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Old 24th September 2001, 09:26 AM   #1
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I'm thinking of building an Aleph 5 because of its power output, but I also like the performance of the aleph 30 into difficult loads.

Is there a good way to increase the bias current ? What would I have to look out for ?

How would I lower the output impedance on the aleph 5 ?

Would the aleph 3 circuit deliver more power with higher rail voltages, adequate heatsinking and higher power parts ?

Is it safe to increase the value of the input resistor (connected to the unbalanced in) to a higher value at the expense of some high freq. roll-off and lower input gain ?

Thanks for your help !
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Old 24th September 2001, 10:19 AM   #2
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My modified Aleph 5 actualy runs with about 2.5A of bias current.
I changed the source resistors value from 1 to 0.47 ohm, acurately trimming the bias current by an appropriate value for R19.
I used a cermet trimmer, in order to vary the current from 1.4A to 3A, depending on the loudspeaker choice.
The output impedance of the A5 is mostly defined by the NFB factor, and should be about 200mOhm, why do you need to lower this?
If you want to increase the input impedance, you may employ the Volksamp A30 or A60 input stage.

Tortello

[Edited by tortello on 09-24-2001 at 07:03 AM]
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Old 24th September 2001, 12:28 PM   #3
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Well I,m also thinking of building the aleph 5, Circuits shows that the amp's voltage is rated at 34V DC

i was wondering that is it ok to use a 2x24V toroid rated at 9.7A/channel to power it, as the output voltage from these transformers will certainly be less than 34V DC after rectification.

If so will it degrade the amp's performance/operation?
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Old 24th September 2001, 03:50 PM   #4
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Hello cp642,
at a first sight a 2X24V, 480VA PER CHANNEL should be a good choice, at least it seems to me.
I think that 1-1.5V more or less for each rail is not a big issue: the Aleph series is perfectly scalable.
My Aleph5 mono's run with a 500VA, 2X25V per channel, and 2X34Vdc, running at 2.5A (the regulation of the power transformer is about 6%): they run cold and noiseless for many hours.
Tortello
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Old 24th September 2001, 11:00 PM   #5
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amplitone,
If you feel that the Volksamp 30 is superior, but want the power of the Aleph 5, why not build a pair of Volksamp 60s?
cp642,
That transformer sounds a bit scant. A good rule of thumb is to double the power the circuit will need, which in the case of an Aleph 5, would indicate a transformer with a 600VA rating. Note also that the 34V rail would collapse (I'm not counting the drop across the rectifier, which would make things even worse) to perhaps 30V under the current draw.

Grey
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Old 25th September 2001, 01:05 AM   #6
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GRollins,

I have thought of a pair of Aleph 60s, but the cost of machining two chassis would be really high.

Are aleph 30s scalable to the point where I can just stick 35V rails to the circuit and uprate all the components for the higher power handling ?
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Old 25th September 2001, 02:15 AM   #7
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Who says it has to be two mono chassis?
Use one larger power supply and put everything in one box. There's no reason that a Volksamp 60 can't be a stereo amp.

Grey
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Old 25th September 2001, 06:19 PM   #8
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Grey,
600VA of toroid per/ch? sounds like an over kill.....

cp
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Old 25th September 2001, 07:07 PM   #9
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cp642,
No, for the Aleph 5 (being a stereo chassis) 600VA would be for both.
Were you talking about building monoblocks? If so, I missed it. Sorry. You'd need a minimum of about 300VA for each channel. If you've got two of the transformers you mentioned above, you should be okay. Yes, the voltage will still be a bit low due to losses in the rectifier, but the rail should be firm. The Aleph circuit has enough 'give' in it that you should be able to use it without modification. Output wattage will, of course, be slightly lower, but not enough to be troubling.

Grey
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Old 25th September 2001, 07:12 PM   #10
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I was looking at the datasheet for some transformers and it seems that the VA rating occurs at the safe thermal limit of the device.

When running a transformer at full load, efficiency is lower because the resistance of the windings becomes significant and generates heat.

Running about 40-50% of full load usually gives the highest efficiency for that transformer. Of course, larger transformers are more efficient than smaller ones and also have better regulation (so the output voltages are more constant)

With all the heat that the output devices produce, I don't think we need a hot transformer in there....
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