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Old 17th April 2009, 01:38 AM   #371
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Nigel,
if you are using BL devices, the source resistor values will necessarily be higher for a certain current. The biasing points are now right (videlicet as desired). R6/7 set the closed-loop gain, you could try a lower value for R6, say 180 Ohm instead of 220 Ohm.
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Old 17th April 2009, 02:18 AM   #372
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Quote:
Originally posted by Lumba Ogir
Nigel,
if you are using BL devices, the source resistor values will necessarily be higher for a certain current. The biasing points are now right (videlicet as desired). R6/7 set the closed-loop gain, you could try a lower value for R6, say 180 Ohm instead of 220 Ohm.
I think I understand this... reducing R6 means more feedback, which mains reduced gain, right? Sounds like an interesting thing to try.

This isn't the same issue I asked about, though - so you don't like the idea, or I misunderstood? And if I msunderstood, why does the schematic I posted have the 240R resistors on the drivers bases ?

Too late to change R6 values tonight.... It'll have to wait till tomorrow.

Cheers, and thanks

Nigel
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Old 17th April 2009, 10:11 AM   #373
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Nigel,
answering your questions:
Quote:
your schematic didn't show the bjts between R3 and R4 and the jfets - but these don't make any difference, right?
Right.
Quote:
I think I understand this... reducing R6 means more feedback, which mains reduced gain, right?
Right. You may want to go even lower, towards 100 Ohm, using 1W resistors.
Quote:
This isn't the same issue I asked about, though - so you don't like the idea, or I misunderstood? And if I msunderstood, why does the schematic I posted have the 240R resistors on the drivers bases ?
There`s no need for base resistors here.
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Old 17th April 2009, 10:59 PM   #374
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Lumba, your suggestion certainly helped to cut gain down. In each channel I paralleled two 470R with the 220R already in the circuit, bringing the total value down to 112R or so. The amp is still a bit loud, but in a more practical range - by which I mean not so low on the volume control it's impossible to get right. Tomorrow (time permitting) I'll get a couple of 100R 1W resistors and do things properly. (At present they're just hacked on to the back of the boards...

Thanks

Nigel
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Old 17th April 2009, 11:28 PM   #375
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Nigel,
to me, the closed-loop gain is more "proper" now for this amp. Could you make a drawing everything specified?
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Old 18th April 2009, 01:45 AM   #376
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nice work chief,

would like to see your circuit now if you have time (please )

-Dan
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Old 19th April 2009, 02:49 AM   #377
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Sorry about the delay in posting the circuit, guys... Lost a whole day to fixing a borked linux installation... and am not finished yet, unfortunately... Will post as soon as poss.

Cheers
Nigel
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Old 19th April 2009, 07:39 AM   #378
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No stress mate

-Dan
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Old 19th April 2009, 09:34 AM   #379
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Hi Dan,

In post #358 you seem to query if the Y grade of fet is correct but in the link (at the bottom of your posts) to the Jean Hiraga Le Monstre you talk about building this amp using the original fets which are shown in the schematic as the Y grade. Can you let me know which grade you used?

thanks, sp
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Old 19th April 2009, 12:52 PM   #380
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the first one i built used y or V grade.

unsure if it is really a Y or a V though the writing on the jfets is so small...

recently i am unsure if it is y like the hiraga article suggests or a v like the data sheet suggests.

also i dont have the amp anymore so i cant check !!!

sorry for no help !!!

-Dan
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