Cascoded LTP input stages

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Has anyone tryied to use it, instead of tradicional ltp, and which advantages this approach has.

I found interesting site and different and interesting input stage approches.

Input stage variants

Now in my power amp, I am using tradicional LTP, but I'm planning to try B example. But I' m concerned about CFP LTP stability.

Maybe someone has more experience :). And is it worth cascoding ltp at all. :)
 
LPT-CFP Input Stage

Hi Andrew,

Thanks for correcting me. Sorry for mistake. I'm interested in LTP-CFP input stage. Is it better than a traditional diff. input stage and why. I used only tradicional diff. input stage in my amp with constant current source and current mirrors.
 
AndrewT,
C and D are real cascodes, they could be called "cascode followers" (like John Linsley-Hood did) or "floating cascodes" : the bias voltage of the common grid is referenced to the base of the input transistor, it could also be to the emitter.
The aim is to maintain the Vcb and Vce voltage of this input transistor nearly constant, this is all the beauty of this kind of cascodes which is lost when using a fixed voltage referenced to 0V or power supply unless there is no emitter degeneration.
 
Hi AndrewT,

I must a bit more clear about emitter degeneration : in the case of a LTP pair, as shown in the schematics (which come, I think, from Peufeu's site), the input transistors without emitter degeneration suffer from voltage variations if the common grid transistor has its grid bias referenced to a fixed voltage.
An often seen cascode scheme is bipolar transistors in common base used as outputs of the cascode circuit with their base voltage referenced to the tail of the input pair.
 
more diff pair hacks in:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=64482


jcx said:
http://db.audioasylum.com/cgi/m.mpl?forum=prophead&n=3169


http://www.analog.com/UploadedFiles/Data_Sheets/524850917MAT04_d.pdf

(fig 6)


Hawksford also explored some related diff pair input linearization techniques

J4 DISTORTION CORRECTION CIRCUITS FOR AUDIO AMPLIFIERS, M.O.J. Hawksford, JAES, vol.29, no.7, 8, July/August 1981

At http://www.essex.ac.uk/ese/research/audio_lab/malcolms_publications.html

Somewhere you might also find Gilbert’s “multi-tanh” proposal

Its not clear that these techniques are necessary in audio amplifier design - high loop gain, emitter degen and bootstrapped cascode appear to be enough for super low distortion in audio amp inputs

do you really need to do better than ExtremA's input stage? (or the converse of the question - is the rest of your amp already better than this)
http://www.diyaudio.com/forums/showthread.php?s=&threadid=96853
 
CFP LTP

Hi !
If I can understand Sefl's book, CFP LTP is only good for analysing input stage. In the whole amp, CFP has no any improvement.

A good paragraph
"With the 1mA current chosen for the cfp stage the gain will be increasing with current, and this will add to the distortion from other sources of non-linearity. What we need is a device with either constant current gain at 1mA +/- 10% or even better a reducing gain with increased current so that other non-linearity can be partly cancelled. The best I found so far is the BC650 (or the higher voltage BC651) which has maximum d.c. current gain around 1mA, but unfortunately appears to no longer be available. Another alternative is the 2SC2911 which has an almost flat current gain from under 1mA up to 30mA, but this is a medium power device with lower gain"
(from http://www.angelfire.com/ab3/mjramp/ampdes1.html)

MikeB said that CFP boost gain--> compensate. Oh! I fear it
 
Hi all,

I have decided to use classical diff pair for my power amp first.
And then I will try cfp pair.

Which transistors I shoud use for the first stage? 2SA872A or 2SA970?

Has anyone tried to compare them (in listening tests)?

Froms specs. 2SA872A looks a little bit better.

May be there are some other better transistors for power amp input stage?
 
Hi Mindaugas

Here is another input arrangement for you.

As with Andrew I use bc 550 and bc 560 for the input pairs. Although on pre amps I have used LM 394 dual pair.

If it is of interest I think I have a better/up to date drawing at home.

Don
 

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forr said:
Expensive ? Where to get them ?

'Expensive' is a relative term in DIY. :D I may be able to afford a couple to play with if they are worthy. I stumbled across it in the Mouser catalogue, but I'm sure there are other outlets. I haven't searched widely. They seem to be constructed differently, with metal oxide instead of pn junction isolation. Thinking of trying the 340 as a constant power diff circuit without degeneration, or very little.

http://www.mouser.com/catalog/629/1933.pdf
 
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