Aleph P 1.7: 2k pot is in the signal path? - 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 19th December 2002, 08:17 AM   #1
swede is offline swede  Switzerland
diyAudio Member
 
swede's Avatar
 
Join Date: Sep 2001
Location: Geneva, Switzerland
Default Aleph P 1.7: 2k pot is in the signal path?

Hello!

I've had some thoughts on the 2k pot between R51 and R66 on the Aleph P 1.7 pre amp board. (see http://www.passlabs.com/pdf/aleph/apserv17.pdf)

Is it to be considered to be in the signal path, hence call for an expensive pot, or could I just go on and use any regular "found it on the street" pot?

Best,
//magnus
  Reply With Quote
Old 19th December 2002, 04:28 PM   #2
diyAudio Member
 
Join Date: Apr 2002
Location: Toronto
I do not use a pot as a gain resistor.

Actually I replaced R51 and R66 with a 2K2 single resistor.
I get plenty of gain from all my sources, becuase I have modified the input resisitor values for each source. The input resistors are before my selector switch and are sized for each different source to give good volume at about 90% of my attenuator setting.

I maximized the gain resistor, because I assummed like the BZLS the distorion would be the least with the largest resistor value.
I think there is also a reference in the P 1.7 manual about trying to maximize the gain setting for best performance.

I am very pleased with the resulting sound quality.
  Reply With Quote
Old 19th December 2002, 04:52 PM   #3
swede is offline swede  Switzerland
diyAudio Member
 
swede's Avatar
 
Join Date: Sep 2001
Location: Geneva, Switzerland
Default Thanks!

Thanks for your info. It is really helpful! I think I'll do the same as you do!

Though, I didn't get an answer to wether the pot is in the signal path or not. ;=) Is it?

//magnus
  Reply With Quote
Old 19th December 2002, 06:32 PM   #4
diyAudio Member
 
Join Date: Apr 2002
Location: Toronto
Default pot in signal path

I think the pot might be thought of as in the signal path because any fluctuation or noise from the source of Q16 will affect the gain of Q19 and vice versa.

Therefore anything connecting the two sources has an affect on the signal.

Maybe the fact that a larger value gain resistor creates less distortion is because it increases the slight isolation from Q19 to Q16. (This is my guess).
  Reply With Quote
Old 19th December 2002, 10:55 PM   #5
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
Look at it this way:
Assume a single-ended input. Drive the left half of the circuit. An output appears at the right side by virtue of having entered through the Source of that MOSFET via the 2k pot.
<i>Ta da!</i>
Yes, it's in the signal path.

Grey
  Reply With Quote
Old 19th December 2002, 11:00 PM   #6
swede is offline swede  Switzerland
diyAudio Member
 
swede's Avatar
 
Join Date: Sep 2001
Location: Geneva, Switzerland
Default In the signal path...

Hence, it is evil. ;=)

//magnus
  Reply With Quote
Old 19th December 2002, 11:06 PM   #7
swede is offline swede  Switzerland
diyAudio Member
 
swede's Avatar
 
Join Date: Sep 2001
Location: Geneva, Switzerland
By the way!

GRollins: Welcome back! I've missed you!

//magnus
  Reply With Quote
Old 20th December 2002, 12:25 AM   #8
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
Evil?
Well...
So are gain devices, if you choose to look at them in terms of the harm they do to the music.
Sooner or later, you're going to need to use a volume device of some kind, whether a pot, rotary switch & resistors, fancy transistor (or chip) array, or whatever. It will either be directly in the signal path, change the operating point of the gain device(s), or change the amount of feedback. Any way you slice it, you're going to have to shake hands with the devil at some point...might as well make up your mind to enjoy the experience.
Go ahead and put it 'in the signal path' (as though there were any other way...) and be done with it.

Grey
  Reply With Quote
Old 28th December 2002, 10:12 PM   #9
Cobra2 is offline Cobra2  Norway
DIY !
diyAudio Member
 
Cobra2's Avatar
 
Join Date: Apr 2002
Location: Stavanger, Norway
Send a message via Skype™ to Cobra2
Default Aleph P 1.7 pre amp Q:

While discussing the "gain" pots...

Why not use (re-construct) these as volume pot? (when not in need of balance).

What I have heard, in a balanced circuit, it is much better to "cancel out" sound between +/- phase w. even a cheap pot-meter, than having the best pot. in the signal-way.
(Ref.: Electrocompaniet).

Other thaughts?

Arne K
NORWAY
  Reply With Quote
Old 29th December 2002, 01:10 AM   #10
diyAudio Member
 
macka's Avatar
 
Join Date: Dec 2001
Location: Australia
HI Grey,

Nice to see you back,

I see the function of R15 in the BOSOZ to be basically the same as the 2K pot in the P1.7

I thought about this and found it easier to use R15 = 450R and parellel with a 2K Pot in series with say 150 - 200 R.

This would appear to make the use of the pot legitimate while keeping the Dale fixed resister in the signal path so it is only shunted by the pot when in use(for higher gain). Hence the effect of the pot on quality is minimised.

This is a bit off topic but I would like your clarification here Grey.

It seems in the Aleph P.17 Preamp the manipulation of the output levels is a bit misunderstood relative to the BOSOZ.

There is no input volume control in the P1.7 , only an internal pad for some 12 db attentuation so to set and forget, the Gain level controls (2Kpots) and and the Master Volume control at the output (a shunt attenuator).

I trying to figure out if the BOSOZ can be set up the same way as the P.17, or does the circuit perform differently in respect the the active use for R15 and of P1 & P2 level controls at the input and its optimum operating point.

In the Pass manual for the P.17 it states for best performance keep the gain level controls counter clock wide (minumum gain) and the Master Volume control as clock wise as possible.

This to me means keep the gain low and it is preferred for best performance not to have the output shunted down too much to enable reasonable voltage swing.

Could this perhaps suggest why there is a tendancy for everyone to use a 5K pot or (or series switched attenuator) rather than the shunt attenuator for the BOZSOZ so the output impediance is constant?

best regards

Ian
  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
Panasonic FM on signal path ? deuginthesky Digital Source 5 22nd July 2008 10:31 PM
How does this sound for a signal path? Fryguy Digital Source 0 25th May 2004 09:15 PM
Help identifying the signal path Eric Solid State 7 24th August 2001 09:10 AM


New To Site? Need Help?

All times are GMT. The time now is 07:59 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