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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 12th November 2017, 01:12 AM   #31
keantoken is offline keantoken  United States
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The Cob of the passive side is divided by the VAS current gain whereas the VAS miller capacitance is not. Usually though, the passive side Cob is dwarfed by the Cob of the drivers so it's inconsequential (except for those odd designs where the VAS and drivers use the same transistors).
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Old 12th November 2017, 03:46 AM   #32
Ian Finch is offline Ian Finch  Australia
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Thanks for the link, Michael. It brings a lot of output stage topology issues together in a convenient read.
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Old 12th November 2017, 08:30 PM   #33
AKSA is offline AKSA  Australia
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Kean,

Could you please explain 'the passive side'?
I'm sure it's good stuff, but I cannot understand your words.

Hugh
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Old 12th November 2017, 09:07 PM   #34
keantoken is offline keantoken  United States
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I was borrowing Ultima's words. By "passive side" he means the CCS transistor in a VAS where a CCS is used instead of a bootstrap. By "cause the passive VAS transistor to counteract the voltage development on the VAS output" he means the collector capacitance of the CCS shunts the VAS output, loading it. I had to read that about 5 times myself.

He suggests that Miller effect can increase this capacitance, but I haven't seen that in practice. Usually the current gain of the CCS arrangement is very low (which is what makes it a CCS), and so there is no multiplying of the collector capacitance due to miller effect.

So what I'm saying is that the CCS capacitance loads the VAS output, whereas the VAS miller capacitance loads the VAS input. Hence, the CCS capacitance is divided by the VAS current gain whereas (from the perspective of the VAS input) the VAS miller capacitance is not.

I'm taking the perspective of the error/drive signal as each component contributes to it at the VAS input.
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Old 12th November 2017, 10:39 PM   #35
Bigun is offline Bigun  Canada
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AKSA's Lender Preamp with 40Vpp Output
Quote:
Originally Posted by xrk971 View Post
Juma, thank you for the suggested schematic.
This is a very clever schematic, I'm going to hazard a guess that the harmonic profile will be very nice.
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Old 12th November 2017, 11:51 PM   #36
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Default New measurements

I switched to a new location with less noise and redid the measurement with a quieter laptop and secondary Focusrite. I still got the same 5th and 9th harmonic peaks. Then in REW, under the "Generator" dialog, I clicked "Apply Dither", and the 5th and 9th peaks went away. I unclicked it, and they 5th and 9th peaks did not come back. So apparently, there is some glitch in the sinewave generator. Anyhow, the new measurements now are presented below and look, well, superb. Here is 4v p-p, 20v p-p, and 30v p-p (all my preamp could muster at this gain setting). But the distortion profile remains excellent and the noise floor is still flat and clean. I think that this is a superb result.

4v p-p into 25.3k ohm load:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-4vpp-25k-test-2-png

20v p-p into 25.3k ohm load:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-20vpp-25k-test-2-png

30v p-p into 25.3k ohm load:
AKSA's Lender Preamp with 40Vpp Output-lender-preamp-aksa-fft-30vpp-25k-test-2-png

Edit: on the 4vpp case I probably need to increase the gain on the audio input as the 10:1 attenuator is putting signal closer to noise floor hence THD actually went up.
Attached Images
File Type: png Lender-Preamp-Aksa-FFT-4vpp-25k-test-2.png (80.2 KB, 7897 views)
File Type: png Lender-Preamp-Aksa-FFT-20vpp-25k-test-2.png (80.3 KB, 9164 views)
File Type: png Lender-Preamp-Aksa-FFT-30vpp-25k-test-2.png (80.9 KB, 8944 views)
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Last edited by xrk971; 13th November 2017 at 12:19 AM.
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Old 13th November 2017, 12:39 AM   #37
Max Headroom is offline Max Headroom  Australia
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Quote:
Originally Posted by xrk971 View Post
.....I think that this is a superb result.
Yes, most certainly....who needs op amps ?.
What is the distortion like into low and very low impedance loads ?.

Dan.
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Old 13th November 2017, 12:45 AM   #38
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
This was designed to be a preamp - so not for low impedance loads. Otherwise it would be a headphone amp. Are you talking 1kohm or 120ohms?

Although adding a pair of power transistors either in push pull, or SE Class A could make this a low impedance amp.
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Last edited by xrk971; 13th November 2017 at 12:48 AM.
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Old 13th November 2017, 12:48 AM   #39
keantoken is offline keantoken  United States
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Yeah, there is no need to broaden the design scope that far, you would just end up with an overbuilt, and likely less optimal design.
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Old 13th November 2017, 01:03 AM   #40
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by xrk971 View Post
I think that this is a superb result.
It is very good.

I use this topology in my amplifier Emprit Amplifier.

Several friends follow the modification that suggested by Mr. Hugh Dean. Some very happy with the modification, some not and stick to the original. By the way, thank you Mr. Hugh Dean.
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