Double buffered B1 - 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 6th October 2014, 03:25 PM   #1
diyAudio Member
 
Join Date: Mar 2014
Default Double buffered B1

Well my old faithful preamp of 18 years is being temporarily retired in order for some major upgrades. so I need a stop gap. With me stop gap probably means about 6 months.Based on what I had lying around in the parts bin a B1 seemed to be a very good starting point. Quick and easy and cheap to build (esp as I have a +/-15v supply lying around) and if nothing else would be better than the unit I am taking out of service.


when planning the build I started wondering about the age old question with line stages, over whether is it better to put the buffer before of after the pot. I realised that, as I have enough transistors for 4 buffers I could build something that, with a couple of extra switches would allow me to select from one of 4 options

Passive
Buffer before pot
buffer after pot
buffers before and after.

Which will allow me to test whether there is any difference and if I would have been OK with a passive line stage all along.

I have a 10K stepped attenuator on its way, mainly as I will need that for the other pre-amp rebuild, so will fit that initially. Once the stripdown of the old pre-amp is underway, then a 50k will become available. I could go really silly and have selectable pots to compare 10k and 50k in passive mode.

Will all the above answer any pressing objective questions...Nope, and I hope that subjectively I can't hear any difference between the 8 modes. But no harm will be done and I will at least know if the outputs of my source components have any limitations for my use cases
  Reply With Quote
Old 7th October 2014, 01:40 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
In general a vol pot cannot drive the audio load that follows. This becomes a much bigger problem when higher value vol pot is used and/or it is used to drive some interconnect cabling.

Keeping to 10k for the vol pot and using <<1m of low capacitance cable may not require a buffer to maximise the audio signal that your want to pass.

Any more capacitance than suggested or more impedance and a buffer becomes mandatory.

A buffer before the vol pot is never required.

It's the Source that may or may not require a buffer to drive the interconnect cable and the paralleled vol pot load.

Source>Buffer>cable>vol pot>Buffer>cable>Amplifier.

This results in 4 Buffers to drive 4 cables in a stereo connection.
__________________
regards Andrew T.
  Reply With Quote
Old 7th October 2014, 01:57 PM   #3
diyAudio Member
 
Join Date: Mar 2014
In general no, but no harm in testing. And if the source cannot drive the cable and load, then a high input impedance is good. I hope to hear no difference with or without the input buffer. I also hope to discover that a fully passive mode is not as good. Whichever way I will learn something, if only that I am cloth eared and driven by preconceptions.
  Reply With Quote
Old 7th October 2014, 02:39 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I quite liked my first attempt at a DIY passive pre (vol pot inside a little metal box).
I had just bought my Tannoy Berkleys (about 1978, the last of the HPDs before the factory got burnt down and they had to make new alloy casting moulds) which are reputed to have a quite strident Treble response.

At low volumes it all sound nice.
As I turned it up the treble became more rolled off. This probably resulted in a "too loud" filter being implemented to partially attenuate the strident treble.
Result: even at very loud, it still sounded nice.

As I learned more, I removed the passive pre.
__________________
regards Andrew T.

Last edited by AndrewT; 7th October 2014 at 02:41 PM.
  Reply With Quote
Old 8th October 2014, 06:48 AM   #5
omishra is offline omishra  India
diyAudio Member
 
Join Date: Nov 2008
Location: PNQ, MH
I have build one

I am using one, double buffer Pass B1. I have stacked PCBs over each other.
B1 Buffer -> Volume Control -> B1 Buffer

It depends upon source whether to sound same or have improvement. Many of my friends happy without source side buffer.
image746.jpg
image748j.jpg

Same with stacked DCB1. I found it get more details for phono stage and Tuner, but ineffective with my DAC.

IMG_20141001_194230-S.jpg

IMG_20141001_194250-S.jpg

Last edited by omishra; 8th October 2014 at 06:50 AM.
  Reply With Quote
Old 8th October 2014, 10:26 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Same with stacked DCB1. I found it get more details for phono stage and Tuner,
I think there is a message in there that you are not seeing.

