Unusual amp from 1987

[egra]

"It doesn't make much sense to use single transistor input (why? to save 50 cents over op-amp?),
and then put op-amp as a servo..."

This structure gives great room for creativity, including applying singleton input. Someone prefer the sound of single tranistor over LTP and opamp, this is why I mentioned that. Two drawbacks need to be solved: low PSRR and offset fluctuations.

 

[minek123]

Q2 only for 2SA1381C, leave Q3 as MJE340

If that's the profile you like - why not..

 

[stanislav1957]

The first two transistors will work well on BF422/BF423. They should not heat up, but if the supply voltage is higher than + / -45 volts, you can put a 100-300 om resistor in the Q2 collector.

 

[metal]

If that's the profile you like - why not..

come on.. this looks OK... Honestly it is the best setup so far..

12 hours later, minek built it

 

[minek123]

Here is sim file for Lomakin/Parshin + LatFets from post #358
Very simple amp - at the cost of LatFets from Exicon.

I happen to have a stash of ECW20N20/ECW20P20 - dual die Exicon Latfets, rated at 200W each (at 25 degrees).
So I guess just 1 pair of these devices should be good for an honest 100W amp, given that proper cooling is used.

 

[stanislav1957]

With the emitter repeater, the operational amplifier will not be skewed.The output of oramp will be 0 V DC.

 

[minek123]

>With the emitter repeater, the operational amplifier will not be skewed.The output of oramp will be 0 V DC.

This is regarding which schematic?
Which transistor is an "emitter repeater" in which schematic?

 

[stanislav1957]

In your scheme, the output of oramp is skewed.To avoid this,you need to enter a repeater emitter.Highlighted in red. And an anti-clip is simply introduced

 

[OldDIY]

Then turn on the second transistor with a common base

 

[minek123]

But this is standard LMK, as built from Radio magazine.
The only change is BJTs => LatFets + few tweaks.
DC at the output of the whole amp is 0V in the one I built (post #97).
You didn't suggest anything like this back then...

 

[stanislav1957]

Then turn on the second transistor with a common base

What for?

 

[stanislav1957]

Then turn on the second transistor with a common base

But this is standard LMK, as built from Radio magazine.
The only change is BJTs => LatFets.
DC at the output of the amp is 0V in the one I built.

As wish.

 

[minek123]

Then turn on the second transistor with a common base

Then it will become Wiederhold amp

 

[minek123]

As wish.

Maxim, are you saying, LMK amp as described in Radio magazine (as in build from post #87), DOES need this emitter repeater?
If, as you say, output DC voltage on the op-amp is "skewed", what problems it is going to cause?
I don't see why this LMK amp (with latfets) would have better simulated results than Wiederhold amp?

 

[stanislav1957]

it's easy to check in the model.

 

[minek123]

What is 'correct' DC voltage at the output of the amp?
What is 'skewed' DC voltage at the output of the amp?

The one I built (LMK) has 1.1 V at the output of the opamp.

Wiederhold I built, has -1.56 V at the output of the opamp.

 

[stanislav1957]

What is 'correct' DC voltage at the output of the amp?
What is 'skewed' DC voltage at the output of the amp?

The one I built (LMK) has 1.1 V at the output of the opamp.

Wiederhold I built, has -1.56 V at the output of the opamp.

In those ancient times, the output stages oramp were mostly low-power and with very low quiescent current, so due to the skew of the ramps, the operational amplifier switched to single-stroke amplification mode.And with a modern oramp, why do you need a constant voltage at the output ?

 

[minek123]

I personally, do not need DC voltage on the output
I guess Lomakin did, or he wanted to save 1 transistor.

Are you saying it should be 0V ?
But they seem to be working fine with non-zero DC output...

 

[stanislav1957]

It will be even better. Anti-clip at least a decent make, and then in the clip at the output of the OU is a very large signal.
Here is a picture, an illustration.
This is the effect of nested OOS.
Anti-clip operation

 

[minek123]

Maxim's amp

Please check out this amp with Maxim's suggestions.
Looks better than before.