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Old 6th May 2004, 05:48 AM   #1
lubbie is offline lubbie  Canada
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Question Long Tailed Pair Modification

Located this mod of a Long tailed pair.. Anybody have any comments??

Is there a formula for Calculating the values for the resistors in a standard LTP??
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Old 6th May 2004, 10:32 AM   #2
AKSA is offline AKSA  Australia
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Lobbie,

You perhaps might tell the forum that this circuit, which is stunning for its originality, comes from the fertile mind of Graham Maynard.

I can assure you it works, and works well.

Cheers,

Hugh
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Old 6th May 2004, 11:50 AM   #3
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In a regular LTP, i.e. no current mirror, the operating point is determined by the VAS transistor. In a non-degenerated VAS, the resistor should be dropping about 0.7 V, i.e. tail_current/2*R=0.7V. In a degenerated VAS, add the emittor resistor voltage drop.

Now this circuit here is quite different. The lower 550 pair form a current mirror. The 4.7 k resistors essentially allow the upper 550 pair to measure the current in each leg. As more current flows, the respective 550 will begin shutting off the corresponding 560 input transistor. It seems to be some kind of local feedback which serves to linearize each input transistor.


I don't have the time now to think it to the end. Anybody else have comments on what the pros and cons of this circuit are?
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Old 6th May 2004, 11:58 AM   #4
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I think that it's actually an example of using CFP:s in the input stage. It will increase current gain, and I suppose it raises inptu impedance in the process.

Rune
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Old 6th May 2004, 05:49 PM   #5
lubbie is offline lubbie  Canada
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You are correct this is Grahams work.. I was doing research on LTP and found his preamp. I was considering using his curcuit as the first stage to a new power amp... Also intended to use a couple of Douglas Self's ideas on the VAS section of the amp. Finishing it off with a Sazaki pair (hmmmm)... Gotta think about that one. LOL


Original Preamp can be viewed at.
http://www.mitedu.freeserve.co.uk/Ci...udio/gmpre.htm
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Old 6th May 2004, 06:05 PM   #6
Alcaid is offline Alcaid  Norway
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Isn't 60dB gain in a preamp way too much?
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Old 6th May 2004, 06:52 PM   #7
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Quote:
Originally posted by Alcaid
Isn't 60dB gain in a preamp way too much?
Presumably the plot is the open-loop response.
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Old 6th May 2004, 06:56 PM   #8
Alcaid is offline Alcaid  Norway
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How much gain have this design then?
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Old 6th May 2004, 07:08 PM   #9
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Quote:
Originally posted by Alcaid
How much gain have this design then?
I never actually checked the fb network, since 60dB sounded
so way off for a closed loop response, but checking the diagram
again I see that it actually was the closed loop repsonse!
I guess something is wrong somewhere. Either it isn't a
preamp, but a microphone amp or something else that needs
a lot of gain (no RIAA network, so that's not the answer), or
the fb resistors are wrong.
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Old 6th May 2004, 07:15 PM   #10
Alcaid is offline Alcaid  Norway
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Strange...

Anyone know what to change in the circuit to have a voltage gain of 2 or maybe 3? It looks really cheap to build it, but what do you think about it's performance? better than a OPA627 buffered with a BUF634?
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