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Old 3rd March 2009, 01:27 PM   #1
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Default scope readings

this is the out put of one preamplifier when i feed it with square wave 10 khz 200mv

please notice the rounding in the corners and if possible explain me what does it mean .....

forum member from tube area said to me that this represents a loss of 0.1db @20khz ....

is that so or is it worst ?????

in a good preamp shouldnt it be perfect square to at least 30KHZ ???

thanks
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Old 3rd March 2009, 02:09 PM   #2
Elvee is offline Elvee  Belgium
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It looks like a nice, clean first order roll-off. The kind of response you expect from an RC placed at the input to limit the ingress of RF signals and similar trouble makers. Nothing abnormal IMHO.
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Old 3rd March 2009, 02:24 PM   #3
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we need to see the input square wave image for comparison...

preferably on the same screen, although not essential.
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Old 3rd March 2009, 02:34 PM   #4
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Default bear .....

of course in the input there is a totally square image ....what you expect ..... ( normaly signals like that when coming from average signal generators are expected to be totally square ....)

in the input ....there is nothing ....since this is a tube preamp there is no r-c to restrict the input signal ..... just 100k resistor to signal connection with 100k resistor to ground .....
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Old 3rd March 2009, 03:07 PM   #5
wg_ski is offline wg_ski  United States
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Default Re: bear .....

Quote:
Originally posted by sakis
in the input ....there is nothing ....since this is a tube preamp there is no r-c to restrict the input signal ..... just 100k resistor to signal connection with 100k resistor to ground .....
Tubes, like semiconductors, have capacitors hidden inside them. And the circuit may or may not have NFB, and therefore makes its own RF/ultrasonic filter - probably some pesky 100k plate resistor and a few pF somewhere.
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Old 3rd March 2009, 03:10 PM   #6
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Default very well .....

in this case can you ensure me about that rounding in the corners ????

does it really represent a los of 0.1db at 20khz ???or is it more ???
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Old 3rd March 2009, 03:29 PM   #7
Salas is offline Salas  Greece
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Looks like being fed by large source impedance, creating a rather large time constant due to Miller. I have made 12AY7 line circuits and they go much higher.
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Old 3rd March 2009, 03:34 PM   #8
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a 3db loss would have rounding that intersected the flat top at about 70% (actually 67%)of the way through the square wave, just as a way of comparison. the 0.1db estimate is probably very close.
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Old 3rd March 2009, 03:36 PM   #9
wg_ski is offline wg_ski  United States
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Default Re: very well .....

Quote:
Originally posted by sakis
in this case can you ensure me about that rounding in the corners ????

does it really represent a los of 0.1db at 20khz ???or is it more ???
You've got a scope and a signal generator - measure it. If you have trouble resolving 0.1dB, measure where it's down 1dB and back-calculate where the 0.1dB point would be assuming it's a singe-time-constant roll off.
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Old 3rd March 2009, 03:43 PM   #10
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Default well salas ....

thanks for join in .... the circuit is fairly simple ...please take a look and give us your sugestion if you have any ....

thanks
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