B1 Buffer Preamp - Page 80 - 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 17th October 2008, 12:25 PM   #791
Tolu is offline Tolu  Germany
diyAudio Member
 
Tolu's Avatar
 
Join Date: Mar 2007
Quote:
Originally posted by AndrewT

If a 100k passive pot is added between the two active stages the Rs, for the next active stage varies upto 25k. This reduces F-3dB to 5400Hz. Nothing short of a disaster in reproducing the sound in anything close to High Fidelity. [/B]

Who uses a 100k pot in a passive pre?

10k should be max! With this you have a good match for input and output imedance if your source is lower than 1k and your amp higher than 25k!

What many people ignore is the fact that output impedance of a passive pre (e.g. a 10k pot) is significant lower than 1k in the most used range of -40 to -20dB!
  Reply With Quote
Old 17th October 2008, 12:45 PM   #792
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally posted by Tolu
What many people ignore is the fact that output impedance of a passive pre (e.g. a 10k pot) is significant lower than 1k in the most used range of -40 to -20dB!
wrong.
with Rs=1k0, the 10k pot will have a total source resistance as presented to the previous stage of 11k.
The output impedance of that 10k pot @ -20dB will be ~1k0//10k0~=910r. That is not significantly lower than 1k.
It is the value I used in the example calculation in para 2 of my post.

If a 100k pot were to be used the Rs @-20dB will be~9k01. That is 9times worse than my example and yet many builders adopt this 100k pot value simply because they are not aware of the compromises that are being foisted upon them.
So much for informed decision making.

If you want to get involved in the hobby of amplifier "design" you MUST be prepared to do the arithmetic.
If not then build exact copies of reputedly good designs.
Willy nilly swapping of components is not design.
__________________
regards Andrew T.
  Reply With Quote
Old 17th October 2008, 01:08 PM   #793
ABO is offline ABO  Netherlands
diyAudio Member
 
Join Date: Dec 2003
Location: Amst
The source resistance presented by a 10k pot at its -20 dB setting is 900 ohm, which is lower than b1 output resistance.
  Reply With Quote
Old 17th October 2008, 01:52 PM   #794
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally posted by ABO
The source resistance presented by a 10k pot at its -20 dB setting is 900 ohm, which is lower than b1 output resistance.
but, you forgot to add the source resistance presented by the B1 preamp.
__________________
regards Andrew T.
  Reply With Quote
Old 17th October 2008, 02:00 PM   #795
sparky6 is offline sparky6  Canada
diyAudio Member
 
Join Date: Oct 2008
Default theory refresher

First of all, I'd like to say thanks for the refresher to Stein2, Steenoe, AndrewT and Tolu - I truly appreciate your time in bringing me up to speed.

One question remains though about the rated input Z of the power amp ->25K. I'm a still a bit confused by this number in the specs. In your eg. you're using values of 100 for input Z ????

I was advised to choose a value close to this for the series stepped attenuator.

Now at about 1 o'clock (which is as loud as I'd want to go) I have 22k5 for Rs and 2k5 to ground (-20 dB). The interconnects are short ~.7m - so what would would be a rough calc for the LP cutoff?
  Reply With Quote
Old 17th October 2008, 02:07 PM   #796
Tolu is offline Tolu  Germany
diyAudio Member
 
Tolu's Avatar
 
Join Date: Mar 2007
@AndrewT
Not wrong!

It's only the definition of significantly! If you compare 910R with 1k than it is 9% lower. A stockholder would say that a 9% lower Dow Jones is significant!
  Reply With Quote
Old 17th October 2008, 02:28 PM   #797
ABO is offline ABO  Netherlands
diyAudio Member
 
Join Date: Dec 2003
Location: Amst
Well, In electronics, I wouldn't normally call a 9% improvement significant.

Andrew, The way I see it is that you want to lower the source resistance as seen by the load of the preamp. With the B1 this resistance is 1k. With a passive pre, this resistance is dependent on the volume setting. With a 10k pot it is a value between 0 and 2k5.

At -20dB it is 900 ohm. This means that for volume settings between 0 and -20dB the source resistance (as seen by the load, a power amp for instance) is lower with a passive pre.

My subjective assessment of B1 concurs with these calculations, i.e. it sounds undynamic with 1k output resistance in my system.

Thanks,

Abo
  Reply With Quote
Old 17th October 2008, 08:33 PM   #798
steenoe is offline steenoe  Denmark
diyAudio Member
 
steenoe's Avatar
 
Join Date: Mar 2004
Default Re: theory refresher

Quote:
Originally posted by sparky6
First of all, I'd like to say thanks for the refresher to Stein2, Steenoe, AndrewT and Tolu - I truly appreciate your time in bringing me up to speed.

One question remains though about the rated input Z of the power amp ->25K. I'm a still a bit confused by this number in the specs. In your eg. you're using values of 100 for input Z ????
You are welcome.
The figures were chosen for the purpose of showing the overall picture of how the impedance issue between two circuits works. It was not a real life example. Illustrative purposes, means just that. Anyway, with so few parts involved i wouldnt shy away from just building it, and see if works out as hoped for.

  Reply With Quote
Old 23rd October 2008, 07:12 PM   #799
diyAudio Member
 
Join Date: Jun 2007
Default Linear taper / no shunt?

Please forgive my simple-minded question.
The recommended pot for P100 / P200 is a 25k linear taper. My previous understanding is that 1) a log taper will give a better usable range for audio, 2) a linear taper can be substituted if a fixed resistor is added (about 5-10% of the pot) as a shunt between the input and the wiper. Most importantly, This would "restore" the usable range and secondarily the shunt resistance may provide some benefits (perhaps when used in a so called "passive preamp" configuration).

So my question is why does the design of the B1 call for a pot with a linear taper and why is no shunt used (at least I was not able to identify an equivalent)?

Alternatively, does the 25 k linear pot provide enough usable range to be used as a volume pot?

Forgive what is probably a simplistic question.
  Reply With Quote
Old 23rd October 2008, 09:58 PM   #800
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Default Re: Linear taper / no shunt?

Quote:
Originally posted by WithTarragon
Alternatively, does the 25 k linear pot provide enough usable range to be used as a volume pot?
Normally you would use an audio taper pot for the range, but this is a
"no-gain" circuit, and most people find that they don't need that range.

  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 06:16 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