Whatta You Think About This Buffer/Line Amp/HP Driver?

[SonsOfThunder]

Wanted to see what you all thought about this ckt

(I thought I wasn't going to be able to figure out how to make this clickable!) :thumb:

Some data for Q1 to Q6 is on page 8 of this selection guide: The part is apparently obsolete now but I found full data in a 1997 Moto Book. This sheet has the necessary data methinks though.

I'd like to preface this discussion by saying that I'm not sure who the designer of this circuit is and I'm certainly not out to slight Mr. Bob Katz who is hosting this. As I said, the part is now obsolete so its kind of a moot point on this ckt anyway.

I think this might have been brought up here before. :?:

Here we go!
Charlie

 

[gyraf]

http://www.digido.com/User/Assets/Active/PDF files/00478-buffer.pdf

[Working link][/url]

 

[SonsOfThunder]

Thanks Jakob!
The thing that was throwing me off was the space between the words "PDF" and "20files". I somehow figured out that I needed to substitute a "%" for the space. :roll:

Let me take this opportunity to say thanks for your many contributions to this community here.

Peace!
Charlie

 

[PRR]

When protection transistor Q2 goes into action, Q5 is going to fry (and Q3 Q6 on the other side). Clamp the Base of Q5 Q6.

It is an old tradition to have a Base-Emitter resistor at places like Q1 Q4. Aside from absorbing transistor DC leakage, it gives a pull-down to turn off the transistor faster.

Output idle current is not specified.

I suspect you don't need a Darlington buffer to mate a 627 to a 100 ohm load, but maybe it sounds better.

I don't see what VR1 does for you.

Servos make me dizzy, but I suspect R14 could be a LOT bigger (and inject less servo-crap into the audio).

> I needed to substitute a "%" for the space.

Actually a "%20". Or better yet, smack any webmaster who puts spaces in URLs.

 

[tk@halmi]

Could VR1 be an attempt at changing the behavior of the OPA627's output transistors?

 

[SonsOfThunder]

[quote author="PRR"]I suspect you don't need a Darlington buffer to mate a 627 to a 100 ohm load, but maybe it sounds better.[/quote]
This was my first thought when I looked at this circuit. The second thought (after looking at the datasheet for the transistors) was what are we gaining? These have an Ic of 50mA vs like 55mA (typical) of the OA! Isolation is all we can say, but if we are going to the trouble of adding this much stuff to a hearty OA like the 627, why not do it up?

[quote author="PRR"]When protection transistor Q2 goes into action, Q5 is going to fry (and Q3 Q6 on the other side). Clamp the Base of Q5 Q6.[/quote]
So this isn't a "protection" transistor...more like a destruction transistor.:green: This was what my a friend first said when I showed it to him, asking him "why do this?" His comment is that you also need a reversed diode in each side to prevent the protection transistors from acting like a limiter.

[quote author="PRR"]I don't see what VR1 does for you.[/quote]
We think it might be an attempt at bias adjustment but its not so clear either.

[quote author="PRR"]Servos make me dizzy, but I suspect R14 could be a LOT bigger (and inject less servo-crap into the audio).[/quote]
Ya but then wouldn't that larger resistor value inject more noise on the input? How bout this for an idea? Dump the servo altogether...? The offset of the OPA627 at the absolute worst case is 250uV...that's MICROvolts! So whatever offset you have is from the input which is probably going to be cap coupled anyway.

[quote author="PRR"]> I needed to substitute a "%" for the space.
Actually a "%20". Or better yet, smack any webmaster who puts spaces in URLs.[/quote]
Thanks for the lesson in html, too! Maybe now other people can comment since I actually corrected the link this time.

Thanks PRR...

[quote author="TK"]Could VR1 be an attempt at changing the behavior of the OPA627's output transistors?[/quote]
I think that's as good of a guess as mine. Maybe trying to do the pull-down thing to put the OA in Class-A? Was that your thought, TK?

This is fun so far...Anyone else?