Boosting signal generator or op amp outputs to >+30 dBu

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mediatechnology
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by mediatechnology »

Thanks!

You were able to test the LTC6090 for us which I appreciate and it seemed to do a good job into a high impedance load.

I guess I shouldn't let a moderator from GroupDIY discourage me.
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JR.
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by JR. »

mediatechnology wrote:
In general the Vbe should be small wrt Re so no need to compensate. The Re will prevent thermal runaway.
Thank you. Yes, the IR drop across Re is ~ 3 to 4 times Vbe and it's stable and free from runaway with a modest heatsink.
The only reason for Iq regulation would be to provide a wider supply range without having to adjust resistors.

In response to my reluctance to post an interim work-in-progress schematic:
Please don't, just one of a hundred ways to skin this cat, and I hear you can buy an opamp that drops in... and works.
Some broader questions:

So are you suggesting that I just end this thread the same way I did with the precision rectifiers?
Is that some other drama I created and didn't know about?
At least you didn't tell us we could do this in DSP. That's refreshing.
Well you could use PWM but technically that isn't digital or dsp, it's actually analog.

I am an old analog dog, but I can not deny that I am drawn to microprocessors as a superior tool for so many jobs. Not because I like digital, I like what I can do with it.
I saw no point in continuing that thread due to cold water being thrown on it.
I should have anticipated that this time and just not bothered.
It always seems to happen as a thread develops life. Why is that?
I worked for years as an engineering group manager. If there is a drop-in solution, often that is the right choice.
I hear you can buy an opamp that drops in... and works.
You mean the LTC6090?
It's HF distortion performance leaves a lot to be desired and its sole-sourced.
It's only good for about 10 mA peak.
Driving a 600R load, the LTC6090 is capable of a whopping +15 dBu before it craps out. We need about 60 mA.
My thought was that someone might want to do this with an off-the-shelf op amp and a couple of transistors they could buy almost anywhere and actually drive 600 Ohms.
I've seen SGs +/-40V op amp but it has a lot of bits and there's really no need for a fully-discrete solution.

Do I correctly infer that you don't think there's any value in posting a refined and working schematic or continuing the thread?
If so, why?
Are we boring you?
Why the flippant comment?
I thought I was seconding your sentiment... but I guess not

I bothered to post a schematic only to find out I made a mistake :oops:
Two people here have asked for an output with +30 dBu capability and I've tried to make a circuit you posted - which had an error - tested and available.
There are tons of circuits capable of +30 dBu with low distortion.
But where are they?
Why are people asking for them if they are so obvious and easily-found?

I wanted to provide something simple and of value.
Once I had something tweaked I thought I'd be generous to share a schematic of it.
Maybe I shouldn't be so gracious with my time now.
Sorry if I hit a nerve...

Most of the trick circuits I have experience with are not very high performance. Even my old power amp was .25% THD at 20 kHz, while I attribute some of that to the dirt slow power devices and quasi-comp topology (and driving a speaker load).

This project is a little like, exactly like, designing a small power amp, but without the high current/power.

The goals get a little confusing depending on which parameter you try to optimize. This too is not unlike power amp design where swinging hard to the rails is a virtue for high output and efficiency.

Common emitter output stages in principle offer this, but are harder to stabilize than common collector outputs (I think Cherry discussed this in an AES paper back in the day).

If you think about it, the driven supply amp design is common emitter. QSC got a patent for their current limiting approach for that topology.

I am not trying to rain on your parade, I even offered some discussion about a related (I thought) output stage biasing circuit, which is probably of more interest to me than you. I am a little busy now or I would draw it up too.

JR

PS: If I wasn't interested I wouldn't lurk and participate here.
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JR.
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by JR. »

mediatechnology wrote:Thanks!

You were able to test the LTC6090 for us which I appreciate and it seemed to do a good job into a high impedance load.

I guess I shouldn't let a moderator from GroupDIY discourage me.
Is that the real problem? I joined that forum because of my interest in "your" DC coupled mic preamps (I still think there is a commercial product there).

They asked me to moderate and i accepted, while I spend more time ranting about economics and politics than electronics over there.

Surprised I haven't worn out my welcome there, like apparently I am here.

Oh well... (other) things to do.

JR
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mediatechnology
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by mediatechnology »

I joined that forum because of my interest in "your" DC coupled mic preamps (I still think there is a commercial product there).
Well, thank you for that.
I've done a lot of work on the DC-coupled preamp since posting but I'm very reluctant to post anything new about it.
I've also thought about pulling it because the information was so old.
But I thought it provided a good diversion. At this point I'd prefer people to look at it and dismiss it as unworkable.

In the detector thread you told us that a $2 micro was the way to go.
I figured "Well, why bother with posting anything else in that thread that's analog."
"There's nothing to see here folks, just move along."

There's another idea that I've been working on that I might have shared here but didn't.
Glad I didn't.

So I started working on this thread to provide some harmless fresh content for the forum.
I figured the LTC6090 I had suggested could be done simply with an opamp and voltage gain stage and drive 600R loads.
I used your circuit.
With some tweaks I was able to make it perform almost as good as the generator that feeds it.
I intrepreted your response to be: "Please don't bother posting what you came up with."

So why post anything?

I appreciate your contributions John but you have no idea how much you discourage me (and perhaps others) from contributing sometimes because you shoot down stuff just as its beginning to develop.
If it's totally bogus shooting it down early is probably a good thing.
But it seems to happen late in threads when progress is being made.

Once I finish it I'll draw this circuit up in case I need it sometime in the future.
Anyone interested in it can PM me.
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JR.
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by JR. »

I need to edit myself (more). My intent is not to be dismissive or offensive.

JR

PS: Audio paths that exceed IC 36V IC technology is a very old problem with a great deal of art.
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mediatechnology
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by mediatechnology »

I need to edit myself (more). My intent is not to be dismissive or offensive.
Thanks John. Apology accepted.
I decided to continue to see what can be done using simple off-the-shelf parts.

Not much is published about the common-emitter push-pull output stage.
I did find the AES Cherry references but did stumble upon US Patent 3,852,678 which has a unique current mirror bias topology: http://www.waynekirkwood.com/images/pdf ... 852678.pdf
ricardo
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Zillion Vout for sign gen

Post by ricardo »

http://www.aes.org/e-lib/browse.cfm?elib=16571 A Simple Reliable Power Amplifier with Minimal Component Count - John Vanderkooy.

This needs a floating PSU but it's likely you need an extra PSU for your zillion Vout anyway. The extra PSU seems a good trade for simplicity & good performance.

No HV devices needed except for the outputs.

We've already got the +/-15 supplies.

I think Prof Cherry first suggested it in an Electronics World article but this might be the first detailed look.
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mediatechnology
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Re: Boosting signal generator or op amp outputs to >+30 dBu

Post by mediatechnology »

Thanks ricardo!

This reminds me that I also need to renew my AES membership which expires this month.
While I'm doing that I'll visit the e-library.

I had very good AC performance from a CE stage driven by current mirrors but thermal compensation was difficult.
The CE stage driven by resistors had OK AC performance but required a lot of emitter degeneration to be thermally stable.
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