Commercial Gainclone kit- building instructions - Page 157 - diyAudio
Go Back   Home > Forums > Commercial Sector > Vendor Forums > Audio Sector

Audio Sector Kits & PC boards from AudioSector

Reply
 
Thread Tools Search this Thread
Old 13th March 2012, 01:00 PM   #1561
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
If you made a little mistake then just fix it or acknowledge Andrew's help. It was not a negative comment. This is a forum that should be based on fact and when a statement is wrong but not corrected most will view it as fact.

The DC offset on this amp is abysmal. I think you will be very unhappy with it as long as you have DC offset levels like those quoted. I enjoy my LM3886 amp with 2 and 3mV offset. Offset pushes out the drivers for 100% of the time the amp is powered. It lowers your dynamic range and you will actually miss much of the music.
I did a little research and found, guess who... AndrewT explaining the problem for us. He is followed by another explanation from another knowledgeable member so read on from the link.
DC offset and gain on GC LM3886
The offset must be fixed. You could place a huge cap at the output or vary some of the resistors values to see if you can reduce it.
Uriah
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 13th March 2012, 01:16 PM   #1562
diyAudio Member
 
Join Date: Feb 2010
Quote:
Originally Posted by dirkwright View Post
That's not right I think. Those headphones have 120ohm impedance and 74dB/volt sensitivity. You should only need a couple of watts to drive them to full output.
i'm telling you the truth this thing is heavier to drive than the hifiman he-6

this is a link to the specs of my cans: Wharfedale ID1 Isodynamic - Wikiphonia
  Reply With Quote
Old 13th March 2012, 01:24 PM   #1563
diyAudio Member
 
Join Date: Feb 2010
Quote:
Originally Posted by udailey View Post
If you made a little mistake then just fix it or acknowledge Andrew's help. It was not a negative comment. This is a forum that should be based on fact and when a statement is wrong but not corrected most will view it as fact.

The DC offset on this amp is abysmal. I think you will be very unhappy with it as long as you have DC offset levels like those quoted. I enjoy my LM3886 amp with 2 and 3mV offset. Offset pushes out the drivers for 100% of the time the amp is powered. It lowers your dynamic range and you will actually miss much of the music.
I did a little research and found, guess who... AndrewT explaining the problem for us. He is followed by another explanation from another knowledgeable member so read on from the link.
DC offset and gain on GC LM3886
The offset must be fixed. You could place a huge cap at the output or vary some of the resistors values to see if you can reduce it.
Uriah
peter daniel does not use capacitor at all in the signal path. i followed his recommendation

as peter daniel suggests, i only soldered R1(BG NX 22uF), R2, R3, & Rf in the amp board. what do you think may cause this problem?
  Reply With Quote
Old 13th March 2012, 01:57 PM   #1564
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
DC coupling, or mixed AC & DC coupling, is likely to result in highish output offsets.
These offsets are very much more likely to vary depending on volume controls and whether the sources are connected or not connected.
__________________
regards Andrew T.
  Reply With Quote
Old 13th March 2012, 02:11 PM   #1565
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
Send a message via AIM to Peter Daniel
On page 5 of this thread I'm showing that offset (in this particular circuit configuration) depends greatly on a chip itself, as well on input impedance (R2): Commercial Gainclone kit- building instructions

To get lower offsets you need to ask me for sorted chips.

