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Old 6th November 2009, 08:12 AM   #21
rem280 is offline rem280  United States
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I had an idea on this. You could use a single volume pot and ac couple it to the pre and power stages then run dc through it and conect the wiper through a low pass filter to comparators that switch on different bais circutry. when they reach their triger voltage. I was thinking of a single ended common source mosfet amp where you could switch on different bias resistors with mosfets controled by the comparators. You could get three currents with two resistors by switching on one the other or both.
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Old 6th November 2009, 10:31 AM   #22
AndrewT is online now AndrewT  Scotland
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just as an example of what could be achieved in the way of power (energy) saving, lets start with a 100W 2.6A bias ClassA amplifier (similar to a Krell KSA100)

volume control setting -0dB to -14dB PSU +-52V Bias 2.6A, dissipation 260W
volume control setting -15dB to -24dB PSU +-30V Bias 2.6A, dissipation 156W
volume control setting -25dB to -34dB PSU +-30V Bias 1A, dissipation 60W
volume control setting -35dB to -44dB PSU +-20V Bias 1A, dissipation 40W
volume control setting -45dB to -80dB PSU +-20V Bias 0.4A, dissipation 16W

This requires 3 voltage settings and 3 bias settings brought in staggered as volume control is advanced/retarded.

The lowest setting of 0.4A bias +-20Vdc allows an 8ohm 95dB/W @ 1m pair of speakers to produce ~85dB maximum (or about 65dB average) at a 2.5m listening distance from ~0.3W of pure ClassA power and staying in class A for ~ three times the nominal output current demand of an 8r0 load. If an unexpectedly high transient came through then the push/pull nature of the output stage would still allow 14W of maximum output power producing a maximum SPL at the listening position of ~101dB and this at just 16W of dissipation/channel.

The energy (heat) saving potential is enormous.
Why has this never been promoted by the ClassA manufacturers?
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Last edited by AndrewT; 6th November 2009 at 10:35 AM.
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Old 6th November 2009, 10:42 AM   #23
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Quote:
Originally Posted by AndrewT View Post
just as an example of what could be achieved in the way of power (energy) saving, lets start with a 100W 2.6A bias ClassA amplifier (similar to a Krell KSA100)

volume control setting -0dB to -14dB PSU +-52V Bias 2.6A, dissipation 260W
volume control setting -15dB to -24dB PSU +-30V Bias 2.6A, dissipation 156W
volume control setting -25dB to -34dB PSU +-30V Bias 1A, dissipation 60W
volume control setting -35dB to -44dB PSU +-20V Bias 1A, dissipation 40W
volume control setting -45dB to -80dB PSU +-20V Bias 0.4A, dissipation 16W

This requires 3 voltage settings and 3 bias settings brought in staggered as volume control is advanced/retarded.

The lowest setting of 0.4A bias +-20Vdc allows an 8ohm 95dB/W @ 1m pair of speakers to produce ~85dB maximum (or about 65dB average) at a 2.5m listening distance from ~0.3W of pure ClassA power and staying in class A for ~ three times the nominal output current demand of an 8r0 load. If an unexpectedly high transient came through then the push/pull nature of the output stage would still allow 14W of maximum output power producing a maximum SPL at the listening position of ~101dB and this at just 16W of dissipation/channel.

The energy (heat) saving potential is enormous.
Why has this never been promoted by the ClassA manufacturers?
Hmmm. Nice idea. The variable power supply would probably not offer a huge power savings in practise (unless it is SMPS), but the bias current would. You could couple a separate pot to the volume pot to set the bias.
It does require an integrated amp rather than a preamp-power amp set of course. Also, one would need to take a hard look at the dynamic range in each setting to find the best/optimum bias point in each case. But interesting it is.

jd
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Old 6th November 2009, 10:54 AM   #24
AndrewT is online now AndrewT  Scotland
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Originally Posted by janneman View Post
Also, one would need to take a hard look at the dynamic range in each setting to find the best/optimum bias point in each case.
that was my starting point for each setting of the volume control. I asked myself "what signal voltage and what worst case peak transient current would be required at each setting?"
Then calculated the bias and PSU voltage requirements accordingly. I based all these on achieving 110dB @ 2.5m using a pair of 95dB/W @ 1m speakers with the volume control @ -10dB.
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Old 6th November 2009, 11:03 AM   #25
Mooly is offline Mooly  United Kingdom
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Andrew,
That's all nicely presented, and it really does put the idea into perspective too...

The use of different PSU rails is another "addition" to my basic idea... certainly for a manufacturer using a SMPS it should present few challenges, might be difficult to implement at a DIY level. A linear regulated supply wouldn't be energy efficient, and however any of it were implemented (bias, supplies) it would have to operate seamlessly with no clicks or noises etc.
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Old 6th November 2009, 11:16 AM   #26
AndrewT is online now AndrewT  Scotland
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Hi,
window comparators (monitoring the third track of the pot) controlling relays would do it.
The variable bias just needs extra parallel resistors just like the first Krell Klone PCB had on board.
The variable PSU voltage just needs three sets of tappings on the secondaries.
ClassH? has something similar, it's only the output stage that needs the variable, high current, supply.
Finally a DC servo to keep the output offset near zero and of course DC detect and speaker isolation if something goes wrong.

Just noticed that a 0-15,25,35Vac dual secondary ~800VA would do this example admirably.
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Last edited by AndrewT; 6th November 2009 at 11:22 AM.
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Old 7th November 2009, 03:56 PM   #27
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to implement an idea like that ( which actually sounds very nice )will be an easy thing to do even in diy level ...

reading the thread, but also hate too many digital things i thought about a simple vu meter led made 3915 style conected in the out for example with some capacitor in the input to create delay

so you ask ammount of volume A so you get 3 leds on and since a capacitor exists in the input leds will not play up and down but stay lited untill you adjust volume higer or lower again ....

so then take this information use it how ever you like through opotocouplers and/or other to adjust any bias you like ...

but on the other hand why use 3915 it can be done also with juts one transitor or two as long as you mange to create this kind of delay so the info you have is stable .... something like an AGC with delay ....sounds fairly simply
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Old 7th November 2009, 04:23 PM   #28
npapp is offline npapp  United States
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What about switching on/off out put pairs such as 2 devices always on and then a switch to add additional pairs on demand? The bias can stay fixed for all devices.
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Old 10th November 2009, 12:32 PM   #29
npapp is offline npapp  United States
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Did my silly noobie suggestion stop the flow of this conversation?
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Old 10th November 2009, 12:46 PM   #30
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by npapp View Post
Did my silly noobie suggestion stop the flow of this conversation?
Not at all Just can't see how you would achieve it transparently and how would you switch the outputs in and out of circuit ? You say "add a switch"... can you eleborate on that, relays ? solid state etc ?

To vary the bias as I proposed at the start means it can all be implemented at a "low signal level" with no power switching.
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