Differential and DC offset

[lumanauw]

In some writings, it is said that one of the advantage of having differential for input stage is that it can hold DC offset. It makes me think. Designs like JLH or NAD3020 power amp doesnt use differential at front end. They just use single transistor.
How do these amps hold their DC, without servo? (and if we dont use cap in the output). Will their DC wonders around when the amp gets hot?

Differential also cancels 2nd harmonic. What is the goal of the designer not to use differential. Is it to get overwheling 2nd order harmonic?

[Richard C]

The DC offset at the output of the later DC-coupled JLH is very unstable, but then it was never really designed to work without an output capacitor. I've built several of these in the past and fail to see or hear why it is still so popular.

[mikeks]

Richard is right....the class_A JLH has nothing to recommend it....if you must have class-A operation in a simple design at reduced cost, i suggest you look at Nelson Pass' early designs:

http://www.passdiy.com/pdf/classa_amp.pdf

http://www.passdiy.com/pdf/a40.pdf


and perhaps something from Rod Elliot:


http://sound.westhost.com/project3b.htm

[traderbam]

Differential also cancels 2nd harmonic. What is the goal of the designer not to use differential. Is it to get overwheling 2nd order harmonic?

I suppose it is possible to make a differential deliberately distort but this is not the conventional reason for it. Rather, a differential pair, used correctly, provides one of the most accurate ways to subtract two voltages.

[lumanauw]

Hi, Tradebam,

I suppose it is possible to make a differential deliberately distort

It is interesting. How can I use differential front end, but have big 2nd harmonic? In what part of differential should be modified?

but this is not the conventional reason for it.

So why those designers use single transistor for front end?

[millwood]

I have to say that my jlh1969 has been very stable, DC-wise. After it thermal-stablizes (in 30 seconds or so), the DC on the output cap doesn't wonder more than 60mv - on a pair of mosfets. the circuit seems to be more stable with BJT output devices.

The single transistor input is fast (faster than ltp). But it does lose some DC stability, tho I doubt it matters that much.

[lumanauw]

This is an interesting example of differential and DC offset. In this X-amp schematic, R19 and R29 forms a voltage divider more than to hold offset. I think the offset is set by the low-value R1/4 and R44/45 at the output.
In this particular schematic, can the amplifier maintain DC offset without R19 and R29?

[thanh]

The single transistor input is fast (faster than ltp).

Why? Can ltp faster than single transistor?

[thanh]

The single transistor input is fast (faster than ltp).

Why? Can ltp faster than single transistor?

[Richard C]

Originally posted by lumanauw

It is interesting. How can I use differential front end, but have big 2nd harmonic? In what part of differential should be modified?

So why those designers use single transistor for front end?

By adding a simple single-ended open-loop pre-amp to any power amplifier you can increase 2nd harmonic distortion.

The use of single transistor input stages is mostly an outdated practice and it's rare to see anything like this in a modern (post 80's design). The ltp beats it in every way without adding complexity.