medium to record test material too, looking at both DXD and custom 1-inch tape machines. Seriously. PCM does not impress me much anymore, even if I use it for multi-tracking every day. Playing a scratchy vinyl is much better.

Martin

 

[Guest]

Martin, I whisper in front of microphone varying a distance in order to test low level detail reproduction quality. However, a real church reverberation is great as well.

 

[burdij]

I am not sure that the following classifies as the "simplest" op-amp circuit, but I was inspired to do this by this thread. What I did was simplify the Jensen topology and tried using a current mirror and discrete input transistors selected for gain and Vbe. Here is the schematic:

I built up a couple of these to make a mike pre. Here is a photo of the completed unit:

This photo shows two units in a prototype servo mike preamp. There is a gain stage followed by an output driver stage. Cinemag transformers for input and output. I have done some recording with a pair of unmodified Apex 460 type tube mikes. I will wait to post audio clips until I get someone to play something other than me (as I am a "lame" musician).

I have done some testing with one of these as an output drive for a 600 Ohm transformer coupled load. It will put out > +20 dbm with less than .01% harmonic distortion (the limit of my analyzer).

 

[Jim Zuehsow]

A long time ago, there was an article in Audio magazine featuring Richard Burwen's 20,000 watt home stereo system. It also had his complete control system which consisted of dozens and dozens of plug-in opamps he designed just for this. I seem to remember there were 4 transistors in each module. I'll look around and see if I can find the article and post the schematics. Burwen, by the way, claims in his resume, that he was one of the founders of Analog Devices, along with Shingold and several others. I still have some of their gigantic early modules, which are probably 12 time the size of a API module.

 

[skipwave]

[quote author="Martin B. Kantola"] One of my other favourites is using a single female vocal, close-miked, it contains so much demanding information[/quote]

This reminds me of a story I heard about the selection of a voice to be used for automated messages to fighter pilots. They found that a seductive female voice commanded the pilot's attention better than a more urgent female voice or any male voice.

My girlfriend's voice contains lots of demanding, but not a lot of information. :twisted:

 

[Freq Band]

[quote author="JohnRoberts"]

There has been a great deal of study and knowledge documented about human perception.
[/quote]

...by whom ?


:razz:

=FB=

 

[JohnRoberts]

[quote author="Freq Band"][quote author="JohnRoberts"]

There has been a great deal of study and knowledge documented about human perception.
[/quote]

...by whom ?


:razz:

=FB=[/quote]

I've seen a lot of work published in the AES Journal over the years. That would be a good place to start.

http://psy.ucsd.edu/~ddeutsch/ Diana Deutsch is one who comes to mind but there are many different researchers who published papers and articles on different aspects of audio perception.

JR

 

[burdij]

I used to work in a laboratory at the University of Wisconsin that worked mostly with human perception studies, auditory, visual, tactial, and motor tracking. It was called the Behavioral Cybernetics Laboratory and was in the Dept. of Psychology. Dr. K. U. Smith was the director.

 

[Freq Band]

[quote author="JohnRoberts"][quote author="Freq Band"][quote author="JohnRoberts"]

There has been a great deal of study and knowledge documented about human perception.
[/quote]

...by whom ?


:razz:

=FB=[/quote]

I've seen a lot of work published in the AES Journal over the years. That would be a good place to start.

JR[/quote]

No, no....
I meant , whom is doing the perceptions....... on human perception ?
Not humans I hope. :wink:
------------

I've never built an op amp. Recently a friend gave me a couple from an "anonymous audio company", +24v, but I've yet to apply them.
Strangely, I recently found (what looks like) two other discrete opamps off of a Grass Valley Group video dist. amplifer.
Hmmm, they seem to be searching me out....for some hintable reason???

=FB=

 

[clintrubber]

I've admired the several discrete op-amp designs that have been posted here and elsewhere. But being a minimalist at heart, and someone who does not consider himself an expert in design with discrete transistors, I wonder if good performance could be obtained with simpler circuits.

Hi,

Was the Studer A101 mentioned here yet ?

2*PNP input & 2*NPN doing SRPP

(attached pic mirrored on purpose to correct for negative-on-top drawing style)


http://recordist.com/studer/A101%20latest%20version-4.pdf

http://recordist.com/studer/A101%20first%20release-1.pdf

More:  http://recordist.com/studer/technical.html  (thanks Jakob)


Regards,

  Peter

 

[bcarso]

In Len Huelsman's book Active Filters: Lumped, Distributed, Integrated, Digital, and Parametric, the section by Kerwin shows a "Positive Gain VCVS" with a gain of times 2 and three transistors (page 19).  Omit the feedback and feedback divider resistors and you get the Studer opamp sans active output load device.

Note that by removing one half of the differential pair you get the complementary feedback pair much loved by Self and others.

 

[JohnRoberts]

For simple, IIRC there was an active filter designed for the old Bose 901  EQ that used a single darlington as the only active element.

Simple can be elegant and good, if all the fluff not used is not really needed, but complexity in circuits is generally to solve specific problems and deliver better performance, so less is not more, it often really is less.

I recall designing a DOA for a console bus amp, back when you couldn't buy off the shelf opamps that quiet. Since it was a simple inverter, I didn't need to worry about CMR. Likewise I could filter PS noise to manage PSR. For that specific application, I would have been adding cost and perhaps noise from more complexity that I didn't need, or could deal with externally more easily. 

JR

 

[thermionic]

edit - just answered my own question, so I'll rephrase it:

I am curious as to whether there have been many 'simple' opamps, i.e. with fewer than 5 transistors, that are unity gain stable and do not implement the LTP at the front.

J

 

[JohnRoberts]

edit - just answered my own question, so I'll rephrase it:

I am curious as to whether there have been many 'simple' opamps, i.e. with fewer than 5 transistors, that are unity gain stable and do not implement the LTP at the front.

J

You need to be more clear about how you define "opamp".  The working definition I have always heard involves 2 high impedance inputs, differential gain, etc etc.  The classic LTP is the simplest way to execute that, especially with the low active device count constraint. 

I recall an alternate IC amplifier topology called a "Norton" amp (LM3900) that used a ground referenced current mirror for the + and - inputs but this was definitely not an "operational amplifier". 

JR

 

[tv]

edit - just answered my own question, so I'll rephrase it:

I am curious as to whether there have been many 'simple' opamps, i.e. with fewer than 5 transistors, that are unity gain stable and do not implement the LTP at the front.

J

So-called "current-feedback" opamps, "CF-opamps", have a "high-impedance" (usually not-so-high anyway) and a low-impedance "input" node, used for the "feedback".

http://en.wikipedia.org/wiki/Current-feedback_operational_amplifier

 

[tv]

IOW, depending on the POW, one could argue that simplistic "gain-stages" like this one:

http://sound.westhost.com/project37a.htm

... can be in theory wieved as a simplistic CFOA variant; ... then yes, a simplistic "current-feedback" opamp can be actually made with less-than-5 transistors.

Elliott's example could be in fact re-done with 3 BJT's (and, of course, a LED in place of Q4).

 

[JohnRoberts]

To repeat myself, it depends on your working definition of an opamp.  I don't see two high impedance inputs on those examples.

JR