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Old 29th September 2012, 09:57 PM   #1361
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by stinius View Post
SW discrete opamp BF862 (post 135).

I have put in bootstrapped cascodes on the front end and 5401/5551 as the cascode BJT's in the "VAS", so there is no problem using the circuit at higher voltages.

As you can see in the attached file I have stepped the source resistance from 50ohm to 2kohm and there is almost no change in THD.

It is possible to improve the circuit, but it is rather pointless to design a perfect front without using a relevant output stage.

So in order to move forward we must, among other things therefore agree on the load.
600 ohm or 30 ohm?

BTW: Maybe the circuit needs a better name than "post 135" or maybe not?

Stein
IC opamps can do 600 ohms. Make it do what a common IC cant do alone and be more flexible.... 30 Ohms. Sure, it's brutal and tough. And can drive anything you need for it to do. Otherwise, why do this if we can get same results with an existing 5532 IC? You want bragging rights? Then do 30 Ohms.
[But then it doesnt matter as the output devices can be replaced by beefier devices at any time. Just need the topology for extending power if needed.]
Name? DOA (Discrete Op Amp)

Last edited by RNMarsh; 29th September 2012 at 10:07 PM.
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Old 29th September 2012, 11:01 PM   #1362
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Quote:
Originally Posted by RNMarsh View Post
opamps can do 600 ohms. Make it do what a common IC cant do alone and be more flexible.... 30 Ohms.
and +24dBu, plus I already said there are plenty of IC's that drive 30 Ohms. Dick, I sent you a rerference on how those CFP outputs can go class B switching I consider it a problem that needs to be rectified.
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Old 29th September 2012, 11:08 PM   #1363
bcarso is offline bcarso  United States
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Quote:
Originally Posted by scott wurcer View Post
and +24dBu, plus I already said there are plenty of IC's that drive 30 Ohms.
And in a DIP8 footprint. Well, after all, no one has yet suggested a height restriction Or a requirement on airflow.
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Old 29th September 2012, 11:18 PM   #1364
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Originally Posted by bcarso View Post
And in a DIP8 footprint. Well, after all, no one has yet suggested a height restriction Or a requirement on airflow.
Yes, but this will hopefully be rectified soon . That AD815 monster package cost almost a dollar, hard to believe these days.
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Old 30th September 2012, 12:09 AM   #1365
bcarso is offline bcarso  United States
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Originally Posted by scott wurcer View Post
Yes, but this will hopefully be rectified soon . That AD815 monster package cost almost a dollar, hard to believe these days.
I was tempted to draw a cartoon, but I'm not very good at that.
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Old 30th September 2012, 12:20 AM   #1366
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Originally Posted by bcarso View Post
And in a DIP8 footprint. Well, after all, no one has yet suggested a height restriction Or a requirement on airflow.
That was my next question.

Anyway 600 ohm was my target.
When I use my own favorite output stage it looks good down to about 150 ohms.
The picture shows THD with load stepped from 300 - 1200 ohms. (at 40V pp)

Stein
Attached Images
File Type: jpg DOA 300-1200.jpg (59.0 KB, 303 views)
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Old 30th September 2012, 12:31 AM   #1367
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
and +24dBu, plus I already said there are plenty of IC's that drive 30 Ohms. Dick, I sent you a rerference on how those CFP outputs can go class B switching I consider it a problem that needs to be rectified.
You did? I missed seeing the reference. I'll look into it if I find the reference... gimmie it again pls. The circuit (non-sim) in real life hasnt that issue that I have found... so I'll see what the conditions are and if they are reasonable for audio app/use. Then tweek it, if necessary. Partly because of the way i built the supplies perhaps? Let me see. I know you could fix it asap - wanna tell me or make me suffer thru it :-) Or, is it something inherant in the topology?

IC's that drive 30 Ohms -- maybe even 8 ohms. I have seen high voltage opamps for decades as well. Are they sota in distortion and noise as well? Comparable cost to your discrete? Analog? Common opamp IC? Low cost? Exotic? which ones?

So we should at least do 30 ohms like the other IC's?

Or was the goal to fit in a dip space like the JE990 could? I dunno. But I wouldnt be interested in a discrete if for the same performance and approx cost could buy an IC.

I think I see the light at the end of the tunnel. I hope it isnt a train.
-RNM

Last edited by RNMarsh; 30th September 2012 at 12:48 AM.
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Old 30th September 2012, 01:29 AM   #1368
RNMarsh is offline RNMarsh  United States
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Default Discrete vs IC performance

Quote:
Originally Posted by scott wurcer View Post
and +24dBu, plus I already said there are plenty of IC's that drive 30 Ohms.
Scott - I am refering to a 'common' IC like the popular 5532/34 type. Not a special IC opamp developed for high voltage or high current use. Like a OPA541 in a 125W TO3 package. A discrete design ought, IMO, do at least better than what can be done in a 'common' audio IC. Or as John Curl said, otherwise just buy the IC. -Dick

[I am not saying there is anything magical or special about doing 30 ohms -- it is a useful audio Z to design for. When done with development here, it will be something to brag about and might find a long and varied life in many products for high-end for years to come. Now i'm not going to say designing for more current output than a 600-100K load needs has any audible benefits,either... does it?]

Last edited by RNMarsh; 30th September 2012 at 01:56 AM.
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Old 30th September 2012, 02:10 AM   #1369
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Originally Posted by RNMarsh View Post
Or was the goal to fit in a dip space like the JE990 could?

-RNM
Not a chance that the JE990 fit in a DIP space.

Erik's paper using a current feedback pair (adding a darlington) for high current output.

http://www-f9.ijs.si/~margan/Articles/Class_B_Dist.pdf
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Old 30th September 2012, 02:25 AM   #1370
jcx is online now jcx  United States
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if you can live with < ~ +22 dBu then DSL driver chips are fine output stages

the TPA6120/THS6012 has audio specs, will put out 400 mA so the +22 dBu could be into 30 Ohms too

smaller than the 2 pairs of TO-126 output Q (not counting necessary added bias, current limit parts) you'd probably use in discrete to get the same current in 2 channels

I know you're going to dismiss the very idea but maybe you should have an idea what the monolithic competition performance level really is today

Last edited by jcx; 30th September 2012 at 02:29 AM.
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