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?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
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.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?
Well, “JT11K8,” which is what it says on the schematic, is a Jensen part number.
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.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.
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.
I actually removed C29 and put the output transformer there. I also replaced the U4 from MC33079 to an OPA1604.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.
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.I actually removed C29 and put the output transformer there. I also replaced the U4 from MC33079 to an OPA1604.
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.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).
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.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?
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.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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