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 25th August 2007, 11:21 AM #3081 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi Nordic, is the schematic in post 3077 the one I need for that link? __________________ regards Andrew T.
 25th August 2007, 11:34 AM #3082 Nordic   diyAudio Member     Join Date: Sep 2005 I think so... Andrew... fwd in PDF format .
 25th August 2007, 12:13 PM #3083 AKSA   diyAudio Member   Join Date: Sep 2001 Location: Melbourne, Australia Bypass Sizing I try to set it up for 6dB loss at around 2Hz. This is pretty conservative. If the cathode resistor is say 1K, then 6dB down at 2Hz corresponds to a capacitive reactance of 1K at this frequency; half signal down the resistor, half down the cap. Formula is Xc = 159000/F x uF, or uF = 159000/Xc x F. So, for 2Hz and 1000R, this is: uF = 159000/2 x 1000, or 79.5uF. NPV would be 100uF in this case. There are other ways to achieve this figure, but this is a good guide, and very broadband. You should set it up so that the time constant, given by the product of the cathode resistor and the cap size (uF x megohms) gives a reciprocal in Hz which should NOT be matched in any other section of the circuit to avoid resonant phase effects. Cheers, Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com
 25th August 2007, 01:13 PM #3084 Graham Maynard   R.I.P.     Join Date: Dec 2003 Location: UK Hi Andrew, Re 22uF across the V-be multiplier. If these multiplier components are all mounted and working in close proximity with the VAS/driers then I am pretty certain that whether any value of C is fitted or not could not be determined by listening tests. However when the V-be multiplier transistor is remotely sensing temperature away from the pcb, then it would make sense to mount a capacitor on the pcb itself. Value ? A sub min 22uF is physically small, cheap and convenient, so will do the job just fine. Maybe a 470nF ceramic would do too, but not as common or cheap. I have used 10uF either side of the slider on my own circuit. It works fine, though I heard no difference when no capacitor at all was fitted, though the circuit might well not then behave predicatably during overdrive or overload. (And I quite often drive my amps beyond 'normal' requirements because if they can take overdrive and overload without probs, then they are not likely to be upset during normal operation either.) Cheers ......... Graham. __________________ http://www.gmweb1.net/ and http://www.gmweb2.net/
 25th August 2007, 01:34 PM #3085 sandyK   diyAudio Member   Join Date: Apr 2007 Location: Sydney VBE multiplier bypass capacitor Thanks for the enlightening replies. Graham's reply is particularly pertinent where the multiplier is mounted externally. Regards SandyK
 25th August 2007, 01:54 PM #3086 ecat   diyAudio Member     Join Date: Jun 2007 Very informative. Hugh, Graham thank you. So, if I have this right : External Vbe pcb consists of the resistors and the transistor. The capacitor should remain on the main pcb. What are the problems with keeping things simple and mounting the cap on the external pcb ?
 25th August 2007, 02:05 PM #3087 Nordic   diyAudio Member     Join Date: Sep 2005 the cap should help counter inductance of those little leads... potentialy cleaning up what they pick up...?
 25th August 2007, 02:13 PM #3088 Graham Maynard   R.I.P.     Join Date: Dec 2003 Location: UK Hi ecat, I would go with your summary. Amplifiers react at RF frequencies not just AF, and as Carlos has commented - at RF a wire is an inductor. The V-be multiplier potential is slewing against 0V heatsink ground - a voltage field load on VAS collector. This is why I prefer to have the VAS device on the heatsink too - minimise voltage field generation on wiring near to the small signal devices. Hi Nordic, I now see you just got in just in front of me whilst I was writing this reply. Cheers .......... Graham. __________________ http://www.gmweb1.net/ and http://www.gmweb2.net/
 25th August 2007, 02:25 PM #3089 ecat   diyAudio Member     Join Date: Jun 2007 And thanks again Graham, Looks like I need a new pcb, lol. I have loads of room over the NFB track
 25th August 2007, 02:49 PM #3090 Nordic   diyAudio Member     Join Date: Sep 2005 As always, this hobby requires occasional tradeoffs..I have seen the cap done on the bias servo board itself.. and from a practical point on the pcb, this will just have to do for now.... standard dx doesn't even have a cap there.......

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