about ccs of Leach Amp

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I would prefer using LED(s) to set the voltage for a CCS transistor.
Like the arrangement you mention.

What I know, LED produces less noise than zenerdiode.
There are also other factors like temperature stability to consider in some applications.

Generally we see zeners used more frequently as current sources in older amplifers.

A third method is to use JFET and a resistor.
This is the most simple way to make a CCS, constant current source.

Another method often used is two bipolar transistors.
One transistor holds constant voltage across the emitter resistor of the other, while limiting base current.
This will give a CCS of Vbe/Re.
 
jaycee said:
He is using the current through a resistor as the current source, with the zener providing a stable voltage reference so that the current is constant. His page explains why he did not use "active" current sources.
From Leach website:

Diodes D13 through D16 are 20 V Zener diodes which regulate the voltages that set the bias currents in the diff amps. Each zener diode is biased at a current of about 3.3 mA. Although a single zener diode could be used in place of two series diodes, I have found that the error tolerance in the voltage is less with the series connection of 2 diodes. C2 through C5 are ac decoupling capacitors which ensure the bases of Q5 and Q6 are at ac signal ground.

R15 and R16 set the tail bias current in each diff amp to about 3.25 mA. This current and resistors R7 through R10 set the differential tansconductance gain of the diff amps at about 1.6 mA/V.

- The diff amps use resistive tail current bias circuits.
- Not only do these generate less noise than active current sources,
but they provide a smooth amplifier turn-on that is free of thumps.

He has thought about alternatives but found this to best solution
mainly for 2 reasons.
 

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Hi lineup,

Sorry, I find Leach's explanation, or excuse, for his simple cascode rather baseless.

2 BJT current sources using low noise small signal BJTs are not noisy. Zeners are.

My SKA and 300W both use 2BJT CCS's because of the superior PSRR afforded by them. Any they have NO turn on/off thump.

More of a throwaway explanation that doesn't survive scrutiny.

I tried both alternatives on my 300W amplifier (always keeping an open mind) and the sim PSRR result is here -

http://www.diyaudio.com/forums/showthread.php?postid=790678#post790678

He has included an RC filter in the early stage supply lines to improve things.

Cheers,
Greg
 
lineup said:
I would prefer using LED(s) to set the voltage for a CCS transistor.
Like the arrangement you mention.

What I know, LED produces less noise than zenerdiode.
There are also other factors like temperature stability to consider in some applications.


Generally we see zeners used more frequently as current sources in older amplifers.

A third method is to use JFET and a resistor.
This is the most simple way to make a CCS, constant current source.

Another method often used is two bipolar transistors.
One transistor holds constant voltage across the emitter resistor of the other, while limiting base current.
This will give a CCS of Vbe/Re.

Yes, amplifierguru
this is what I have learned, too.

I am always surprised to see zeners in audio amplifiers.
I have seen Nelson Pass using zeners in some earlier amps.
Do not know if he still.

I would never consider put zener diodes into where noise should be avoided.
 
john-china said:
Leach Amp v4.5 uses two zener and some resistors as tail of differential input circuit .

i take the tail as ccs , in normal amplifier , tail ( ccs) is consisted of a transisitor and diode or LED


so the question is which one is better?

imho it is more of convience than anything, imagine stringing what? 20 led's in series to get 40 volts! so a zener in this case is more practical.

where a transitor is used with a current source or sink, an led is more practical imho.
 
Earlier versions did not have the zeners. IIRC, they were added at Ver 4.4 along with some other value tweaks along the way to the current version. The amp will run fine without them, and seems to fit Guru's preference for resistive biasing of cascodes.

If you eliminate the zeners R13/14 change, since they no longer have to pass the zener current. From the Leach page I saved a while ago: R13 = R14 = (V - 38.2)/5.42, where V is the power supply voltage.

Sorry Jacco, this gives you another option to try ;)
 
acenovelty said:
Any chance that this subject is about "noise" only birds can hear?
Once again, bashing the Professor is still not a good idea.
But then, please carry on. It's yur amp.

Regards

No, bashing a professor in electronics about an amplifier detail in one of his designs
is something you normally should try to avoid :D

In this case I think he did smash 2 flies in one smash.
The zener diodes serves 2 purposes in a good way:
- provide a constant current to input stage
- setting operation voltage for cascoding transistors in input stage

There are more sofisticated and maybe better ways to do these both things,
but no other such simple way, as Leach is using.
 
BobEllis said:
Earlier versions did not have the zeners. IIRC, they were added at Ver 4.4 along with some other value tweaks along the way to the current version. The amp will run fine without them, and seems to fit Guru's preference for resistive biasing of cascodes.


I recall that he used the zeners in Ver. 4.2, dropped them in the 4.3, and then brought them back again. Never was able to find out what led to these changes.
 
Hi,

Leach Low TIM 1976 and on have Zener referenced tail current resistor with capacitor bypass.
Version II combined the cascode with the tail resistor from the common Zener reference.
Version 4 introduced the dual series Zeners on each half.
Version 4.5 still follows this.

Can you post V4.2 & V4.3 schematics showing the changes you refer to?
 
Hi Jacco & others,
the zener reference can be eliminated and replaced with a pure resistor string or LED string BUT the reference voltage does not have to remain at +-40V.
You could reduce down to a 2, 3 or 4 LED string or equivalent resistor string ( about 4V to 7V).
The CB capacitance of the input transistors would rise slightly at this lower voltage.
Q. Is CB capacitance an issue?
Q. Is more benefit gained by reducing the CE voltage across the input transistors?
 
You're precious Ace,

Some of us couldn't wait to get out of acadaemia and into the real world designing amplifiers for a living, rather than dabbling on a state wage.

Not to demean Mr Leach but if I went for medical treatment I would rather a 'hands on' experienced clinician than an acadaemic.

I hung my career on my ability to design amplifiers. Still do.


Cheers,
Greg
 
Not to demean Mr Leach but if I went for medical treatment I would rather a 'hands on' experienced clinician than an acadaemic.

afaik, these leach amps were used by prof. leach in teaching his classes and from the late 70's there could have been hundreds of students of his who have built these amps, and a countless hundreds around the world, all under his guidance and supervision! myself have exchanged emails with him, very accomodating and unselfish guy. not driven by how many amps he can sell, just a plain desire to share.!

if this is not "hands on" i do not know what is!

how many of his students that learned from him could have gone on designing their own amps.:D

there are many who can build great sounding amps from others' designs, but ask them how the thing works and they do not know a thing!

theory and practice always go together! otherwise there will be blind leading blinds, and lames trying to boogie with fellow lames!



;)
 
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