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Old 18th September 2006, 05:09 PM   #1
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Default Open loop line stage pre-amp design revisited

In the past months open loop line stage designs have been discussed in general in the Blowtorch thread known to many frequent forum visitors and members.

Some topics have been explored to a certain extend and others just scratched on the surface.
My intention is to explore this kind of design with others in a constructive way which enables us to gain new insights on the design in general, as well as specific topics related to it. Apart from that I will try to post links with basic theoretical information so that the less academic schooled diy-ers, including myself, can have a better understanding of how the circuit works and what are key-issues. The Blowtorch thread already has a wealth of information thanks to John Curl, the designer of the well know Blowtorch pre-amp, so you might want to check that thread for additional information on the Blowtorch design itself. It may serve as a guide on our journey here.

Here we will deal with a similar design in general and explore its potential together. So your valuable input, experience and knowledge is welcomed.

We will start with the basic circuit which Justcallmedad a.k.a. JMCD has posted in the BT thread and which I use with his permission in this renewed thread (Thanks JMCD).

I cannot predict the course this thread will take, but would like to ask all participants to stay on topic as much as possible. Given the many areas this design encompasses, it may be mandatory to start a new thread for specific topics directly related to open loop line stage designs to keep the thread-flow as natural as possible and avoid information fragmentation. In this respect I welcome ideas from participating members.

Topics to explore or discuss are for instance:

- Determining gain
- Proper biasing
- Methods of achieving proper frequency response
- DC-Servo offset methods
- Noise-free power supplies, which are mandatory - for this design
- Improving the overall design
- Any other related topic which I may have forgotten

The following schematics serve as a basis: First the open loop cascade line stage.
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Old 18th September 2006, 05:12 PM   #2
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Here is a possible configuration of the DC Servo:
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Old 18th September 2006, 05:16 PM   #3
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Hi Franklin,

no resistors between sources of J1J2 -- J3J4 ??
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Old 18th September 2006, 05:20 PM   #4
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You are right, it slibbed my attention. But here is another schematic quite similar:
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Old 18th September 2006, 05:22 PM   #5
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Would a constant current source work in the differential amps to help the PSRR?
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Old 18th September 2006, 05:24 PM   #6
MikeB is offline MikeB  Germany
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Have you already simulated with ~22k in series to the signal supply and observed rise of distortions ?
I'd say, this should be no1 discipline for a preamp, insensitivity to source impedance.

Mike
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Old 18th September 2006, 05:25 PM   #7
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Who would design a perfect CCS ??
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Old 18th September 2006, 05:25 PM   #8
MikeB is offline MikeB  Germany
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Quote:
Originally posted by davidallancole
Would a constant current source work in the differential amps to help the PSRR?
This jfet constellation already forms a ccs. But psrr is anyway degraded to ~-6db by the use of R6/4/20/16.

Mike
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Old 18th September 2006, 05:28 PM   #9
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Quote:
Originally posted by MikeB


This jfet constellation already forms a ccs. .

Mike
No. Should be instead of 203 ohm drain resistors.
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Old 18th September 2006, 05:29 PM   #10
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So putting a ccs in the differential amps would do jack squat.
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