What Class do 2@5@2@0, 9@9@0, etc run in?

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strangeandbouncy

Well-known member
Joined
Aug 8, 2004
Messages
2,112
Location
West Sussex. UK
Dear all,


Hope this is not a shmucko question! Perhaps you can settle a debate for me. I was always given to believe that 2520 was running in Class A. Also the 990, Forsell, etc. Someone was telling me yesterday that the 2520 runs as ClassAB. Do I owe him a pint, or are the brews on him? - HA!


All the best,


ANdyP
 
Well, all (push-pull) class A gets class AB at some point - it's a question of load impedance.

Example:
Opamp X runs with 10 mA quiescent current in the output stage. If it's a push-pull-stage, it can deliver 20 mA in class A, i.e. 10 Vpeak into 500 ohm. At this point, one output transistor is right on the edge with switching off, so if you increase the load and keep your output voltage, we'll enter class AB.

So if you don't ask too much from your 2520 or 990, they'll be class A, otherwise AB.

I don't remember a discrete opamp which is not class A/AB. Probably because class B (and other switching classes) is more elaborate to implement (at least if you ask for low THD).

Note that a class AB output stage, once it leaves class A, distorts more than a properly biased class B.

Samuel
 
[quote author="featherpillow"]I thought I remembered John Hardy telling me once that the 990 was class A.[/quote]

It probably is for most loads/feedback networks, but Samuel's discussion is spot on.
 
If you read Fred Forssell's white paper about the simple Jfet opamp he say that the output stage is class A up to a reasonable output current then it turns into class AB.

chrissugar
 
Wow! guys!

So much info. i am even more confused than ever tho', yet simultaneously much better informed! I thought that Class A meant just that . . . Just to add another angle, I was visiting the Tonelux site this evening, and i found this:-

http://www.tonelux.com/opampinfo.html

Paul Wolff discribes the 2520 and 990 as Class AB! So there you are. From the mouth of the designer himself. Ho Hum!

Thanks for everyones input,

guess we'll have to split the cost of the beer . . . .


ANdyP
 
Paul Wolff discribes the 2520 and 990 as Class AB! So there you are. From the mouth of the designer himself.
Paul is the person to ask about the 2520, but it was Deane Jensen (Jensen Transformers) who designed the 990. The 990 is a class-A op-amp for most load impedances. At some point, if the current required by the load exceeds the class-A operating current of the output stage, the 990 will be working in AB mode. Thank you.

John Hardy
The John Hardy Co.
www.johnhardyco.com
 
Thank you John for the info.
What is the lowest impedance for A class operation for the 990C.

chrissugar
 
thank you John, for your input on this! I have learnt so much from all the folks round here. So little time, so many questions . . . like, does it turn into Class AB with a low impedance, or a higher one? Forgive the question, but i am trying to understand, is it a higher current draw that causes the change?

Many thanks,

ANdy P
 
One thing I've pondered is why not just use a JH990 with a 2503 style output transformer since it's cheaper and more available than a 2520? Especially considering that they both would operate in class AB with this kind of load. I'm sure several have tried a 990 here. How does it sound?
 
One thing I've pondered is why not just use a JH990 with a 2503 style output transformer since it's cheaper and more available than a 2520? Especially considering that they both would operate in class AB with this kind of load.
An output transformer does not, by itself, look like a 600 ohm load (or whatever impedance you wish to call it). If the transformer sees a high-impedance load, the 990 driving the transformer sees a high-impedance load.

Better yet, replace the 2503 with a Jensen JT-11-BMCF or JT-11-DMCF (the "DM" is one size smaller than the "BM", and the "DM" is the same size as the 2503). Of course, the 2503 is probably a contributing factor to the API sound, as is the 2520.

John Hardy
The John Hardy Co.
www.johnhardyco.com
 
[quote author="John Hardy"]...An output transformer does not, by itself, look like a 600 ohm load...[/quote]Yes, I was assuming secondary termination for <667 ohm load (via Samuel's calculation), but thanks for clarifying that.
 


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