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Old 20th October 2013, 03:02 AM   #1671
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Old 20th October 2013, 03:04 AM   #1672
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Old 20th October 2013, 03:18 AM   #1673
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by bimo View Post
Acording to LC, diamond buffer on input sound worse. Of course it is not about THD, at least in simulation. I want to know why?

I modified Bonsai's schematic. (Don't laugh!). I think it is OK, using cap on input. But I do not know about DC offset stability and if this compensation is correct or not.
No comment about my schematic? Is it too stupid?
According to my LTSpice, THD at 20kHZ, 23,7Vp 8Ohm = 0.001913%

Last edited by bimo; 20th October 2013 at 03:22 AM.
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Old 20th October 2013, 03:42 AM   #1674
Bonsai is offline Bonsai  Taiwan
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Esperado and others, you have added COMPLEXITY. The circuit I posted up has six transistors and no decoupling caps for the feedback network or bias networks.

With these low component count designs, don't expect stirling distortion performance. However on bandwidth and SR they are very good.
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Old 20th October 2013, 02:11 PM   #1675
sonnya is offline sonnya  Denmark
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One suggestion.

Add a transistor pair on the R3, R5 and R4,R6 to stabelize standing current the VAS stage. BC847DS and BC857DS This will form an modified wilson current mirror.

This is only 4 transistors more, but would keep the bias circuit simple (resistor and CAP R19, C3).

Change Mosfets to LFET from Semelab. The have selfcontrolled bias with an constant vgs voltage.

Add gatestoppers too.
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Old 20th October 2013, 04:08 PM   #1676
forr is offline forr  France
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Quote:
Originally Posted by Esperado View Post
Q13 &Q14 are just voltage amplifiers with voltage gain fixed by 470 ohms in collector and 23.5 Ohm in emitters. And lower the impedance seen by the VAS for free. Exactly like in VSSA. The current part in the VAS base can be neglected (< 50µV ?)
Considering the negative branch, the hfe of Q18 (2SC3503C) is between 40 & 80.
( http://www.cordellaudio.com/book/datasheets/KSC3503.pdf )
With a quiescent current of 5.6 mA, the base current is at least 70 µA (not µV...). The input impedance at the base of Q18 should be around 350 Ohm and is not very linear. Being in parallel with the R4 (470 Ohm), the voltage gain of this transistor is not defined solely by 470/23.5.

Quote:
Getting their emitters at ground DC, to get rid of any cap in feedback, their base voltages will be at an offset fixed by their base-emitter diode..
The schematics would be more clear with the Q7 & Q13 set in the lower part and Q10 & Q14 in the upper part.

I have doubts about the DC stability of the circuit.
The input stage is nothing else than a diamond input stage circuit but with the usual resistors defining the bias in the inverting input transistors (Q13 & Q14) omitted.
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Old 20th October 2013, 04:24 PM   #1677
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Quote:
Originally Posted by Bonsai View Post
Esperado and others, you have added COMPLEXITY.
I don't see-it that way. I don't care about the complexity around the loop, like regulation of rails, CCS, or my first emitter follower, as long as they don't add any pole in the closed loop. I believe my version is still very fast and low enough HD. (<0.0006 at1kHz 50W).
I will compare with your as soon i'll get some time for it.

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Originally Posted by bimo View Post
No comment about my schematic? Is it too stupid?
No, Bimo. Just we are looking for DC coupled ideas, at this time.
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Originally Posted by Petr1951 View Post
Petr
Yes, Petr, i had understood your diamond buffer point. The pro is better DC behavior. The con is we loose some AC gain in the first transistor with the 47Ohms+css AC attenuation. so little increased distortion, variable with trans GM and sonic influence of the 47 Ohm. Are the pro > than the cons ? We should try-it in real world with our ears.

Well, adding to my last schematic a Schottky to the second input stage collector move the VAS bias point in a nice way. Enough to give margin for VAS current up to 100mA, and/or VAS emitter degeneration. Time now, to play with this and currents to optimize at the best. With the actual values, my lying cartoon designer tells me 0.000547% with 11mA in the VAS and 22 ohms of VAS degeneration.
I like-it that way, and i believe it will sound as good as the VSSA with better basses. What about thermal drift without servo ?
The pro is very healthy bandwidth and nicer little signal square waves, more stability margin with less comp. The cons is little added HD.
Attached Images
File Type: gif mc-dc-coupled-VASS.gif (36.0 KB, 226 views)
Attached Files
File Type: asc VSSA-last-no cap2.asc (10.1 KB, 17 views)
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Last edited by Esperado; 20th October 2013 at 04:52 PM.
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Old 20th October 2013, 11:10 PM   #1678
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Esperado, I'm glad you understand the work the equivalent of the current mirror transistors of different conductivity. Through diodes and resistors in the emitter of the second transistor stage is now working as a large-scale current mirror. Thermal stability is also significantly higher. With regard to the servo, the voltage drift will depend on whether in the same temperature conditions are opposite transistors circuit
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Old 21st October 2013, 06:24 AM   #1679
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Originally Posted by Petr1951 View Post
Esperado, I'm glad you understand the work the equivalent of the current mirror transistors of different conductivity.
We have a proverb for this, in French: "Connu comme le loup blanc".
In fact, i only worry about offset variations. Variations due to temp will only have an influence in VAS bias, as the OPS bias is set by a TLVH431 voltage source, very stable vs temp and laterals have a negative behavior.
Attached, a version with your suggestion:
75W HD: 0.000681%
50W HD: 0.000568% (Original version : 0.000742%)
50W 20kHz: 0.002714% (Original version, 0.001257%)
Slew-rate > 1000µs

In the same conditions, original VSSA shows 0.01%hd at 20kHz.
Attached Files
File Type: asc VSSA-last-no cap-diamond.asc (11.3 KB, 44 views)
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Last edited by Esperado; 21st October 2013 at 06:45 AM.
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Old 21st October 2013, 03:21 PM   #1680
Bonsai is offline Bonsai  Taiwan
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I'd say we have made excellent progress on this thread.

Dadod has demonstrated some very high performance designs, both low distortion, and very high slew rates and bandwidth.

I think these latest iterations of yours Esperado, also give a flavour of the excellent performance CFA's are capabe of.

Hopefully, as we move forward, this topology will start to become more mainstream.
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