992 opamps application, doubt

Hi Fred,

I'm finishing a micpre using your schematic and 1 of your 992 opamp. JT16-B as input, servo circuit, as the one in your page. I have other 2 992 to use so I have a couple questions if you don't mind.

http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF

1) Can I use put a 600:600 output transformer on that servo micpre and use both outputs at same time?

2) Can I use the 992 in substitution of 2520 on API micpre, using Profile transformer as output? It's a 75R load instead of 600R, would it burn the 992?

thanks for you time. I have to say that you opamp is the most beatiful sounding amp I ever heard. No matter what I try, the 992 always freak me out. Shame on me, I just got 3 of them

cheers!
Fabio

Hi Fabio,

I'm glad the JFET-992 are working well for you. I think they are a good combination with the JE-16 input transformer and no transformer on the output. I think the JFET-993 also works well in that application, but honestly, I tend to not use transformers in audio signal paths so I can't speak to the differences.

Regarding your other questions... all I can say is that the JFET-992/993 opamps are NOT designed to drive loads lower than 600 ohm. That doesn't mean they can not do it, only that this is not an operating condition for which they were designed. I typically use them with loads far greater than 600 ohm, which is by far the most commonly encounter operating condition (for me anyway) in modern pro audio.

Because I don't know anything about the API stuff or the Profile output transformer, I can't say how the JFET-992/993 opamps would work with that type of a load. 75 ohm is a really low load Z for a non-power amp circuit. But not being a transformer guy (I almost nevery use them, and haven't for 30 years) I can't say how common such operating conditions are. It is fairly easy to design and build a JFET-99x type opamp and replace the output JFETs with MOSFETs. You need to run a ton of current in the output stage and the circuit will run really hot (and actually cooler when a heavy load is connected), but you can drive 75 ohm loads all day long. Perhaps such a design is another topic that we should cover some time.

Sorry if I haven't been much help to you on this. My final suggestion to you is to try the JFET-992 in your intended application and test the results. It would be helpful to all here if you (or someone else) did that and posted the result here for the benifit of all. If I have the time, I may do this myself. What is the turns ratio of the Profile transformer and what load imedance is it designed to work into? I better stop now (the morning coffee has kicked in and I'm rambling).

Cheers,

Fabio,

According to Tim Ryan, the Hardy 990 op amps will work in the API circuit, but isn't anywhere close to optimal - and performance will reflect that.

I know the 992 is a different design, but i would expect similar results. I would use the Avedis, real 2520, or SCAs new SC25 op-amp instead in your API circuit (which looks especially nice btw!).

Shane

P.S. What ever came of your 2520 and 1731 op-amp projects? Have you found a way to pot them?

Fred,

Matter of fact I've plugged the 992 in the API and it works. It sounds ok but it gets hot. :grin: I dont wanna risk to burn it anyway...

I dunno yet how it sounds on your transformeless out circuit, but will see on the end of this week. I was just curious, but do not plan to use the 992s in the API application. I'll use the circuit that's on your page with different input transformers. :wink:

The interesting thing to me would be to have 1 of the 3 transformeless channels with a 600:600 transformer as an option, I'll try that.

Thank you!


Shane,

Thanks for the compliments!
The 2520/M1731 are being stuffed... man, you got to have patience...

I'll show some progress with pics on the weekend.

cheers!
Fabio

Can you please explain the basics theory of the output stage of your JE16 based preamp? I can imagine how the negative output voltage is taken but it's not really clear in my head...
What are the advantages/disadvantages of this configuration compared to transformer or active output buffer?
Many thanks,
Frank

[quote author="FredForssell"]It is fairly easy to design and build a JFET-99x type opamp and replace the output JFETs with MOSFETs. You need to run a ton of current in the output stage and the circuit will run really hot (and actually cooler when a heavy load is connected), but you can drive 75 ohm loads all day long. Perhaps such a design is another topic that we should cover some time.
[/quote]

I have been working on this sort of stuff for some time, but I am not an experienced designer and my progress has been rather slow.
I have successfully substituted the output JFETs with ZVN3310A/ZVP3310A MOSFETs using a 2k trimpot in place of the 10 ohm resistor to bias the the MOSFETs (the gate connections are reverse of the JFET ones). Also, added 100 ohm gate stopper resistors, but lower values has worked as well. These MOSFETs have twice the transconductance and can dissipate 50% more power than the JFETs. I must say that these ZETEX parts are very sturdy little parts as they have stood up to some nasty things I have put them through.

A major barrier to building a JFET/MOSFET opamp that drives 75 ohm transformers well has been finding complementary MOSFETs that have:
1. Low Vgs (<2.5V)
2. Low input capacitance (<100pF)
3. Dissipate at least 10W of power
4. Small enough to fit on a 990 compatible board.

Cheers,
Tamas

[quote author="Bauman"]
2) Can I use the 992 in substitution of 2520 on API micpre, using Profile transformer as output? It's a 75R load instead of 600R, would it burn the 992?
Fabio[/quote]

I think even though you are not getting an instant burn-up of the 992 you will be reducing the lifespan of the part significantly as they are seeing too much of a short. You need something like 60ma to drive the 75 ohm transformer. The JFETs on the output can supply 20ma at best.

Tamas

Fabio, try the profile transformer wired as 2:2 (300Z:300Z). It would be a nicer load...

Can you please explain the basics theory of the output stage of your JE16 based preamp? I can imagine how the negative output voltage is taken but it's not really clear in my head...

Click to expand...

