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Old 25th October 2005, 06:41 PM   #1
Buhl is offline Buhl  Denmark
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Default Aleph "something-in-between"

Hi there
mabye this have been covered before, but I did not manage to find anything via the search...

I have 4 pcs. of A30 boards with all nessecary components, but the trafos I have have found (big, good and cheap) have to high secondary voltage for a 30, and to little for a 5 or 60 - I calculate that I have about +/- 30 volts after my C-R-C filter.

That mean that I would get a little over 40 Watts at 8 ohm (??) - but without enough BIAS ? - When I compare the diagrahms over the 30 and 60 the difference is R19 and R21 - and of course the number of output devices and their source resistors.

I have enough heatsinking (well you never have enough, but then plenty....) - can I just leave R19 open ??


Sound quality and low end control are more important than theoretical wattage @ 8 ohms....

Cheers !
Buhl
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Old 25th October 2005, 07:14 PM   #2
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put pots in for r19 and r21 and tweek away until you have the highest standing current you dare, then adjust the AC gain so you don't clip. Then listen, adjust by ear, then pick a compromise. You can always add a pair of fets or two, and the bias will remain constant in each fet, but the total bias will rise.

Check out my thread on my Aleph 2+ with 55v rails and 8 pairs of fets.

At last, My Aleph 2+
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Old 25th October 2005, 07:34 PM   #3
Tweeker is offline Tweeker  United States
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If you have the heatsinks for it 30V rails and pulled R19 will be fine on an A30, the transistors can take this. I have read in a thread here an otherwise stock A30 with pulled R19 gives about 2.4A bias, ~20% more.

More devices will help the low end control, too many and you lose at the top end. The circuit gets additional bias for each pair added, bias per transistor remaining the same.
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Old 25th October 2005, 09:34 PM   #4
Buhl is offline Buhl  Denmark
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Okay, so I think I will try and leave R19 open - but what does R21 do then ? - and how do I set AC gain ??
so, am I right in the assumption, that just upping the supply voltage 5 volts will leave the BIAS pr. device unchanged, thus lowering the output in 4 ohms ? and leaving R19 out will get me some of the 4 ohm output back ( and some more heat ;-) +/-30 volts at 2 amp gets me 120watts, and with 2,4 amps its 144 watts, or about 50 % more than standard... this should bring the dispassion pr. device up to 24 watts, close to the original A3?

Thanx helping me out !
/Hans
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Old 26th October 2005, 12:18 AM   #5
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Up it to 8 outputs, this will raise you to 2.67A without changing r19 or r21 and allow you to safely run the sinks a few degrees hotter. But that's just me, I like extra fets.
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Old 26th October 2005, 12:21 AM   #6
Tweeker is offline Tweeker  United States
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Just raising the rails you dont lose any power into 4 ohms, stock is 40 watts, with 30V rails is 40 watts, its current limited in either case. Just pulling R19 wont raise it much as AC current gain drops, but some preffer the sound somewhat lower AC current gain. Changing R21 doesnt effect power into 8ohms at all. Reducing R21 will increase AC current gain and power into 4ohms.

This tool from wuffwaff might be useful to you. Aleph-Power.
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Old 26th October 2005, 12:26 AM   #7
Tweeker is offline Tweeker  United States
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Quote:
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.
-Nelson Pass
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Old 26th October 2005, 02:08 AM   #8
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There's no one right answer to this. It's a balancing act between the voltage across the devices, the current through them, the power dissipation...and so on and so forth.
Most folks who think about solid state all the time will tell you that high bias current is the ticket. Someone from the tube side of things is likely to tell you to put as much voltage on the device as it can stand. Both reduce distortion, but in the real world the device burns up.
In any amplifier design, you usually design from the back end towards the front. First pick a given wattage into your load. That will tell you what your rail voltage needs to be. The bias current will determine how low an impedance you can drive. Everything else will just sort of fall out naturally from those first couple of decisions...including what transformer to buy.
In this case, you appear to have leapfrogged that step, choosing the transformer first. 30V rails aren't too outlandish for an Aleph. Yes, the heat dissipation will be higher, but if your heatsinks are big enough, that need not be too much of a problem. I regularly run TO220 output devices at around 30W/device, but my amps are water cooled, which dramatically increases my heat dissipation capabilities. With big enough heat sinks, you'll be fine. If they're too small, you might want to use forced air cooling to help things along.
Another option is to drop more voltage through your CRC filter. If you want to get really fancy, you can use a regulator. Yet another option is to use a thermistor on the transformer to reduce the inrush current--a side effect will be that it will slightly reduce the voltage coming out of the secondary.
Oh, and take a look at the schematic for the original Aleph 3. If memory serves, it ran at a slightly higher output bias than the 30. It will give you another perspective.
You can play with the tuning of the current source if you want, but I'd say to just start with the regular circuit and wait until you get used to having a nifty amp before you start playing with the details. That'll get you music faster.
You've got plenty of options. You can even combine a couple of them if you want, just for fun.

Grey
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