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adjusting current gain for Aleph30 necessary?
adjusting current gain for Aleph30 necessary?
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Old 15th February 2007, 11:22 AM   #1
h_a is offline h_a  Europe
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Default adjusting current gain for Aleph30 necessary?

Hi!

I read quite some about the importance of having 50% current gain on the current source, but - please excuse my lacking knowledge - there's no hint which resistor is to change?

I read about Aleph-X, Aleph5, even Aleph2, but not the Aleph30...

I'm sure you have already the feeling that with my lacking knowledge it would be smarter to use the stock values from the schematic anyway, so how much deviation of the 50% I have to expect when using well matched parts (9610er input pair matched to within 2mV@20mA)?

Would be great if one could help me here! I really don't want distortion!

BTW when does the amp start to clip? Given a 3V source, a 25k log pot as preamp, does the Aleph30 clip at full throttle?

How many watts@8Ohm has the Aleph30 with stock rails really?

Thanks a lot for helping me out!

Cheers, Hannes
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Old 15th February 2007, 11:41 AM   #2
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adjusting current gain for Aleph30 necessary?
Default Re: adjusting current gain for Aleph30 necessary?

Quote:
Originally posted by h_a
Hi!

I read quite some about the importance of having 50% current gain on the current source, but - please excuse my lacking knowledge - there's no hint which resistor is to change?

I read about Aleph-X, Aleph5, even Aleph2, but not the Aleph30...

I'm sure you have already the feeling that with my lacking knowledge it would be smarter to use the stock values from the schematic anyway, so how much deviation of the 50% I have to expect when using well matched parts (9610er input pair matched to within 2mV@20mA)?

Would be great if one could help me here! I really don't want distortion!

BTW when does the amp start to clip? Given a 3V source, a 25k log pot as preamp, does the Aleph30 clip at full throttle?

How many watts@8Ohm has the Aleph30 with stock rails really?

Thanks a lot for helping me out!

Cheers, Hannes

30W/8E
40W/4E

for AC current gain ,vary resistor in series with 220Uf connected from output rail to base of bjt in output CCS
hehe
your searches are lousy

and-yes-if you stick with stock values,you'll be in ballpark of everything
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Old 15th February 2007, 12:24 PM   #3
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So...R21 with 820 Ohms?

Well...you know..if some terms yet mean nothing to you, it's a bit hard. The specific number of the resistor in the schematic helps a lot.

But being not even 2 months in the DIY business I think I do quite well

Concerning current gain:
Sorry, but being in the ballpark is not enough for me - I would greatly appreciate a more numerical guess, something like 45-55%?

Thanks a lot! Hannes

BTW I thought the 30W is a rounded number? I read Pass' gear is underrated powerwise, so I expected something like 35Ws?

Don't forget, this info is added to the wiki so your chances are good, nobody has to ask that again ;-)
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Old 15th February 2007, 12:25 PM   #4
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Ah...and by the way: what about clipping?
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Old 15th February 2007, 07:04 PM   #5
Nelson Pass is offline Nelson Pass  United States
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adjusting current gain for Aleph30 necessary?
If you build from my stock designs, you will get accurate enough
results from the specified parts.

Any yes, they can be clipped.

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Old 15th February 2007, 07:58 PM   #6
wuffwaff is offline wuffwaff  Netherlands
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Hi,

clipping will occur at max current (two times the bias current at 50% ac-current-gain) or at max. voltage (about 4-5V under the rail voltage) depending on the load.

There are quite a few threads on setting ac current gain. Replacing the ac-c-g resitor with a pot, setting the amp at 50% ACCG and then replacing it with a fitting resistor seems the easiest way to do it. Theres quite some tollerance in the bjts and fets so sometimes you will need different resitors for both channels.

William
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Old 16th February 2007, 10:39 AM   #7
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Thanks for all your replies!

However I'm a bit worried clipping wise...

The A30 has a gain of 26dB for unbalanced input, that means input voltage is multiplied by a factor of 20, right?

So a 2V source (connected directly) drives the amp into the wall?

Consequently I can turn up the volume only to half of the max position (0.5x2V=1V times 20 gives 20V, 5V below rail)??

Turning it up further the A30 clips?!

This would practically mean that I can only turn up the volume to half of its maximum?!

Sorry for a truly stupid question, but at that position (half of max) do I get then already the full 30Ws?

Maybe somebody could take the time to reply to my little calculation?


Please help me out of this!

Thanks a lot!

Cheers, Hannes

EDIT: position half of max would mean roughly 2/3 for a log pot.
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Old 16th February 2007, 11:41 AM   #8
wuffwaff is offline wuffwaff  Netherlands
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Hi,

yes that is true but only if the 2V source puts out 2Volts. Normal CDs are recorded with a peak value much lower than 2V.

So if the max output is -6dB the CD player will "only" put out 1V and your A30 wont clip.

And since your volume pot is logarithmic half of the voltage will occur when it is almost in the 5 o clock position.

Im not shure I understand what you are afraid of. This will happen with every 30 watt ampifier.

William
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Old 16th February 2007, 12:06 PM   #9
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Oh yes I know.

I was just wondering wether I get the whole 30Ws before clipping or not?

Cheers, Hannes
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Old 16th February 2007, 01:35 PM   #10
Babowana is offline Babowana  South Korea
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I don't need to worry about
the distortion,
the current gain rate,
and the clipping
when I don't build it.

Why worry . . . ?


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