Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 25th January 2013, 09:28 AM   #51
DrRick is offline DrRick  United States
diyAudio Member
 
Join Date: Nov 2004
Location: New Jersey, US, Third Stone From The Sun
Ian,
I meant to post both. Thanks!


Quote:
Originally Posted by ruffrecords View Post
I don't think I am THAT good but thanks for the kind words nonetheless.

People might also like my DIY site too:

http://www.ianbell.ukfsn.org/EzTubeMixer/


Cheers

Ian
__________________
Peace,
DrRick
  Reply With Quote
Old 25th January 2013, 11:24 PM   #52
popilin is offline popilin  Argentina
diyAudio Member
 
popilin's Avatar
 
Join Date: Oct 2010
Location: Traslasierra
Hi Ian, hi DrRick, thanks for the links !

Now try to show that putting a pot on the input of a high gain amplifier, in this case a 100W class A hybrid, is not a very good idea.

Sim#1 - Amp without pot.

Sim#2 - Amp with 100K pot.

Sim#3 - Amp with 50K pot.

Sim#4 - Amp with 10K pot.
Attached Images
File Type: jpg ScreenShot001.JPG (103.5 KB, 178 views)
File Type: jpg ScreenShot002.JPG (101.8 KB, 177 views)
File Type: jpg ScreenShot003.JPG (101.2 KB, 174 views)
File Type: jpg ScreenShot004.JPG (104.4 KB, 176 views)
__________________
I do not suffer from insanity, I enjoy every minute of it. -Edgar Allan Poe
I'm not crazy about reality, but it's still the only place to get a decent meal. -Groucho Marx

Last edited by popilin; 25th January 2013 at 11:30 PM. Reason: Tarzan-English
  Reply With Quote
Old 26th January 2013, 02:22 AM   #53
diyAudio Member
 
Join Date: Jun 2011
It may surprise you, but I did read the whole thread. While I note there are some 50 posts, a mere handful relate to pots, and not a single one proposes how to control the output volume in the absence of a pot, or a switched attenuator.

Pots, or variable attenuators, are a necessary evil in any practical music reproduction system intended for use in a domestic environment. Many systems moreover, manage to have a pot without any audible noise. Even in those systems where there is audible noise, it is masked by the music in most circumstances, and noise is regarded by many as the most tolerable of flaws in a reproducing system.

So what's your point?
  Reply With Quote
Old 26th January 2013, 03:55 AM   #54
popilin is offline popilin  Argentina
diyAudio Member
 
popilin's Avatar
 
Join Date: Oct 2010
Location: Traslasierra
Quote:
Originally Posted by counter culture View Post
It may surprise you, but I did read the whole thread.
I can vouch for this.

Quote:
Originally Posted by counter culture View Post
While I note there are some 50 posts, a mere handful relate to pots, and not a single one proposes how to control the output volume in the absence of a pot, or a switched attenuator.
What you say is correct, there are only a few posts referred specifically to the pot, some joke among friends, and as any exchange of ideas, several interesting issues arise.

Quote:
Originally Posted by counter culture View Post
Pots, or variable attenuators, are a necessary evil in any practical music reproduction system intended for use in a domestic environment. Many systems moreover, manage to have a pot without any audible noise. Even in those systems where there is audible noise, it is masked by the music in most circumstances, and noise is regarded by many as the most tolerable of flaws in a reproducing system.
In the world of the SS, the pot presents no difficulties, being low impedance devices, you can put very low pot values.
However, with valves, it gets more difficult, and the noise starts to be an issue.
We also discussed the effect on bandwidth and phase shift.

Quote:
Originally Posted by counter culture View Post
So what's your point?
As you say, the pot is a necessary evil, my point is to put it where do the least damage.
__________________
I do not suffer from insanity, I enjoy every minute of it. -Edgar Allan Poe
I'm not crazy about reality, but it's still the only place to get a decent meal. -Groucho Marx

Last edited by popilin; 26th January 2013 at 04:04 AM.
  Reply With Quote
Old 26th January 2013, 10:17 AM   #55
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
Originally Posted by popilin View Post
As you say, the pot is a necessary evil, my point is to put it where do the least damage.
It is fairly obvious that a large value pot contributes more noise than a smaller value one. It is also fairly obvious that the noise contribution of a pot will be greater if you put more gain after it. So to answer your question it will create the least noise if it is last in the chain. So what you really need is a 100W constant impedance T pad connected between the 100W amp output and the loudspeakers. This may well affect the damping factor but that is a whole other debate.

What you really need to look at are ALL the noise sources in the system including the source material and compare their values relative to the pot at each proposed placement of the pot.

Something to consider: just about every professional mixer uses 10K faders so just about every source you feed through your pre-amp has been through at least one and probably several 10K pots. If you use a 10K pot at your pre-amp input, how is its noise contribution likely to compare with that of all the 10K pots that preceded it?

