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Bricolo 16th October 2004 07:51 AM

In what case can we go for open loop, and when should we use feedback?
Hi all,

I'm working on an active crossover project (2 ways, variable freq, variable slope, maybe variable Q too)
Such a project can be done in various ways, one of them is using tons of opamps. And I'm a bit reluctant to that.

For example, the input stage. I'll need something like 6 to 12dB of gain, between the source and the filter stages, in order to prevent the filter stages from working with too low signals.

What should I use for that? A classical non inverting opamp, with gain=4? A discrete opamp (still a "heavy" feedback configuration. That's the point, I'm wondering if I need this high level of feedback and "high" parts count, simply for a x4 gain stage that will (at most) output a few mAs), an open loop gain stage, or even a simple common emitter with emitter degeneration resistor?

The problem is that I have no idea what topology I can use in this situation. But in fact it's a wider topic, not only specific to this application.

Opamps ICs are often criticized here, but are discrete opamps relly better in the same applications? Is there a reason not to go for an open loop stage, or a low feedback one, instead?

I'm all ears :)

Jocko Homo 16th October 2004 04:58 PM

The answer is........
There is no easy answer.

It depends on personal taste, space and cash limitations, and a whole bunch of other things that usually have little to do with science.

But......having said that:

Followers work well as buffers/filters for active crossovers. Don't take up lots of space, are cheap, and easy to make.

For some obtuse reason, the nature of which I have no idea why, op-amps configured as buffers never seem to "sound right". could put your gain stage in front an array of followers (not that you really need that many) as a practical suggestion.

As for open loop, or closed loop...............

A gain of 12 dB is only a gain of 4. Something that you could easily do with a handful of discrete components, and no GFB, at line levels.

Seems that would have some intrinsic value as a learning experience...............


Bricolo 16th October 2004 05:14 PM

Thank you for your answer, Jocko ;)

I've been re-reading the threads about active XO here, this afternoon (Grey's ones, Fred's one, Moamp's ones...).
And I came to one conclusion (please someone tell me if I'm mistaken): I can't go with a follower in the filter stages (but I'd be pleased to can!) because there's no easy way to set the Q without an opamp.

So, I'm thinking about discrete opamps for the filter stages, a discrete input gainstage, and discrete output buffers.

Jocko, when you wrote that 12dB of gain can be easily done with a handful of components, did you mean a common emitter stage, or something more complicated?


sss 16th October 2004 05:26 PM

discrete shmiscrete , use op amps!! theres nothing wrong with 'em

Bricolo 17th October 2004 07:59 AM

I'm not 100% sure about that ;)

P.Lacombe 17th October 2004 01:12 PM

In theory, there's no difference...

Bricolo 17th October 2004 01:22 PM

I think I've already heard this somewhere... :D

Jocko Homo 17th October 2004 01:58 PM

Theory, schmeory..........
Problem is..........

They all sound different. You expect him to go out and a handful of every audio nerd approved op-amp and try them????

The kid is trying to learn something. He will learn a lot more by building a discrete gain cell than throwing a bunch of op-amps together.

Anyone can do that............just look at P**** D******.


Bricolo 17th October 2004 02:46 PM

Opamp ICs are already out of choice, my question was more about the use of discrete opamps or discrete open loop gain stages (or low feedback) for relatively low gain applications (<20dB)

Mr Evil 17th October 2004 03:40 PM

Well, it depends what you're trying to do. The best discrete designs will outperform op-amps, with lower THD, wider bandwidth etc (yes P.Lacombe, you need to go back and look at those theories, as they don't say there will be no difference!). Of course good op-amps already offer extremely good performance, so don't expect huge benefits, or even truly audible ones at all.

On the other hand, they are much more complex, requiring many times more components. You can build much simpler discrete amps, but they will not come close to op-amp performance. However, they would still be good to build for learning purposes, as Jocko Homo suggested. Having said that, apparently some people prefer the distorted sound offered by some of the various simple designs.

As for feedback. In my opinion there is never a reason to go without overall negative feedback. You will get better perfomance by doing so. However, as above, apparently some people prefer the more distorted sound offered by some designs so you'll have to experiment for yourself.

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