...OK we all seem to agree that the high frequency PSRR of the class A output stage is important to suppress the distorsions from the class B part.
And of course it is a combination of the parasitic capacitances and the transistor transconductance. But transconductance might be poorer than many people expect. In my Rookie amp examinations especially most PNPs did not really show great properties, - already in DC examination.... together with capacitances...
That's why I was considering to adjust AB instead of B. Your heat sinks could probably handle that.
And of course it is a combination of the parasitic capacitances and the transistor transconductance. But transconductance might be poorer than many people expect. In my Rookie amp examinations especially most PNPs did not really show great properties, - already in DC examination.... together with capacitances...
That's why I was considering to adjust AB instead of B. Your heat sinks could probably handle that.
Okaayyy.....the error corrected class A power module is now added to the list:
http://users.picknowl.com.au/~glenk/MAIN.HTM (previous)
http://users.picknowl.com.au/~glenk/CLASSA.HTM (+new one)
Haven't got time to do a technical write up ATM though.........
To be continued.........
http://users.picknowl.com.au/~glenk/MAIN.HTM (previous)
http://users.picknowl.com.au/~glenk/CLASSA.HTM (+new one)
Haven't got time to do a technical write up ATM though.........
To be continued.........
...uahu... even if I am a fan of simple designs. This design I do like even with that giant component count.
Hope you get it running properly.
... I can't resist to ask:
Do you really need a tripple darlington for the output stage? I mean your main amp is sophisticated enough to be modified without additional components to drive a little bit more current.
Don't bother, it is my passion to strip down circuits
And there are also people here, who clearly state that I tend to strip down my circuits to much for their taste. 😉
Hope you get it running properly.
... I can't resist to ask:
Do you really need a tripple darlington for the output stage? I mean your main amp is sophisticated enough to be modified without additional components to drive a little bit more current.
Don't bother, it is my passion to strip down circuits
And there are also people here, who clearly state that I tend to strip down my circuits to much for their taste. 😉
ChocoHolic said:...uahu... even if I am a fan of simple designs. This design I do like even with that giant component count.
Hope you get it running properly.
... I can't resist to ask:
Do you really need a tripple darlington for the output stage? I mean your main amp is sophisticated enough to be modified without additional components to drive a little bit more current.
Don't bother, it is my passion to strip down circuits
And there are also people here, who clearly state that I tend to strip down my circuits to much for their taste. 😉
It's actually a quad darlington output! 🙂
And yes, it is required, as the error correction circuit requires a high input impedance to work as best as possible. The EC circuit develops an exact voltage across the 750 ohm input resistor to counter and compensate for any deviation between the input and output voltages. This input resistor should be presented with little loading from the output stage.
The output current is limited to 64A, which is a nice safety factor of 100% over the peak current at the rated power of 1000Wrms into 2 ohms.
All BJT's used are high fT devices (especially the pre drivers), so speed isn't a problem.
Cheers,
Glen
Hi Glen,
I just caught your thread. Nice project.
Look up the Carver Lightstar amplifier. They used about 13 VDC differential between the outputs and the tracking power supply. Now, if you use switching mosfets for the PWM "down convertor" you will end up with a simplified, reliable amplifier. The high frequency switchers are easier to filter out than class B types. I think they ran about 40 KHz.
Don't forget, Carver left the class B supply tracking for PWM voltage control at high frequency. You could lose most of that heatsink material.
Just a thought.
-Chris
I just caught your thread. Nice project.
Look up the Carver Lightstar amplifier. They used about 13 VDC differential between the outputs and the tracking power supply. Now, if you use switching mosfets for the PWM "down convertor" you will end up with a simplified, reliable amplifier. The high frequency switchers are easier to filter out than class B types. I think they ran about 40 KHz.
Don't forget, Carver left the class B supply tracking for PWM voltage control at high frequency. You could lose most of that heatsink material.
Just a thought.
-Chris
G.Kleinschmidt said:
Glen,
I haven't had time to go over the design thoroughly, so quick
question - does amp have global FB or is OP EC sole means of
linearising OP stage?
Also, WRT OP stage only and at full power - what do you envisage
the EC voltage swing will be. IOW the total non linearity from IP to
OP (of the OP stage), that the 750R IP R will be correcting for?
It is quite complex but should be very linear.
cheers
Terry
G.Kleinschmidt said:
Ooohps, right.
You are crazy 😉
What do you smoke? I am just on chocolate....
But it is a fantastic project! Go on, I am curiously reading.
BTW,
Do you find the Hawkford EC effective for "class A"
Any idea how much distortion reduction?
Do you find the Hawkford EC effective for "class A"
Any idea how much distortion reduction?
Glen, you might have in mind that achieving high performance means also a compact design. A huge amp might have unwanted inductances, making it hard to reach a good bandwidth. An another drawback is sensitivity for RFI.
150 years ago:
-- Mr Berlioz, I have heard that you sometimes write for 100 musicians.
-- I am sorry, but you are misinformed. I do, however, sometimes write for 500 musicians.
Today:
-- Mr Kleinschmidt, I have heard that you sometimes design class A amplifiers with a power of 100 Watts.
-- I am sorry, but you are misinformed. I do, however, sometimes design for 500 Watts.
-- Mr Berlioz, I have heard that you sometimes write for 100 musicians.
-- I am sorry, but you are misinformed. I do, however, sometimes write for 500 musicians.
Today:
-- Mr Kleinschmidt, I have heard that you sometimes design class A amplifiers with a power of 100 Watts.
