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Old 23rd January 2013, 08:50 AM   #3801
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Quote:
Originally Posted by Esperado View Post
Example from a CFA i'm working on.
First image:
1 -no cap. the amp is not stable.
2 -7pF paralleled with feedback resistance of 5.9 k: Amp stable.
3- 1Mhz square wave with 1K/50pf of low pass filter.
(Yellow is the feedback )

Second image: idem, Cdom: 0.6pF (for no peak in bandwidth) low pass: 1K/55pF
Thanks for posting. Squares are 100 kHz. OK, so 1,2,3 has 7 pF without Cdom. Will try this in VSSA since this is on the bench. 100 kHz resistor loaded squares in VSSA are the same.
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Old 23rd January 2013, 10:17 AM   #3802
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Hm... seems my problem is troublesome, it is not consistent. I was going to try input filter, then everything went to nice even without filter. Will try more to see whats the problem.

Anyway, I notice something strange. I trimmed the pot to make output zero without any source, but as I connect to my netbook, the output immediately turn to 0.17mV. Is this source-related problem ?
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Old 23rd January 2013, 10:49 AM   #3803
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Quote:
Originally Posted by guitar89 View Post
Hm... seems my problem is troublesome, it is not consistent. I was going to try input filter, then everything went to nice even without filter. Will try more to see whats the problem.

Anyway, I notice something strange. I trimmed the pot to make output zero without any source, but as I connect to my netbook, the output immediately turn to 0.17mV. Is this source-related problem ?
Is it 0,17 mV or 0,17 V?

If it is 0,17 mV variation, that's is nothing as we speak about output DC offset.

Input bias current makes voltage drop on 10 k resistor, and when source is connected this is changed, causing different output DC offset. Solution is input DC offset compensation but for 0,17 mV difference it is absolutely not necessary.
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Old 23rd January 2013, 11:12 AM   #3804
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Quote:
Originally Posted by Lazy Cat View Post
Is it 0,17 mV or 0,17 V?

If it is 0,17 mV variation, that's is nothing as we speak about output DC offset.

Input bias current makes voltage drop on 10 k resistor, and when source is connected this is changed, causing different output DC offset. Solution is input DC offset compensation but for 0,17 mV difference it is absolutely not necessary.
haha, srry, typing error. It is 0.17V, which is significant. But will it be detrimental ? extra 7mW across 4ohm speaker for nothing. Anyway, is there any other way to make the amplifier input more RF-resistant ? ( other than RC filter )
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Old 23rd January 2013, 12:05 PM   #3805
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Low pass filter at input will not decrease the listening quality, on the contrary, it reduce any high speed signal, (Including high order distortion from the source) thus reduce induced IM distortion. It provide better immunity to RF (same remark). I use to set the best value for square waves, as said, then try to lower the corner frequency with careful listening.
It is more important with BJTs in input, as they are less RF immunity than FETs, may-be the reason why some prefer FET's sound for their input devices.

Ps: Sorry for the typo, yes, it was 100Khz square waves. Caution, the amp simulated is not *the* SSA, but one i'm playing with to experiment some alternatives: don't use my caps values.
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Last edited by Esperado; 23rd January 2013 at 12:24 PM.
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Old 23rd January 2013, 12:49 PM   #3806
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How to do follower from SSA?
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Old 23rd January 2013, 06:16 PM   #3807
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Emitter follower?
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Old 23rd January 2013, 07:00 PM   #3808
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I try to keep any cap on the input below 47pF, so the amp can be used with a volume pot. Otherwise you'll have an LP filter that changes with volume. I rather put LP filtering before the pot, where source impedance is much more predictable. My amps typically have a simple input buffer which makes them much more tolerant to a variety of source impedances. This solution is less expensive than creating an amp without an input buffer and then having to buffer the volume pot/attenuator.

Another option is to employ a fancy bootstrapped input LP filter to cancel input impedance, but this must be done carefully or you'll have an oscillator.
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Old 24th January 2013, 01:12 AM   #3809
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Yesterday, I apply the input filter using 1 ohm & 1 uF, doesn't changes much for me. IF I TOUCH the input with my finger, it sound really badly, is that normal ? (filter doesn't help)
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Old 24th January 2013, 02:22 AM   #3810
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Quote:
Originally Posted by Lazy Cat View Post
Emitter follower?
Amp with K=1.
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