twin servo build questions

[berkleystudios]

looking at to to proto a few jensen twin servos since I have 6 jt-123-sl..... not what it calls for in the schematic but it has a low turns ratio and just a few db of less head room so i think ill be fine.

http://www.jensen-transformers.com/as/as083.pdf



but has anyone ever built one of these :

http://www.jensen-transformers.com/as/as082.pdf


ive never hear or used an ad797 and i amp not against using IC, in my circuits if it sounds great..
http://www.mouser.com/Search/Refine.aspx?Keyword=ad797





also anyone know any alternatives to this
http://www.jensen-transformers.com/datashts/oli2.pdf

 

[ppa]

AD797 is specified for +-15V of powe supply, with these voltages and an 1:1 out trasformer the headroom is not so great for an uncheap preamp.

 

[berkleystudios]

AD797 is specified for +-15V of powe supply, with these voltages and an 1:1 out trasformer the headroom is not so great for an uncheap preamp.

So the 990s would be better because of the high voltage the require? Which in turn increases headroom?

 

[millzners]

A while back Presonus made the MP20 which was basically this exact same circuit built with Jensen transformers.  They used cheaper IC's than the AD797, for example 5534, and eventually even moved on to a cheaper transformer vendor than Jensen.

I have one I swapped the cheaper IC's out for the AD797 and I couldn't tell the difference although I was never good at detecting these types of differences.  I even tried phase inverting and nulling out AD797 channel 1 vs. unmodified channel 2 and if I remember correctly the resulting signal was high end like a HP filter...  However I will say this:  In non-music (audio-band) applications I've used the AD797 and it is as low noise as they say; it's a very high-performing op amp and you can certainly measure the difference between it and a cheaper IC.

If you search around this site there's a few threads about modifying the MP20, and you'll find a thread where they also indicate substitutes for each op amp in the circuit including the output IC and the insert IC that doesn't appear in that Jensen schematic.  You can also find out on the internet other sites where a guy named Jim Williams, who modified these things, suggests a list of IC's for the MP20. 

Suggested substitutes are LT1357's or AD8597 or AD797 or OPA211 or OPA627 or OPA827 for the op amp in question.  Most of those you can get 4-6pcs of as free samples shipped to you if you go to the manufacturer's website and find the "free samples" section.

 

[abbey road d enfer]

AD797 is specified for +-15V of powe supply, with these voltages and an 1:1 out trasformer the headroom is not so great for an uncheap preamp.

So the 990s would be better because of the high voltage the require? Which in turn increases headroom?

Indeed, headroom would increase. Let's say you go from +/-15 to +/-18V, you get about 2dB increase in headroom, going to +/-24 gets you another 3dB. Great.
What are you gonna do with this headroom? Most likely this will go to an A/D converter which has not enough input headroom to take advantage of it. The 797 is a wonderful design, the 990 too. Potentially, they may yield a marginally better noise figure than a 5534, a somewhat more extended HF response, and a slightly better THD performance. Provided you are an expert at laying out a PCB that avoids all the traps with capacitive and inductive coupling, ground current circulation and RFI/EMI protection.

 

[ppa]

AD797 is specified for +-15V of powe supply, with these voltages and an 1:1 out trasformer the headroom is not so great for an uncheap preamp.

So the 990s would be better because of the high voltage the require? Which in turn increases headroom?

No, I did not say this, but only to bring to the attention there is an headroom that is lower than the classic famous twin servo preamp.
Personally I prefer a twin servo preamp with 990's than AD797 inside but "de gustibus non disputandum est".

 

[offkey_99]

Very old thread, but original link is dead. The schematics from the original posts can now be found here:

https://www.jensen-transformers.com/wp-content/uploads/2014/08/as083.pdf

and here:

https://www.jensen-transformers.com/wp-content/uploads/2014/08/as082.pdf



Also, I couldn't find a link to OLI2 (output isolator 2) on Jensen's site anymore but found it here:
https://www.yumpu.com/en/document/read/27221328/jt-oli-2-jensen-transformers

A nearly (or maybe exactly) identical OLI3 is available here though: https://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-oli-3.pdf

The Yumpu link has lots of ads, etc.; I prefer the Jensen links for sure.


Figured I'd this info since I was curious myself, maybe will help others.
-Chris

 

[offkey_99]

Also, since I have some personal experience now with both MP20 (did opamp swaps and swapped out the no-name transformer for the recommended Jensen models) and John Hardy Jensen Twin-Servo in my own studio (having also built a clone-like circuit of that preamp) I just wanted to give my opinion comparing them in response to @millzners comment above.

The transformers used in the John Hardy Twin Servo are basically the best Jensen offers (bandwidth is widest, frequency response is thus the flattest, linearity is the highest) and the use of the 990 opamps (discrete opamp with low-voltage noise, BJT based input stage) being noise optimized with these transformers (not to mention higher voltage rails) sounds much nicer to my ears compared to the MP20 (even with mods) having the JFET source-follower embedded in the circuit front-end (with "IDSS" actually adjustable drop in degeneration making the gm less linear - not super musical "warmth") and decent-enough but technically less proficient transformers along with IC opamps. For the MP20, about 10 dB of the gain is coming from the transformers step-up ratio. If I recall correctly, the transformers in the Hardy/Jensen Twin Servo are 1:1, which apparently results in less compromises when winding, etc. resulting in the better specs.

The MP20 ("dual servo") is not bad, but the original Twin Servo circuit is definitely a step up in life-like clarity (to my ears). Having said that, the headphone amp and mixer feature on the MP20 are useful at times and I definitely used the 80Hz HPF quite often. Not a bad product at all, just not truly "high end" in my opinion. Have since sold it, don't really miss it. But...I learned a lot messing with the opamps and recalibrating the servos.

I should also point out that the Hardy is much more money than the MP20 - not really "a fair fight"!