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Old 15th November 2014, 04:05 PM   #1
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Default help with long tail pair bandwidth

i have a problem with a long tail pair bandwidth

i dont know where is the problem , the amp roll off at 7khz.

help me please
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File Type: jpg long tail pair.jpg (107.3 KB, 182 views)
File Type: jpg bode.jpg (262.9 KB, 173 views)
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File Type: asc logtail-bode.asc (2.8 KB, 8 views)
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Old 15th November 2014, 04:29 PM   #2
maton00 is offline maton00  Mexico
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Why do you have C1 connected in that way?
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Old 15th November 2014, 04:30 PM   #3
jane is offline jane  Norway
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The 100 k gridstopper (R6) will reduce bw.
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Old 15th November 2014, 04:32 PM   #4
DF96 is offline DF96  England
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Miller effect in U2? 100k is quite a large value for a grid stopper, except in a guitar amp where full audio bandwidth is not a requirement and people want to be able to:
1. adjust treble response by swapping valves instead of using a tone control,
2. get away with an appalling layout wth long wires everywhere.

NB I assume C1 is to stop Miller effect in U1?

Last edited by DF96; 15th November 2014 at 04:33 PM. Reason: add NB
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Old 15th November 2014, 04:35 PM   #5
nec3 is offline nec3  Turkey
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Reduce C1 down to 1P, or delete.
Also, C2,R7 NFB network needs correction.

Last edited by nec3; 15th November 2014 at 04:39 PM.
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Old 15th November 2014, 05:56 PM   #6
DF96 is offline DF96  England
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Removing C1 will narrow the bandwidth further, by introducing Miller effect in U1. "1P" I assume should be 1pF? I can't see any R7 or NFB.
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Old 15th November 2014, 06:08 PM   #7
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Quote:
Originally Posted by DF96 View Post
Miller effect in U2? 100k is quite a large value for a grid stopper, except in a guitar amp where full audio bandwidth is not a requirement and people want to be able to:
1. adjust treble response by swapping valves instead of using a tone control,
2. get away with an appalling layout wth long wires everywhere.

NB I assume C1 is to stop Miller effect in U1?

Well even though I had a good laugh at this, I don't know of any guitar players who swap valves to adjust treble response.........must be a British thing?

+1 on the long wires and crappy layout though
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Old 15th November 2014, 06:22 PM   #8
nec3 is offline nec3  Turkey
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Quote:
Originally Posted by DF96 View Post
Removing C1 will narrow the bandwidth further, by introducing Miller effect in U1. "1P" I assume should be 1pF? I can't see any R7 or NFB.
C1 10uf is too high.
C2, R7 are in asc. file.

Edit: Q2 has a Vce voltage 149v. Spice model of BC547 is defined with a 50V max. I have no idea how LTspice response this though.

Last edited by nec3; 15th November 2014 at 06:45 PM.
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Old 15th November 2014, 06:27 PM   #9
kevinkr is offline kevinkr  United States
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V3 does not need to be more than about -12V. If you build the circuit consider using a cascode CCS with DN2540.
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Old 15th November 2014, 07:18 PM   #10
Merlinb is offline Merlinb  United Kingdom
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The problem is R1, not C1. This is a cathode-coupled amplifier, not a phase splitter. Reduce R1.
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