Hi,
already have posted this in the Pass forum in the middle of some another
schematics...
The virtual grounds of the 7V rail single ended classA amps are driven by the 18V rail classB amps.
May I expect that this sounds good ?
Is this similar to some Technics ClassAA or so ?
Anything that I can do better ?
already have posted this in the Pass forum in the middle of some another
schematics...The virtual grounds of the 7V rail single ended classA amps are driven by the 18V rail classB amps.
May I expect that this sounds good ?
Is this similar to some Technics ClassAA or so ?
Anything that I can do better ?
To me, the whole reason for using class A is to avoid crossover/switching distortion. At least go with fidelity or efficiency. You have a class B stage producing distortion and an inefficient class A output amplifying that distortion? I don't get it. If anything you should make a class A drive a class B.
Okay, may be the title is a little bit wrong.
The classA amp runs @ low voltage rails and thus has low power dissipation.
But also will start clipping soon and allows only little voltage swing.
To prevent this clipping, the classB/AB amp ( outer fets ) modulates the virtual ground of the classA amp ( inner fets ).
When the signal is positive, the B amp shifts the positive rail of A amp up and shifts the negative rail of A amp down.
The waveforms of the gain fet and the current source behave/look just like normal classA amps like Aleph.
I found the basic schematic here: http://home.kimo.com.tw/skychutw/Circuits/Idea/ClassAD.html
They use another driving op amp for the B amp.
The classA amp runs @ low voltage rails and thus has low power dissipation.
But also will start clipping soon and allows only little voltage swing.
To prevent this clipping, the classB/AB amp ( outer fets ) modulates the virtual ground of the classA amp ( inner fets ).
When the signal is positive, the B amp shifts the positive rail of A amp up and shifts the negative rail of A amp down.
The waveforms of the gain fet and the current source behave/look just like normal classA amps like Aleph.
I found the basic schematic here: http://home.kimo.com.tw/skychutw/Circuits/Idea/ClassAD.html
They use another driving op amp for the B amp.
As far as I understand Technics Class AA :
The class A amplifier is low voltage but is biased to the full
ouput current. Defining this is tricky so a full complementary
stage that can enter Class B for low impedance loads is
used.
Its most straightforward to drive the Class B amplifier from the
output of the class A amplifier, but direct input is possible.
The simplest way of arranging the floating supplies is the class B
amplifier drives the floating OV point of a fully isolated transformer
secondary that provides the class A power rails.
The power supply to the class B amp providing the overall OV
reference point.
Quote :When the signal is positive, the B amp shifts the positive
rail of A amp up and shifts the negative rail of A amp down.
For Class AA :When the signal is positive, the B amp shifts the
positive rail of A amp up and shifts the negative rail of A amp UP.
Trying to move between fixed power rails is more like Class G
IMO and there are easier ways of doing than this.
/sreten.
The class A amplifier is low voltage but is biased to the full
ouput current. Defining this is tricky so a full complementary
stage that can enter Class B for low impedance loads is
used.
Its most straightforward to drive the Class B amplifier from the
output of the class A amplifier, but direct input is possible.
The simplest way of arranging the floating supplies is the class B
amplifier drives the floating OV point of a fully isolated transformer
secondary that provides the class A power rails.
The power supply to the class B amp providing the overall OV
reference point.
Quote :When the signal is positive, the B amp shifts the positive
rail of A amp up and shifts the negative rail of A amp down.
For Class AA :When the signal is positive, the B amp shifts the
positive rail of A amp up and shifts the negative rail of A amp UP.
Trying to move between fixed power rails is more like Class G
IMO and there are easier ways of doing than this.
/sreten.
Think we talk about the same...
Thats what I did.
The 7V rails are floating, their middle point is connected only to the output of the B amp.
I made a mistake with description "shifts the negative rail of A amp down", what I meant was that the voltage across the current source decreases, while the voltage across the gain fet increases, as needed to prevent the A amp from clipping.
My question is, if this is worth to built 
Does this kind of circuit have disadvantages, known to the public, like we do
build that kind of amp because 
Greetings, Bernhard
sreten said:
Thats what I did.
The 7V rails are floating, their middle point is connected only to the output of the B amp.
I made a mistake with description "shifts the negative rail of A amp down", what I meant was that the voltage across the current source decreases, while the voltage across the gain fet increases, as needed to prevent the A amp from clipping.
My question is, if this is worth to built
Does this kind of circuit have disadvantages, known to the public, like we do
build that kind of amp because Greetings, Bernhard
OK if I accept it works as advertised :
the technics class AA circuit used floating 15-0-15 V
supplies and and a fixed 50-0-50 V supply.
