Which transformer for 4 Ohm speakers? - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 7th January 2007, 02:04 PM   #1
bluegti is offline bluegti  United States
diyAudio Member
 
Join Date: Nov 2004
Location: Northern Virginia
Default Which transformer for 4 Ohm speakers?

I've read numerous times the 25V transformers are good for 8 Ohm speakers. I have a pair of definitive technology BP-10's (original model) that are 6 Ohm. I want my amp to be safe with 4 Ohms if needed (I'm using the chipamp.com LM38886 kit).

Here are options I'm considering from Parts Express and a guy on Ebay:
  • 160VA 18V+18V
  • 250VA 18V+18V
  • 200VA 20V+20V
  • 400VA 20V+20V
  • 160VA 22V+22V
  • 300VA 22V+22V

Which one would be most appropriate?

Can someone point me to a place where I can find out why in layman's terms? I know how to solder and follow directions but don't know anything about the science of why 25V is bad for 4 Ohm speakers.
  Reply With Quote
Old 7th January 2007, 02:14 PM   #2
diyAudio Moderator
 
pinkmouse's Avatar
 
Join Date: Apr 2002
Location: Chatham, England
Default Re: Which transformer for 4 Ohm speakers?

Quote:
Originally posted by bluegti
Can someone point me to a place where I can find out why in layman's terms? I know how to solder and follow directions but don't know anything about the science of why 25V is bad for 4 Ohm speakers.
Download the datasheet, and work your way though the design guide for voltage and heatsinking. It may look daunting, but it is broken down into fairly basic maths and is about as basic as you can get.
__________________
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
  Reply With Quote
Old 7th January 2007, 02:35 PM   #3
phn is offline phn  Sweden
diyAudio Member
 
phn's Avatar
 
Join Date: Oct 2004
You also might want to look into PSU design. 2x22V might not be enough to give you +/-25. At least not if you want to be on the safe side.
  Reply With Quote
Old 7th January 2007, 02:58 PM   #4
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
your chipamps will give a maximum safe output of about 60W.
This will be into 4ohms if you select the transformer for that duty.
You will get less power into 6ohm, but I guess still adequate.

The VA should be about 1.5times the total output power.
The 160VA 18-0-18Vac
or
200VA 20-0-20Vac would do for two channels.
__________________
regards Andrew T.
  Reply With Quote
Old 7th January 2007, 03:09 PM   #5
phn is offline phn  Sweden
diyAudio Member
 
phn's Avatar
 
Join Date: Oct 2004
Andrew is right. 18-0-18 is roughly +/-25V.

But if you want a regulated PSU you will need, I would say at least, +/-30V for +/-25V.
  Reply With Quote
Old 8th January 2007, 02:37 AM   #6
diyAudio Member
 
Join Date: Jan 2007
Send a message via AIM to fitchmicah
but the plans at chipamp.com call for a power supply with two rectifiers.

if bluegti chooses a center tapped transformer like 18V-0-18V or 20V-0-20V, wouldn't there need to be two of them?
  Reply With Quote
Old 8th January 2007, 02:49 AM   #7
phn is offline phn  Sweden
diyAudio Member
 
phn's Avatar
 
Join Date: Oct 2004
He will need two secondaries, or four leads. For example 2x18VAC, which will give +/-25V, or +25-0(ground)--25.
  Reply With Quote
Old 8th January 2007, 02:54 AM   #8
diyAudio Member
 
Join Date: Jan 2007
Send a message via AIM to fitchmicah
those are hard to find and expensive!

wouldn't it work to use two center tapped transformers in place of a single transformer with two secondaries?

I'm honestly thinking about just buying a T-Amp and building something else for my first DIY cause the power supply is going to be like twice as much as the amp, and I only have like $100 right now!
  Reply With Quote
Old 8th January 2007, 08:24 AM   #9
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
a chipamp reduces the total cost of the amplifier.
You may find that the amp part is now only 5% of the total cost.

Many chipamp PCBs are designed to suit dual secondaries with dual rectifiers. As far as I know all can be modified to work with a centre tapped secondary and single rectifier. If you need support for the modification then come here or ask the designer.
__________________
regards Andrew T.
  Reply With Quote
Old 8th January 2007, 12:51 PM   #10
diyAudio Member
 
Join Date: Aug 2002
Location: Toronto, Canada
Default centre-tapped transformer

Hi Micah,

You can find an example of the modification to use a
CT transformer in Brian's original LM3875 kit documentation:

http://www.audiosector.com/nigc_kit-users_guide.pdf

Section #3.1 contains a wiring diagram for a CT transformer.

Cheers,
Dennis
  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
DIY Electrostatic speakers Audio Transformer??? pmscompany Planars & Exotics 49 22nd February 2009 04:28 PM
Built differences between Guitar Speakers and PA Speakers? agadis PA Systems 7 24th September 2007 09:22 AM
Transformer Hum leaks to Speakers kff322 Tubes / Valves 16 18th January 2007 12:01 AM
toroid Transformer as power Transformer for tube tone Tubes / Valves 7 11th February 2003 09:57 AM
toroid Transformer as output Transformer for tube preamp? tone Tubes / Valves 15 2nd February 2003 05:36 AM


New To Site? Need Help?

All times are GMT. The time now is 03:14 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