Jung mic pre - anyone tried it?

[peterc]

Browsing through the Schematics META, I found the Walt Jung mic pre schems.

http://home.hetnet.nl/~chickennerdpig/FILES/EDN/Jung-Markell_micpre_pg1_EDN-19900913.jpg

http://home.hetnet.nl/~chickennerdpig/FILES/EDN/Jung-Markell_micpre_pg2_EDN-19900913.jpg

Has anyone built this? Looks like a cool DIY project.

Regards
Peter

 

[Carl_Huff]

Looks very interesting! I have nothing but respect for Walt Jung. This circuit is also in the Notes for the LT1115 device that can be downloaded on Linear Technology's website.

An interesting experiment might be to work up a circuit that substitutes a Burr Brown INA103 instrument amp for the Jensen input transformer and LT1115 gain block. With a little work you might end up with an interesting transformerless mic pre.

Hmmm .... Is that what's in the Grace mic pre product???

 

[dale116dot7]

Yes, I built it. It sounds very nice, at least for the gains I use it at. I used Sowter iron in mine, though, which gives a different character than the Jensens. I find it very clean, and very useful on anything with lots of HF such as overheads, hat, acoustic guitar. Highly recommended.

I probably would not try making the input transformerless - the critical part is not the LT1115 but rather, the first stage of amplification - the balanced input. One of the attractions (and features) of this amplifier is the lack of coupling capacitors - the servo loop accomplishes this beautifully. You immediately introduce coupling caps when you go transformerless unless you are going to use mics that do not need phantom. Well, that's not too bad - tube mics, ribbons, and dynamics only. I could live with that most of the time.

The Sowter transformers are really quite reasonable in price and sound good. And this forum's host sells them in the US.

 

[clintrubber]

from Dale:

Yes, I built it.

Whoa, now already ? :shock: :wink:
Can't be of those scans I uploaded, you must have seen it before right ?

 

[Flatpicker]

[quote author="dale116dot7"]...The Sowter transformers are really quite reasonable in price and sound good...[/quote]Which Sowters did you use?

 

[skipwave]

[quote author="dale116dot7"]Yes, I built it. It sounds very nice, at least for the gains I use it at. I used Sowter iron in mine, though, which gives a different character than the Jensens. I find it very clean, and very useful on anything with lots of HF such as overheads, hat, acoustic guitar. Highly recommended.
[/quote]

Can you estimate the total cost to build?

The Linear ICs are cheap, I came up with $17 for 2 of each. If the other components are affordable, this is definitely one I want to build.

 

[dale116dot7]

Yea, it's in the LT1115 application note; I've been using the one I built from that schematic for a about a year and a half now.

Sowter 9145 for input, 4652 for output.

-Dale

 

[featherpillow]

Looks like it uses that 4304 to force the LT1115 into class A operation.

An interesting experiment might be to work up a circuit that substitutes a Burr Brown INA103 instrument amp for the Jensen input transformer and LT1115 gain block. With a little work you might end up with an interesting transformerless mic pre.

What about a FET or transistor-based differential? You could easily put one together with a simple diode-based CCS on the emitter for a high CMRR.

And then on the output, use a simple push-pull stage.

Maybe I'm making it too complicated?

 

[skipwave]

I'd very much like to keep this thread alive, as I'm intent on building this one. Anyone else onboard?

On the transformer front; I'm feeling cheap [being poor] so I want to use the least expensive trafos possible. Has anyone used the Edcor WSM Series M6X Grade Silicon Steel trafos? Perhaps I should start a seperate thread.

They've got to be the least expensive new trafos out there, but maybe you really do only get what you pay for, though no one has been able to convince me of this.

 

[peterc]

Skip

I think one of the objectives of the pre was to have the best transformers possible, hence the Jensens.

However, I would also be keen to hear what it sounds like with OEP's & Edcors. The OEP's have always surprised me. I have some Lundahls that I think would work, I just have to re-check the numbers....

I cant get hold of the chips locally, so if anyone would be prepared to buy me 2 sets, I'll send them 2 Green mic pre PCB's.

Regards
Peter

 

[skipwave]

[quote author="peterc"]I think one of the objectives of the pre was to have the best transformers possible, hence the Jensens.[/quote]

A fine point to make. I do have some vintage trafos around that might work, but I was reserving those for a tube pre. I'd like to have the Jung pre built soon, so I might have to settle for the Edcors and upgrade later.

[quote author="peterc"]I cant get hold of the chips locally, so if anyone would be prepared to buy me 2 sets, I'll send them 2 Green mic pre PCB's.[/quote]

I'd be happy to add yours to my order, but I'm not really interested in building the Green pre. Maybe the What Compressor or Mic Pre PSU boards instead? Let me know what sounds like a fair value trade.

 

[Carl_Huff]

I see that 2N4304 FET is no longer in production. Can anybody here suggest a suitable replacement?

 

[tk@halmi]

Alsmost anything with enough Idss will work there. The PN4393 should be available readily. You will need a larger resistor to get 2ma. Something in the 200 ohm range perhaps. I have no idea if it is pin compatible though.

If you need a really quick solution you can always use a current source "diode" like the 1N5304, 1N5305 or 1N5306. They are actually JFETs connected for constant current source, sorted for Idss and stuffed in to DO-35 packages.

 

[skipwave]

Thanks tk@halmi!! You are quick on the draw.

Is it this DC current being applied that is "the 4304 is forcing the LT1115 into Class A operation."

Thanks to everyone who has chimed in thus far!

 

[tk@halmi]

You could use a resistor possibly with decreased performance.

Yes, the current source (FET or resistor) forces the top transistor of the output to stay open. As longs as the load is light (>20k) the output will remain single ended Class-A. If you load it some it comes out of Class A into a crippled push-pull and complements you signal with an assortment of distortion products.
You always want to use a buffer after the opamp when you mess with the output of an opamp in this manner.

 

[featherpillow]

Is it this DC current being applied that is "the 4304 is forcing the LT1115 into Class A operation."

Yes, this is the function of that FET. Any current source will work, as long as its supplying 2ma.

You could even use a 9k resistor as a current source, although I would advise against it. The opamp will see that 22 ohm, the 100 ohm, and the current source all in parallel as a load it has to drive. Also, it only gives a constant current level while the op-amp's output signal remains constant. Since we're amplifying music, the bias level will only be constant during the silent parts. The resistor bias isn't useless, it just doesn't work as well as other methods.

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

Is it just me, or does a 2ma bias seem a little low here?

*edit*--looks like I was beaten to the punch :wink:

 

[clintrubber]

from TK:

You always want to use a buffer after the opamp when you mess with the output of an opamp in this manner.

Sounds like you've had troubles otherwise. Please tell.
Anything related to problems with driving certain loads now ? (with that sink present)

Thanks,

Peter

 

[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]

That's what a Focusrite tech guy told me, anyway, when I commented that the high-level stages of their Platinum series were more transparent than most 5534-based stages. I wasn't terribly knocked-out by the mic preamp stages of that stuff, but the high-level part was damn near a straight wire.

Peace,
Paul

 

[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.

 

[featherpillow]

I know that most of you are interested in the input/ouput transformer idea, but I'm interested in exploring a transformerless version of input. I suppose the INA217 et al could work, but I've spent the last 24 hours doodling with a BJT differential based input with a constant current source and an emitter follower to lower output impedance. It's been adapted to +/-18vdc, and I think it could work for this app.