Don't go changing opamps willy nilly

[user 37518]

} ;D

 

[JohnRoberts]

Circuit design is not a contest to see who can spend the most money on a given task (pretty much the opposite), while some audiophool products look like they may try to impress customers with an expensive BOM or high profile components.***

Second guessing competent design engineers over component choices is not a good use of time, IMO.

Of course I am biased. 8)

JR

*** I once put gold plated jacks on a phono preamp because my customers would think they were better, and they didn't actually cost much at all. After that product I escaped the hifi business for a more sensible one (live sound reinforcement).

 

[Tubetec]

There is a tendency to be consumed by numbers once you start down the metrology path ,can become obsessive .When Im faultfinding its ears first ,then eyes ,if the fault hasnt shown itself then meter . Second guessing a designer rarely bears fruit unless corners were cut to begin with .

 

[Tubetec]

John pipped me to the post 

 

[Tubetec]

I couldnt really agree with that Dual ,you could easily have a high spec piece of ic op amp gear thats good uptill a certain point once you hit the overload margin its game over ,where a vintage unit might have a much more distortion but a gracefull overload and less higher order garbage once you hit the redline .
Being able to anticipate the levels you expect is the key to good sound engineering , hitting a 20k pa or a digital recorder system with square waves never ends up good .A good tube circuit will gently round your waveforms and even under extreme overload will still not require a take 2 scenario , As you ascend to the Nth harmonic the subjective nastiness increases exponentially ,its not all about the numbers.

 

[JohnRoberts]

Once the amplifier is driven into its non-linear region of operation all bets are off, however I have yet to encounter an opamp based amplifier that measures low THD and IMD but sounds bad, considering it is used in its linear region, however I agree that THD is not the only parameter at stake.. To me its a good practice to keep the levels down, nowadays there's little to be gained by runnning amplifiers at +24dBu levels. I do have to admit I prefer tube amplifier distortion when it comes to guitars, but thats a whole different thing.

An analogy I use is that how amplifier stages behave when clipped is like how race cars handle after bumping into the guard rail. A well designed race car will handle better than a street ride when bumped.  In power amplifiers some design effort is spent on recovery from clipping. Anti-sat diodes across intermediate gain stages will prevent hard saturation so the device can recover linear operation immediately after the overdrive is removed. If allowed to saturate it will take a longer finite time to recover, AKA sticking to the rails, creating more audible perturbations than a brief clean clip.  I learned a ton of subtle stuff like that from Jack Sondermeyer at Peavey (he designed the CS800 among other old industry standards). I would only half joke that he forgot more than I know.

Tubes OTOH are a completely different animal when overdriven, in fact Peavey (and others) developed solid state circuits to mimic tube overload and the sundry patents are interesting reading for anyone interested. They never sounded 100% like a real tube but close enough to fool most players in an unblind A/B test.

Since I do not consider clipping normal operation I do not worry beyond, keeping unintended side effects to a minimum.

JR 

 

[JohnRoberts]

The diode thing sounds like a good idea, however I usually try to avoid adding limiting diodes, they tend to add distortion in normal operation.

This is probably TMI information but an anti-saturation diode is placed between base and collector of a common emitter gain stage. When operating normally the diode is reverse biased so does not conduct and has zero affect on the output.

When the transistor is turned on so hard the collector voltage drops lower than the base the anti-sat diode conducts and pulls current away from the base to prevent hard saturation. If allowed to saturate charge carriers (?) build up and prevent the device turning off again quickly which can cause more audible perturbations after clipping . 

This is kind of inside baseball for discrete circuit design. Feel free to disregard this is not likely to come up on the test.

JR

Tubes do sound good when overdriven, as an effect such as in electric GTR, they are hard to beat.

 

[gridcurrent]

This is probably TMI information but an anti-saturation diode is placed between base and collector of a common emitter gain stage. When operating normally the diode is reverse biased so does not conduct and has zero affect on the output.

When the transistor is turned on so hard the collector voltage drops lower than the base the anti-sat diode conducts and pulls current away from the base to prevent hard saturation. If allowed to saturate charge carriers (?) build up and prevent the device turning off again quickly which can cause more audible perturbations after clipping . 

any examples other than the Neve BA-440 ?
what are the audible perturbations ?
are they measurable ?

