Can anyone tell me what this JFET does in this circuit?
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It is a high impedance buffer aka "source follower"..
You boxed two JFETs. The top one is the buffer the bottom one is a current source.
JR
You boxed two JFETs. The top one is the buffer the bottom one is a current source.
JR
ah ok, so its strictly for the hi impedance input on the front of the preamp where it says Hi-Z? Im assuming for a guitar or something right?
The input impedance is effectively 1M so it would definitely not load down a lead guitar pickup plugged into the strip, not that it's a good idea to track guitars that way. 
JR
JR
No, it’s for both. If you look at the signal coming from the secondary of the input transformer (T1), it passes through the jack - so when nothing is plugged into the DI input, signal comes from the mic input, and when something (like a guitar or bass or keyboard) is plugged into the jack, it replaces the signal from the mic input. In other words, everything runs through that JFET.
It’s the first active component in the signal path of the device. Its purpose, as JR put it, is to provide a high impedance input for both the DI jack and the input transformer. I doubt replacing it with a different JFET would improve anything, unless it was faulty/failing for some reason.
BTW what is this device? From what we can see here, there’s not much room for “improvement.” It uses a nice Jensen input transformer, has a JFET front-end with a current source, and feeds a 5534, which is a good opamp. This circuit probably doesn’t have much “color,” but it appears well designed.
john12ax7
Well-known member
Looks to be a Presonus mic pre. You can upgrade the JFETs by removing them altogether, although that would be a bit involved. It was mostly a marketing thing for their IDSS control. Was a cool idea, but in practice did not sound very good.
For a simple upgrade in these changing the opamp can make a noticeable improvement.
For a simple upgrade in these changing the opamp can make a noticeable improvement.
Hi guys, its a Presonus Eureka. It doesn't have a Jensen transformer, just some generic no brand transformer. I am going to replace it with a Tamura I got out of a Yamaha mixing board. It's a Tamura 80071, and I am also putting a 600:600 tamura on the output after the quad mc33079.
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Well, “JT11K8,” which is what it says on the schematic, is a Jensen part number. After some googling, it looks like there was a version of the Eureka (maybe called the “hot-rodded Eureka?”) that did in fact come with a Jensen JT-11k8. Better make sure that’s not the version you have, or you would be seriously downgrading your transformer.
That Tamura, typically wired 600:3K, would be around a 1:2.25 step up, while the stock transformer is 1:3.5, so you would have less gain out of it, which may result in more noise (due to needing to turn the preamp up more). Probably not ideal.
As for the output, it appears it’s already being balanced in the master section by U4 sections D and A (section C feeds the strangely unbalanced TRS output). Are you planning to replace those with the transformer? If so, how? You might be better off taking a tap from C27/R36 where it feeds the unbalanced out. To be clear though, I don’t think that would be much of an improvement.
That Tamura, typically wired 600:3K, would be around a 1:2.25 step up, while the stock transformer is 1:3.5, so you would have less gain out of it, which may result in more noise (due to needing to turn the preamp up more). Probably not ideal.
As for the output, it appears it’s already being balanced in the master section by U4 sections D and A (section C feeds the strangely unbalanced TRS output). Are you planning to replace those with the transformer? If so, how? You might be better off taking a tap from C27/R36 where it feeds the unbalanced out. To be clear though, I don’t think that would be much of an improvement.
john12ax7
Well-known member
Presonus did indeed use Jensen originally in their mic pres, but then switched to generic versions at some point. So it depends what era the unit is from.
For those with the generic version you could call Jensen and they would sell you a transformer at a slight discount if it was to upgrade a Presonus pre.
This product was the source of some outrage when I owned Jensen. We contributed heavily to its design. The overall design is a lower-cost version of the famous Jensen TSMP design. The Twin-Servo Mic Preamp uses two cascaded gain stages (to achieve higher gain-bandwidth) and DC servo feedback around each stage (to avoid using coupling capacitors). It won many preamp "shootouts" for transparency and low distortion and is also made, with Jensen's blessings, by John Hardy see The John Hardy Company Jensen Twin Servo 990 Mic Preamp). One of my techs came up with the "coloration" circuit using the JFET with a control to move its AC operating point and, hence, even-order distortion (the kind made by triodes and usually considered "warm'' and "musical." But, when they started using a knock-off input transformer and still "riding our coat-tails" by mentioning Jensen in their ads ... well, let's just say I wasn't at all pleased! We offered the JT-11K8-PC (the design-in transformer) at near-cost pricing to customers who discovered the knock-off and, in the beginning, actually installed quite a few for customers until the volume of work just got too large.
I had no idea of all the history behind this channel strip. It really does sound great, I do have a JT-11k8 I installed on one of them. In total I have about 6 of them, some of them came broken, I bought real cheap 2nd hand, in order to try and repair/learn on. None of them came with the Jensen transformer. I ended up buying a Jensen transformer from Jensen a while back, and I bought a Cinemag 3.5 as well. I am experimenting on this really broken one, with a few Tamura transformers I have on hand.
Yeah, its a generic transformer, there isn't Jensen written on it. I have a genuine Jensen 11k8 installed in one of them already. THis one was completely dead. The U441 is shorting from pin 1 and 2. The sockets for the op amps are really bad condition, as in the op amps socketed in them are loose. So i will be changing those sockets out too. Also, the VCA THAT4301 chip has a broken leg on it, but it's a N/C so dodged a bullet on that one. I bought a bunch of these real cheap 2nd hand, broken/for parts...and decided to revive them.
I actually removed C29 and put the output transformer there. I also replaced the U4 from MC33079 to an OPA1604.
If that’s all you did, you still have U4A hanging off of it, connected to nothing, not to mention DC on the primary of the OT.
At the very least, you should lift R48 and put C29 back (leading to the primary of the OT).
Oops I forgot to mention, I actually removed C29 AND C31. And had the primaries of the OT in there and the secondary of the OT going to the XLR.
How is that? Still no good?
Without a coupling cap, that still leaves DC on the transformer. OpAmps typically operate with some amount of DC offset, which means that at R37, the DC voltage is not 0. Having DC on the primary of the transformer is likely to cause distortion. You definitely want to have a coupling cap between the opamp and the transformer. As for lifting C31 - that's actually worse. You now have an opamp connected to you output (through R48) with no load. There's a good chance that will cause oscillation.
Your best bet is to put C29 and C31 back in the circuit and keep the electronically-balance XLR out as-is. Connect the primary of your transformer to the C27/R36 junction that leads to the TRS jack (disconnect it from the jack), and connect the secondaries to the tip and sleeve of the jack (removing R28). That will give you the option of using the XLR output if you want transformerless (electronically balanced) operation, or using the TRS jack for transformer balanced operation. It's also the easiest mod to undo if you don't like it.
This made alot of sense and thank you for teaching me. I learned something new today. I was under the impression that transformers pass only AC and block DC. I will do as you.
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Newmarket
Well-known member
Transformers transfer only AC energy from one winding to another eg primary to secondary. But DC can still flow through the windings producing heat and more complex 'magnetics' issues depending on the application and design.
Just as @Newmarket said, transformers don't pass DC through to the output - that's not the issue here. The issue is that the DC will flow through the primary coil to ground, which will likely cause distortion, and in some extreme cases, can burn out the winding (they are often low DCR and low wattage ratings because the wire is so thin). Let's say you had 2V of DC offset (an extreme example), and the primary DCR was 10ohms - that's 200mA of current at 2V, or 0.4W. If the magnet wire used in the transformer is only good to 1/4W, you may burn out the coil.
