On Sun, 13 Mar 2005 14:01:19 GMT, Patrick Turner
wrote:
At the end of the day, what "score" of music sounds best is the
final decider, surely.
Indeed, although if as claimed 'the first watt is the most important',
I'll lay odds that the various options are sonically indistinguishable
up to that limit.
I have just had a geek at Pinky's provisional SS amp.........
OK, we have a complementary emitter follower pair of class A bjts presumably
biased with a couple of amps idle
current from an RC smoothed supply, so they'll run in class A.
1 amp quiescent current, giving a maximum output into 4 ohms of 8
watts. The voltage limitation also dictates a maximum of about 8 watts
into 8 ohms.
Let us suppose the current gain was 30 between the output current and the base
inputs. Therefore if there is 1 amp rms in the output current load, threre will have to
be 0.033 amps base input current.
It's typically about 200 for the MJL4281A/MJL4302A devices chosen for
this design, hence about 5mA base current.
Now the Gm of the output bjts would be about the usual 30A/V,
It is in fact something like 150 A/V at 1 amp for these devices, so
far as can be seen from the Vbe/Ic curves.
and open loop gain would be approx Gm x RL, so if the load is 8 ohms,
each output bjt sees approximately 16 ohms, depending on the current sharing
between the two
bjts.
Therefore open loop voltage gain of the output bjts is 30 x 16, or 480.
Actually, about 2500.
But because Pinky has a EF output stage, closed loop voltage gain is 480 / (1 +
480 x beta),
or very close to 1.0.
So there is a gain reduction due to NFB = approximately 1/480 times
So providing the input voltage can be kept linear, thd should also be reduced
from say 10% for common emitter connection to
0.021% for emitter follower connection, and at 8 watts.
I'd say closer to 0.0015%, but let's just agree that it's negligible.
So the amount of series voltage NFB applied in the output stage is 20 log 480,
so about 54 dB of NFB is applied locally in the output stage.
Its not uncommon for the emitter follower connection to automatically
give between 30 and 60 db of series voltage NFB.
We can neglect the 0.22 ohm emitter bias stabilizing resistors for the present.
You can? Without loop NFB? Intriguing..................
For an 8 volt swing on the output, there is just slightly more than 8v
applied to the output bases, and a 0.033 amp swing,
so the Rin of the output stage is 8 / 0.033 = 242 ohms.
More like 8/0.005 = 1600 ohms.
One has to remember that with BJT output stages, the Rin to an output stage
varies in proportion
to the output load, so that if 4 ohms were connected, Iin would be 0.066 amps,
if the
voltage swing was the same,
and Rin would be 8 / 0.066 = 121 ohms.
Actually, about 800 ohms.
Now the driver transistor will have a much higher Rc than the load, so
the only thing that tends to keep the drive voltage constant is the R5, 150
ohms.
Correct.
Now the open loop gain of the driver stage also = Gm x RL, since it is a
*current*
source, and RL with 8 ohms is 150 // 242 = 93 ohms, and with 4 ohms its 67 ohms
Actually, about 150//1600 = 137 ohms, and about 126 ohms for a 4 ohm
load.
So the gain of the driver with 8 ohms will be nearly 3 dB greater than
when 4 ohms is connected, and a rough calculation
makes me think the output resistance of the amp will be 6 ohms, due to the huge
variation
in driver gain due to reflected load variation..
No, in fact it will be more like 0.72dB, and the Zo of the amp will be
less than 1 ohm.
This is a perfectly lousy result, and well below what the 300B is capable of.
Bull****, since your assumptions are incorrect. Check your facts.
Also, if any load caused distortion currents are generated, they too
escape the voltage emitter follower FB clean up, because they flow through to
the bases,
and appear in the input wave form largely unsupressed by any NFB
action.
But let us consider the driver stage, a lone transistor, presumably
some sort of T0220 driver type of bjt with a current gain of maybe a 100,
and Gm = 20 A/V, typical of what is available.
