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Old 27th February 2002, 06:59 PM   #1
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Default the (in-) famous mini-Crescendo by elektor

i'm about to begin my mini-crescendo project, i have all the parts needed.
The big question is, are there any 'must-do' modifications to the original, to improve stability, to get rid of oscillation, ripple rejection, and so on...

-any suggestions to what kind of volumecontrol/drivestage/inputselector i should use?

-any comments or help greatly appreciated!
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Old 27th February 2002, 07:52 PM   #2
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Hi Triatic,

I donīt know if you already have a pcb but if not try to use 2pairs of Fets per channel. The 7A max current with just one isnīt very much. Get rid of the coupling cap in the feedback and use a dc-servo with an OP77 (described in a few other elektor projects).

Donīt use wirewound source resistors cause this will def. lead to oscillation. Use MJE340/350 as drivers. These can be used in place of the BF469/470 without changes and do sound a lot better. ( BD139/140 are a second choise if the first is not available)

Try to change the input filter to something like 100-160kHz (-3dB).

If you want to do more use a regulated supply for the driver section (LT137AT/LT337AT) at about 3-4 volt above main supply level.

if possible use a double mono configuration for the (unregulated) power supply.

Iīve removed the output inductance in all of the crescendos Iīve build without any problems, you can use 33 and 330pF ceramic caps for the 2SK135 (33 between D and G, 330 between G and S)


william
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Old 28th February 2002, 03:56 AM   #3
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Default Crescendo

Hi Triatic

Since Wuffwaff suggested you to try 2 pairs of Fets, you might as well look at the Crescendo, which is the "big brother" giving about 150W if I'm not mistaken. The circuit, if I remember correctly is "similar" with different component values, that's all. Just solder in the component values of the bigger unit into the Mini Crescendo layout and you'll have a mini version of the big Crescendo.
I have built both and the big brother sounds definitely better. As Wuffwaff had pointed out, you'll be very much better off with the dc servo. It opens up the sound whereas the vocals in the original arrangement can sound somewhat compressed at times. Please remember the voltage ratings of the caps need to be much higher if you build the bigger version.
If oscillation happens(which some of my friends experienced!), the DC offset will wander and the 10R resistor in the Zobel network becomes really hot!

ckt
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Old 28th February 2002, 08:20 AM   #4
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I have built both the Crescendo and Mini-Crescendo with output Mosfets being 2SK1058 and 2SJ162. I designed my own boards, eliminating wirewound Source resistors and replacing these with paralleled 1ohm 1watt resistors. I had oscillation problems with the big brother. The input section was powered separately through a regulated supply - 4 volts higher than the output section. I moved the Zobel network after the 'output choke' rather than the more usual 'before' the choke configuration. This move alone stopped the oscillations. However, I further added caps as already stated by 'wuffwaff' just to be doubly sure. The sound quality is good but not upto the standard of more modern designs.
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Old 28th February 2002, 08:28 AM   #5
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Hi Diyman,

when building the big brother (70-75V supply) maybe 3 or better 4 paralled fets would do. For my self I build one with 3 paralled fets and the supply voltage of the mini crescendo. This gives about 80W/8Ohm and almost double into 4 Ohms and again into 2 Ohms (with 2x 500VA power supply).

With the lower voltage power supply (42V) you can use BC550/560 in the driver stage (instead of 546/556) and the choice of caps for your power supply is a lot easier.

Since I donīt need the extra power (I measured about 240W/8Ohm with 3 paralleled fets and 75V) Iīm very happy with this configuration.
It drives my pair of thiel cs6 without any problems.

william
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