JLH1969 MOSFET Class A/AB amp

DarkHorse

原帖由 millwood 于 2008-4-23 19:59 发表


that may happen if you have cross-over distortion - the idle current is too small and the amp works in Class C.

and most amps have higher distortion at very low input signal.



that i ...

thanks for your reply!
I have build one channel amp like ALEX,replaced with diffent CCS constrution,speaker is 3" SONY full-range unit,it sounds very sweet,but only short of the bass,I don't know the reason at that time...and I also think that is the amp's problem,now I know,3" SONY unit is no mach good to proceed such suitable bass to our ears...my mistake.
that is to say,ALEX's MOSfet amp,which circuit he designed for his dougher is suceed,and ALEX is the CREEK AUDIO No.1 Designer at that time.
all above, X.G. & I knew about it---the thing happened in DIYAUDIO web.

millwood

not sure why the bass could be bad, darkhorse.

if you can post your schematic, we can go over it.

htwlhu

小弟英语太差了，哪位大哥帮忙翻译一下楼住的话，谢谢了

qjdj

楼主的大意是建议你如果用5200,推荐33v单电源供电.
1815的功率太小了,不推荐用在第二级,第二级推荐用中功率管,比如d669.

htwlhu

我买的是成品板，现在手头上也没管子可以换，在不换管子的情况下，双33V的变压器可以用不？我不想拆变压器

millwood

原帖由 htwlhu 于 2008-5-2 01:31 发表
在不换管子的情况下，双33V的变压器可以用不？我不想拆变压器

sure, just use one winding.

qjdj

翻译"可以,只用其中一个绕组"
可能末级发热会比较大,注意散热.

htwlhu

谢谢，就是说没问题了，我一个声道用一个绕组，两声道独立供电，散热方面我加装风扇。静态调小

millwood

原帖由 htwlhu 于 2008-5-2 10:59 发表
静态调小

the BJT version (the original jlh1969) has to work in Class A with large idle current, because it is current coupled.

the mosfet version can work in either Class A or Class AB, with smaller idle current, because it is voltage coupled.

while the two versions look very much the same, the topology is entirely different.

DarkHorse

原帖由 millwood 于 2008-5-1 21:12 发表
not sure why the bass could be bad, darkhorse.

if you can post your schematic, we can go over it.

I have research this JLH MOS VERSION before.you can found the schematic below(#61):
http://bbs.hifidiy.net/viewthrea ... p;extra=&page=4

only for your reffence!!!

millwood

darkhorse:

the two amps are quite similar but works quite differently, all due to the inclusion of R10. the inclusion of R10 requires a lot more current go through the CCS (Q4), as it runs more closely to the original jlh1969 BJT.

issues I see for your amp:

a) too much gain: 4.7k / 120ohm = 39x. I would set R5 to 1k.
b) two things are determining the idle current in the phase splitter: the CCS load (Q4) and the Vgs on the mosfet. you will need to relax one of them. I would replace the CCS with a resistor or a bootstrap network (aka JLH1969 style).
c) R2, R3 and RP1 are too big. shrink them by 5 - 10x.
d) R10 and R9 are too big. try 220ohm - 330ohm for faster response from the mosfet.
e) Q2 should be a medium power mosfet, idling at 30ma or so.
f) C5 usually isn't required, unless stability becomes an issue.

after that, the amp should work well.

DarkHorse

millwood,you are hight hand!
ho-ho-ho!
now I know Q4(2SK170BL) choosing isn't right,I have test it's curve of Uds/Ids,too many difference between the low Uds(2V) & Hight Uds(40V) .2SK373 Vgs is ok but it's low power can not fit the ciruit topology.
as a new born bird,I also doing wrong with the CD's output signal value - max Valtage(Peak-peak),so the gain I seted was too much.now I think 10X~15X of gain perhapes more suitable for our daily-using,do you think so?
I accept all of your advise uppon,your circuit analysis is correct and clear,good job!

millwood

darkhorse: you can run at higher gain.

the issue with your particular topology is that the addition of R10 means that your phase splitter is current-coupled with your output devices, aka the original JLH1969 BJT version, in spite of the fact that you are using mosfets as output devices.

for that to work, you will have to idle the phase splitter at 2x of the current for a voltage-coupled topology (aka my design).

while mosfets are voltage controlled devices requiring very little current in either on or off state, changing their states from on to off or off to on can require substantial current. for IRF540 class mosfet (2000pf - 3000pf Ciss), that current at the top end of the audio frequency is typically 1 - 3 ma. as a rule of thumb, you will need to idle the drivers (aka the phase splitter here) at 10x of that current. Thus, the phase splitter will need to idle at about 10 - 30ma for the voltage coupled design, and for your design, it will need to idle at 20 - 60ma.

that means

a) the CCS has to deliver 20 - 60ma;
b) R10 / R9 should be between 3.5v/10-30ma = 110ohm - 330ohm, thus my recommendation.

If you chose a resistor too big (like 1k in your design), you will NOT have enough current to drive the mosfets, bad for frequency response and for higher gain, or thd performance.

your design is foundamentally sound. you just need to right value to make it sound beautiful.

yp100yp

支持外国朋友来玩

htwlhu

求救，我的HOOD 1969每次开机的时候输出端都有直流，大约20秒后正常，并且噪音很大，但在输入电容两断串一个电阻噪音消失，但我感觉声音没以前后，并且音量也小了，有人说是管子配对问题，请高手指点一下，万分感谢

qjdj

你具体使用的那个电路？
最好注明当下你所用元件的参数。
第二级i管子可能需要一个小pf电容防自激。

htwlhu

我在输出电容后对地接了个小电容，噪音问题基本没了，不知道这样对音质有什么影响没有。影响多大，这个电容需要不需要好的电容，但开机有直流还是依旧，顺便问下怎么上传电路图呢

millwood

the capacitor to the ground essentially formed a low-pass filter with the output impedance of the amp.

most amps would go crazy when you do that. the JLH, being an extremely fast amp, can still remain stable, thanks to its speed and wide open loop frequency response.

I would try to find the root cause of the noise / DC offset though.

qjdj

这个电路有输出电容，怎么会输出有直流呢？“htwlhu”朋友是指开机瞬间对输出电容充电的现象吗？
上传图片可以看论坛页面右上脚的   帮助  。

htwlhu

电路图在这里，大家帮忙看下，现在在输入电容对地接了一个电容，噪音很小了，但开机的时候还是有直流输出，大概20秒后正常。我的管子用的是2SC5200 一般声道2个，小管是AC970和C1875