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CarlosT 9th December 2006 04:20 PM

Less Negative Feedback Is Better? broken Search button came up with ancient threads that spanned 50 pages but I thought I would bring it forth again.

In coming up with decent resistor values for my LM3875 IGC, I can choose to keep the feedback resistor nice and high to hedge for errors in the external unpredictable input impedances as well as other resistor "errors" or I can go with lower feedback and supposedly nicer sounding lower values (less resistor noise, etc) and just deal with some uncontrolled variability in the gain and DC offset.

For example, if I'm aiming for a nice high gain of 50 (because my input is an MP3 player with maybe .5V worth of output max) and we assume a low input impedance which can then be disregarded, we have various combinations (kohms):

R2 R1 R3

50 1 1
60 1.2 1.2
70 1.4 1.4
80 1.6 1.6
90 1.8 1.8
100 2 2
110 2.2 2.2
120 2.4 2.4
130 2.6 2.5
140 2.8 2.7
150 3 2.9
160 3.2 3.1
170 3.4 3.3
180 3.6 3.5
190 3.8 3.7
200 4 3.9
210 4.2 4.1
220 4.4 4.3
230 4.6 4.5
240 4.8 4.7
250 5 4.9
260 5.2 5.1
270 5.4 5.3
280 5.6 5.5
290 5.8 5.7
300 6 5.9
400 8 7.8
500 10 9.8
600 12 11.8
700 14 13.7
800 16 15.7
900 18 17.6

What is the "sweetspot"? Are there combinations that just won't work?

CarlosT 9th December 2006 04:22 PM

BTW I don't know if this helps but most MP3 players put out about 5 to 10 mW into about 32 ohms (I guess the deemed impedance for most earbuds kids put in their ears nowadays).

That's my input.

Nordic 9th December 2006 04:33 PM

time to get an ipod shuffle, 30mA at 1V !!!!!!!!!!!

CarlosT 9th December 2006 04:44 PM

I just ordered one for my mom for Xmas. Where did you find specs for that? I could only come up with decent specs for Creative Labs Zen/Muvo/Rhomba stuff. I had to use all the Ohm's Law stuff to back into the voltages.

By comparison, most home DVD players and CD players put out about 2V per channel into something like 75 ohm or so.

My Adcom preamps go up to like 10V at max!

So I'm right? If the average MP3 player puts out .5V max and I have an LM3875 IGC with a power supply of 35V, I need a gain of like 70:1 to get the full output outta the chip?

I had considered doing like a DPDT switch with two different sets of R1/R3s to go from high gain to low gain just in case I hook up some input with serious juice. Does this even make sense to you? :D

Steve Eddy 9th December 2006 04:46 PM


Originally posted by Nordic
time to get an ipod shuffle, 30mA at 1V !!!!!!!!!!!
Yeah, but is that 1 volt peak, peak-to-peak, or RMS? :D


CarlosT 9th December 2006 04:47 PM


Originally posted by Nordic
time to get an ipod shuffle, 30mA at 1V !!!!!!!!!!!
Is that per channel? That would be like 30 mW into 33 ohms! You sure a device that is 1" x 1.5" and juiced up to run for 12 hours really put out this much?

adason 9th December 2006 04:56 PM

hi CarlosT,

I can not help with your calculations, but one thing is escaping...efficiency of your speakers ..... and why do you need to achieve full output from your amp?
Do you consider the fact that for normal listening level we do not use more than one watt? If you have highly efficient speakers, you might never need more than few miliwatts.
I suggest that your gain should be such that you will never need preamp for normal listening.
If you like it loud, than efficiency of your speakers matters even more, for no matter what your gain, you will be cliping poor amp.

Nordic 9th December 2006 05:02 PM

yes, per channel

The reason for this sterling performance is that the left and right channels each have two transistors, one pushing, one pulling, and no capacitor that gets discharged over time.

You wont get 12 hours at that level.

CarlosT 9th December 2006 05:04 PM

That's funny that you should mention this...I've been all over the map with driver choices and had initially bought some really cheap ($14) Audax 4 ohm coaxials as part of my old school simple look for the retro cabinet (no crossover, no separate tweeter, one hole to cut and look at). Now that I'm spending more and more on the amp stuff and cabinet finish and doing more scary max power calculations into 4 ohms, I chickened out and went the high $$$ route and got some super spiffy yellow fibre 8 ohm Vifas and 3/4" Vifa tweeters in a homemade 1st order xover with Solen caps BLAH BLAH BLAH :D

The Vifas are supposed to be about 87 dB/m sensitivity.

What would be a good gain in your mind?

Nordic 9th December 2006 05:10 PM

More than enough, My current drivers are about 86db and it works fine without a preamp.... (the later being under construction and will be intgrated.)

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