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Old 4th December 2003, 01:48 PM   #1
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Talking My 50W class A design!

Hello everyone, I`ve gathered a lot of useful hints and tips on this forum, so I though I`d share my project (me proud!).

I`m currently etching the PCB`s and waiting for heatsinks and output transistors to arrive, so it will probably take a couple of weeks before I can realize it in the real world. It would have been fun if someone else also has buildt this, in this way I could get some feedback on possible improvement and so on...


Okidoki, here are the facts

It`s a 50W class A balanced power amplifier.



-Differential input with cascode loading and current mirror
-All current sources are modified wilsons, the rolls royce of current sources
-2-pole compensation in the VA-stage(local feedback)
somewhat close to the ones I got in multisim.
-3 sets of push-pull output transistors.
-A heatsink with thermal resistance less than 0.3C/W per channel should do.
The reason I achieved these S/N, and THD levels, where the use of current sources, cascode at input , degeneration resistors, tail current tweaking (with respect to the CC), two-pole compensation, and of course the current mirror in the input stage.
The current sources and current mirror alone improves the THD+N by over 5000 times, the cascode then improves the THD by 40-50% (increased linearity).
In multisim, the simulation results where far beyond the spesifications I set as a minimum before I began designing the amplifier. (However, the results where even better when I drove the input unbalanced. The reason to this is probably differential input inbalance, I`ll look in to that in the next days).

(interpolation level was set to 12, and sampling frequenzy to 128kHz for more accurate results):

THD>0.0002% (3rd. harm: 0.4uV@1kHz, 7.5uV@20kHz
2nd. harm: 1uV@1kHz, 21uV@20kHz)
All harmonic distortion simulations with 8ohm load and full voltage swing (degeneration resistors where changed to 10ohm)!!

Due to the balanced input, noise analyzis isn`t possible (S/N>115db in unbalanced mode)

Pout 8ohm=52W RMS (with 1V loss in powersupply)

DC operating point=-6mV


I`ll post my pre-amp project in a couple weeks..

Give me some feedback on this one...
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Old 4th December 2003, 07:12 PM   #2
PRR is offline PRR  United States
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> Give me some feedback on this one...

You asked....

Looks like current in Q19 Q20 is undefined. I don't see a bias diode.

I assume Q25 Q26 are "protection". If that is your "bias diode", I never saw that scheme and will have to think about it. For a few months.

If Q25 Q26 are bias: you have no short-protection at all. For a home-brew with $1 transistors, that's fine, but don't loan it to your clumsy friends.

If you short the output, and feed signal, Q10 will die instantly (before the output smoke).

Some preamps may not like the low input impedances.

Input impedances are not balanced for both common-mode and differential inputs (many "balanced" input designs have this flaw).

Base bias for Q15 Q16 is referenced to the supply rail. This is good for the lower mirror, bad for the input diff-pair. If I had to choose, I'd try to keep the input diff-pair at constant collector voltage, and let the mirror suffer.

Triple-zero distortion is not going to happen outside the fantasy world of the simulator. Layout coupling will introduce more distortion than the simulator can simulate. If you commit no grave grounding sins, this amp will probably be double-zero THD through 1KHz.

The signal current variation in Q10 is quite large due to high load current and only 2 stage buffer. Many amps work fine this way, but if you are playing the numbers game you might consider either higher standing current in Q10 or a triple buffer.

As a practical matter: do you really want driver stage power supplies higher than output stage supply? It adds another power supply problem and cost. Generally cheaper to run both on the same supply rails and accept a little waste. Also it may be possible to forward-bias the C-B junctions of Q19 Q20 and blow them.
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Old 4th December 2003, 07:26 PM   #3
sreten is offline sreten  United Kingdom
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I can't see what sets the standing current through the output
stage - at all - am I confused ?

/sreten.
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Old 4th December 2003, 08:25 PM   #4
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Hi,

I like it very much, very spectacular , lots of features and proffesional looking... I only have some doubts about the CCS.. I'm not used to this arangement.. Seems like the Iq is refferenced to the Vsupply.. what if there's some 100Hz humm.. Seems like that humm will 'only' be attenuated by about 40dB .. but I could be wrong.. .. have been before

Succes!!,
Thijs
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Old 4th December 2003, 08:45 PM   #5
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R11, C6 D1 connected wrong? Constant voltage ACROSS emitter-collector of Q3 and Q4.

