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Hi all
Just wondering what the real-world differences are between the M-1 and the Twin Servo. I know what the technical differences are, but I'm wondering if the more expensive type is better, differerent or whatever.



FifthCircle Mon, 07/04/2005 - 21:59

I asked John that exact question at AES last year and he didn't have a particularly good answer. He said they sound very similar, but the twin servo is the orignal Dean Jensen design. The specs are a bit better and he prefers the sound a bit more, but even he said they were close.

FWIW, I use the original Boulders (twin servo) here all the time and I think they are pretty amazing sounding pres.


JWHardy Tue, 07/05/2005 - 20:42

FWIW - The Seventh Circle J99 is a clone of the twin-servo Hardy/Jensen design

This is not entirely true. I could be mistaken, but I believe the J99 is based on the schematic that is posted on the Jensen web site:

That schematic does NOT include "input bias current compensation" circuitry. The specific circuit and schematic that is used in the Jensen Twin Servo 990 Mic Preamp that I manufacture is proprietary and not available to the general public. It DOES include the input bias current compensation circuitry. The Jensen Twin Servo that I manufacture was a co-development of Deane Jensen, Steve Hogan and Bill Whitlock of Jensen Transformers, and myself. It was a replacement for the Jensen Twin Servo that was made by Boulder.

The J99 also appears to use a smaller output transformer, the Jensen JT-11-DM, rather than the JT-11-BMQ.

The J99 uses the Lundahl 1538XL input transformer instead of the Jensen JT-16-B that is used in the actual Jensen Twin Servo. The 1538XL has a 1:5 turns ratio (150:3.8k), the JT-16-B has a 1:2 ratio (150:600 ohms). The optimum ratio for use with a 990 op-amp is 1:2. But, as the saying goes, your mileage may vary. Thank you.

John Hardy
The John Hardy Co.

JWHardy Wed, 07/06/2005 - 21:13


Choosing which of my preamps is best is not easy. As mentioned earlier, they should all sound very similar because they have the same input transformer (Jensen JT-16-B, Jensen's best input model), same op-amp (990C), no coupling capacitors in the signal path, same output transformer (Jensen JT-11-BMQ, Jensen's best output model), although the JT-11-BMQ is an option on the M-1 and M-2, standard equipment on the Jensen Twin Servo.

The Jensen Twin Servo might have an edge if you need very high gain much or most of the time, thanks to the 2-stage design. I always use the example of 60dB of overall gain: The input transformer provides the first 5.6dB of gain, the one 990C of the M-1 or M-2 provides the remaining 54.4dB of gain. With the Jensen Twin Servo, there are two 990C op-amps providing 27.2dB each, or 54.4dB total for the op-amps. Each of the two 990C op-amps is working at a lower, more relaxed gain than the single 990C of the M-1 or M-2, so the distortion may be somewhat lower.

Even the best op-amps start to show an increase in distortion when operating at very high gains, at the highest frequencies first, progressing to lower frequencies as the gain goes up even further. But, the 990C does a fine job in the M-1 and M-2 at 54.4dB of gain. I have modified many M-1 and M-2 preamps, and some of the Jensen Twin Servo preamps, for higher than the standard 60dB maximum gain, and the results have been very good.

So, your mileage may vary. I am a bit partial and biased in favor of the M-1 because it is my first rack-mounted preamp. But the M-2 is almost identical, and the Jensen Twin Servo may have its advantages thanks to the 2-stage design. Thanks.

John Hardy
The John Hardy Co.

John Stafford Thu, 07/07/2005 - 20:39

Thank you for your response.

It is the fine reputation of the M-1 that made me interested in your preamps in the first place, and I must admit to have been ignorant of the Jensen Twin Servo. The information you have provided is very useful indeed.

Thanks again for taking the time to explain the differences.


Randyman... Thu, 07/07/2005 - 20:48

Point taken, Mr. Hardy :) .

Believe me, I wish I could afford 4 channels of your Twin-Servo pre's, but Seventh Circle is all I can afford (And I still have to save up for that!).

Thanks for pointing out the exact differences. What kind of difference would I notice if I built the J99, but use the Jensen 1:2 input trafo opposed to the Seventh Circle Lundahl 1:5 trafo?


JWHardy Thu, 07/07/2005 - 21:51


What kind of difference would I notice if I built the J99, but use the Jensen 1:2 input trafo opposed to the Seventh Circle Lundahl 1:5 trafo?

I don't know! But, if you look at the specs of the various mic input transformers on the Jensen site, you will see the laws of physics at work: the lower the impedance ratio of the input step-up transformer, the better it performs in terms of having a wider bandwidth, more linear phase response and lower distortion. The trade-off is, there is less voltage gain from the lower ratio transformer.