But your design has floating 7-0-7 V, fixed 18-0-18 V and an
additional floating 15-0-15 V supply that should be superfluous
and will require the costs of an additional transformer and
associated components ?
The technics class AA arrangement though technically sound is
expensive for the output power, no-one else could actually
(Well class A is very expensive for the output power )
use the topology and costs as well a technical difficulty go
through the roof if regulated supplies are considered.
Having said that, i reccommended a Technics class AA amp to
a friend who wanted to buy a good second-hand amplifier.
Serious piece of technology, good sounding, good price.
/sreten.
the technics class AA circuit used floating 15-0-15 V
supplies and and a fixed 50-0-50 V supply.
But your design has floating 7-0-7 V, fixed 18-0-18 V and an
additional floating 15-0-15 V supply that should be superfluous
and will require the costs of an additional transformer and
associated components ?
The technics class AA arrangement though technically sound is
expensive for the output power, no-one else could actually
(Well class A is very expensive for the output power
)use the topology and costs as well a technical difficulty go
through the roof if regulated supplies are considered.
Having said that, i reccommended a Technics class AA amp to
a friend who wanted to buy a good second-hand amplifier.
Serious piece of technology, good sounding, good price.
/sreten.
Hi, sreten
For sure the Technics was not bridged
Thats why I have lower rails.
Do you know a link to a schematic ???
My 15V rails are for the op amps only, and they are modulated too, yes, so the op amps can swing +/-17V with +/-15V rails thats magic
I do not care for component cost, but for heat...
Now I liked to know if the distortion of the classB amp will affect sound.
Below 7V 0-peak of the sine source all waveforms are absolutely clean
The ugly graphs show effect of impossible high distortion.
By the way, I did not build that yet, it is only sim everything, maybe it will oszillate like hell
I think the waveforms of the currents not depend on feedback, just the gain fet is a voltage follower and as long the supply voltage is high enough, it doesn't care.
Normal waveforms below 7V "distortion":
For sure the Technics was not bridged
Thats why I have lower rails.
Do you know a link to a schematic ???
My 15V rails are for the op amps only, and they are modulated too, yes, so the op amps can swing +/-17V with +/-15V rails thats magic
I do not care for component cost, but for heat...
Now I liked to know if the distortion of the classB amp will affect sound.
Below 7V 0-peak of the sine source all waveforms are absolutely clean The ugly graphs show effect of impossible high distortion.
By the way, I did not build that yet, it is only sim everything, maybe it will oszillate like hell
I think the waveforms of the currents not depend on feedback, just the gain fet is a voltage follower and as long the supply voltage is high enough, it doesn't care.
Normal waveforms below 7V "distortion":
I only have a very basic schematic for Class AA - similar
to your link - from Doug Self's "Self on Audio" book.
I'm not really qualified to comment on your circuit -
its simply not my kind of power amplifier - I don't like
IC's in power amps and I'm not keen on power FETs.
As far as I understand there are quite a few
op-amps that can run off 18-0-18V rails.
/sreten.
to your link - from Doug Self's "Self on Audio" book.
I'm not really qualified to comment on your circuit -
its simply not my kind of power amplifier - I don't like
IC's in power amps and I'm not keen on power FETs.
As far as I understand there are quite a few
op-amps that can run off 18-0-18V rails.
/sreten.
Sure, you gets lots of help. But you know how it is, you ask 10 diyAudio members, you get 11 opinions. Sometimes you just have to get your own feet wet.
The modulated supply will not exactly follow the signal. When it deviates, its a bit like (not completely) ripple on a normal supply. So, it is back to the class A amps PSRR to see if these anomalies on the supply get to the output as distortion. One difference with ripple is that in your case there can be hi-freq jumps and pulses on the class A supply as a result of the main supply having trouble to follow the fast envelope. Since at hf the PSRR normally is (much) worse then at 50/60Hz plus some harmonics, it may in the end sound worse because of this. To avoid this, I would put some soft-clipping in the class A to avoid that it runs up to the main supply limit with the described possible problems. And no, I don't have a ready-made soft-clip circuit. Life's a [female dog], isn't it?
Jan Didden
The modulated supply will not exactly follow the signal. When it deviates, its a bit like (not completely) ripple on a normal supply. So, it is back to the class A amps PSRR to see if these anomalies on the supply get to the output as distortion. One difference with ripple is that in your case there can be hi-freq jumps and pulses on the class A supply as a result of the main supply having trouble to follow the fast envelope. Since at hf the PSRR normally is (much) worse then at 50/60Hz plus some harmonics, it may in the end sound worse because of this. To avoid this, I would put some soft-clipping in the class A to avoid that it runs up to the main supply limit with the described possible problems. And no, I don't have a ready-made soft-clip circuit. Life's a [female dog], isn't it?
Jan Didden
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.