Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 10th July 2004, 06:25 PM   #11
johnf is offline johnf  Germany
diyAudio Member
 
johnf's Avatar
 
Join Date: Jun 2004
Location: Bremerhaven
To add another possible gotcha to the list, I wonder if mixing in a potentiometer with the gain-setting resistors might shorten its life. Some pots, such as those in the Alps RK27 series, expect to only see AC. In a DC circuit, their sliders can suffer silver ion migration and a shortened life. The Alps application engineer I talked to couldn't get any numbers for an excessive DC bias. I got the impression people are careful not to apply one.
  Reply With Quote
Old 10th July 2004, 08:45 PM   #12
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally posted by sss


what if i'll do i in a pre amp , i think most op amps will be stable from unity gain to ~ 27db

Preamps are there to attenuate the signal, NOT to increase it. Even with the preamp gain at 1, and the power amp at the usual 30, your sound level will be much too high for comfortable listening. It will probably blow out your speakers, if the amp doesn't clip beforehand. Imagine a CD player with a 2V output amplyfied 30 times, that 60V in 8 ohms = 450Watts.
One reason the pot is normally at the input is to let you attenuate the signal 30 or 40 dB to prevent our ears from being blown out.

Jan Didden\
__________________
Never explain - your friends don't need it and your enemies won't believe you anyway - E. Hubbart
Check out Linear Audio Vol 7
!
  Reply With Quote
Old 11th July 2004, 01:15 AM   #13
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Quote:
To add another possible gotcha to the list, I wonder if mixing in a potentiometer with the gain-setting resistors might shorten its life. Some pots, such as those in the Alps RK27 series, expect to only see AC. In a DC circuit, their sliders can suffer silver ion migration and a shortened life. The Alps application engineer I talked to couldn't get any numbers for an excessive DC bias. I got the impression people are careful not to apply one.
How many mA can pass through 1 small potentiometer?
  Reply With Quote
Old 11th July 2004, 01:58 AM   #14
diyAudio Member
 
Hybrid fourdoor's Avatar
 
Join Date: May 2003
Location: Ft. Worth, TX
Here read this thread, I asked the same thing....notice how i did the series resistor and pot to eliminate any chance of a short.
__________________
I enjoy my organic shapes.....
  Reply With Quote
Old 11th July 2004, 04:38 AM   #15
BStrum is offline BStrum  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Cabin John - MD
Default Inverted Op Amp Pre

I recently completed a preamp using an inverted op amp circuit with a 50K Nobel stereo pot controlling the gain in the feed back leg. The topology whihch always uses maximum input signal does optimize the signal to noise.

I used ADC 825s and set the maimum gain of the Preamp at 6 db (2X input).

I also added a mono L + R output with volume for sub woofer control.

The unit runs off +/- 12 V using AA batteries.

It is very quite and sounds great.

It is currently diving an intersting system.

Bill Strum
__________________
Bill Strum
Bstrum@imagematics.com
  Reply With Quote
Old 11th July 2004, 07:25 AM   #16
sss is offline sss  Israel
diyAudio Member
 
sss's Avatar
 
Join Date: Jul 2003
Location: Israel , haifa
Send a message via ICQ to sss
Default Re: Inverted Op Amp Pre

Quote:
Originally posted by BStrum
The topology whihch always uses maximum input signal does optimize the signal to noise.
thats what i'm trying to say
  Reply With Quote
Old 11th July 2004, 08:06 AM   #17
johnf is offline johnf  Germany
diyAudio Member
 
johnf's Avatar
 
Join Date: Jun 2004
Location: Bremerhaven
Quote:
Originally posted by lumanauw

How many mA can pass through 1 small potentiometer?
I couldn't get an answer on that. You might get some ideas if you click on the link marked "Caution" under "Related Informations" on this this Alps page.
  Reply With Quote
Old 11th July 2004, 09:56 AM   #18
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
I built an inverting preamp with a 100K Alps pot.

While it sounded great, the pot wore out in a very short time.

Commercial designs like this capacitor couple the wiper and then add an input bias resistor.
  Reply With Quote
Old 11th July 2004, 12:08 PM   #19
mskeete is offline mskeete  United Kingdom
diyAudio Member
 
Join Date: Oct 2003
Location: London
I've built a few preamps with a pot in the feedback loop of an inverting opamp. The pot used to become noising after a few months but I realised one day that I hadn't bothered to add a input capacitor. Once I added the input capcitor and used a fet opamp everything has been OK (so far)

I've also seen a nice non inverting circuit in electronics world december 1995 pg 1012. This uses a 10klin pot. The advantange of this circuit is that the opamp gain is very low when the signal is reduced to 0. See attached

Mark
Attached Images
File Type: jpg gain.jpg (5.9 KB, 125 views)
  Reply With Quote
Old 11th July 2004, 02:50 PM   #20
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
Just a point of interest/curiosity. There is a commercial powrr amp, I think it is AccuPhase, that has switchable gain. It's not a volume control, but rather is supossed to let you match the gain with speaker sensativity. Anyway this shows that there is at least one respectable commercial design that does not consider gain to be completely "untouchable"
  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
feedback loop rmenger Instruments and Amps 4 21st October 2006 07:04 PM
feedback loop rmenger Parts 0 29th July 2004 01:24 PM
ZEN V4 without feedback loop Selfmade Pass Labs 8 20th February 2003 03:38 PM
Feedback Loop help KevinLee Solid State 4 1st December 2002 01:39 AM
Shunt volume controll (scaling the shunt resistors) Freddie Solid State 7 30th August 2001 09:47 PM


New To Site? Need Help?

All times are GMT. The time now is 02:05 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