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Old 1st December 2003, 07:12 AM   #1
byteboy is offline byteboy  Netherlands
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Default Power dissipation control

What would be a good way to switch to a low power / low dissipation mode for an Aleph-X?

I am thinking of some kind of (remotely controlled-) switch wich puts the amp in a low power / low dissipation mode, eg. by switching off some of the power stage MOSFETs, reducing the bias, lowering the supply voltage or maybe a combination of these.

I am aware of the possibility of sonical consequences in the low power mode (lowered current/temperature of the devices)

Maybe this switch could also be combined with a (remote-) off/standby mode.

I know this looks like a job for a small control processor but I am more interested in the implementation aspects in the amplifier circuit itself.

Any thoughts?
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Old 1st December 2003, 09:45 AM   #2
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byteboy,

there are a few ways to do this.

The first would be to lower the supply voltage but this is rather difficult. You need a transformer with multiple secondaries wich you can switch. For example 2x15V and 2x5V for 10/15/20V secondaries (just put them in series in or out of phase)

You could also use a few relais and switch bias and active current gain. Say a setting 8A and 50% ac -current gain for serious listening and 4A with 60-65% ac-current gain for background or not so serious listening. In this case you need to switch 4 resistors (bias setting and AC-C-G for each side). This way you can halve the dissipated power. Raising the ac-current-gain will see to it that you donīt end up with only 25% of the power.
You can also combine the setting for high bias and high ac-current-gain (when driving low impedances (paralleling speakers at parties?) Or low bias/low ac-c-g for driving horns.

Since the X seems to be somewhat sensitive concerning dc offsets I would opt for the second possibility. Here the bias current through the diff pair will stay the same.

william
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Old 1st December 2003, 05:31 PM   #3
sreten is offline sreten  United Kingdom
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Just a tentative suggestion :

The netherlands mains voltage is 127 or 220 Vac.

If you run at 127 perhaps you could put in a switch for the primaries to run at 240 - thus halving the secondary voltage.

Of course your power amplifier - which I'm not familiar with -
would need to be able to run correctly at half supply voltage,
albeit at ~ 1/4 normal power output.

/sreten.
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Old 1st December 2003, 08:48 PM   #4
byteboy is offline byteboy  Netherlands
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Default Lowering the supply voltage

Hi sreten,

We use 240 Volts in The Netherlands, but it would have been a nice and cheap way to achive a low power mode.....
I think there are hardly any places left in Europe that use 127 Volts these days......?

The power amp wich I am going to build is the Aleph-X as described in another thread here on this forum and this amp can run at lower voltages.

I am starting from scratch, so a possibilty would be to buy a transformer with a number of extra secondaries like wuffwaff is suggesting.

Or another option might be using 2 identical transformers with say 2 X 12 Volt secondairies / half the VA-rating and use some clever way (maybe using the trick you mention) to get the lowered V+ and V- supply voltages.

This depends of course on the price of the special (?) high VA-rated transformer with the extra secondary windings versus the price of two standard, lower VA-rated transformers.

Would this have any consequences for the correct working of a capacitance multiplier?

Another question is if the bias has to be adjusted when the supply voltage is lowered?

Standby mode
I understand that an Aleph-X can be switched to stanby by just disconnecting the V+ supply? Could this be combined with the power transistor of the capacitance multiplier by using it as this switch?

I have to think a little bit more about the possibilities with lowering the output power by adjusting the bias and/or current gain as indicated by wuffwaff but I do see that it could be done rather simply with a relay.
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Old 5th December 2003, 05:23 PM   #5
Yonnat is offline Yonnat  France
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Hi Byteboy,

on my Aleph3 I am using a stanby mode wich is done by a relay who puts a resistance in parallele with R113(47K), it works perfect but I don't know if the same is usable on a X.

Regards.
Yonnat.
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Old 7th December 2003, 12:19 AM   #6
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"The netherlands mains voltage is 127 or 220 Vac"

I wonder how they ended up at 127 volts??? I can understand the 220 but 127?

Mark
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Old 15th December 2003, 09:58 PM   #7
byteboy is offline byteboy  Netherlands
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Default Variac to control supply voltage / dissipation?

Hi guy's,

Stil thinking of ways to control the dissipation of these "hot" amps....

I can get some fairly reasonable priced Variac's with medium/high VA ratings ( up to 1500 VA), so this could be a relatively cheap and simple solution to control the dissipation when using low volume for background listening.

Would it be a good idea to use one or more of these permanently to control the power supply voltages by adjusting the primary voltage of (a number of-) an Aleph-X('s) within a limmited range or for two or three "fixed" primary voltage settings?

Or would this mean an adjustment of the bias for each setting of the supply voltage?

An added advantage could be a controlled soft "switch-on" of a number of amps?

Any thoughts?
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Old 15th December 2003, 10:56 PM   #8
Cobra2 is offline Cobra2  Norway
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Variacs also waste energy.
Would it not be easier, and just as cheap/expensive to get a custom-wound trafo, with 4 secondarys, 2 of them may be half the voltage?

Arne K
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Old 15th December 2003, 11:30 PM   #9
byteboy is offline byteboy  Netherlands
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Default Variacs

Hi Arne,

Quote:
Variacs also waste energy.
I assume Variac's are equally efficient as "normal" transformers, (96 - 99 % efficiency or so?) so much energy isn't wasted this way. Or am I wrong assuming this?

Quote:
Would it not be easier, and just as cheap/expensive to get a custom-wound trafo, with 4 secondarys, 2 of them may be half the voltage?
For one AlephX you may be right; if you want to control, say a 3-way stereo active speaker system (6 amplifiers with one controlling switch/device....), things are quite different!
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Old 16th December 2003, 12:29 AM   #10
sreten is offline sreten  United Kingdom
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Quote:
We use 240 Volts in The Netherlands, but it would have been a nice and cheap way to achieve a low power mode.....
IMO forget the variac.

Get a transformer with 120/240V windings, however with
half the secondary voltage that you need for full power.

So on 240 you get 1/4 normal output power. Dissipation
will only be half though if its constant current Class A, as
the current will be twice whats actually needed, but then
again it will handle low impedance loads with impunity
compared to the full power setting.

Putting in a switch to use the 120V windings on 240V will
then give you full power.

sreten.
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