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-   -   about ccs of Leach Amp (http://www.diyaudio.com/forums/solid-state/69781-about-ccs-leach-amp.html)

john-china 15th December 2005 08:18 AM

about ccs of Leach Amp
 
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?

lineup 15th December 2005 09:37 AM

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 15th December 2005 10:40 AM

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.

lineup 15th December 2005 11:28 AM

2 Attachment(s)
Quote:

Originally posted by jaycee
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.
Quote:

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.

amplifierguru 16th December 2005 12:23 AM

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/showt...678#post790678

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

Cheers,
Greg

lineup 16th December 2005 12:45 AM

Quote:

Originally posted by lineup
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.

amplifierguru 16th December 2005 06:46 AM

Hi Lineup,

My experience has been that simple resistive bias with bypass capacitors gives the best audition results, THD and noise (all)compared to zeners for cascode bias.

I used this on the Magnet 300F which was rebadged and won TAS Golden Ear.


Cheers,
greg

jacco vermeulen 16th December 2005 10:22 AM

Quote:

Originally posted by lineup
I would never consider put zener diodes into where noise should be avoided.
Reliability is a second grudge for me.
Already have 6 Leach amplifier boards layed out for the Zeners, but thinking of replacing those by a string of LEDs.

AJT 16th December 2005 12:21 PM

Re: about ccs of Leach Amp
 
Quote:

Originally posted by john-china
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.

BobEllis 16th December 2005 12:39 PM

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 ;)


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