Pre- amp gain and LM3886 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 19th February 2008, 12:58 PM   #1
decky is offline decky  Australia
diyAudio Member
Join Date: Sep 2006
Default Pre- amp gain and LM3886

I have just finished a pre-amp kit based around Lite audio A-15 board and MV-01 switching relay board, both obtained from

I tried to pair it with my gainclone based on the BrianGT kit (gain 33) but on my disappointment I realized that the pre-amp gain is far too high. I could not turn the pot (100K alps) more then a degree or two without pushing 3886s into clipping.

The first question is regarding the pre-amp configuration. I wanted to have the A-15 board after the pot so it can act as a signal buffer. Is that a good idea or should I put it before the pot.

My second question regarding the pre-amp gain. I attached the schematics of the preamp (one channel only) and the accompanied regulated power supply board. Does anyone have an idea how to cut down the pre-amp gain.

I have quite sufficient gain out of the power amp - I need pre-amp only as a "signal conditioner" and impedance matching unit.
Attached Images
File Type: jpg a15small.jpg (72.4 KB, 926 views)
  Reply With Quote
Old 19th February 2008, 04:43 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Hi Decky,
your pre-amp has a gain of 11times (+20.8db).
You are right this is very high.

If you reduce the gain you will alter the gain margin and the phase margin.
The pre-amp may become unstable or at least become very trebley or sibilant.
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 19th February 2008, 05:00 PM   #3
diyAudio Member
Join Date: Dec 2007
It looks like the gain of the preamp is controlled by R117. Raising the value of that resistor should lower the gain of the preamp, but like AndrewT said, reducing the gain may or may not have adverse effects on the performance of the preamp.

That said, your schematic shows a pretty classic approach to a BJT amplifier, and I *think* you should be ok with putting the gain somewhere in the 2-4 range. That corresponds to a value of R117 from 200k-400k.
  Reply With Quote
Old 19th February 2008, 05:46 PM   #4
BWRX is offline BWRX  United States
diyAudio Moderator Emeritus
Join Date: Jan 2005
Location: Pennsylvania
You have four options: change the gain of the preamp, change the gain of the power amp, change the gain of both, or insert a voltage divider somewhere in the signal path. The gain of that preamp should be equal to (1+R120/R121), so you can change either or both of the resistors to change the gain but you may encounter stability issues as Andrew mentioned. The gain of the amp is set in a similar manner. The safest option would be to insert a voltage divider somewhere in the signal path, either before the preamp or between the preamp and the power amp.
  Reply With Quote
Old 19th February 2008, 06:24 PM   #5
diyAudio Member
Join Date: Dec 2007
Oops, they're right. I was looking at the wrong feedback loop.
  Reply With Quote
Old 19th February 2008, 09:39 PM   #6
diyAudio Member
DECKY999's Avatar
Join Date: Jul 2007
Thanks guys heaps.

I have to repeat my first question - if someone can comment - am I better off with the current configuration (pre-amp after the pot to act as a buffer) or pre-amp before the board (then I am having problems of 100K pot looking at 25K input at the gainclone.

Also - regairding voltage divider - is that just a T resistor network that acts as an attenuator - say 10K in the line and 5K between the line and the ground would give me 9.55dB attenuation - A=20*log(R2/(R1+R2)).

And finally - more regarding the impedance matching. It is not super easy for me to change anything on the gainclone side but I can easily modify the pre-amp board. Should I change anything to better match output and input impedances. At this stage I am measuring ~11kohm at the output of the pre-amp but that is only the resistive component. Should I be bothered with this at all considering 25kohm at the gainclone input.

..and one more - is there any real benefit in having this preamp board if I have to eat all the gain with resistors. This is more in the big picture by taking into account impedance matching issue (pre-amp as a buffer), or this extra transistors are just adding noise to the base signal.

Uf too many questions - sorry guys
  Reply With Quote
Old 19th February 2008, 11:01 PM   #7
diyAudio Member
danielwritesbac's Avatar
Join Date: Nov 2007
Some of the amplifier gain settings that I've tested result that you can use 1k at the NFB instead of the stock 680R.
Just take the 1k off the input and recycle it over to the NFB.
That way, you can also change your input in-series resistor to 2.2k.

That adjustment will cut your amp gain a bit, and I think it sounds nice that way.

Problem: A 100k pot usually has a fairly low resolution when its nearly all the way down. Solution: Just swap it out for a voltage divider, also called "fixed l-pad" which is just 2 resistors per channel. Chances are good that a 50k here will be more quiet than a 100k. In that case, the sum of the 2 resistors (per channel) will equal (or almost equal) 50k. Its my best guess.
Anyway, a resistor is usually higher resolution than a pot.

A voltage divider looks like

ground source

Usually, you just read off the pot's preferred settings with a multimeter and then replace it with the same value resistors doing the same job in the same way.
(To change from 100k to 50k, just use half values).
  Reply With Quote
Old 19th February 2008, 11:20 PM   #8
BWRX is offline BWRX  United States
diyAudio Moderator Emeritus
Join Date: Jan 2005
Location: Pennsylvania
If I were in your shoes I would try changing the gains of the pre and power amps. For the preamp I would change R120 from 10kohm to around 5kohm. That will bring the gain down to around 6V/V. For the power amp I would change R3 from 680ohms to around 1.1kohm (while leaving R1 in place), which will bring the gain down around 21V/V. That will lower the total gain of your system from about 363V/V (51dB) to around 126V/V (42dB). That's still rather high but should allow you to change the volume pot setting a bit more.
  Reply With Quote
Old 20th February 2008, 12:13 AM   #9
diyAudio Member
DECKY999's Avatar
Join Date: Jul 2007
That is exactly what I was planning to do. Also I will have voltage divider as a backup option.

However I am still a bit puzzled with the impedance matching issue.

I am correcting myself - what I was planning on the preamp side is to go down to gain 3 by replacing R121 from 1K to 5K.

Thinking loudly - this preamp could be quite a nice headphone amp as well.
  Reply With Quote
Old 20th February 2008, 07:18 AM   #10
Nordic is offline Nordic  South Africa
diyAudio Member
Nordic's Avatar
Join Date: Sep 2005
Plese do try the amp without a preamp... they often sound better with just an attenuator.... less stages of distortion....
  Reply With Quote


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
choosing gain in LM3886 jarthel Chip Amps 30 5th April 2016 12:33 PM
Need your opinion, how small gain can I use with LM3886 (how close to "flat amp")? danielwritesbac Chip Amps 20 30th November 2008 12:27 AM
FS: LM3886 Gain Clone Chips (19) bolserst Swap Meet 5 26th October 2008 06:35 PM
Heretical Unity Gain line stage PLUS a little gain ? yagas Tubes / Valves 7 24th January 2006 01:30 PM
LM3886 gain clone vs STK4048XI for 4 ohm load filgor Chip Amps 0 25th September 2004 12:05 PM

New To Site? Need Help?

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

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2