MosFet Class A amp, better compensation, TMC ?

[Joachim Gerhard]

Sure, it looks weared.
The noise is a kind of humm. Yes, i think it is oszillating.

 

[Joachim Gerhard]

Dadod, is it possible that i get a pair of PCBs from you ?
I really whould like to try your design.

 

[dadod]

Dadod, is it possible that i get a pair of PCBs from you ?
I really whould like to try your design.

Joachim, you have PM.
Damir

 

[Joachim Gerhard]

Damir has send boards so i will build his design.

 

[Bangla H]

Hallo Joachim.
Hopefully we can later read more here about your successful assembled Damir Boards.

My Circlophone was shipped to you today.

Cheers Bangla.

 

[Joachim Gerhard]

Thanks

 

[rsavas]

Damir has send boards so i will build his design.

Good choice
It can be used as your reference and then compare anything else that you build or have against it.
There are little things that can add performance, fast rectifiers, snubbers, filters, wiring etc.

Good Luck

 

[knutn]

In post #9 the plus and minus input of the operational amplifier is interchanged. I think that's why it's oscillating.

It's a good idea to use a cap multiplier for the Nostalgia amp, mine have some hum without it.

 

[Joachim Gerhard]

I see that too. Linear Audio had an errata that i implemented. I will look up what i build.

 

[Joachim Gerhard]

I build this version :
https://linearaudio.net/sites/linearaudio.net/files/May 28-Figure 9 - Full Amp.pdf

 

[wahab]

I build this version :
https://linearaudio.net/sites/linearaudio.net/files/May 28-Figure 9 - Full Amp.pdf

+-50V supply voltage doesnt make sense given that the op amp is limited to +-18V, but well, i guess that such details do not matter much nowadays..

 

[Joachim Gerhard]

There are zeners before the opamps supply. Rather primitive but.,...
The output stage contributes with voltgae gain. Other wise the amp whould not work ( maybe, somewhat..).

 

[wahab]

The OS is a follower so its voltage gain is somewhat less than one, at most there will be +-18V at the gates and as much in the sources, and still, it would be the case only if there was a bootstrap, in the current configuration there can be this drive voltage only with a low current once the op amp has reached its max swing, rest will be supplied by the resistors that come from the zeners...

As such it s limited to 18V peak or so, you could reduce the supply voltage to say +-22V and reduce the two 1.5K that feed the zeners, you could also use a high voltage op amp instead and increase the zeners voltage accordingly.

 

[Joachim Gerhard]

That is not what is told in the magazine... how can that slip the " Experts ".
there was no complain
I asume, i am the only one that build it from the supplied schematic in good faith-
High End is dead, i said that many times.

 

[vzaichenko]

The OS is a follower so its voltage gain is somewhat less than one, at most there will be +-18V at the gates and as much in the sources, and still, it would be the case only if there was a bootstrap, in the current configuration there can be this drive voltage only with a low current once the op amp has reached its max swing, rest will be supplied by the resistors that come from the zeners...

As such it s limited to 18V peak or so, you could reduce the supply voltage to say +-22V and reduce the two 1.5K that feed the zeners, you could also use a high voltage op amp instead and increase the zeners voltage accordingly.

Hi Wahab,

The OS is not the follower, it's common source with gain - the sources are grounded, output signal goes through PSU. Clever arrangement

Cheers,
Valery

 

[wahab]

They believe that it s so because it use a floating ground like some QSC and Amcron amps, but still, they apparently didnt notice that those amps have adequate voltage amplification up to the rails levels since they use a pair of transistors (supplied with the full voltage) as voltage amplifiers between the op amp and the power devices, wich is not the case here.

As it work the outputs transistors are connected to the amp ground, so this latter is floating while the power supply ground is at fixed value and the speaker is connected between said amp ground and the PSU ground, given that the amp is limited to +-18V swing its ground will be floating by as much..

Hi Wahab,

The OS is not the follower, it's common source with gain - the sources are grounded, output signal goes through PSU. Clever arrangement

Cheers,
Valery

There s no gain, only detail is that the amp ground is floating but it wont float more than what is allowed by the op amp supply voltage.

Edit : An exemple of such a design :

http://www.hifiengine.com/manual_library/qsc/1200.shtml

 

[vzaichenko]

That is not what is told in the magazine... how can that slip the " Experts ".
there was no complain
I asume, i am the only one that build it from the supplied schematic in good faith-
High End is dead, i said that many times.

Good one - performance/simplicity ratio is enormous

 

[vzaichenko]

They believe that it s so because it use a floating ground like some QSC and Amcron amps, but still, they apparently didnt notice that those amps have adequate voltage amplification up to the rails levels since they use a pair of transistors (supplied with the full voltage) as voltage amplifiers between the op amp and the power devices, wich is not the case here.

As it work the outputs transistors are connected to the amp ground, so this latter is floating while the power supply ground is at fixed value and the speaker is connected between said amp ground and the PSU ground, given that the amp is limited to +-18V swing its ground will be floating by as much..



There s no gain, only detail is that the amp ground is floating but it wont float more than what is allowed by the op amp supply voltage.

If you draw it in more "visible" way or simulate it, you will see that the output MOSFETs provide voltage amplification in this arrangement. I played with this kind of circuits a lot some time ago.

See a good example, based on the same principle, attached (Japanese design). This amp would not work if the output stage would not increase the swing - the gain of input jFETs is simply not enough.

 

[vzaichenko]

They believe that it s so because it use a floating ground like some QSC and Amcron amps, but still, they apparently didnt notice that those amps have adequate voltage amplification up to the rails levels since they use a pair of transistors (supplied with the full voltage) as voltage amplifiers between the op amp and the power devices, wich is not the case here.

As it work the outputs transistors are connected to the amp ground, so this latter is floating while the power supply ground is at fixed value and the speaker is connected between said amp ground and the PSU ground, given that the amp is limited to +-18V swing its ground will be floating by as much..



There s no gain, only detail is that the amp ground is floating but it wont float more than what is allowed by the op amp supply voltage.

Edit : An exemple of such a design :

QSC 1200 - Manual - Stereo Power Amplifier - HiFi Engine

QSC is a good example, but then you contradict yourself, saying "There s no gain...".
How would you achieve 100W @ 8 ohm with +/-15V_rails OpAmp, if the output stage would not provide additional voltage gain?

 

[wahab]

If you draw it in more "visible" way or simulate it, you will see that the output MOSFETs provide voltage amplification in this arrangement. I played with this kind of circuits a lot some time ago.

See a good example, based on the same principle, attached (Japanese design). This amp would not work is the output stage would not increase the swing - the gain of input jFETs is simply not enough.

I ll simulate the thing out of curiosity, but that being said i wouldnt waste the Exicon in such a half baked design since this amount as common source configuration if it provide some voltage gain, a LME49830 driver is not that expensive and would provide much better perfs.