| Peter Daniel |
I was wondering if it would make much of an improvement to use separate regulated power supplies for front end of Aleph power amps. I have some custom transformers from Plitron (I was using them for A75) with separate windings for that purpose.
Peter |
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| jam |
Peter,
You can only regulate the tail of the differential of the Aleph(positive rail). This might have some sonic benefits the negative rail cannot be regulated.
Jam |
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| Peter Daniel |
| Why not regulate the negative rail? |
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| BrianGT |
I see no reason that you cannot regulate the negative rail. If you look at the Balenced Zen Preamp power supply schematic, the positive and negative rails are regulated.
I see no reason to run seperate power supplies for each. As noted in the below post, this possibly cause problems.
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Brian
gte619j@prism.gatech.edu |
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| HarryHaller |
| The current source for the front end of the Aleph must come up at the same time as the rest of the circuit or you will get a big turn on thump. I lost some tweeters in a biamp setup this way by putting a Cap across the zener reference. Messing with the decoupling caps for the Dynanic current source in the output will most likely get you in the same trouble. Getting a power amp to power up without a transient on the output is a challenge. But if the 9.1V zener was replace by a couple of series LM329 and the source resistor increased to give the same current you would have better regulation of the reference for the current source. They can be biased by a current source for even greater Power supply rejection You also could bias the zener with a 5 ma current source for the same effect. A larger source resistor for the current source will give a higher output impedance for the Mosfet current source as will cascoding. I have 5 series green LEDs bypassed by a small cap in place of the zener in my Aleph 3. |
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| Peter Daniel |
From my past experiences I have learned that the more separate power supplies you have the better the sound: better defined bass and more texture overall. One way to avoid turn on thump would be to leave the amp on all the time and disconnecting output stage from PS only, avoiding inrush currents as well.
I was hoping that maybe someone has already experimented with supplying separate voltage to amp's front end and would share his observations.
Harry, this is tweaking at it's best. BTW does your Aleph has a thump at turn on? My A75 doesn't.
Peter |
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| BrianGT |
Doesn't the Pass Labs X series keep the input stage on during standby mode and turn off the output stage?
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Brian
gte619j@prism.gatech.edu |
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| jam |
Peter,Brian,
If you look at the drain resistor of the differential you will see that the output mosfets are turned on by the voltage drop across this resistor. Now if you regulate the front end how is the output mosfet going to track the voltage across the drain resistor when the outputs rail falls or rises. The only solution would be to regulate the whole supply.
Regards,
Jam |
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| jam |
Brian,
In standby mode the amplifier stays on the input is muted and the bias to the outputs is shut off.
Jam |
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| BrianGT |
I was initially referring to regulating the entire supply. I see now what was initially meant that you could not regulate the negative cannot be regulated. Would any benefit be obtained from regulating the entire supply?
Also, as far as a standby mode, could you place relays on the rails between the input and output stage, to turn off the biasing of the output stage, on the Aleph 2? Would this be all that is needed for standby mode?
Thanks for you input,
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Brian
gte619j@prism.gatech.edu |
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| jam |
Harry is correct about turn on thump and correct design of the regulator is essential. He offers an excellent solution to the problem.
Jam
P.S. Look Ma, no heatsinks! |
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| jam |
Brian,
To regulate the entire supply your regulator would have to be bigger than your amplifier, and I have found that they usually hurt the sound and therefore I only regulate the front end.
If you could reduce the bias without causing a transient and at the same time shorting the input would solve your problem of a standby mode.
Jam
P.S. Mr. Pass might have a solution for us. |
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| HarryHaller |
Ah, the irony....... jam getting me out of a jam. Improving the current source on the Aleph might be worth doing as increasing power supply rejection usually pays pretty good divendends. Checkout how Erno Borbely does his voltage references at in his construction articles at http://www.borbelyaudio.com/index15.htm
H.H. |
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| Peter Daniel |
Jam,
I don't quite follow your way of thinking. If I regulate the front end the voltage drop across drain resistor will always be between 4-5V and this will allow the output mosfets to be turned on. Now, what does it have to do with changing the voltage on the output rail?
Peter |
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| jam |
Harry,
With all those led's in your circuit your pc board must be lit up like a Christmas tree.
Jam:) |
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| jam |
Peter,
If you regulate the front end the voltage on
the front end will stay constant, on the other hand the voltage ont the output supply rail will bounce up and down with demand and line voltage flucuations.
Jam
P.S. The voltage drop that turns on the output mosfet is between the junction of the differential mosfet and drain resistor and the negative rail attached to the output mosfet. |
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| Peter Daniel |
| From what I see that voltage drop is dependent only on the value of the resistor (392ohm) and not on the supply rail voltage. I would supply Q4 with the output stage voltage rail. |
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| jam |
Peter,
That is only true if the outputs and differential share the same supply rail.
Jam |
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| HarryHaller |
| Hey! you have come up with an interesting idea........ |
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| jam |
Nice one Harry!
I wonder what Mr.Pass would have to say about this idea?
Jam |
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| GRollins |
Leave the front end on all the time...
Grey |
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| BrianGT |
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| BrianGT |
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| HarryHaller |
Christmasy transistors to use with your green LED voltage references......
H.H. (Happy Holidays)
P.S. That is actually the color they come in. |
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