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Another very fast CFA concept
Another very fast CFA concept
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Old 22nd September 2017, 09:17 PM   #11
miro1360 is offline miro1360  Europe
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thanks Paul, I post progress here ...
good to know that you was very satisfied with result from similiar stages and it was confirmed by musicians ...
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Old 23rd September 2017, 03:06 AM   #12
kct is offline kct  Canada
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Another very fast CFA concept
Hi miro1360,

well your DAC looks great. I did something very similar you can find some images of my build here. I used the TPA6120A2.

kct Gallery - My Photo Gallery

Well, I was bored today and messed around with my sim, this is what I am getting. Altho the amp easily performs well into the 150kHz range I am not overly impressed with THD. I don't have the proper spice models for the driver and end stage transistors so I used something generic.
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File Type: jpg VFA_4_Rev.8.jpg (348.3 KB, 740 views)
File Type: jpg VFA_3.jpg (482.9 KB, 731 views)
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Old 23rd September 2017, 04:07 AM   #13
miro1360 is offline miro1360  Europe
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I saw your builds, they are nice work!

this tuning looks nice, dont give up


the fun is, that THD (11Vp/8R/10kHz) just only with resistors in place of CCS looks like this:
https://ctrlv.cz/shots/2017/09/23/98N5.png
https://ctrlv.cz/shots/2017/09/23/9CIF.png


and THD (22Vp/8R/10kHz) looks like this:
https://ctrlv.cz/shots/2017/09/23/XRuV.png


... maybe you can create own models in multisim (just take spice info from .asc file which I posted few post above) ...
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Old 26th September 2017, 02:31 PM   #14
miro1360 is offline miro1360  Europe
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Default Differential input

possible differential input for this CFA
50W/8R, modification for SMD input transistors (BC856/46)
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File Type: jpg cfa_50w_smd_diff_miro1360.jpg (50.2 KB, 636 views)
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File Type: asc TC1_01_HV_diff.asc (12.2 KB, 35 views)
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Old 27th September 2017, 03:19 AM   #15
kct is offline kct  Canada
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Another very fast CFA concept
Hi miro,

well, I messed around some more and achieved a slight improvement with THD.
But my simulation result are not nearly as good as what you get.
I guess my transistor models are perhaps not good enough.
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File Type: jpg VFA_4_Rev.9.jpg (382.7 KB, 620 views)
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Old 27th September 2017, 05:34 AM   #16
sajti is offline sajti  Hungary
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Why R28-31 are so high value? That stage is not a real current mirror using such high values. One tenth of them would be better.

Sajti
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Old 27th September 2017, 07:03 AM   #17
HI-FI PRO is offline HI-FI PRO  China
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Quote:
Originally Posted by miro1360 View Post
possible differential input for this CFA
50W/8R, modification for SMD input transistors (BC856/46)
This means your input impedance approx 2.2kohms, too low.
Impedance should be at least 22k. Peferablly 68k-100k

EDIT: You'd also want to change gain of tarnsistors, and current and mismatch the amp on purpourse in circuit simulator.
If the operating point changes allot with small gain changes or current changes. That indicates that your amp may not work when partically built

I had considered building this circuit, hower this circuit seems to have a low impeance and has the input on emitter. Therfore I gave up on the idea in building this circuit.
I also considered using its vas stage. But I don't like current mirrors in amps.

at q7 q8. you can chagne the diodes to a vbe multiplier or install any type of current gain stage you want. It would work. If the gain stage operates properly
Presonally I can think of a very practicall way of adjusting speaker dc voltage to zero. without using your complicated method having a+-1v at the nfb part. However I do not want to give it away, I do not want to see any one compete with the amp I sell If I ever do sell one.

Last edited by HI-FI PRO; 27th September 2017 at 07:26 AM.
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Old 27th September 2017, 09:38 PM   #18
HI-FI PRO is offline HI-FI PRO  China
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Quote:
Originally Posted by miro1360 View Post
possible differential input for this CFA
50W/8R, modification for SMD input transistors (BC856/46)
Guys if you want my method for dc voltage adjustment. make r16 r14 adjustible with pot.
Or use a adjustible constant current source in place of r16 r14
The orignal circuit is designed for high frequency, I do not see it being hifi at all.

Last edited by HI-FI PRO; 27th September 2017 at 09:40 PM.
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Old 28th September 2017, 01:17 AM   #19
kct is offline kct  Canada
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Another very fast CFA concept
Hi Sajti,

well it seem to give better THD with higher values.
I will try with lower values again and see if this gets better yet.

Quote:
Originally Posted by sajti View Post
Why R28-31 are so high value? That stage is not a real current mirror using such high values. One tenth of them would be better.

Sajti
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Old 28th September 2017, 03:40 AM   #20
HI-FI PRO is offline HI-FI PRO  China
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Quote:
Originally Posted by kct View Post
Hi Sajti,

well it seem to give better THD with higher values.
I will try with lower values again and see if this gets better yet.
This should be becasue of more emitter feedback, emitter resisance larger means the gain changes less when transistor internal impedance changes due to change in transistor emitter current.

Increaseing bias current has a similar effect. internal resistance is 25mv/emitter current.
If current higher impedance internal small, changes in internal resistance caused by change in current is less
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