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Old 11th April 2016, 05:22 PM   #7011
mrmax063 is offline mrmax063  Serbia
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Originally Posted by xrk971 View Post
Speaking of caps - is there a reason we need a 10uF electrolytic input cap? I would rather not have a 'lytic in the signal path. I swapped it out for a 1uF film cap and two 1uF ceramic caps in parallel and listening to it now. Will it suffer lack of bass extension with just 3uF on the input? Sounds nice to me so far. Overall sound quality seems more accurate and textures are clearer. Forgot to mention I also added a 47pF ceramic disc bypass across the feedback resistor per earlier suggestion to get rid of overshoot on square wave. I wonder if that is also changing perception of clear sound?
1uF ceramic caps that you used for AC coupling are probably made using X7R of Y5V dielectric, which is not temperature stable. This results in considerable fluctuation of capacitance and has impact on the cut off frequency and associated phase shift. If you really insist on not using electrolytic capacitor for AC coupling, use physically smallest 4.7uF film capacitor that you can find.

47pF capacitor across feedback resistor is way too much. For this amplifier only a very small capacitor in the range of 2.2pF-4.7pF can be used for lead compensation.
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Old 11th April 2016, 05:22 PM   #7012
gaborbela is offline gaborbela  Canada
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Originally Posted by Dacz View Post
thanks apex & jay.. i'll fix it soon.
-After you tested can you post the final (layout or schematic) to us
Thank you
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Old 11th April 2016, 06:34 PM   #7013
egra is offline egra  Hungary
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Hi xrk! Did you measure the VAS current in FX8 (on 47 Ohms)? Simulated value is 3,6mA only, which (theoretically) is too small current for driving Laterals' input properly, i.e. less able to charge-discharge the input capacitance of FETs. Nonetheless can not be detected any slowing of squarewaves. In VSSA I measured 13,8mA in VAS.

Meanwhile I simulated many of similar circuit (Sansui AU-317, 417, 517, Pioneer A30) and all of them have a small overshot on the top of their squarewave beyond 2 kHz. It seems impossible to remove these spikes. Another interesting observation is that these circuit don't show rounded squarewaves in higher frequencies rather trapezoids with very sharp edges. Very interesting... Sansui Au-317 an up have nearly perfectly flat squarewave at 5Hz!

All in all this is a very mysterious topology. I can't wait to arrive the ordered parts.
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Last edited by egra; 11th April 2016 at 06:53 PM.
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Old 11th April 2016, 08:41 PM   #7014
bimo is offline bimo  Indonesia
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Originally Posted by egra View Post
Hi xrk! Did you measure the VAS current in FX8 (on 47 Ohms)? Simulated value is 3,6mA only, which (theoretically) is too small current for driving Laterals' input properly, i.e. less able to charge-discharge the input capacitance of FETs. Nonetheless can not be detected any slowing of squarewaves. In VSSA I measured 13,8mA in VAS.

Meanwhile I simulated many of similar circuit (Sansui AU-317, 417, 517, Pioneer A30) and all of them have a small overshot on the top of their squarewave beyond 2 kHz. It seems impossible to remove these spikes. Another interesting observation is that these circuit don't show rounded squarewaves in higher frequencies rather trapezoids with very sharp edges. Very interesting... Sansui Au-317 an up have nearly perfectly flat squarewave at 5Hz!

All in all this is a very mysterious topology. I can't wait to arrive the ordered parts.
It is depend on the compensation. My simulation did not show an overshoot on 100kHz square wave. Very high slew rate
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Old 11th April 2016, 08:51 PM   #7015
egra is offline egra  Hungary
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It is depend on the compensation. My simulation did not show an overshoot on 100kHz square wave. Very high slew rate
Thank You bimo! It's very impressive. I also like the high SR.
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Old 11th April 2016, 11:10 PM   #7016
Jay is offline Jay  Indonesia
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Originally Posted by xrk971 View Post
That 22k (or 20k) resistor does indeed see a lot of current. It feels fairly warm almost hot. <snip> Should it be 1/2watt?
Yes, I think it should be specified 1/2 Watt or even higher. Is it not? Well, if its already done...
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Old 11th April 2016, 11:20 PM   #7017
apexaudio is offline apexaudio  Serbia
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Yes, I think it should be specified 1/2 Watt or even higher. Is it not? Well, if its already done...
44V/22k=2mA, 44Vx2mA=0,088W...
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Old 11th April 2016, 11:30 PM   #7018
Jay is offline Jay  Indonesia
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Originally Posted by xrk971 View Post
Speaking of caps - is there a reason we need a 10uF electrolytic input cap? I would rather not have a 'lytic in the signal path. I swapped it out for a 1uF film cap and two 1uF ceramic caps in parallel and listening to it now. Will it suffer lack of bass extension with just 3uF on the input? Sounds nice to me so far. Overall sound quality seems more accurate and textures are clearer.
LF cutoff can be calculated (to find the minimum capacitance), but I found that bigger capacitance ("theoretically" bigger than necessary) brings better bass or short term dynamics.

My standard is 4.7uF minimum. Polarized cap is very bad, but with bipolar (BP) I found that capacitance is not an issue. I mean, even 47uF is fine when it is a BP. And BP has less footprint than MKP (if interference is an issue). MKP might be "clearer" but also "harsh" compared to cheap BP. Good BP (such as the Black Gate N or something from Nichicon) is my preference.

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Originally Posted by xrk971 View Post
I also added a 47pF ceramic disc bypass across the feedback resistor per earlier suggestion to get rid of overshoot on square wave. I wonder if that is also changing perception of clear sound?
As mrmax said, that's too high. It should be done with oscilloscope or simulator, to remove UHF bump. You risk lowering the HF extension, which might result in "clearer sound" when the HF is exaggerated or contains HF garbage (or the tweeter is incapable of producing HF cleanly).
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Old 11th April 2016, 11:38 PM   #7019
Jay is offline Jay  Indonesia
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44V/22k=2mA, 44Vx2mA=0,088W...
Yes, I think it is not the issue with power (may be noise?), but I haven't done my HW checking the issue, but I doubt it will make any audible difference so when it is already done... But I will always use 1/2W here before I have the chance to conduct the experiment...

My FB series resistor is also 1/2W. I don't know if 1/4W will make audible difference or not. Just a piece of mind knowing that the Johnson noise is lower, beside, there is long distance from output to input transistor.
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Old 12th April 2016, 01:33 AM   #7020
xrk971 is offline xrk971  United States
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I only used 47pF on FB resistor based on what Egra said about results of Bimo's simulation. It sounds fine - but a measurement of speaker frequency response will be made.

I have 47uF non polarized electrolytic cap to try as input cap. Is that better than polarized one? You called them BP caps (bipolar ?)
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