-- I am sorry, but you are misinformed. I do, however, sometimes design for 500 Watts.
anatech said:
A switching amplifier suffers from non linearities other than clock frequency hash on the output. They are just not as linear as a class B amplifier. I don't know much about Carver amps, but I'm sure I've never seen one rated at 1kW RMS continuous sinewave per channel with peak output currents of 64A.
As stated already, I'm chasing very low thd and noise figures, so a switching design is ruled out.
Terry Demol said:
Both class B and class A amplifiers have lots of global NFB, but only the class A amplifier is augmented with EC as well.
The correction voltage is well under 2V at the onset of curent limmiting at 64A. Most is this voltage drop is produced across the emitter ballast resistors for the output transistors, which do not adversely effect the linearity.
peranders said:
There are rather simple ways around these problems. I've worked on solid state linear transmitters bigger than this, which work at 10's of MHz without being hindered much by stray inductances.
My heatsinks may be big, but the actual individual BJT power output modules are not particularly large. The heatsinks also have to dissipate the power of other things, such as the multiple bridge rectifiers for the high current power supplies. The critical parts will be grouped togeather.
Compact design in an amplifier such as this can also be a disadvantage, due to the need to isolate the high power stages from the low voltage, high gain amplification stages.
myhrrhleine said:
Thanks for the compliments.
I've built a low power prototype I eventually got the EC working well, but I won't know exactly how well it performs on my high power class A stage though until I've built it.
Playing around in spice though shows >50 reduction in THD. This is probably a bit optimistic though. EC works fine with class A.
I don't know where Bob got the low voltage idea from. The EC circuit Bob modified slightly for his MOSFET amp was originally designed by Hawksfors and described in his paper for Darlington BJT output stages.
I'm using a different EC circuit (figure 4 in Hawksford's paper) to do something different.
Cheers,
Glen
Cheers,
Glen
G.Kleinschmidt said:
Excuse me, which non-linearities are these? Also, how could carrier residuals at the output be considered a non-linearity when they are not related to the components of the signal amplified itself (and outside the passband)?
Switching amplifiers (modulators) have better open loop linearity than a single bipolar transistor or MOSFET operating class A in common emitter/source without NFB. The main difference is that much less global feedback (or nothing at all!) is employed in class D while lots of feedback is employed in linear amplifiers (either local or global).
By the sound of your assumptions I think that you are quite unfamiliar with switching electronics and high power PA stuff (like I once was).
BTW: Not to mention that starting the designing process of an amplifier by buying huge heatsinks and transformers is like building a house and placing nice carpets and furniture before the walls and the roof are done.
Eva
What is the highest available slew rate of a switching amplifier AFTER THE FILTER ?
Now, don't you think the supply modulator should be as fast or faster slewing than a main amplifier?
What is the highest available slew rate of a switching amplifier AFTER THE FILTER ?
Now, don't you think the supply modulator should be as fast or faster slewing than a main amplifier?
Hi Eva,.........this is solid state part of the forum.........it's like chip amp vs. tube vs. solid state vs. class D vs. super state...............................you know 😉
Hartono
Hartono
darkfenriz said:
It makes no sense to define slew rate for switching amplifiers (for example, it would be very high near the resonance of an unloaded output filter and almost 0 well above resonance). What makes sense to define is maximum output amplitude versus frequency, and this is just the frequency response of the (loaded) output filter multiplied by the supply voltage.
Correction to my Class A power output stage schematic:
I made a wiring error while redrawing my schematics from my scribbled notes. The collectors of the over current protection transistors should have been connected to the “error voltage” side of the input resistor R1 via a pair of diodes. This prevents the EC circuit from “fighting” the over-current protection transistors when they begin to conduct at a load current of above ~64A.
The schematic has been updated:
http://users.picknowl.com.au/~glenk/CLASSA.HTM
I made a wiring error while redrawing my schematics from my scribbled notes. The collectors of the over current protection transistors should have been connected to the “error voltage” side of the input resistor R1 via a pair of diodes. This prevents the EC circuit from “fighting” the over-current protection transistors when they begin to conduct at a load current of above ~64A.
The schematic has been updated:
http://users.picknowl.com.au/~glenk/CLASSA.HTM
Eva said:
Well sorry if I caused offence. I was quickly responding to a lot of questions and my wording wasn't particularly good. Carrier noise itself is not a non-linearity, no. I was just meaning to say that there are other things (linearity issues) to worry about besides carrier noise. You know, things such as dead time of the switching devices, the output network (big inductors requiring magnetic isolation from each other, high voltage capacitors, etc)...the limited amount of nfb that can be applied, in contrast to a linear amplifier.......
Eva said:
I don't think I said anything about open loop linearity. Pehaps I sould have just stated my reason for not choosing to build a switching amplifier more simply and clearly:
In terms of radiated noise and THD, switching amplifiers hold less potential than do linear amplifiers.
Eva [/i]By the sound of your assumptions I think that you are quite unfamiliar with switching electronics and high power PA stuff (like I once was).)[B][/QUOTE] If you say so. [QUOTE][i]Originally posted by Eva said:
That sure doesn't make a lot of sense. I started the design process well before ordering anything. The heatsinks were chosen based on the desired power output and the style of construction I had in mind. While I have since upgraded the power output specification, I have done so with a change in topology which gives the same dissipation (as discussed previously).
As for the transformers (not to mention the heatsinks as well), I actually need them to build and test the thing, don't I?
Now if you have any good reasons why my project being discussed here is technically incomptent or misguided (or any constructive comment on the circuit details I have posted thus far) I'd much rather hear them instead of lots of bickering about switching amplifiers, which I am NOT interesting in for this design - which is intended to be a high power version of the Technics A+ concept (to clarify for those who still don't get it.)
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