B1 Buffer Preamp - Page 129 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 8th February 2009, 07:54 PM   #1281
BFNY is offline BFNY  United States
diyAudio Member
 
Join Date: May 2004
Location: Saratoga NY
Quote:
Originally posted by rhysh


I can scrap the B1 all together, and just use the volume control but then i will be sending 50K over an interconnect, which is not a good idea...


Says who? This is DIY! Experiment and learn. To get a 50K max. output impedance you would need to use a 100K pot. Why so high? Use a 25K-50K pot then. What is the source impedance?

I made a simple passive attenuator in a small metal box with hardwired in and out connections (no RCA jacks, cables direct wired to the pot with RCA connectors). I made sure the output cables were short (about 20cm or 8") and low capacitance. It works fine and sounds very transparent. Others have made attenuators that mounted directly to the RCA input jacks (no output cables)

http://www.tweakaudio.com/Ultimate%20Attenuators.html

You still need to answer the question - "how much preamp gain do I *really* need for the loudest comfortable listening level?"

This is the whole premise of the B1 article. Try to build the preamp with that much gain, and no more, so that max level is achieved at max. rotation of the volume control. *Not* at 11'oclock, is more common.
  Reply With Quote
Old 8th February 2009, 08:15 PM   #1282
rhysh is offline rhysh  United Kingdom
diyAudio Member
 
rhysh's Avatar
 
Join Date: Sep 2008
Location: Southampton
Send a message via MSN to rhysh
A 50K signal over an interconnect is prone to interference with pretty much anything, it will pick up noise Very easliy, especially from AC lines. I have tried this in the past, the same with many others and got an undesirable result.

It may be DIY, but spending $300 on a attenuator project which may not produce desirable results is still a hole in my pocket, DIY or not.

The purpose of the B1 is to convert this into a 1K signal, which is much less prone to interference over an intrerconnect.

This will then be inputted into my preamp.

The purpose of the B1 in my case is to avoid as many interference problems as possible, not to be used as my 'preamp', just in the attenuator.
  Reply With Quote
Old 8th February 2009, 08:20 PM   #1283
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
the maximum output impedance of an unbuffered attenuator is one quarter of the pot value plus the source output impedance.

50k pot and Rs=1k0 gives Max Zout=[50+1]/4=12k75

10k pot and Rs=1k0 gives Max Zout=2k75

100pF of input and cable capacitance will roll off the treble @ ~125kHz using a 50k pot.
If the power amp is fitted with 1nF RF filter then the treble is rolled off @ ~12kHz.

Unbuffered pots need very careful selection of component values and the listener must accept compromise.
__________________
regards Andrew T.
  Reply With Quote
Old 8th February 2009, 08:22 PM   #1284
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally posted by rhysh
A 50K signal over an interconnect is prone to interference with pretty much anything, it will pick up noise Very easily, especially from AC lines. I have tried this in the past,
I think this experience is not due to using a 50k passive attenuator, but to some other problem.
__________________
regards Andrew T.
  Reply With Quote
Old 8th February 2009, 08:24 PM   #1285
rhysh is offline rhysh  United Kingdom
diyAudio Member
 
rhysh's Avatar
 
Join Date: Sep 2008
Location: Southampton
Send a message via MSN to rhysh
Quote:
Originally posted by AndrewT
I think this experience is not due to using a 50k passive attenuator, but to some other problem.

So do you think i would be fine, using a passive attenuator before my preamp with no buffer, if done properly? We are talking 1 - 2ft of interconnect.
  Reply With Quote
Old 8th February 2009, 08:38 PM   #1286
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
buffer all sources that have a high output impedance.
__________________
regards Andrew T.
  Reply With Quote
Old 8th February 2009, 08:40 PM   #1287
rhysh is offline rhysh  United Kingdom
diyAudio Member
 
rhysh's Avatar
 
Join Date: Sep 2008
Location: Southampton
Send a message via MSN to rhysh
Quote:
Originally posted by AndrewT
buffer all sources that have a high output impedance.

The output impedance of my DAC (main source) is 3R. If i dont need to use the B1 after my attenuator then i wont bother.
  Reply With Quote
Old 9th February 2009, 01:02 AM   #1288
BFNY is offline BFNY  United States
diyAudio Member
 
Join Date: May 2004
Location: Saratoga NY
Quote:
Originally posted by BFNY
To get a 50K max. output impedance you would need to use a 100K pot.
Sorry, this should read "To get a 50K max. output impedance you would need to use a 200K pot."
This would be way to high for a passive unbuffered attenuator.
  Reply With Quote
Old 9th February 2009, 08:54 AM   #1289
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally posted by rhysh



The output impedance of my DAC (main source) is 3R. If i dont need to use the B1 after my attenuator then i wont bother.
but you attenuator has a high output impedance.

If you want Rs<500r then the maximum pot value would be 2k.
Can your source equipment drive 2k//cable capacitance?
__________________
regards Andrew T.
  Reply With Quote
Old 9th February 2009, 09:50 AM   #1290
rhysh is offline rhysh  United Kingdom
diyAudio Member
 
rhysh's Avatar
 
Join Date: Sep 2008
Location: Southampton
Send a message via MSN to rhysh
My source will drive a normal 50k pot easily. What i am wondering is if i need the b1? Or could i just use a passive attntr.
  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
buffer/preamp? TheDriver41 Chip Amps 2 3rd October 2004 08:40 AM


New To Site? Need Help?

All times are GMT. The time now is 04:40 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