Destroyer x Amplifier...Dx amp...my amplifier - Page 309 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th August 2007, 11:21 AM   #3081
AndrewT is offline AndrewT  Scotland
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.
  Reply With Quote
Old 25th August 2007, 11:34 AM   #3082
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
Join Date: Sep 2005
I think so... Andrew... fwd in PDF format .
  Reply With Quote
Old 25th August 2007, 12:13 PM   #3083
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Default 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
  Reply With Quote
Old 25th August 2007, 01:13 PM   #3084
diyAudio Member
 
Graham Maynard's Avatar
 
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.
  Reply With Quote
Old 25th August 2007, 01:34 PM   #3085
sandyK is offline sandyK  Australia
diyAudio Member
 
Join Date: Apr 2007
Location: Sydney
Default VBE multiplier bypass capacitor

Thanks for the enlightening replies.
Graham's reply is particularly pertinent where the multiplier is mounted externally.
Regards
SandyK
  Reply With Quote
Old 25th August 2007, 01:54 PM   #3086
ecat is offline ecat  United Kingdom
diyAudio Member
 
ecat's Avatar
 
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 ?
  Reply With Quote
Old 25th August 2007, 02:05 PM   #3087
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
Join Date: Sep 2005
the cap should help counter inductance of those little leads... potentialy cleaning up what they pick up...?
  Reply With Quote
Old 25th August 2007, 02:13 PM   #3088
diyAudio Member
 
Graham Maynard's Avatar
 
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.
  Reply With Quote
Old 25th August 2007, 02:25 PM   #3089
ecat is offline ecat  United Kingdom
diyAudio Member
 
ecat's Avatar
 
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
  Reply With Quote
Old 25th August 2007, 02:49 PM   #3090
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
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.......
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 03:36 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2