Jung mic pre - anyone tried it?

[clintrubber]

I've heard a rumor that Focusrite uses the resistor-as-current-source method in some of their products. But I digress...

Can't find it right now, but there's this F*-I/O-circuit around, someone (likely of here) has it on his site. There were a few resistors added, but don't remember if they were in the last stage before going outside, or earlier (as suggested by TK, that youy shouldn't use it in a last stage, for reasons yet unknown to me :wink: ).

 

[featherpillow]

IIRC, the reasoning behind the need for buffering here is that, because the current level that must be passed by the current source depends on what load the opamp is driving, we obviously want to keep that load to a minimum.

To keep the op-amp in class A, the CCS has to be passing more current all the time than the load would take on its own. You can imagine what would happen if the load is demanding some of that current. As TK says, the opamp's output pair comes out of class A into a crippled push-pull.

Imagine also that you are driving a low-impedance load using a CCS set-up like this. You have to bias the opamp with a buttload of current to keep it in class A. Then you run the risk of a couple of things: tempature problems and current-limiting problems.

For example, let's say that our opamp is only capable of delivering 40mA maximum. By putting something like a 22mA current source on the output of the opamp, you will likely make the chip perform worse than with no biasing at all. At this level, you'll most likely activate the current limiting circuitry of the opamp, plus you're raising the operating temperature of the chip significantly.

So, buffering it is! :grin:

 

[tk@halmi]

[quote author="clintrubber"]...shouldn't use it in a last stage, for reasons yet unknown to me...[/quote]

Here is my reasoning. You are running 2ma single ended using an active source. The amount available to the external load is less than 25% of that claimed by much smarter people than myself. The rest goes to warm your :sam:.

What sort of line, let alone a transformer, can you drive with 0.5ma and stay in Class A? Switching between single ended Class A and asymetrically loaded push-pull Class B is less than glorious. It is not like going between Class A push-pull and Class B push-pull operation of common output stages. I mean you are changing the functionality of components on the fly, not just changing their opmode. Aural mayhem.
Even if the next stage has a 10k input voltage swing will be restricte due to the additional capacitive load of the cable (dependent on the cable length).
Contrast this with the fact that many designers are reluctant to drive lines with ordinary opamp outputs despite their capacity to produce 20ma to 30ma these days.

 

[featherpillow]

The amount available to the external load is less than 25% of that claimed by much smarter people than myself. The rest goes to warm your :sam:.

That's interesting--I hadn't thought about that aspect of it.

It would be interesting to see how Ficusrote handled things.

 

[pstamler]

[quote author="skipwave"][quote author="pstamler"][quote author="featherpillow"]I've heard a rumor that Focusrite uses the resistor-as-current-source method in some of their products. But I digress...[/quote]
I commented that the high-level stages of their Platinum series were more transparent than most 5534-based stages.[/quote]
By high-level, you mean line-level inputs? In the +4dbV range?

I haven't used any Focusrite pres, so I'm not familiar with their layout/staging.[/quote]

The Ficusrote pieces I was working on were the Voicemaster and the ToneMaster. Each has a mic/instrument preamp stage followed by various processing stages (EQs, filters, compression, gating, etc.) that work at higher operating levels, followed by an output stage. It was the processing stages to which I was referring.

Peace,
Paul

 

[skipwave]

Thanks for the info TK, featherpillow, and pstamler.

In the Jung-Markell circuit, with the 1010 as a buffer, the load to be driven by the 1115 is low enough, and with no cable capacitance in the equation, that it won't have the issues described? Is this also why the bias current needed is so low, at 2ma?

OT, but is it common practice around here to misspel the names of gear manufacturers? Such as Ficusrite? Obviously, I'm quite new to the forum.

 

[featherpillow]

In the Jung-Markell circuit, with the 1010 as a buffer, the load to be driven by the 1115 is low enough, and with no cable capacitance in the equation, that it won't have the issues described? Is this also why the bias current needed is so low, at 2ma?

That's correct. I thought bias was a little low at first sight--ususally you'll see bias going into a buffer @ ~5ma or so.

OT, but is it common practice around here to misspel the names of gear manufacturers?

I don't know, actually. I spelled it "Ficusrote" because I thought it was amusing. It reminds me of that article in The Onion "The G*dd*mned Ficus Tree Needs to Come With Instructions"

 

[peterc]

Hi all

I'd like to try an Edcor 1:1 on the output, should I be using the 10k:10k or the 600:600 units?

