useme2305

Passive Jensen Stereo DI questions
« on: July 15, 2019, 09:25:49 AM »
hi,

i am building a diy version of the jensen DI and the jensen sheet suggests a switchable pad of -10db and -20db.

i  want to make it a fixed -15db pad and i'm struggling to find a correct solution to this simple task as i have no idea how the rest of the components in this design influence the choice of the pad components.

do i omit the R4 15k resistor in the non-switchable design and what values should i choose? does the input impedance of the jensen tranny have impact on the pad design?
« Last Edit: July 16, 2019, 07:57:39 AM by useme2305 »


PRR

Re: Jensen DI pad question
« Reply #1 on: July 15, 2019, 05:10:53 PM »
Take R2 out, cut it in half (or replace with two half-value resistors), the half-point will be very-near halfway from -10dB to -20dB.

Re: Jensen DI pad question
« Reply #2 on: July 15, 2019, 05:24:40 PM »
Take R2 out, cut it in half (or replace with two half-value resistors), the half-point will be very-near halfway from -10dB to -20dB.
That saw  is killing me ;D

moamps

Re: Jensen DI pad question
« Reply #3 on: July 15, 2019, 05:46:48 PM »
does the input impedance of the jensen tranny have impact on the pad design?
Transformer in a passive DI design has high step down ratio so its input impedance is high what is BTW needed in order to not load too much passive guitar or such pickups. So most probably this transformer doesn't have a substantial impact in ratio of a low impedance attenuator. But you can check it. Just do the math and verify accuracy of the pad positions  without connected R15. 
You can use just two resistors for -15dB pad, 10k and 2k2 for example.

useme2305

Re: Jensen DI pad question
« Reply #4 on: July 15, 2019, 06:57:58 PM »
thanks everyone. i went the 10k (series) and 2.2k (shunt) route and my test resulted in perfect padding of exactly -15db.

can anyone tell me what the 15k resistor is good for tho? i implemented it of course but i don‘t know why.

squarewave

Re: Jensen DI pad question
« Reply #5 on: July 15, 2019, 08:25:08 PM »
That saw  is killing me ;D
Yeah, that took effort. You can tell it's a coping saw. The blade is fixed up with a line. I'm kinda impressed.

squarewave

Re: Jensen DI pad question
« Reply #6 on: July 15, 2019, 08:47:44 PM »
can anyone tell me what the 15k resistor is good for tho? i implemented it of course but i don‘t know why.
Because there is a source impedance that is ideal for the transformer primary. If the source impedance is not "ideal" you can get a wonky frequency response (bump in bass maybe). With a 1K load, the reflected impedance on the primary is 140K. So it "likes" to be connected to something with higher impedance. Thus R4 is inserted to help with that.

Specifically, when the pad is out, the source is probably a guitar which has a source impedance of around 30K. But when the pad is in, then it's probably a keyboard or some other equally low impedance source in which case the source impedance seen by the transformer is just R1 in parallel with R2+R3 (for the -10dB position) which is ~2.5K and then that is in series with R4 15K. So source Z is 2.5K without R4 and 17.5K with. That is closer to 30K which the transformer "likes".

Re: Jensen DI pad question
« Reply #7 on: July 15, 2019, 10:04:15 PM »
The 15 k series resistor is to make sure that the source impedance (seen on the transformer primary) is never less than 15 k.  Lower source impedances affect high-end frequency response adversely.  It has to do with internal winding resonances (their frequencies and Q factor) ... which are part of the "black art" of transformer design.  I hope this helps!

Bill Whitlock
(former Jensen owner/chief engineer for 25 years)

boji

Re: Jensen DI pad question
« Reply #8 on: July 16, 2019, 12:52:06 AM »
Quote
Take R2 out, cut it in half

 ;D ;D ;D

Edit: Woah, straight from the horses mouth, useme2305.  Welcome (back?) to gdiy, Mr. CMRR! 
« Last Edit: July 16, 2019, 01:32:32 AM by boji »

useme2305

Re: Jensen DI pad question
« Reply #9 on: July 16, 2019, 03:39:27 AM »
thanks again everyone for answering my question with patience. very enlightening.


useme2305

Re: Jensen DI pad question
« Reply #10 on: July 16, 2019, 07:43:22 AM »
quick follow up question.

i want to implement maximum connectivity  in this DI (it will be a stereo diy duplex version).

would it be resonable to add some 15k series resistors in other places of the unit like say the RCA (cinch) inputs, the 1/8 stereo mini jack or even the after the balanced to unbalanced xlr inputs i plan on putting in it or are these fine without the 15k series resistor?

the RCA inputs and the 1/8" iputs stereo jack will typically see -10db line level from an iphone/ipad or laptop and stuff like that.
the balanced to unbalanced XLR Ins will see +4db balanced line level from interfaces or preamps and stuff like that.

i attached a pic of the fixed ballanced to unballanced input pad i will put behind the switchable XLR inputs.
i also attached a pic of the radial JDI duplex diagram that is my reference in the following post

as you probably noticed i want to basically clone the JDI duplex.

any other suggestions to perfect this build are welcome.
« Last Edit: July 16, 2019, 08:38:43 AM by useme2305 »

useme2305

Re: Jensen DI pad question
« Reply #11 on: July 16, 2019, 07:44:25 AM »
here's my reference for the unit i am building just to clear things up. it's the JDI Duplex MK4.
« Last Edit: July 16, 2019, 07:52:04 AM by useme2305 »

squarewave

Re: Jensen DI pad question
« Reply #12 on: July 16, 2019, 10:58:49 AM »
would it be resonable to add some 15k series resistors in other places of the unit like say the RCA (cinch) inputs, the 1/8 stereo mini jack or even the after the balanced to unbalanced xlr inputs i plan on putting in it or are these fine without the 15k series resistor?
You need 15K in series if the source is low impedance (meaning not a guitar) because driving the primary of JT-DB-E with low impedance will cause high frequency peaking / resonance in the output. But I would not switch in more than one if that is what you're asking.

At this point you really need to post a schematic of your specific circuit. Find a schematic capture program like KiCad maybe and make a schem. Or just draw it on a piece of printer paper and take a picture of it with your phone (in v. bright light). Even something passive like this is probably harder to get right than you might think. You need a schem.

useme2305

Re: Passive Jensen Stereo DI questions
« Reply #13 on: July 17, 2019, 06:14:59 AM »
ok, cool. thanks for the hint! =)

it took a moment to understand the whole KiCad/Eeschema thing but here's the schematic i came up with. it's pretty much just the jensen version with mergeable in/thru and an added balanced XLR input.

any suggestions? i'm still not sure if i nailed the balanced xlr input. the merge section is fine though, right?
« Last Edit: July 17, 2019, 10:43:51 AM by useme2305 »

useme2305

Re: Passive Jensen Stereo DI questions
« Reply #14 on: July 17, 2019, 06:33:12 AM »
attached some build in progress pics of my little box of dreams :P



squarewave

Re: Passive Jensen Stereo DI questions
« Reply #15 on: July 17, 2019, 11:00:37 AM »
Lot's of problems with that circuit.

Signal output on common of pad is just grounded. That's not going to work.

You would need to switch both pins 2 and 3 of XLR in to the primary or pin 3 hot will not work. Also you don't need ground there to make a pad. Use a balanced pad. And you need 15K in series with each leg for reasons previously explained. So make an H pad with 15K output legs and two pole switch to switch to transformer.

The "Thru" connection isn't in if "Merge" is engaged? Just connect tip-to-tip and move RCA directly to 1K.

But other than that, I'm sure all of your dreams will be fulfilled once completed.



useme2305

Re: Passive Jensen Stereo DI questions
« Reply #16 on: July 17, 2019, 05:02:27 PM »
ok, thanks so far.

i don't understand why the XLR pad wouldn't work. it's supposed to unbalance the signal because i was out of DPDT switches. are you saying it wouldn't work that way either?
i know you explained it....i just don't get it.

anyways, maybe i'll just get another pair of DPDT switches for this task as you suggested unless anyone else here says it's fine either way...anyone?

attached is the new version with 15k series resistors and double switch...please review it as i have no idea what i'm doing, really.


the RCA directly to 1K and tip to tip thing i don't understand at all, sorry. could you elaborate on that?
direct Thru is of course bypassed if merge is activated. if i do tip to tip i wouldn't be able to merge via the 1K resistors anymore on the 1/4" ins which might come in handy if i have two stereo synths on a stage or in the studio that connect via two (L/R) jacks that i want to send as two mono synths to the desk.

thanks so far. this is fun.

EDIT: already found an error in the transformer wiring. corrected that and reuploaded the schematic.
« Last Edit: July 18, 2019, 04:27:07 AM by useme2305 »

useme2305

Re: Passive Jensen Stereo DI questions
« Reply #17 on: July 18, 2019, 10:40:35 AM »
anyone else wanna chime in on this? i'm not very confident in my own designs but i wanna finish this thing.

just looking for verification of my latest schematic. especially the -20db XLR pad and the merge section. am i doing it right?

squarewave, maybe you can specify what you said about the merge section? i don't really know what you mean and what i achieve by doing it your way.
« Last Edit: July 18, 2019, 10:46:55 AM by useme2305 »

squarewave

Re: Passive Jensen Stereo DI questions
« Reply #18 on: July 18, 2019, 04:16:00 PM »
There's a lot going on in that circuit. It's hard to say what is "right". Just build it and see I guess.


 

Related Topics

  Subject / Started by Replies Last post
12 Replies
3369 Views
Last post July 20, 2004, 06:06:10 PM
by clintrubber
3 Replies
1576 Views
Last post November 12, 2005, 09:14:12 AM
by maxime
0 Replies
1063 Views
Last post October 04, 2011, 08:19:01 AM
by Thomasdj
0 Replies
359 Views
Last post October 23, 2018, 08:51:20 AM
by MaxDM