If you DO use the JT-16-B instead of the Lundahl, make certain that you use the proper components across the secondary winding. The JT-16-B should have a 6.19k resistor across the secondary, and a series combination of a 620pF and 4.53k resistor across the secondary. You can look at the pdf files of any of the mic preamp products on my site to see these components.

Also note that the J99 circuit uses one pole of a rotary switch as an attenuator after the secondary of the input transformer. The JT-16-B may be able to handle higher levels without saturating, and it has less gain than the Lundahl, so an attenuator at that point may not be the best idea with the Jensen.

Make sure that there is physically enough space to mount the JT-16-B. It is a large transformer.

BTW, what is a "trafo"? (ha ha)

John Hardy
The John Hardy Co.

Randyman... Thu, 07/07/2005 - 22:18

Thank you very much! :mrgreen:

So, the Jensen would be more linear with wider bandwidth, but with less voltage gain? I'm still "green" on the whole complex circuits thing. Where would the attenuator go IF I decided to use the Lundahl (if it will fit)?

I realize I am asking you for info not related to your product, so I can't expect much in return. You have already shed some light my way, and I appreciate that!!! You are a gentelman, sir.


JWHardy Thu, 07/07/2005 - 22:36


Where would the attenuator go IF I decided to use the Lundahl (if it will fit)?

I'm guessing that you meant the Jensen?

The "attenuator" that I mentioned consists of R1, R2, R3 and R4 on the J99 schematic on the seventhcircle site.

This attenuator attenuates the signal at a point after the input transformer but before the 990 op-amp, to avoid an excessively high signal level at that point in the circuit (I did not design it, so I cannot be sure, but that is what it looks like to me). It only becomes active at the lower gain settings of the rotary switch, since that is when the signal level is probably at its highest and you would be trying to reduce the levels to avoid distortion.

But, since the Jensen JT-16-B has less gain to it, the level would not be as high at that point when using the JT-16-B, compared to using the Lundahl which has a higher impedance ratio (therefore, higher gain). But it probably wouldn't hurt anything to leave that part of the circuit alone. However, the components that I mentioned are important. The series total resistance of R1, R2, R3 and R4 is 6185 ohms, almost the same as the 6.19k (6190 ohms) resistor that I mentioned, so that is already taken care of if you use the JT-16-B. You would still have to add a 620pF and 4.53k resistor as mentioned, for proper operation of the JT-16-B.

So, what exactly is a "trafo"?

John Hardy
The John Hardy Co.

Randyman... Thu, 07/07/2005 - 23:13

Doh. I have been getting all kinds of stuff mixed up today! I've got Laptops, 802.11b, and a new router on my brain, so I'm short circuiting :lol: ...

Trafo is just my slang for transformer (I'm a 2-finger typer :wink: )... It is not a widely used abbreviation, I see.

I'll probably go ahead and get the J99 w/stock Lundahl, and see about swapping out to the Jensen if I'm not satisfied with the J99's "as-is". Either way, I'll be buying 4x J990C's from you at some point!

The attenuator on my A12 and N72 are both continuously variable pots, so I thought you might have been referring to the stepped gain control. But it now makes sense after you explained it.

Thanks for your expertise, and back to your regularly scheduled thread :cool:

Cucco Fri, 07/08/2005 - 06:59

Randyman... wrote: Trafo is just my slang for transformer (I'm a 2-finger typer :wink: )... It is not a widely used abbreviation, I see.

Trafo is undoubtedly an uncommon slang. However, I find it better than "Trannie" which conjurs up images of my dad prancing around the house in a summer dress and size 14 ladies pumps.... :cry:

Randyman... Mon, 10/16/2006 - 21:59

"Trafo" IS CORRECT Aferall!!!

Sorry - but I had to dig this thread up out of hiding. Apparently, I was Flamed for using the abbreviation "Trafo" for Transformer in this thread, and I knew I had seen it used elsewhere.

Kev informed me that trafo was indeed a standard abbreviation for Transformer...

Kev said:

Tech Talk (now defunct) concluded that trafo was the world wide accepted short for for transformer ... not TX
The term is used all the time at The LAB

So - In Your Face! :D

Sorry to bring up old sh*t - but I HAD to point this out ;)

PS - 2 More A12's on the way to complete my Seventh Circle Rack. My drums will be in hog heaven! 4 Channels of A12, 2 channels of N72, and 2 Channels of J99. Throw in a HHB Classic-80 high-tension tube pre for fatness, and an Avalon U5 for DI Duties, and I have most of my bases covered IMO. Now, I need a few good rooms :mad: