alhe

Jensen Twin Servo or M1 DIY
« on: February 23, 2019, 11:06:43 AM »
I recently got hold of a tape deck module using two 990 opamps and two Jensen output transformers. I am now debating whether to build the classic Twin Servo circuit (JLM99 pcb) or two pieces of the Hardy M1 (Mitsos 312 pcb). If I build the Twin Servo I could sell one of the Jensens to finance the correct input transformer, or I can have two channels of the M1 - which is probably excellent sounding, too. Have anyone heard both and is the extra parts count for the Twin Servo worth it?  :)


Recording Engineer

Re: Jensen Twin Servo or M1 DIY
« Reply #1 on: February 27, 2019, 01:08:47 PM »
From my understanding (I think it comes directly from John Hardy himself), it’s going to be a matter of how much gain you need/want; the sound difference being pretty negligible. So if you’re going to be using it on quiet sources with low output mics with any kind of distance, you’ll definitely need the Twin Servo’s gain.

dmp

Re: Jensen Twin Servo or M1 DIY
« Reply #2 on: February 28, 2019, 02:57:23 PM »
I built the single 990 design (M1) which was Deane Jensen's original preamp design. Using the JT-16 and JT-11BM transformers. 
I had the gain go up to 70 dB and I've never noticed any negatives. I've been using these for years now in my studio and they are fantastic. 
The twin servo splits the gain into two stages and might spec out a little better but I never felt the need to try it.

Here is my 8 channel build using Mitsos PCBs:
https://groupdiy.com/index.php?topic=54082

L´Andratté

Re: Jensen Twin Servo or M1 DIY
« Reply #3 on: February 28, 2019, 05:20:44 PM »
Here is my 8 channel build using Mitsos PCBs:
https://groupdiy.com/index.php?topic=54082
Totally badass box, I even remember seeing it from 5 years ago ;D
Strictly amateur since 1973...

L´Andratté

Re: Jensen Twin Servo or M1 DIY
« Reply #4 on: March 01, 2019, 06:36:15 AM »
Maybe interesting, found this: Walt Jung Opamp applications handbook,Very Low Noise Mic Pre (M1 style with AD797 opamp)
Strictly amateur since 1973...

Recording Engineer

Re: Jensen Twin Servo or M1 DIY
« Reply #5 on: March 02, 2019, 02:27:30 PM »
I built the single 990 design (M1) which was Deane Jensen's original preamp design.

For historical-accuracy, that’s not right. The M1 is John Hardy’s design and not Deane’s original design. His has always been the Twin-Servo design.
https://m.youtube.com/watch?v=YR3_fJGlYzE

dmp

Re: Jensen Twin Servo or M1 DIY
« Reply #6 on: March 04, 2019, 10:51:47 AM »
For historical-accuracy, that’s not right. The M1 is John Hardy’s design and not Deane’s original design. His has always been the Twin-Servo design.
https://m.youtube.com/watch?v=YR3_fJGlYzE

Interesting - I remembered wrong - I went and dug out Deane's original paper introducing the 990 from 1980. I thought he introduced the schematic of the M1 at the end, but his schematic (photo attached) did not have the same bias and servo circuit as the M1. It did have compensation to eliminate a capacitor before the output transformer but it was not adjustable. 

alhe

Re: Jensen Twin Servo or M1 DIY
« Reply #7 on: March 10, 2019, 08:40:37 AM »
Great work, @dmp  :)

If the M1 really is good for 70dB clean gain I think that would do nicely. I have plans to build the AD797 version as well, but with Lundahls I think (to save on $).


 

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