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Jensen Twin Servo 500: Preliminary Information

Discussion in 'Pro Audio Equipment' started by JWHardy, Oct 5, 2009.

  1. JWHardy

    JWHardy Active Member

    I have finished the design for the Jensen Twin Servo 500, a variation of the original Jensen Twin Servo 990 Mic Preamp for the Lunchbox/500-series format. Here is the link to the pdf with preliminary information:


    I have not ordered circuit boards yet, but will do so when I return from the Audio Engineering Society convention (booth #329) after October 12th.

    Thanks to all of you for your patience. If you have any questions, please let me know.

    John Hardy
    The John Hardy Co.
  2. Kev

    Kev Well-Known Member

    always good to hear from John

    a quick look at the pdf
    Jensen's best in and out
    pair of 990C DOA ( John's decrete opamps)
    NO caps in the signal path
    72db gain
    a low imp mic feature
    Lunch Box mounting

    I'm guessing this is fully assembled and not a kit ?
    projected cost ?
  3. JWHardy

    JWHardy Active Member


    Fully assembled. Tentative price is the same as adding a channel to the original Jensen Twin Servo: $895.

  4. Kev

    Kev Well-Known Member

    thanks John
  5. audiokid

    audiokid Staff

    Good to see you here John, what booth number?

    Feel welcome to talk more about things, how your gear is developing etc if you like.
  6. JWHardy

    JWHardy Active Member

    Booth #329.

    I'm always willing to answer questions. Any specific areas or directions in mind?

    Thank you.

  7. jammster

    jammster Active Member

    John, after looking at your website, I was stunned at the look of the circuit board for the twin servo 990.

    This preamp looks very impressive. What is equally impressive is what others have said about how they perform.

    Is there any differences between the lunchbox design and the rack mounted design?

    Do both offer the same specifications?

    Thanks again for your time,
  8. JWHardy

    JWHardy Active Member


    The schematic is proprietary, so I cannot provide certain details. But the new version is essentially the same as the original, with various things done to adjust for the lower supply voltages. The original operates on +/-24VDC, the 500-series around +/-16VDC.

    So, it should have the same performance, subject to limitations in the rack that it is plugged into. There could be grounding issues, supply issues. Of course, it will not have as much headroom due to the lower supplies. But until the clipping point, performance should be the same.

    The output transformer is the "DM" size rather than the larger (largest) "BM" size of the original Twin Servo. But this is fine for two reasons. First, the lower supply voltages mean that the 500-series version will not be able to produce output levels high enough to warrant the use of the larger transformer. Second, the larger one wouldn't fit anyway.

    Thank you.

  9. Link555

    Link555 Well-Known Member

    John first let me thank for providing so much detail with the Jensen Twin Servo 990 Mic Preamp. I learned a great deal from your efforts.

    I do have a few basic questions;

    If using transformers and they block DC, why do we need caps on the input at all?

    If your opamp has a DC offset pin(pins) will that mess with your servo feedback idea?

    Why is the gain adjustment never done in the negative feedback path? I mean why is the gain adjustment always Ri and not Rf?

    Thanks very much for all your efforts!
  10. JWHardy

    JWHardy Active Member

    There AREN'T any caps on the input in my preamps. I'm not sure what you are getting at with this question. If you DON'T have an input transformer, you almost always DO need caps to block DC when using the Phantom supply for condenser mics. If you DO have a transformer at the input, you DON'T need blocking caps. You can still use them, but what's the point? The transformer does the DC blocking. This is an advantage of a transformer input. You can eliminate the capacitors, and the audio degradation that they cause.

    The 990 does NOT have a DC offset pin. Even if it did, there are factors that can change the DC offset of my preamps so a set-it-once DC offset adjustment would only be optimum for one fixed gain setting. The elimination of the traditional cap in series with the gain control in the path from the inverting input to ground means that the gain of the 990 includes DC gain. That traditional cap is there to make the DC gain = UNITY to reduce DC offset problems AND to keep input bias current of the inverting input from reaching the gain control where it could cause noise during adjustment of the control. The elimination of that cap means that the DC impedance in that leg is variable, so the input bias current flowing from the inverting input will cause variable DC voltage/current there while the conditions at the NON-inverting input remain fixed, resulting in differences in DC voltage at the inputs, which get amplified, resulting in DC offset at the output of the 990 that could change significantly. The "input bias current compensation" circuitry exists to null the bias currents at the inputs, greatly reducing this problem. But having a DC servo at the output keeps the DC offset at the output well below 100 microvolts regardless of the gain setting. It constantly nulls any offset that appears at the output of the 990 regardless of the gain setting. It is the best approach, guaranteeing extremely low and constant DC offset under a wide range of (DC) gain conditions. If some other op-amp does have a DC offset pin, it is generally a separate issue from the DC servo circuit. The DC servo circuit nulls DC at the output. If an op-amp has a DC offset pin, and the surrounding circuitry supports it (not on my boards), you would be deliberately (or accidentally) changing the DC offset, but the DC servo would still null it out (within design limits).

    It CAN be done in the feedback path, but the bandwidth of the 990 (or any op-amp) will change as the gain is adjusted because the feedback capacitor is a constant value while the feedback resistance is being changed. You also want a low value for the shunt resistance (inverting input to ground path) when at high gains so the noise of the op-amp will be reduced.

    Thank you.

    John Hardy
  11. Link555

    Link555 Well-Known Member

    Thanks Very much, I understand now, when you say a cap-less design you mean the gain pot cap.

    So you use the input XFMR to block the phantom power.

    Thank You!
  12. JWHardy

    JWHardy Active Member

    When I say "no coupling capacitors in the signal path", I mean NO COUPLING CAPACITORS IN THE SIGNAL PATH!! This means the following:

    1. There are no coupling capacitors related to the inverting input.

    2. There are no coupling capacitors related to the non-inverting input.

    3. There are no coupling capacitors related to the output.

    I use the JENSEN JT-16-B input transformer because it is the BEST INPUT TRANSFORMER IN THE WORLD, AND IT DOES AN EXCELLENT JOB dealing with signals from a microphone, regardless of the type of mic. Part of that job is the elimination of input coupling capacitors and the problems they cause. The JT-16-B also has a much higher common mode impedance than a transformerless preamp, providing the opportunity for higher common mode rejection. An input transformer can also withstand much higher common mode voltages than the typical transformerless circuit, around 300V.

    John Hardy
  13. Link555

    Link555 Well-Known Member

    Great thanks very much.

    EDIT: Please check you PM's I hope my message clarifies my vaque questions. Thanks Again!
  14. audiokid

    audiokid Staff

    here's an inside look


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