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PostPosted: 26 Oct 2018, 00:45 
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Joined: 23 Feb 2017, 02:02
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I'm not supprised because high gain means higher gain at high frequency without the use of cascoding.

Its configured as non inverting op amp.

The 4 stage pre amp is also running on +-12v

192v/uS with 12au7


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PostPosted: 26 Oct 2018, 01:17 
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Joined: 23 Feb 2017, 02:02
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Thing with higher gain op amps is that the NFB corrects most errors including poor PCB layout.
NFB fix most errors but not all errors that will hamper sound quailty.

More NFB more open loop frequency response


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PostPosted: 26 Oct 2018, 02:54 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
It's hard to talk without knowing what you are doing. No, NFB is not going to fix pcb layout error. At high frequency, oscillation is the main issue, then cross talk. War is won or lost on the pcb layout. Been doing this for years to say this. That's why I layout my own pcb, layout pcb is 40% or more of the design.

Actually the NFB is only used for final touch only.


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PostPosted: 26 Oct 2018, 03:53 
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Joined: 23 Feb 2017, 02:02
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All that is massive slew rate is by removing the huge bottle neck the slow output transistors.
Virtually same schematic as my slow head amp but with one of the stages modified for valve gain to get hifi sound.

This shows the 4 voltage gain stage head amp using same transistors as the pre amp bc547b, has the capability of being wide gain bandwith for low distortion and the ksd882 are fake transistors with very low transistion frequency


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PostPosted: 27 Oct 2018, 12:25 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
I don't think you can conclude that. Big transistor has higher input capacitance, it depends on your driver stage. As I said, you keep talking, but no schematic. From my understanding, you don't use emitter follower as output. If you use common emitter output stage, larger transistor means larger miller caps and load the driver a lot more. If you are using common emitter, that's bad idea as a starter. Nobody use common emitter output stage other than nelson Pass using common source. It's just very inferior.

But, it's your amp, just get some other transistors.


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PostPosted: 27 Oct 2018, 12:42 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
You have to look at other parameters other than fT. For me, there are faster power transistor by Sanken, they are 60MHz. Problem is Cob is very high, that tells me their Ccb and Ceb are high. That will load the driver stage and add capacitance at the output. I decided to use TTC5200 and TTA1943 for it's very low capacitance.

It's not that simple that you change to a TO-92 and see it's faster and claim the KSD is fake. Small signal transistor has much lower capacitance and make everything a lot easier. You have to actually test the fT of the transistor with the specified condition to tell.

Again, no schematic, not much to talk.


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PostPosted: 04 Nov 2018, 15:30 
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Joined: 23 Feb 2017, 02:02
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Theres no way a KSD882 can have higher capacitance and lower transition frequency than a tip41c.
Geniune KSD882 is TO-126 smaller and faster than TIP41C TO-220


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PostPosted: 05 Nov 2018, 15:46 
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Location: Bayarea
Don't know, you have to test it the right way, not putting into the amp and judge.


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PostPosted: 10 Nov 2018, 18:36 
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I've been moding a sound luster latley, before mods this is what is looks like starts clipping at arround 24.5v pk-pk into 8r
Sound qualitywise before mods is same as ma9s2 clone at medium output levels


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PostPosted: 10 Nov 2018, 22:58 
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Joined: 23 Feb 2017, 02:02
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Heres the thing modded to absolute limits just to show you how much distortion a single ended valve amp has.
The pk-pk V swing has been increaced by 2v after modding and open loop distoriton greatly reduced and has NFB to reduce gain from 21x to 12.5x.

I really like the valvy electoron sound after the mods worth listening as orginal one just all over the place and not worh listening at all.

The one also very poor opt where the high inductance causes 20khz voltage to be only 40% of 1khz when no feedback is applied, after nfb the voltage about 80% of 1khz

Fairly clean up untill 18v pk-pk at 10khz very little visible distortion and below that distriton is hard to see.


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