英国谷之声NVA AP30前后级功放

millwood

the folded cascode is a topology widely used in high speed voltage feedback opamps. it is a VAS using current transmission but outputs a voltage signal.

in this particular case, the front end is a standard differential pair, followed by a folded cascode VAS - notice the two pnp transistors on top driving into a folded wilson current mirror? the output stage is a typical class B complimentary ops - ths4012 used a diamond buffer.

this will be a very fast amp, thus the designer used a few capacitors to slow it down.

some key performance stats for ths4012:

290mhz bandwidth, 310v/us slew rate, -80db ths+n (at 1mhz!), 110ma output current (into a 20ohm load - it will drive your headphone to ear bleeding levels).

hjshan0001

原帖由 最爱孙悟空 于 2008-11-4 14:38 发表
不错的DD，设计师是典型的线材无用论者，高价喇叭线接上搞不好会烧鸡！ 至于飞线，有的人是学大师的噱头乱飞，和有技术含量的区别！

真是典型的断章取义，人家只是说不适合接高电容量的线材而已，这是因为电路设计的原因。设计师所说的机壳和螺丝钉影响音质，这不更玄乎吗，你又如何理解？

millwood

here is the simplest folded cascode opamp.

1) when a positive input signal goes to Q1's base, Vin goes up. this opens up Q1 and forces its collector current to go up and its collector voltage to go down. and the opposite happens to Q2.
2) since Q3/Q4's base is fixed by V5, the changes in their emitter potentials cause their emitter current to change: I3e goes down, and I4e goes up.
3) however, Q3 and Q4 drive into a current mirror fomed by Q7/Q8 which is trying to hold I3 and I4 constant. as such, Q5 and Q6's base voltage goes up, and this causes a positive voltage on the output.
4) that positive voltage is then divided by the feedback network of R12/R11 to Q2's base, counter-acting out the positive voltage at Q1's base.

the key here is the VAS (Q3/Q4 and Q7/Q8). They act on a current input signal (I3e/I4e) and output a voltage signal (V5b/V6b). this is the foundamental reason that a folded cascode amp is so fast.

Potential improvement:
1) replacing R3/R4 with a constant current source.
2) fold Q7/Q8 current mirror to improve gain.
3) replacing R2 with a constant current source.
4) double the output stage for better current gain.
5) use diamond buffer as the output stage.

But this should get you started.

millwood

the performance of the above amp is about -80db ths.

the following, replacing r3/r4 with a ccs can improve that by another -5db.

d1/d2 can be replaced with a led of your choice, and you just need to recalculate the resistor values for r3/r4 to make sure that the desired current is achieved.

millwood

here is a more buildable version, with a diamond buffer as the output stage.

obviously, with a DB output stage, you don't need the Vgs generator (V4).

I have also replaced V5 with a r/c filter. when you do that, R5 can be replaced with a diode (1n4148).

this particular amp has a flat frequency response well beyond 1 mhz.

millwood

here is the step response for the circuit in post #62.

the stimulus is in green, and the output in blue. the amp's slew rate is about 12v/0.1u = 120v/us! and probably faster if I could run a faster stimulus to it.

either way, this is one fast amp.

millwood

for those of you liking numbers:

for the amp in #62:

1) open loop frequency response: to 100Khz and beyond. excellent for any amp.
2) open loop gain: 1.8vp output for 1mvp input, or 1800x. again, excellent. in the chart, the blue trace is the output, and the green trace is the 1mv input.

this would be a great preamp or a headphone amp, with little modification.

it can also be a great power amp with an upgraded output stage.

abcd

咋写那么多E文呢？谁能看懂翻译下。

syhkiss

我懒~~支持抄板

最爱孙悟空

原帖由 hjshan0001 于 2008-11-9 22:35 发表

真是典型的断章取义，人家只是说不适合接高电容量的线材而已，这是因为电路设计的原因。设计师所说的机壳和螺丝钉影响音质，这不更玄乎吗，你又如何理解？

你可以直接说不同意设计师的观点就行，没有必要和我这种人较劲！

黄军

谁能说说他用的是什么电容，好像很少见哟

ARGO

谁有清晰的大图啊?

fengxi

这台机真是奇妙，有些形容怎么听着像广告词，什么，声音比不上你做的小甲？那是你的线材问题，没有使用24k纯金线，并且至少6平方。中低音不好？怪你不会听，请脱掉眼镜避免油垢。
不好意思，说了一通胡言。
在机器没脱衣服之前很有好感，但是看了裸体之后却感觉南博了。

发烧龙

顶起来，＝人抄板。

aspll

上面是什么机器?漂亮

情轩西雅士

NVA,,,我喜欢!!!没钱买只有帮顶的份了!希望有机会与NAIM一较高低,大家就不要再争论眼见为虚的事了!一切让声音作主吧!

rainer

家电的老兄也来了

红楼无梦

原帖由 发烧龙 于 2008-11-12 20:34 发表
583554顶起来，＝人抄板。

欣赏这个机器，电源占了一半的面积。信号输入和输出都很紧凑。比第1楼那台机好多了。

marcotamkt

超喜欢!!!!!!!!...........
晶体管也搭棚!! 跟我的理念一样!!!!!!!!!!!!!