R11 to -35 V and C6, D1 to ground. You could also do a little overkill in adding a NPN instead of ground and then connect base between the emitters of Q3, Q4 (via two resitors) and C6 and D1 to it's emitter. Then you really will create constant voltage across Q3 and Q4. The collector of this NPN is connected to +35 V or something lower.

Why don't you build your VAS stage with small signal transistors and then add an extra emitter follower?
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Old 4th December 2003, 10:00 PM   #6
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Quote:
R11, C6 D1 connected wrong? Constant voltage ACROSS emitter-collector of Q3 and Q4.
Cascode load.

Quote:
I like it very much, very spectacular , lots of features and proffesional looking... I only have some doubts about the CCS.. I'm not used to this arangement.. Seems like the Iq is refferenced to the Vsupply.. what if there's some 100Hz humm.. Seems like that humm will 'only' be attenuated by about 40dB .. but I could be wrong.. .. have been before
The power supply for the input stage is seaparated and will consist of a bootstrapped regulator, and, the psrr with the wilson CCS is far better that with the standard diode, just try it out in your school lab!

Quote:
Looks like current in Q19 Q20 is undefined. I don't see a bias diode.
the emitter resistors sets the value of Vbias (the value of these must be changed to 0.91ohm->0.7volts/ca0.8 amps), if you study the curcuit loong enough, you`ll understand that this will work. This type of biasing is used in the class A amp in R. Slone "High-Power Audio Construction Manual.

Quote:
If Q25 Q26 are bias: you have no short-protection at all. For a home-brew with $1 transistors, that's fine, but don't loan it to your clumsy friends.
study the schematic a bit longer, more on protection to come

Quote:
s a practical matter: do you really want driver stage power supplies higher than output stage supply? It adds another power supply problem and cost. Generally cheaper to run both on the same supply rails and accept a little waste. Also it may be possible to forward-bias the C-B junctions of Q19 Q20 and blow them
See above (the trick is to build a voltage doubler at the diode
bridge, then attach a ultra low noise regulator after the doubler, better than the one used in the A75 at www.passdiy.com)

More to come (sorry about the bad english), I was in a hurry writing this....
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Old 5th December 2003, 06:27 AM   #7
Steven is offline Steven  Netherlands
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Hi Panzerlord,

At first glance (no time for details yet) I would say it looks good. The trick with Q25 and Q26 for biasing is also used in some Yamaha pre-amps. This works only for class A output stages and I wonder what happens if your speaker impedance drops and the amplifier needs to shift into class AB operation for higher output currents. It seems to me that at that moment the output stage will get into current limiting by the bias circuit.

Steven
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Old 5th December 2003, 07:11 AM   #8
sreten is offline sreten  United Kingdom
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Q25 and Q26 looked like protection to me, I've never seen
that form of bias circuit before - so I was confused.

Probably still confused but what sets the Vas standing current ?

/sreten.
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Old 5th December 2003, 10:54 AM   #9
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The current through R19 and R13 sets a voltage across Vbe on Q25 and Q26 (-> VbeQ26/R19 ), just stare at the coupling for five mins or so and you`ll get it. I`ve been lying all night thinking about.

However:
Quote:
This works only for class A output stages and I wonder what happens if your speaker impedance drops and the amplifier needs to shift into class AB operation for higher output currents. It seems to me that at that moment the output stage will get into current limiting by the bias circuit.
Is a good point, thanks for bringing this up Steven, I have been so exalted about my project laltely that I`ve forgotten this.

I`ve also thought about what PRR said about input impedance, and I will look into that in the next couple of hours.

I`m thinking about replacing Q26 and Q25 with a modified Vbe multiplier....This is the first time I actually design and build an amplifier, before I have buildt from other peoples schematics, so you have to be a bit paicent. I want to put most of my effort in the design before I actually build it, in that way I`ll avoid falling into traps (I hope ).
And I`m very grateful for your questions and for your criticism, it makes me think twice about the various soulutions regarding the topologies I`ve used, and hopefully make my design closer to perfection..(if it at works that is....)

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Old 5th December 2003, 10:58 AM   #10
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Default Input impedance in balanced mode

Yeah, what should the input impedance be in balanced mode, I was thinking about 10kohms, but is 5kohms to low?
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