Passive pre.... what's going on with this one? - Page 3 - 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 11th July 2007, 09:41 PM   #21
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default Re: Re: Re: Re: Re: Re: Passive pre.... what's going on with this one?

Quote:
Originally posted by gootee
Would it be: Source, then a series 47uF, series 100R, 32pF to gnd, 20k (or 5K) pot, wiper out, to 10pF to gnd, series 470nF, 42K to gnd ?

And if there was an RF filter, I assume that it would go either before or after the 470nF (and might consist of a few hundred series ohms and a few hundred pF to gnd, and then maybe another few hundred series ohms).
Does anyone know if AKSA fits an RF filter to the front of his amps?
Hugh, where are you?

Tom,
I think you have interpreted correctly.
__________________
regards Andrew T.
  Reply With Quote
Old 11th July 2007, 10:05 PM   #22
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Default Re: Re: Re: Re: Re: Re: Passive pre.... what's going on with this one?

[QUOTE]Originally posted by gootee
[B]

Would it be: Source, then a series 47uF, series 100R, 32pF to gnd, 20k (or 5K) pot, wiper out, to 10pF to gnd, series 470nF, 42K to gnd ?



With those values above the first RF filter (100r series and 32pf to ground) it is -3db @ 49mhz
The second RF filter which is half the value of the 5k pot at worst 2.5k (2500r series and 10pf to ground) it is -3db @ 6.3mhz.

If you can hear these filters you must be a bat!

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 12th July 2007, 08:03 AM   #23
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi George,
we know you meant to say that the worst Rs of the pot is it's value divided by 4, so bat's we are not.
The more important issue when short, low capacitance cables are used, is whether an RF filter is fitted inside the power amp input.

For the 20k pot, I would suggest that the RF filter capacitor should be less than 150pF (0.8uS) and for a 5k pot we can tolerate just 680pF. Some would fit caps of only one third of these values (0.25uS) and say they can hear the difference from using anything higher.

AKSA,
please come in and talk to us.
__________________
regards Andrew T.
  Reply With Quote
Old 12th July 2007, 08:19 AM   #24
diyAudio Member
 
georgehifi's Avatar
 
Join Date: Sep 2003
Location: Manly Australia (Jewel of the Pacific)
Send a message via MSN to georgehifi
Looking at the bandwidth of his Aksa 55 he has limited it to -1db @ 35khz I would say this was determined by his input filter, good practice is to have all filters even on the feedback after this faster and faster.
Yes if his cap to ground on the input rf filter is 1000pf to ground then with the output of the passive pot at 2500r that creates a lowpass of -3db @ 63khz this is still ok, but any more than 1000pf at it will start cutting into the top end 20khz. But I have yet to see an amplifier with a input rf cap over 1000pf!

Cheers George
__________________
Avatar : Production Lightspeed Attenuator
www.lightspeedattenuator.com
  Reply With Quote
Old 12th July 2007, 08:41 AM   #25
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
Quote:
Originally posted by gootee
Last King
Maybe Idi Amin is the Swahili translation of Sawney Bean ?
__________________
2015 : World Cup Cat Hacky Sack
  Reply With Quote
Old 12th July 2007, 08:55 AM   #26
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
1000pF and a 20k pot is a killer (~=5uS).
F-3db=31.5kHz. This is clearly audible top cut.
Even changing to 5k pot only brings the top cut down to 1.3uS. This is still audible.
Many recommend the RF filter be in the range 0.5uS to 1.5uS but some argue that these too affect the top end and suggest we go even lower to about 0.2uS to 0.3uS. I use 0.68uS and it works for me.
__________________
regards Andrew T.
  Reply With Quote
Old 12th July 2007, 01:56 PM   #27
rabbitz is offline rabbitz  Australia
diyAudio Member
 
rabbitz's Avatar
 
Join Date: Jul 2003
Location: Newcastle, Australia
AFAIK, there is a low pass filter on the input and well under 1000pF.

The 47uF + 100R in series between the opamp and RCA on the CD player.
Cable 32pF
Pot 20K
Cable < 10pF
RCA of power amp to 470nF and then a mystery. All I know is the input impedance is 42K.
  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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Passive Crossover for Passive Subwoofer? Toast_Master Subwoofers 23 30th April 2006 12:14 AM
passive pre amp mschwilson Solid State 10 27th July 2005 01:43 PM
dB loss by using passive crossovers? Active vs Passive and 1st vs 4th order Hybrid fourdoor Multi-Way 3 11th July 2004 09:16 PM
Help with passive pre sejarzo Everything Else 9 27th April 2004 06:11 PM
Passive EQ? Johan_Ch Everything Else 1 4th September 2003 04:11 AM


New To Site? Need Help?

All times are GMT. The time now is 03:58 AM.


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