 

[Tubetec]

My view really when it comes to topology is that the more feedback you use the longer the recovery time from a transient event,
Once a high feedback topology overloads it doesnt know its arse from its elbow ,triodes without feedback act more like an instant  attack and recovery  limiter /compressor ,5% thd was the normal spec for tube/valve gear long ago and its tollerable to the ear,5% thd in a modern amp especially a class d switcher topo is going to make your ears bleed ,of course you can employ  quality comp/limiters and run your amps well withing their range .,
Downside of the tubes is power hungryness /low power. Momentary overloads can easily excede a power transistors junction tempreature ,where in tubes this isnt possible ,it'll fuzz out sure ,but wont go up in a puff of acrid smoke .
We cant foretell the future and in real life situations tubes suck up the punishment better at the front end ,transformers with all the extra cost and bandwidth limitations offer similar benefits ,a kind of spongeyness, like the core energy fights against the short circuit thats being applied to the rails .
Anyway great to discuss the pro's n con's of various methodologies ,each of course has their place ,there is no right or wrong in it ,only in the way its used.

 

[iampoor1]

My view really when it comes to topology is that the more feedback you use the longer the recovery time from a transient event,
Once a high feedback topology overloads it doesnt know its arse from its elbow ,triodes without feedback act more like an instant  attack and recovery  limiter /compressor ,5% thd was the normal spec for tube/valve gear long ago and its tollerable to the ear,5% thd in a modern amp especially a class d switcher topo is going to make your ears bleed ,of course you can employ  quality comp/limiters and run your amps well withing their range .,
Downside of the tubes is power hungryness /low power. Momentary overloads can easily excede a power transistors junction tempreature ,where in tubes this isnt possible ,it'll fuzz out sure ,but wont go up in a puff of acrid smoke .
We cant foretell the future and in real life situations tubes suck up the punishment better at the front end ,transformers with all the extra cost and bandwidth limitations offer similar benefits ,a kind of spongeyness, like the core energy fights against the short circuit thats being applied to the rails .
Anyway great to discuss the pro's n con's of various methodologies ,each of course has their place ,there is no right or wrong in it ,only in the way its used.

I think you bring up some good points. THD is not a great spec to use when discussing the finer points of distortion. Good for spec sheets, bad for communicating what kind/what harmonics are present.

 

[user 37518]

This is probably TMI information but an anti-saturation diode is placed between base and collector of a common emitter gain stage. When operating normally the diode is reverse biased so does not conduct and has zero affect on the output.

When the transistor is turned on so hard the collector voltage drops lower than the base the anti-sat diode conducts and pulls current away from the base to prevent hard saturation. If allowed to saturate charge carriers (?) build up and prevent the device turning off again quickly which can cause more audible perturbations after clipping . 

This is kind of inside baseball for discrete circuit design. Feel free to disregard this is not likely to come up on the test.

JR

Thats a great tip JR, I will definitely "steal" it

 

[alexc]

There's certainly many places where the more modern, expensive opamps can be worthwhile .... in ad/da conversion stages.

The TI lme49xxx series are pretty amazing - I can't hear the differences over an ne55xx, no way ...  but I can measure it for sure, even with my 'cost effective' measurement setup.

For those lucky enough to be making their first converter investment in todays world, it must be quite nice to have super performance right off the bat!

 

[user 37518]

There's certainly many places where the more modern, expensive opamps can be worthwhile .... in ad/da conversion stages.

The TI lme49xxx series are pretty amazing - I can't hear the differences over an ne55xx, no way ...  but I can measure it for sure, even with my 'cost effective' measurement setup.

For those lucky enough to be making their first converter investment in todays world, it must be quite nice to have super performance right off the bat!

Im a big fan of the LME49xxx opamps myself, I dont even how can they measure distortion for the opamp, its spec is at 0.00003%, im not sure what the distortion of the 553x is, but yeah, hearing the difference is not an easy task.

 

[JohnRoberts]

any examples other than the Neve BA-440 ?

What is a BA-440?

what are the audible perturbations ?

I am surely repeating myself but gain stages that are allowed to saturate (hard clip) do not recover immediately when the overload is removed, so instead of a brief momentary clip, the brief event is stretched out generating lower frequency content, that is generally more audible that the brief clip that some people do not even perceive as distortion (as evidenced by modern practice for some musical genres. )

are they measurable ?

Yes, but I am not sure why...generally this phenomenon is very visible with an oscilloscope.

A simple measurement would be a null test between the clean path and saturated path. The path with anti-sat diodes, will reveal a much shorter null error output from the clip event. Without the anti-sat circuitry the clip event error will persist longer after the clip event ends and generates more error energy. (FWIW  I have never attempted to quantify this.)

Unless you are doing discrete design this is probably too much information. If you are careful about levels and avoid path overload, you do not have to worry about this.

JR

 

[JohnRoberts]

My view really when it comes to topology is that the more feedback you use the longer the recovery time from a transient event,

Be careful about too broad sweeping generalities.