Cheers

Ian
  Reply With Quote
Old 26th January 2013, 02:37 PM   #56
popilin is offline popilin  Argentina
diyAudio Member
 
popilin's Avatar
 
Join Date: Oct 2010
Location: Traslasierra
Quote:
Originally Posted by ruffrecords View Post
It is fairly obvious that a large value pot contributes more noise than a smaller value one. It is also fairly obvious that the noise contribution of a pot will be greater if you put more gain after it.
It is fairly obvious to you, that you know a lot and have all the experience.
Besides my lack of knowledge, I think slowly, and nor with the equations in front of me I could see the problem.
Judging by the countless number of circuits that adopt the traditional scheme, even with 500K pot, probably I am not the only one.
When I noticed the problem, it was a revelation to me ! and I want to share it.

Quote:
Originally Posted by ruffrecords View Post
So to answer your question it will create the least noise if it is last in the chain. So what you really need is a 100W constant impedance T pad connected between the 100W amp output and the loudspeakers. This may well affect the damping factor but that is a whole other debate.
I believe that without deep bass, musical experience would be incomplete, and given the not very low output impedance of valve amps, I designed my hybrid.
Anything that reduces damping factor, is not a good deal, especially when you do not use global feedback.

Quote:
Originally Posted by ruffrecords View Post
What you really need to look at are ALL the noise sources in the system including the source material and compare their values relative to the pot at each proposed placement of the pot.
That is precisely the idea, but as I said before, we have no control over the source material, for each format, analog or digital, it can vary greatly depending on the recording company, not just blame the recording engineer.

Quote:
Originally Posted by ruffrecords View Post
Something to consider: just about every professional mixer uses 10K faders so just about every source you feed through your pre-amp has been through at least one and probably several 10K pots. If you use a 10K pot at your pre-amp input, how is its noise contribution likely to compare with that of all the 10K pots that preceded it?

Cheers

Ian
Good point, but it gives me chills just thinking about it, is not a trivial issue, fortunately I do not need a mixer.
Another approach is to use transformer based attenuators, but the magnetic hysteresis curve is far from a straight line, and the use of exotic materials is far from my pocket.
__________________
I do not suffer from insanity, I enjoy every minute of it. -Edgar Allan Poe
I'm not crazy about reality, but it's still the only place to get a decent meal. -Groucho Marx
  Reply With Quote
Old 26th January 2013, 02:47 PM   #57
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
Quote:
Originally Posted by counter culture View Post
Pots, or variable attenuators, are a necessary evil in any practical music reproduction system intended for use in a domestic environment.
The new Benchmark DAC2 uses 32 bit internals to keep volume control enitely in the diigtal domain. As they put it, "digital inputs never pass through an analog potentiometer."
__________________
Ears aren't microphones.
  Reply With Quote
Old 26th January 2013, 03:38 PM   #58
DrRick is offline DrRick  United States
diyAudio Member
 
Join Date: Nov 2004
Location: New Jersey, US, Third Stone From The Sun
Ruff Sez:
"So what you really need is a 100W constant impedance T pad connected between the 100W amp output and the loudspeakers. This may well affect the damping factor................."
__________________
Peace,
DrRick
  Reply With Quote
Old 26th January 2013, 04:37 PM   #59
diyAudio Member
 
ruffrecords's Avatar
 
Join Date: Jun 2007
Location: Holt, Norfolk
Quote:
Originally Posted by popilin View Post
It is fairly obvious to you, that you know a lot and have all the experience.
Besides my lack of knowledge, I think slowly, and nor with the equations in front of me I could see the problem.
Judging by the countless number of circuits that adopt the traditional scheme, even with 500K pot, probably I am not the only one.
When I noticed the problem, it was a revelation to me ! and I want to share it.
Fair enough, but I think calling the volume pot a 'villain' is a little premature.

Quote:
That is precisely the idea, but as I said before, we have no control over the source material, for each format, analog or digital, it can vary greatly depending on the recording company, not just blame the recording engineer.
And the point is there is a whole range of noise sources in the pre-amp that you can control and pot noise is just one of them. You need to examine and quantify the others before you brand the volume pot as the 'villain'.

Cheers

Ian
  Reply With Quote
Old 26th January 2013, 04:46 PM   #60
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by popilin View Post
As you say, the pot is a necessary evil, my point is to put it where do the least damage.
Hmm. Not exactly news to anyone who has ever designed a radio receiver.

Perhaps a brief recap of Friis' noise equation might have got us to the point a little quicker.

Quote:
Originally Posted by rdf View Post
The new Benchmark DAC2 uses 32 bit internals to keep volume control enitely in the diigtal domain. As they put it, "digital inputs never pass through an analog potentiometer."
Good point. Not the one the OP was making, I think, but a good point nevertheless. Shame you couldn't have made it at the beginning of the thread, then we might have short-circuited some 50-odd posts...
  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
50K volume pot in place of a 20k volume pot? robgilmo Solid State 8 17th December 2012 11:34 PM
Need to Upgrade Tube Preamp Volume Pot MrKTrout Tubes / Valves 6 29th December 2011 03:01 AM
single push volume pot, volume up/down via a button. dragonxwas Analogue Source 4 2nd June 2011 06:56 PM
Volume control : - pot' or pot' variable feedback resistor (gain) ? ash_dac Solid State 5 1st August 2006 05:07 AM
Effect of using 50k pot instead of 100k pot in tube preamp? KT Tubes / Valves 5 17th February 2006 09:56 PM


New To Site? Need Help?

All times are GMT. The time now is 08:33 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