What is the Aleph4's true power capability?

[Jmertz]

Hello all,

I recently did some SPICE simulations of the Aleph 2 and 4 circuits and found some disturbing results...........

Here is what I did: I ran a transient analysis of the amp into different resistive loads. I increased the input voltage until clipping occurred. At clipping I recorded the peak output voltage delivered by the amp into the various loads. Here are the results...........

Aleph4 (bias came out to be 2.25 amps total)
(R29=619 ohm)
81 Wrms @8 ohm (162 peak)
60 Wrms @6 ohm (121 peak)
40 Wrms @4 ohm (81 peak)
20 Wrms @2 ohm (40 peak)
(R29=523)
100 Wrms @8 ohm (200 peak)
92 Wrms @6 ohm (184 peak)
62 Wrms @4 ohm (123 peak)
30 Wrms @2 ohm (60 peak)

Aleph2 (bias came out to be 2.5 amps total)
100 Wrms @8 ohm (200 peak)
133 Wrms @6 ohm (266 peak)
140 Wrms @4 ohm (280 peak)
71 Wrms @2 ohm (142 peak)


So what does this mean???
It appears that according to simulation the Aleph4
is not exactly capable of it's ratings. Of course, simulations can be wrong (those damn computers!)

I have found that only slight changes to the bias current can affect the power output a great deal. Maybe the simulation is just off slightly, I'm not sure (I would think DC bias currents would be pretty accurate).

Has anyone tested their Aleph constructions or commercial offerings for maximum power output??

I would be interested to know if they really do perform as outlined in the user manuals.

If no one knows I guess I will have to wait until I finish mine.



I will run some tests (when I get the thing built of course) and post when I get some experimental measurements to back my simulations.

Perhaps Mr. Pass can help us with this one.
I am very interested in this.

Happy Listening

Jake

 

[GRollins]

The Aleph 2 actually biases at about 3A.
There is some question as to what the bias current is on the Aleph 4. Being a stereo amp with a shared power supply and less heatsinking available per device, I believe that the bias was somewhat less than the 2. Nonetheless, based on scratch calculations I just did, I think you'll find that the 4 will deliver 100W into an 8 ohm load as per the specifications. Looks like it'll deliver peak power around 6 ohms, then begin to progressively current limit below that. The 2, with more current available, continues to increase power down to a lower impedance before it runs out of steam.
If nothing else, keep in mind that you can increase the bias somewhat as long as you've got the heatsinking to handle the increased power dissipation in the output devices.

Grey

 

[blschuler]

The service manual for the Aleph 4 says the output is biased at slightly greater than 3 amps. This doesn't seem right however, and likely occured as the result of Passlabs adapting the service manual form the Aleph 2.
With 0.6 volts across the 1.5 ohm source resistors wouldn't the biasis be 2.4A?

 

[wayne]

Aleph 4 Power

The Aleph 4 is actually capable of about 115 Watts per channel.
This is what they measured at as shipped. The supply rails are higher than indicated in the manual. The current source on the Aleph series needs to be looked at closely and may not simulate properly at higher levels.

Wayne

 

[Nelson Pass]

I checked, and the Aleph 4 figure is about .6 volts
across the source, resistors, which by measurement
and calculation gives a 2.5 amp bias and a 5 amp
peak output, for 200 watt peak, 100 rms into 8 ohms.

As discussed on a previous thread, if you base your results
on the schematic, you suffer from the cut and paste error
of the Aleph 4 schematic pulled from the pre-existing
Aleph 2. We corrected the X6 vs X3 figure, but did not
adjust the NOMINAL figures on the Source resistors.

The Aleph 2 comes in more like 3 amps bias, which
gives about 144 watts rms into 8.

 

[Jmertz]

Thanks guys,

I guess the simulations may have some trouble with calculating the bias currents. I will see how my version does when it is finished.

Jake

Wayne..............

You mentioned the simulations being off at higher current levels. Is this typical of SPICE? Can you elaborate?? Do you have some experience with this type of thing?
You got me interested..........

 

[Nelson Pass]

I've gotten on this soap box before on simulations,
but part of the problem is simply that there is quite
a bit of variation from part to part on MOSFETs, and
of the models available, the MOSFET models seem to
be the most problematic.

In simulating an Aleph 4, I would screw around with the
bias values until you get the .6 volts on the soource
resistors, and adjust the value of R29 until you get the
best results, which is approximately where the current
source supplies 1/2 the AC output current.

 

[blschuler]

Questions.

To increase the bias on an Aleph 4 would one simply decrease the value of the source resistors and then adjust R29?

The value of R29 being where the current source supplies 1/2 the AC output current. How is this measured/comfirmed?

Brett

 

[cp642]

Brett,
Perhaps this might help...

As usual, Grey nails it down. At 25 watts or
so, the transistor "never" fails. At 50 watts,
we see some failures.

As a rule, the higher bias figures per device
are desirable, and you could optimally look for
1 amp bias per device with supplies at +-25 to
30 volts or so. At the same time, paralleling
devices at somewhat lower bias can give you greater
transconductance, and this was chosen in designs
like the Aleph 60 as it gives more bottom end control.

When you parallel devices in an Aleph but want to
vary the bias per device, you look to the value
of the Source resistances on the current source
(the bank on the positive half), and also (referring
to the Aleph 60 schematic) resistor R19.

After you get the DC figure you are looking for, you
want to adjust the AC gain of the current source so
that the current source provides about 50% of the
output AC current.

The easiest way to do this is to build the circuit
without R21 and operate it at 10 watts or so into a
load while measuring the AC voltage across R46-51
which are the Source resistors on the negative half
of the amp. Put in a value for R21 which halves
the AC voltage across R46-51, and you'll know that
the current source is doing half the work.

Another issue that comes up with more devices in
parallel is that the capacitance of the circuit
goes up, and with it the nonlinearity of this
capacitance, which at high frequencies starts
showing up in the distortion curve. Somewhere
around 12 devices in parallel, you have to start
modifying the circuit to deal with this, depending
of course on the devices. With IRF250, this would
be around 6 devices.

 

[Jmertz]

Thanks for the responses guys.

So Nelson.......................
I can tweak the values in the simulation to get the circuit to perform as intended, but when constructing the circuit is it necessary to tweak bias and the value of R29? Or have you already done that in the design?

Will the MOSFET device tolerances require the DIY'er to tweak R29? Perhaps devices are different from batch to batch?

Did the original 'tweaked' design use the same output devices?

It seems logical that if the simulation models are not exactly right, then couldn't the real devices be different from when the circuit was designed?

What would you recommend?

JAke

 

[Nelson Pass]

Usually it was not necessary to tweak the values
in production as the devices were selected within
a range of Vgs in addition to being matched.

You'll have to consider tweaking the values of the
resistances mentioned if you substitute different
values of Source resistance, different MOSFETs, or
change the number of MOSFETs.