It's already been specified as a MJL4281A, with typical current gain
of 200 at the quiescent operating point of 100mA, and even higher Gm
than at 1 amp.
Since its collector load is 93 ohms when 8 ohms is at the output,
then its gain with the 24 ohm emitter load will be about 93/24 = about 4,
so 2 vrms is about what's needed to produce 8vrms at the collector
to drive the output bases..
Nope, your assumptions are incorrect, and it has a voltage gain of
pretty close to 6.
snip of lots of handwaving and false assumptions
BJTs have current linearity, but make poor voltage amps unless
huge amounts of current NFB are used, and as I have explained above,
Pinky needs both lots of current AND voltage NFB to make his 4 stage
amp work far better.
He really needs to have a a pair of darlington connected N and P
devices for the class A output, then perhaps another darlington pair connected
gain stage fro the input. That'd be 6 devices.
Nope, it will work just fine as designed, for the reasons given above.
Get your head out of RDH4 and look at what modern multiple-emitter
poer BJTs can do.
Now Anrdre's amp uses a lone 300B to do the business, and as I have explained to
all
before, there is local electrostatic NFB acting at all AF within the tube.
Its about as local and inimtate an application of NFB as one could ever wish
for,
and it was put there by the God Of Triodes when the tube was made,
and lets not argue with a God in charge of so much high voltage.
**** 'Im, if for some deranged reason you want a *real* SET amp, use
an 845 or 211, and play with proper Frankensteinian voltages! :-)
The mosfet SE amp with OPT will be able to easily make 8 watts,
and with only 23 db of series voltage NFB it will have a very
good DF, and low thd, and measure better than the 300B amp.
The Nelson Pass Zen amp pushes things harder by
making a single mosfet dissipate 50 watts from which 17 audio watts are
available,
and then another mosfet is a CCS also dissipating 50 watts,
but producing no power.
There is 12 dB of shunt voltage NFB applied, and the result
in terms of Ro and thd about resembles the triode amp
with no NFB.
A friend got nothing but smoky silence from his Zen attempt,
and after 3 expensive mosfet changes he gave up.
There's no substitute for skill! :-)
If properly built, the Zen does what it says on the tin, and the magic
blue smoke stays inside the tin.
Expecting any T03 device to safely dissipate 50 watts is total BS imho.
Agreed, without heroic and costly cooling arrangements.
Its like saying an EL34 is happy dissipating 32 watts.
It will, but sooner rather than later it will fail.
In my AM radio I use in the kitchen, I have an SE EL34 in triode
with a paralleled 12AX7 driver tube and 12 db of global NFB.
I have a sensitive speaker, and about all I use is around
1/2 a watt max average power.
The sound is utterly blameless.
Aside from the speaker, of course.......... :-)
The rule for simple *low power* power amps is that
the average levels should be 1/10 the maximum levels for clipping,
so if an amp clips at 5 watts, then only expect a nice average 0.5 watts and no
more.
Except of course properly conceived amps like the 1969 Linsley Hood
10-watter, which is just fine all the way from half-power downwards.
0.5 watts into a speaker with 95 db efficiency with 1 watt means you
get 92 dB, and that's way too loud for me, so
0.05 watts for 82 db is about all I need.
5 watts allows a 102 db ceiling, quite enough in the kitchen.
Shame that there are *very* few decent 95dB/w/m speakers, and they
tend to cost many thoiusands of dollars.
Andre's requirement for 3? watts is a bit low, but he intends for horns, so its
OK.
But a simple amp is still simple if it merely means that to quadruple the power
we just use 4 parallel output devices and use a more current able driver device,
or use a much larger output tube.
AFAIK, there are not many cheap easily available gigantic transistors or mosfets
rated
for safe continuous dissipation of 200 watts.
As you point out, not a problem if you just parallel them up.
Alternatively, grow a brain and build a 100 watt Class AB BJT design
with negligible distortion at *any* level up to 3/4 of full power.
--
Stewart Pinkerton | Music is Art - Audio is Engineering
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