The 6moons review of my first amp shows that GC is actually quite a good match for AKG K1000 (which I own myself as well): 6moons audio reviews: Audio Zone AMP-1 by Audio Oasis This amp was using only BG STD 1000/50 in PS filtering

You can still get better results in balanced configuration and one particular amp I've built recently was specifically dedicated to heaphone use: Commercial Gainclone kit- building instructions

BTW, you will find a lot of additional info browsing links thread: Useful links for Audiosector (and not only) projects
__________________
www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
  Reply With Quote
Old 13th March 2012, 02:23 PM   #1566
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Beats me. In PD's manual he states in the last sentence on pg 45 that he doesnt know how to fix offset. He also says dont go to low with Rin which you dont have.
In the datasheet for LM3875 on page 16 toward the bottom Nat Semi suggests high values of feedback resistors to beat DC offset. For instance they mention 100k and 5k1 for Rf and R3.
I have done well over an hour or so of research and have not found ANY results that specifically address removing offset on a non-inverting lm3875 that already has a DC blocking cap in the signal.
Wish I had more for you.
One thing to consider before you go ahead..
Your headphones need 30mW to go to 95dB. They are 120 Ohms. You need about 2V into your cans to get more than 30mW. 2V/120R=.01667 amps multiply that by your voltage and you have .032W or 32mW. You are connecting it to an amp capable of driving more than 10W into your cans and completely rendering you deaf at the same time destroying your cans. Its possible to alter the input section of this amp to keep it in the range of normal and safe listening but I dont think you should be attempting this at your current level of DIY expertise. At least not with these expensive phones and never actually placing them on your ears until you do get it working. Also the gain is much to high right now to be using this as a headamp.
Uriah

Edit : From Peter's last post I see he is using similar cans. My math seems right to me but I could be wrong. It doesnt make sense to me to use a full blown chip amp for this job as its to easy to overdrive them. Maybe I can be set straight about this.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com

Last edited by udailey; 13th March 2012 at 02:26 PM. Reason: new post shedding light on my headphone statement
  Reply With Quote
Old 13th March 2012, 02:34 PM   #1567
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Peter I read your headphone amp post. So, we can drive HD800 at safe levels with a normal input voltage like 1-2V by only changing R3 to 1k2? Is this the only change and this headphone amp does not damage the phones at max volume?
Uriah
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 13th March 2012, 02:44 PM   #1568
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
Send a message via AIM to Peter Daniel
Quote:
Originally Posted by udailey View Post
Beats me. In PD's manual he states in the last sentence on pg 45 that he doesnt know how to fix offset. He also says dont go to low with Rin which you dont have.
You can drive any headphones to a safe level as long as you are using volume control. More info: LM3875 as a headphone amp (another try)



To be accurate, I present few ways to decrease the offset by means of selecting parts or adjusting components values. In a minimized GC circuit, there's no other ways to control the offset, which will mainly depend on chip itself:

In real life, the offset is quite low, especially with amp connected directly to preamp stage with low impedance buffered output. I often measure chips for offset and it's quite possible to select them for less than 5mV offset in a given configuration. Ci cap is in a signal path, and it will influence the sound basically same way as input coupling cap. That's why I prefer not to use it. In a minimized GC circuit, I don't see a way to adjust offset with a trimpot. Lowering the input resistance will decrease the offset, but you might not want go too low with it.

OTOH, I had a Gaincard amp for testing and that amp, although using datasheet schematic and input coupling caps, was still showing the offset in a range of 80mv.
__________________
www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
  Reply With Quote
Old 13th March 2012, 02:48 PM   #1569
udailey is offline udailey  United States
diyAudio Member
 
Join Date: Mar 2008
Thanks for the link Peter.
__________________
purchase LDRs anytime Also try my Resistor Replacers or LDR based Input Selector Email me. diyldr@gmail.com
  Reply With Quote
Old 13th March 2012, 02:49 PM   #1570
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
If the DC coupled chipamp input sees a near constant source resistance, then it is possible to adjust the offset downwards.

The NFB upper and lower leg resistances load the -IN PIN.
Match the -IN PIN resistance to the +IN PIN source resistance.

If the +IN PIN sees a variable source resistance (eg volume control or disconnected source, or relay muting), then that matching method does not work well.
__________________
regards Andrew T.
  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
Commercial complete Gainclone kit for a beginner? gychang Chip Amps 365 4th October 2011 08:19 AM


New To Site? Need Help?

All times are GMT. The time now is 03:34 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