I guess with "negative output voltage" you mean the inverting pin 3 out? There's no voltage on pin 3 in this design. For a balanced transmission you don't need signal symmetry, you need equal impedances to ground - note that R11 and R13 are equal.

This is called a "impedance balanced output" and is found on many consoles on the less important outputs (direct out, aux etc.).

The advantages over a XFMR-balanced one are the lower price, less distortion and in many cases higher bandwith (which might not be an advantage).

Samuel

> your opamp is the most beatiful sounding amp I ever heard.

So why do you want to muck it up with transformers? They have their place, but if you don't NEED one, don't USE one.

> It's a 75R load instead of 600R, would it burn the 992?

Burn or just distort like mad.

The 75Ω thing comes either from low-voltage transistors (early Neve) or large systems that might need VERY high signal levels into multiple loads (Jensen seems to have been thinking this way). The original Jensen 990 indeed would drive 14V RMS into 75Ω, 2.6 WATTS output. Such demands turned out to be uncommon, and restricting operation to 600Ω-up eases design, possibly allows a better design, at least for a given price or available parts.

The Forssell 992, while inspired by the ancient 990 and fitting many 990 sockets, is (according to Fred) intended for the usual 600Ω-up loads, not the extreme 75Ω loads. Truck or Ferrari.... well, the Jensen 990 is no truck, but Fred didn't design his 992 with heavy-load capacity, opting for a more lightweight plan. Which may be what you like about the Forssell 992!

And at probably 14V RMS or +25dBm in 600Ω, the 992 is hardly "lightweight". No sane reason to strap a 1:4 transformer on its back and make it strain. It -probably- will drive a high-quality big-core +28dBm 600Ω:600Ω transformer, if you really need galvanic isolation, and it will probably drive two 10K loads or one 10K and one 600Ω load fine. But maybe not as sweet: more load means more power and strain.

If you need a "sweet" output for yourself and a "ugly" output for monitor-mix, video-sound, etc, take the 992 output for yourself and add any half-decent buffer (that OPA604 is fine, and can share the supply rails) to drive the ugly-out. That way even if the outside feed wire gets dead-shorted, you still have your good feed.

Fabio, try a AT600/600 (the ungapped version) from Guilherme. I bet it will work fine. Guilherme likes big cores and high inductance windings...

His 600 ohms windings are 10H or more. At least in the 10k:600 one.

If the 600:600 has this high inductance also, I bet a 992 will drive it easy.

Ok, since we?ve gone this far, why not close the loop and make more fodder for the API312 META. :wink:
Besides, this may help to further explain the original question.

[quote author="PRR"]So why do you want to muck it up with transformers? They have their place, but if you don't NEED one, don't USE one...[/quote]
So, we don?t want to use the 990 or 992 in this configuration, but why do we want to use the 1731 or the 2520 here? If the 992 already sounds great servo-coupled without the transformer, why build an op amp like the Melcor 1731 with a class A/B output stage, and push a 75:150 ohm transformer with it?

why ? you ask ...
that's what makes this project (my project 2) such a cool project to experiment with.

To use the output trafo or not
to use an output cap before the trafo or not
voltage rail
chose an opamp/gain block

such a simple project and a small number of parts BUT lots of options to chose from.
Then you will use this in a variety of configs and applications ... 600ohm to bridging to multiple outputs in live and recorded situations.

You have the oppotunity to make a few of these and make them all different.
You can learn so much from this one and yet it is so very simple.

input trafo + gain block + output (trafo)
then add some power
= good

you don't have to know the complete details of what is inside each bit BUT use the different bits in combination with each other to bring different results.

young skywalker - you must un-learn what you have learned

I guess with "negative output voltage" you mean the inverting pin 3 out?

Click to expand...

Yes, pin 3, sorry if I wasn't clear.

For a balanced transmission you don't need signal symmetry, you need equal impedances to ground - note that R11 and R13 are equal.

Click to expand...

So what's the function of R12 (10k)? It looks to my ignorant mind that it causes the impedances with respect to ground to not be equal...

Many thanks,
Frank

[quote author="Kev"]...you don't have to know the complete details of what is inside each bit BUT use the different bits in combination with each other to bring different results...[/quote]I was just trying to get a discussion going on why we would want to do this as opposed to why we would not. You mentioned one example: 600ohm to bridging to multiple outputs. I assume another example would be because of the character that a 75ohm output transformer adds to the audio.

The elephant is strong and the gazelle is fast, but sticking an elephant's head onto an gazelle is not a going to work.

Now, if I could just push my cranium through my pyloric sphincter... :sam:

tamas

Now, if I could just push my cranium through my pyloric sphincter...

Click to expand...

:shock:

If I didn't know better Tamas, I'd say you have your head up your...

:green: :thumb:

But would that be appropriate?

sticking a koala bear's head on a chinchilla would be pretty badass.

Especially after you shaved it up all nice.

dave

> your opamp is the most beatiful sounding amp I ever heard.

So why do you want to muck it up with transformers? They have their place, but if you don't NEED one, don't USE one.

Click to expand...

I´ve just realized that PRR, all 3 Forssell amps with be servo out.
What they do to the sound I can´t archieve with the other combinations I´ve tried...

So, we don?t want to use the 990 or 992 in this configuration, but why do we want to use the 1731 or the 2520 here? If the 992 already sounds great servo-coupled without the transformer, why build an op amp like the Melcor 1731 with a class A/B output stage, and push a 75:150 ohm transformer with it?

Click to expand...

It´s the poor homestudio guy API sounding preamps I´d say that in the beginning I´ve prototyped with OEP transformers + Melcor and I also like the sound of it very much, it have its space in my taste.

cheers!
Fabio