jensen twin servo basic schematic - anyone built one?

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outoftune

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i read through the big post about the 990 DOA's and am trying to wrap my head around it. i know it was concluded that the jensen twin servo "basic" schematic isn't quite finished.

the thread says that there needs to be 1000uf/35v caps where the 24v goes into the DOA. is that all it needs? any other additions?

ive also been looking at the John Hardy M1 schematic, which is similar. should i just try to build something based off of that?

any pics of your twin servos would be great...
 
Or how simular is the JLM or SCA versions ?

seems a nice variation to have for the balence of input headroom and output drive .

regards Greg
 
I made one, though I rolled my own discrete Opamps. I did use caps (470uF) instead of the bias compensation scheme after running into a few issues with the initial design.
It is a big and accurate sounding pre, one of the better sand state designs.
 
analag,

did you just follow the design on the jensen site adding the 470uf's? id guess they would be after the 2nd doa and before the output in the signal path? anything else you added or took out or modified etc? any pics? :razz:
 
For reference: as083.pdf

What I would add:
* input bias compensation (for both opamps) as shown in the John Hardy M1 schematic
* rotary switches instead of pots
* a JT-OLI-3 between second opamp and transformer (that's implicitly shown in the schematic as they use the load output)
* 220 uF decoupling pair for each opamp

There's probably a better opamp for the servos out there, perhaps the OPA627 if you have the money.

Replacing the 990s with arbitrary opamps will definitely run you into issues regarding DC precision, but that's not your intention I think. Adding coupling capacitors somehow kills the basic idea of the Twin Servo.

Samuel
 
[quote author="outoftune"]analag,

did you just follow the design on the jensen site adding the 470uf's? id guess they would be after the 2nd doa and before the output in the signal path? anything else you added or took out or modified etc? any pics? :razz:[/quote]

470uF after R6 and R12. I kept the servos as shown in the schemo, I didn't bother with a rotory switch as I did not feel is was absolutely necessary (think it is more important in differential topology and stereo units) these caps are in parallel with the audio not in series as in the case of coupling caps. Square wave response is beautiful with this pre.
 
I'm sure you could ask Mr. Hardy about this design. He has been willing to share info pretty well here. You should probably be decent about it and buy a thing or two from him while you are asking. He has the pot you need for sure and would probably sell switches for gain if you wanted that instead.

I have had a plan to build at least an M1 for a while on a Baby Animal board, but I'll use Cinemag trafos. I think the servo is part of the sound or lack thereof.

HTH!
 
[quote author="SonsOfThunder"]I'm sure you could ask Mr. Hardy about this design. [/quote]
i had emailed him about this but he said

"The real Jensen Twin Servo 990 Mic Preamp is significantly different
from the schematic on the Jensen site. However, the schematic is
proprietary and not publicly available. I cannot comment any further due
to the proprietary nature of it. Sorry about that."

i can understand this so that is why i decided to ask here. planning to buy the 990c doa's from him if i actually decide to give this a shot...

sons...care to share a bit more about that M1 on a baby animal board?
 
John Hardy has provided the most important pieces of information you need in his M1 schematic—the bias compensation and servo. Samuel is pretty much right on target, you need bias compensation for both 990's, and trim the DC offset on the servo as well (if your meter can measure that low). The main adjustment you need to make is the gain—the "standard" Jensens were designed for 17 to 60db, so with a feedback resistor of 1k (shunt with 560pf), your gain setting resistor would be 43.2 (plus the 1k dual rev log pot). Output isolator to the output transformer (11-BMCF), 100 ohm 2W resistor between first and second stage. The 990's are typically power supply bypassed with 470uf plus 1uf film.

Personally, I think the only reason to go with the Twin Servo instead of a single 990 (Hardy M1) is if you need very high gain (>60).
 
>>>care to share...M1 on a baby animal board?

Okay, I hadn't looked at it close enough. It would be possible on the BA board IF you used an external board for the servo. It would be more easily implemented on the JLM99*MB which is when I had the idea first.

But I am just talking at this point. I have had most of the stuff to build a dual 99V pre since they fist came out and haven't had time or money to finish it yet. I am trying to get that done so I can sell off some stuff from my studio to a friend who is doing some recording too.

HTH!
Charlie
 
I've got a (well 2) JLM dual 99v boards that I've been doing the mods to make conform to the Jensen twin schematic. I've been out of town for the last month and with the backlog who knows when I will get back to it.

The plan is to use the Mnats/ptownkid 990 and the JLM input xformer. First is to make the trace/wireing mods to conform to the twinservo and see what it sounds like. Then add in bias etc from The hardy M1 to see how that improves it.

The current state is a slightly tested base configuration..... Maybe I should post a photo....
 
[quote author="jamesfei"]

Personally, I think the only reason to go with the Twin Servo instead of a single 990 (Hardy M1) is if you need very high gain (>60).[/quote]
I and a friend did a comparison of both (15 years ago) and chose the single, hands down.
 
I'm looking for the documentation for the changes to the JLM Dual boards. I've scanned 2 boards, not very clear but good enough for now.

http://arafel.org/audio/twin-servo/twin.jpg

Just looking over the boards I've made these changes:

Deleted C23, C24, R24, R25, R26, R27, C9, C19, R17

C9 and C19 are jumpered

Second DOA output jumpered to Output A

Gain Jumper configured for second opamp

As I said I did not have much time to work on this. The plan was to wire the boards to a Twin Servo topology and get them working. This involved removing the Electrolytics in the gain and output sections. I changed some resistor values and I think I rewired the pot. I'll have to get my notes out and refresh my memory.

See Joe's schematic:

http://www.jlmaudio.com/JLM99XMB.pdf

Then the plan is to add in the bias network from the John hardy MPC-1.

The reality is that I will play with this for a while and learn a bunch and then convert it back to the original JLM Dual configuration. It would be nice to buy a pair of Hardy MPC-1....

Got that great River to build shomewhere in there.

--ethan
 
I have somewhere a twin servo schematic that I got from Jensen around twenty years ago. If I can find it, I'll post it.

I remember it had bias and offset trimmers, and that the 990 outputs went to the servo (LM11C) non-inverting in and the servo out the 990 inverting in (as opposed to the current Jensen sheet).
 

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