The Aleph 3Turbo: HotRodding the A3

[Blues]

There are about 5 things to consider when going for the evil twin of the Aleph 3. After the changes are made we will find that our A3s have specs similar to its older brother the Aleph 5. We get to keep the toasty DC bias of the A3 only toastier. I think this is what keeps the magic in the Aleph 3. Here’s how we can do it:

1. We need to have a beefier PS transformer. About 600-800VA; 2-28V secondary CT. This will deliver about +/-34Vdc loaded with the A3Turbo.

2. We need to increase the built-in current limiter’s current limit. As called in the A3 schematic, the limit is about 4Aac. We will increase this to about 5Aac. The limiter circuit is made up of Q104, R109 and R110. By lowering R110 to about 75Ù the current limit increases to about 5Aac. How it works: At lower load impedances, by virtue of Nelson Pass’s another paradox the Aleph dynamic constant current source, higher voltages and currents develop across and through the output devices and their Source resistors. At about 5Aac the voltage across R122 increases to about 2.35Vac. The voltage divider circuit that includes R109 will drop some of this voltage while about 0.6V develops across R110. 0.6V across the B-E of Q104 will be enough to turn it on limiting input currents to the Gates of the output MOSFETs thereby setting their output currents to a maximum of 5Aac.

3. For the positive going signal to keep up with the increase in negative going signal as the current limit was increased, the 66% of AC current gain (ACIG) as set by R114 will have to be increased. We will set the ACIG to about 78% by replacing R114 with about 634Ù of resistance. Both + and – signals will now start to distort almost at the same time.

4. If we are going to load up our A3Turbo with speakers that dip to 4Ù or 2 Ù at a bunch of frequencies in the audio band you might want to monitor your Source resistors if they heat up rapidly. We should replace them with a higher wattage ones if it did. Start with 5W.

5. As we did with the Source resistors, monitor our heatsinks. The touch and go method as explained by Mr. Pass is effective as it is.

Let’s fire it up. Oops! Nice output signal. That’s SIMetrix though. I’ve tried this at home but only on a computer. You can put into hardware if you find it appealing.

Please feel free to add to it if I'm missing something.



_____________

"For sounding good, I'll take the heat."

[wuffwaff]

Hi,

just a few things:

1. if you up the voltage from 25 to 34 you´ll have an increase in power dissipation of almost 40%. Watch your fet temperatures!

2. Such a high voltage with such a low peak current will only be suitable for loudspeakers of 8 ohms or more

3. 78% ac-current-gain will probably not be very pleasing to the ear......

4. The mean current through the source resistors will not change with different speaker loads so no need to change source resistors.


William

[Cobra2]

For hotrodding & optimal adjustment, would anybody please post a circuit that can use say a automotive amp-meter with needle in centre, (like the X-series meter), so bias / current gain can be tuned "in flight".
As many may have experienced, your 8 ohm speakers are most often some other value...

Arne K

[uli]

Originally posted by Blues
For the positive going signal to keep up with the increase in negative going signal as the current limit was increased, the 66% of AC current gain (ACIG) as set by R114 will have to be increased. We will set the ACIG to about 78% by replacing R114 with about 634Ù of resistance. Both + and – signals will now start to distort almost at the same time.

Hi Blues,

why don´t you simply beef up bias current to increase current
capability of the current source as its limit is a multiple of bias
current?

Uli

[Blues]

Hi Uli,

I had wanted to keep the A3 DC bias current...which seems to be the magic number among the Alephs. And also to keep the power dissipation of the mosfets to <50W.

Hi WuffWaff,
"1. if you up the voltage from 25 to 34 you´ll have an increase in power dissipation of almost 40%. Watch your fet temperatures!"

Yes...at 36% increase the fearless DIYer might go for it.

"2. Such a high voltage with such a low peak current will only be suitable for loudspeakers of 8 ohms or more"

As I stated, its specs will be similar to the Aleph 5.

"3. 78% ac-current-gain will probably not be very pleasing to the ear......"

A great number of people love the Aleph 5's sound at 78%.

"4. The mean current through the source resistors will not change with different speaker loads so no need to change source resistors."

Okay..now it'll be an easier upgrade.

[wuffwaff]

Hi Blues,

I don´t think a standard A5 has 78%. You get 70% for 2A bias and 90 watts into 4 Ohms. Look at a few test and you will see that A5´s mostly reached around 45-55 watts into 4 Ohms wich would mean around 60% ac-current-gain.

William

[Blues]

Hi William,

Please refer to A5 schematic. Here's how I got the numbers:

ACIG=((0.1175/0.333)*(1000/453))*100=77.9%

Here's how I verified it using voltage divider formula at the current limiter circuit:

Vac at ref Source resistor=0.6/(75/(75+221))=2.368V

Imax=2.368V/1=2.368A

ACIG=(1-(0.5/2.368))=78.9% the slight difference in percentages is our assumed 0.6V of turn-on voltage for Q4. It looks more like about 0.57V.

The A5's ACIG is maxed out with it's current limit to let the + and - signals start to distort almost at the same time.

Please point out any miscalculation I made or missed.

Thanks,
Allan

[Blues]

Originally posted by wuffwaff
Hi Blues,

I don´t think a standard A5 has 78%. You get 70% for 2A bias and 90 watts into 4 Ohms. Look at a few test and you will see that A5´s mostly reached around 45-55 watts into 4 Ohms wich would mean around 60% ac-current-gain.

William

William, the A5's total DC bias is 1.5A and to get it to about 90W in 4ohms the ACIG should be at 78%. If the ACIG is set at 60% you're not maximizing both + and - signals.

Allan

[wuffwaff]

Hi Blues,

the A5 will have a bit more than 1.5A. Mine had over 2A with the specified parts.
I just wanted to tell you that 78% ac-current-gain won´t sound very nice so be prepared to dial in some more bias.

William

P.S. I still don´t understand peoples obsession with symetrical clipping.......

[Nelson Pass]

Originally posted by Blues
Please refer to A5 schematic. Here's how I got the numbers:

ACIG=((0.1175/0.333)*(1000/453))*100=77.9%

Here's how I verified it using voltage divider formula at the current limiter circuit:

Vac at ref Source resistor=0.6/(75/(75+221))=2.368V

Imax=2.368V/1=2.368A

Please point out any miscalculation I made or missed.

We don't seem to be operating from the same schematic, but:

The first formula does not include the limited gain of Q5 circuit
and the transconductance of the output devices.

The result is that the current source gain is less than your
calculation (I arrived at the original figures by comparing the
actual AC current through the output vs the current through
the current source and not through calculation).

Regarding the limiting circuit, the original circuit already used 75
ohms in the divider, so I'm not sure why you need to change
the resistor. The original limiting on the negative side is about 7
amps peak, which is your 5 amp average figure.

Or have I miscalculated?