Con someone look at this buffer/mixer for me?

aletheian · 30th January 2006, 04:02 AM

I am making a little 4 channel PA out of an few LM3886 chips, and I need to rig up a simple buffer, so I threw this idea down. I only included the buffers... they will be fed from 4 discrete preamps with gain controlls, and then sent out to an "insert" for processing... then into these buffers.

Is there a better way to do this? Do I even need the buffers? I figured they would keep from loss from input impedance and maybe kill any ground loops a bit.

lineup · 30th January 2006, 05:57 AM

In same way as when paralleling OP-amps or LM3886,
you need to use a series resistor from output of each of 4 transistors.
They are all connected to the output capacitor.

This way each input can be different and voltage difference will be across these resistors.

aletheian · 30th January 2006, 07:03 PM

Quote:

Originally posted by lineup
In same way as when paralleling OP-amps or LM3886,
you need to use a series resistor from output of each of 4 transistors.
They are all connected to the output capacitor.

This way each input can be different and voltage difference will be across these resistors.

Crap... I forgot those... thanx. Is it OK other than that? Pretty simple.. I just wanted to bounce it off someone else.

clem_o · 31st January 2006, 12:13 AM

It would seem simpler to use one op-amp configured as a summing amplifier to do the job. If you use high values for the input resistors to the summing amp, the source impedance(s) of the previous stages should become pretty much negligible. A FET-input opamp should be able to work with those impedances properly...

Cheers!

pinkmouse · 31st January 2006, 12:16 AM

Yup. Google "virtual earth mixer" for the schematic.

aletheian · 31st January 2006, 02:28 AM

Quote:

Originally posted by clem_o
It would seem simpler to use one op-amp configured as a summing amplifier to do the job. If you use high values for the input resistors to the summing amp, the source impedance(s) of the previous stages should become pretty much negligible. A FET-input opamp should be able to work with those impedances properly...

Cheers!

That's "plan B" in case the fets did not work out. I really wanted to use discreets, and I thought that I might avoid ground lops that way, which might be a problem for a virtual earth setup... meybe not in this context though.. I dunno...

clem_o · 31st January 2006, 04:05 AM

Howdy - don't see how using discrete FETs can avoid ground loops. Each FET still has a ground, which will get common'd through the power supply...

Cheers!

30th January 2006, 05:57 AM	#2
lineup diyAudio Member Join Date: Dec 2005 Location: the north	In same way as when paralleling OP-amps or LM3886, you need to use a series resistor from output of each of 4 transistors. They are all connected to the output capacitor. This way each input can be different and voltage difference will be across these resistors. __________________ lineup

31st January 2006, 12:16 AM	#5
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Yup. Google "virtual earth mixer" for the schematic. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
DIY DJ mixer	Bricolo	Analog Line Level	34	7th January 2011 12:00 PM
Diamond Buffer - Super Buffer	peranders	Solid State	34	16th May 2010 01:46 PM
mixer or pre-amp	adan888	Solid State	1	18th January 2005 07:00 PM
Opamp + buffer, how to bypass buffer at HF for unity gain?	Christer	Solid State	10	29th October 2003 06:50 AM
DJ Mixer	Samuel Jayaraj	Solid State	4	2nd February 2002 09:07 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

31st January 2006, 12:13 AM	#4
clem_o diyAudio Member Join Date: Jun 2005 Location: Manila	It would seem simpler to use one op-amp configured as a summing amplifier to do the job. If you use high values for the input resistors to the summing amp, the source impedance(s) of the previous stages should become pretty much negligible. A FET-input opamp should be able to work with those impedances properly... Cheers!

31st January 2006, 04:05 AM	#7
clem_o diyAudio Member Join Date: Jun 2005 Location: Manila	Howdy - don't see how using discrete FETs can avoid ground loops. Each FET still has a ground, which will get common'd through the power supply... Cheers!

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)