A Buffer at the input should have absolutely no effect on the system performance.
The fact that you are hearing a change tells me that something is actually changing.

I would conclude that these two Sources are struggling to drive the combined load of the cables and the input impedance of the Receiver and the RF filter in the Receiver.

Taking that conclusion to what you did, we arrive at isolating the Receiver input impedance and the Receiver RF filter from the load that the Source has to drive, you have now got the Sources just driving the cable and a very high Rin without an RF filter.

This then tellls me that your two Sources are the problem.
Sort the sources and make them able to drive the Cables AND the Receiver AND the very necessary RF filter.

Move the Buffer to the output of the two Sources. Build two Buffers if that is what is needed.
Sort the problem, don't just plaster over the crack.
__________________
regards Andrew T.
  Reply With Quote
Old 8th October 2014, 10:51 AM   #7
omishra is offline omishra  India
diyAudio Member
 
Join Date: Nov 2008
Location: PNQ, MH
okay I forgot to add one thing. I have LDR based volume control which as impedance of ~13KΩ. I faced loss of LF and details HF. So people who are happy with only buffer after volume control are using volume control either 25KΩ or 47KΩ.
  Reply With Quote
Old 8th October 2014, 10:59 AM   #8
diyAudio Member
 
Join Date: Mar 2014
So you recommend extra buffers on everything? When raising input impedance in one place achieves the same. If raising input impedance is the cure why is that 'plastering over the crack'. In an ideal world all sources have vanishing small output impedance and good current draw and all receivers have very very high input impedances (at least in the single ended world).

Mind you, an active cable has certain audiophool pretentions and would impress visitors.
  Reply With Quote
Old 8th October 2014, 11:06 AM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
A Buffer is a 1x gain amplifier with a high input impedance and a low output impedance.
Most Buffers are designed to perform a particular duty.
Line driving Buffers are probably the most common.
They are designed to drive line level signals into interconnect cable with a Receiver circuit at the far end.
They are so useful that literally dozens of designs are posted on this Forum.
Choose one to suit the duty.


When do you need a Buffer?

When the Source is incapable of driving the interconnect cable and the load on the far end of that cable.

Most Sources can drive short low capacitance cables, resulting in very few Sources needing a Buffer.

When a Buffer is needed, it should not be placed at the wrong end of the cable.
Quote:
In an ideal world all sources have vanishing small output impedance and good current draw
We are not in an ideal world.
Sources are usually designed and manufacturered down to a price. That price shaving usually results in a just adequate current sourcing capability from a barely low source impedance.
When those two stack up together with cable capacitance, the Source proves to be not capable of sourcing sufficient current.

If all sources were designed to drive 50ohm transmission line from a source impedance of 50ohms or less then we would be in an ideal world.
That is not what exists.
__________________
regards Andrew T.

Last edited by AndrewT; 8th October 2014 at 11:11 AM.
  Reply With Quote
Old 8th October 2014, 11:28 AM   #10
diyAudio Member
 
Join Date: Mar 2014
I think we will have to agree to disagree. My sources are fixed and not in line for replacement short term. My pre-amp is on its last legs and IS. I have control over that and the cables. If the source cannot drive the combined R and C presented, you suggest removing C from the equation and I am testing increasing R. Neither are invalid, but I have the parts to hand to build it the way I want to test and don't to build it the way you suggest.

I am hoping to find the input buffer to be surplus to requirements and will be happy if that is that case. If it isn't then I will have to decide if I need to prioritise other changes in the system.
  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
Pot > B1 > step up transformer > B1? sharpi31 Pass Labs 19 20th October 2013 02:45 PM
Double, Double D Hole maker? Destroyer OS. Equipment & Tools 4 13th May 2013 03:45 AM
Mesmerize B1 vs Original Pass B1 - please guide me! KT Pass Labs 9 6th September 2012 06:57 PM
FS: pass labs buffered B1 deepgroove Swap Meet 6 18th February 2010 06:40 PM
Double rail PSU, double ground rail connexion ? youyoung21147 Chip Amps 5 6th November 2005 06:12 PM


New To Site? Need Help?

All times are GMT. The time now is 08:21 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