Data is here:

http://www.edcorusa.com/transformers/audio/matching.htm

Thanks for the help
Peter

 

[featherpillow]

Oh, 600:600, definitely. 10K:10K is too large of an output impedance.

 

[skipwave]

[quote author="featherpillow"]I don't know, actually. I spelled it "Ficusrote" because I thought it was amusing. It reminds me of that article in The Onion "The G*dd*mned Ficus Tree Needs to Come With Instructions"[/quote]

Any reference to the Onion deserves a :guinness: .

Peter raised the issue that the Edcors are unshielded. How much of an issue would that be in this circuit? Is it worth fashioning a shield and, if so, what could be used? :green:

I looked into the OEP input trafo, and it seems like it would be a pain to get in the US.

 

[chrissugar]

[quote author="skipwave"]
OT, but is it common practice around here to misspel the names of gear manufacturers? Such as Ficusrite? Obviously, I'm quite new to the forum.[/quote]

People here misspel not only for fun but because we dont want manufacturers to watch us. With a search function it is easy to find discussions about specific products.
Probably you already saw Moonlee, Masenberg, Cranetune, Ficusrote, N*ve, Dickidizone, Bergenbier and the list is long. If you want to ask about some some current product it is a better idea to use some similar name.

chrissugar

 

[chrissugar]

[quote author="skipwave"]
I looked into the OEP input trafo, and it seems like it would be a pain to get in the US.[/quote]

I think Newark has OEP transformers. If they don't have in their catalog you can still ask Newark, because they are part of the same company with Farnell.

chrissugar

 

[featherpillow]

I think Newark has OEP transformers

I've heard it's faster to order them directly from the UK supplier. Shipping is more, but you circumvent the lead time. I thought I read something about this in the transformer meta...?

Peter raised the issue that the Edcors are unshielded. How much of an issue would that be in this circuit? Is it worth fashioning a shield and, if so, what could be used?

This is true, and while it makes things more difficult, it's not the end of the world. I've used Edcors in a mic input application without problems--I used a fully shielded, fully encapsulated supply, and an aluminum chassis. That's no guarantee, of course. You should contact Brian at Edcor's tech support department and ask him for recommendations. He's told me in the past that customers have used the 150/10k and the 150/600 as mic input trafos.

 

[clintrubber]

Thanks people for the added info on the 2mA-stage & the subsequent buffering. :thumb:

 

[skipwave]

[quote author="chrissugar"]People here misspel not only for fun but because we dont want manufacturers to watch us.[/quote]

I thought that might be the case. I especially like N*ve, because it looks like f*ck. As if it were a profane word. :shock:

 

[bcarso]

[quote author="skipwave"][quote author="chrissugar"]People here misspel not only for fun but because we dont want manufacturers to watch us.[/quote]

I thought that might be the case. I especially like N*ve, because it looks like f*ck. As if it were a profane word. :shock:[/quote]

Not that it comes up much in pro discussions, but I feel the same way about B*s*.

 

[skipwave]

[quote author="bcarso"][quote author="skipwave"][quote author="chrissugar"]People here misspel not only for fun but because we dont want manufacturers to watch us.[/quote]

I thought that might be the case. I especially like N*ve, because it looks like f*ck. As if it were a profane word. :shock:[/quote]

Not that it comes up much in pro discussions, but I feel the same way about B*s*.[/quote]

B*s*? If that's the division of the Ralond corporation, I have to agree. Yuck.

 

[skipwave]

If I could veer back to the topic at hand (with apologies for the diversion);

Is there really any value in using an input trafo with a 150 ohm primary? I don't have any mikes that require this low load impedance, and most recommend a 600-1k load.

I was thinking about simply using a 600:600, with the only expected side effect of slightly less gain. Will this impact the circuit in any other way?

 

[bcarso]

quote: "B*s*? If that's the division of the Ralond corporation, I have to agree. Yuck."

I was thinking of the consumer audio company in Massachusetts myself, the one whose overpriced table radios fill a whole room with awesome sound... :razz:

BTW, I love the Duchamp quote.

 

[featherpillow]

I was thinking of the consumer audio company in Massachusetts myself, the one whose overpriced table radios fill a whole room with awesome sound...

I've always wondered why people pay so much for those paper coned things and their crap-tacular crossovers. I bought a pair of b*s* speakers from a hobo for $10, and I feel I overpayed.

They make auto suspension systems now, too. They're very mid-rangey. Plus they allow you to pretend like you're Knight Rider.

I don't have any mikes that require this low load impedance, and most recommend a 600-1k load

I recommend the 150 ohm inputs, though bcarso may say otherwise. It goes without saying that he knows much more than I know...