Input stage idea - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th July 2005, 01:35 AM   #11
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> it resembles a differential, but it is not.

Sure it is.

Because it drags an awful lot of transistors from its mid-point, it may work best with one input grounded. But the other input clearly senses difference-from-ground.

> In a differential, total current (I1+I2) will always be the same.

Not true. A fixed-current LTP is very popular, because it gives a good approximation of perfection at low cost, but this is NOT the only way.

Ponder this:
Click the image to open in full size.

It wants to be AC-coupled (it has an input offset voltage of several volts). You can take output at either collector. You can (maybe should) ground an unused collector. The output voltage will be a function of the difference between the two input bases. If that isn't differenial, what is it?

BTW, I found this in a 1967 book and it was not new then.

Here is a quarter-baked scheme as a "balanced" mike input preamp:
Click the image to open in full size.

An even simpler differential input is the old single input transistor, source to base, feedback to emitter. It isn't balanced or low-offset, but it sure is differential.
  Reply With Quote
Old 26th July 2005, 02:11 AM   #12
Mr Evil is offline Mr Evil  United Kingdom
diyAudio Member
 
Join Date: Aug 2004
Location: Behind you
It looks very similar to the "long tailed pair" seen at the bottom of this page. The difference is that an NPN + diode-connected-transistor are added to the inverting input in your version.
__________________
https://mrevil.asvachin.eu/
  Reply With Quote
Old 26th July 2005, 05:34 AM   #13
Tom2 is offline Tom2  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Central CA
Quote:
lumanauw said,
There seems a thing that bothers me about this CCT, that is the high-order harmonics. The harmonics below 10khz is OK, but why it also generates harmonics in frequency over 10khz, and quite big too?
I'm guessing they are artifacts of the fft and the method I used.

Tom
  Reply With Quote
Old 26th July 2005, 10:14 AM   #14
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
Hi PRR
Hi Mr. Evil

This circuit is very interesting. I was considering using stuff like this a little time ago and came to conclusion, that this is not that good, because of voltage/current characteristics. A single transistor is well described by Eberse-Moll model assuming exponential current as a function of voltage. Now take a vintage LTP, the current is (more or less or actually) constant, one transistor is turning-on, secong off. Much distortion is cancelled. In this strange circuit nothing like distortion cancellation can ever happen, it adds distortions of both transistors.
Agreed?

best regards
  Reply With Quote
Old 27th July 2005, 04:28 PM   #15
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Quote:
In this strange circuit nothing like distortion cancellation can ever happen, it adds distortions of both transistors.
You see the point It is designed so it does not cancel harmonic, but leave or add harmonic (in controlable way). I always wonder what an amp will sound if has a property of this.

You must see X600 or X1000 from Passlabs. He doesn't use differential, but 2 common emitors connected a with resistor.
  Reply With Quote
Old 27th July 2005, 05:27 PM   #16
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Hi, PRR,

Long time no see

How careless of me. Off course !. Even a single common emitor sees a differential between its base voltage and emitor voltage.

Hi Darkfenriz,

There is power amp(s) that leave the harmonic by using common emitor transistor. NP's Zen and F2 is good example (F1 is differential, although). He keep digging this, there must be something different in the sound

Do you have any idea, why JLH (the famous classA one) uses singleton input stage, not differential input? Is it possible that he try to advoid the differential VBE canceling effect?
  Reply With Quote
Old 28th July 2005, 06:28 AM   #17
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> A single transistor is well described by Ebers-Moll model assuming exponential current as a function of voltage. Now take a vintage LTP, the current is (more or less or actually) constant, one transistor is turning-on, secong off. Much distortion is cancelled.

Distortion NEVER cancels completely. And the scum that remains is often more offensive than simple honest curvature.

In another world, people rave about WE-300B firebottles with no cathode resistor and no negative feedback (other than a triode's internal field effects). The THD number is huge, but almost all simple curvature, nearly no 9th or 13th harmonic such as comes out of high-feedback "distortion-cancelled" amplifiers.

Note that my plan as shown (no overall feedback) is not at all practical without that emitter resistor. That reduces distortion a lot. Interestingly, for the intended use (various input levels all trimmed to a standard output level) the distortion does not change much with gain: it depends on output level, not so much input level. I can get lower THD numbers, sure. Will it sound better? I don't know, I'm no golden-ear. The specific point was to get a "balanced" (differential) input withOUT canceling the distortion too much.

And with the NPN/PNP pairs, a little emitter resistance reduces distortion a lot. (But the real point was that in a mike-amp, this emitter resistance adds to input noise, so must be "small".)

Back to the LTP. The "distortion cancels" only for VERY small input voltages. Even 10mV of input differential will be sour. So you have to follow it with a lot of gain and wrap the whole thing in feedback. Feedback re-injects output for additional intermodulation distortion. If you keep the input levels down in the low-distortion range, there is no big difference between the LTP and a single-ended and that wacky double-single-ended plan. A lot of factors of two one way and another: the SE has twice the naked gain of the LTP so if you are doing feedback you get more feedback for your money (actually less money).

lumanauw has some points about "good" designers clinging to simple SE plans.
  Reply With Quote
Old 28th July 2005, 11:04 AM   #18
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
Hi PRR
Your point is absolutely right
but if one is looking for low distortion, the best thing to do is to use self-linearising, distortion-cancelling circuits. Of course this will cancel lower order harmonics by much higher amount than dreaded non-musical 7th or 11th or..., but it is still better idea than using feedback.
That's philosophically a bit like comparison beetwen SE and P-P.
P-P naturally cancels huge amount of even distortion, leaving odd harmonics, which one may like or not. But if I'd like to have <0.1% THD, I would go for low feedback P-P, not high feedback SE.
regards
  Reply With Quote
Old 28th July 2005, 02:17 PM   #19
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Hi, PRR,

Quote:
If you keep the input levels down in the low-distortion range, there is no big difference between the LTP and a single-ended and that wacky double-single-ended plan.
How low is that?
  Reply With Quote
Old 28th July 2005, 09:09 PM   #20
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
By connecting two transistors by their emitter, you can make a differential pair.

If they are of the same polarity, and the emitters see a (preferably high) resistance or a constant current source, you've got a long tail pair : it's a parallel differential pair.

If the transistors are of opposite polarity, you've got a series differential pair. One gets rid of he constant current source. I think it's sometimes called a Rush amplifier. It is interesting because it has gain without inverting.

In his famous paper in Wireless World 1977, Taylor gives the following distorsion figures :
1% for each mV at the input of a single bipolar
and thirty times less for a parallel differential pair with a 5% current mismatch.

However an overlooked problem with a differentail pair is that its distorsion components, albeit low, are depending on its DC equilibrum which continuously change in an amlifier to correct the output offset.
Is that audible ?

The distorsion components of a single transistor probably behave in a more constant manner. And amplifiers using a single active device at the input have a more simple HF behaviour.

Some people think it would be a good idea to go back to the capacitively coupled output of input stage of amplifiers (as used in tube amps) to avoid its depedance of the DC condition of the whole amplifier.


~~~~~~~ Forr

  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Adding a third input to a differential input stage? maudio Solid State 15 11th October 2006 02:16 AM
current bias j-fet input 2sk170 ,2sj74 at input stage YUTK Solid State 11 2nd June 2005 03:34 AM
Input stage idea DarkOne Solid State 20 19th January 2004 07:11 AM


New To Site? Need Help?

All times are GMT. The time now is 03:39 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2