Yes in an amplifier using negative feedback, there will generally be loop gain compensation (lag) to scrub off gain before the open loop path delay/phase shift flips that negative feedback positive.  If-when that overall gain stage saturates the input stage loses feedback and the compensation (generally dominant pole, but there are multiple variant compensation schemes) capacitor will charge in the direction the input stage is still telling it to go.  When the path saturation finally clears, it will take a finite time for the cap to re-establish  its correct equilibrium operating point. Note, this is not just during voltage clipping, but a similar lack of feedback condition exists during slew rate limiting, because the output is not keeping up with the input when slewing.

In high speed op-amps especially those designed to expect transient overloading and requiring fast recovery (like sample and hold buffer op amps) there are often anti-sat diodes across the input long tail pair so it will recover promptly and not over charge the internal compensation pole.

Once a high feedback topology overloads it doesnt know its arse from its elbow ,triodes without feedback act more like an instant  attack and recovery  limiter /compressor ,5% thd was the normal spec for tube/valve gear long ago and its tollerable to the ear,5% thd in a modern amp especially a class d switcher topo is going to make your ears bleed ,of course you can employ  quality comp/limiters and run your amps well withing their range .,

Sorry I do not follow all that, but interesting you mention class D switchers. Peavey has been making class D amps for decades and they have their own unique issues. In fact when the output stage involves saturated switches recovery time is a dominant  source of distortion. For that reason bipolar devices have not been used for class D outputs since MOSFET switches (different on/off technology) became available. For even more TMI Bruno Putzey's novel class D topology uses the lag from NF compensation to create the class D switching frequency (very elegant design).

Downside of the tubes is power hungryness /low power. Momentary overloads can easily excede a power transistors junction tempreature ,where in tubes this isnt possible ,it'll fuzz out sure ,but wont go up in a puff of acrid smoke .
We cant foretell the future and in real life situations tubes suck up the punishment better at the front end ,transformers with all the extra cost and bandwidth limitations offer similar benefits ,a kind of spongeyness, like the core energy fights against the short circuit thats being applied to the rails .
Anyway great to discuss the pro's n con's of various methodologies ,each of course has their place ,there is no right or wrong in it ,only in the way its used.

NF gets a bad rap but is significantly responsible for the linear (clean) audio paths we take for granted today. Of course anything can be applied improperly, and/or used to separate audiophools from their money.

JR

 

[ruffrecords]

What is a BA-440?
JR

One of the very first Neve discrete op amps.

Cheers

Ian

 

[Tubetec]

Thanks John,

Theres no denying the benefits of the switchers ,light weight ,awesome power ,cheap to produce .

I did recently take a job to repair one of the modern compact Yamaha mixer -amp and speaker setups,it turned out to be way above my head , I dropped it over to a buddy with more than 50 years in electronic service ,according to him he has seen lots of issues with switcher topo's ,not so much with the output stage ,but more to do with the control ic's that run the show , in the case of the Yamaha it just wasnt economical to repair without specialist test jigs ,and at any rate there was dozens of posts online from people who had trouble with that series of unit .Some sent their units back to yamaha who said it was ok ,but odd ball problems continued , Yamaha even suggested only using the original speaker cables for correct operation ,which is bizzare.

Sorry ,slight diversion from the original topic.

 

[JohnRoberts]

Thanks John,

Theres no denying the benefits of the switchers ,light weight ,awesome power ,cheap to produce .

it is only relatively recently that class D delivers cost savings commensurate with the reduced iron and aluminum demands. For decades it cost a bunch more just to make a class D amp. (I know because I was marketing them back then).

I did recently take a job to repair one of the modern compact Yamaha mixer -amp and speaker setups,it turned out to be way above my head , I dropped it over to a buddy with more than 50 years in electronic service ,according to him he has seen lots of issues with switcher topo's ,not so much with the output stage ,but more to do with the control ic's that run the show , in the case of the Yamaha it just wasnt economical to repair without specialist test jigs ,and at any rate there was dozens of posts online from people who had trouble with that series of unit .Some sent their units back to yamaha who said it was ok ,but odd ball problems continued , Yamaha even suggested only using the original speaker cables for correct operation ,which is bizzare.

Sorry ,slight diversion from the original topic.

I have not been competing with yamaha since last century so haven't checked under the hood lately, but they had a decent reputation for product reliability, while perhaps not top of heap for audio integrity in budget MI lines.

I recall a value topbox Yami amp last century that used some sort of crude switcher variable rail on top of a linear amp (for more efficiency)... Not hifi but AFAIK it worked adequately well. Good enough for that market.

JR

 

[okgb]

there's the other side of the coin , especially in the building construction trades , to get from contractors
" ah , you'll never notice , or everyone does it that way ,  It doesn't make a difference or [ insert your own ,
and indeed , you get it inserted !  ]

usually , cause they want to get out as quick as possible with as much profit, with the least amount of effort
there is an element of this in every industry

 

[JohnRoberts]

One of the very first Neve discrete op amps.

Cheers

Ian

No wonder I don't recognize it...  ;D

JR