Yes...my 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?
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?
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?
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?
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.
ed
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.
ed
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.
http://home.comcast.net/~machrone/playertest/playertest.htm
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.
http://home.comcast.net/~machrone/playertest/playertest.htm
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
The Vifas are supposed to be about 87 dB/m sensitivity.
http://www.tymphany.com/datasheet/pdf/vf/MG14WK09-08.pdf
What would be a good gain in your mind?
The Vifas are supposed to be about 87 dB/m sensitivity.
http://www.tymphany.com/datasheet/pdf/vf/MG14WK09-08.pdf
What would be a good gain in your mind?
the unswer to your question is....
yes, no and maybe
definitely yes, even with unefficient speakers, if you are going to listen at normal listening levels
definitely no, if you are party animal and you want to shake the house /I can make 150 watts amp constantly into clipping after some significant alcohol consumption/
go for it, you will see later
ed
I can appreciate that you are trying to design for a player with a low output, but I'm wondering how many here believe the subject line: "Less Negative Feedback Is Better?" as it pertains to power opamps?
This from the LM3875 datasheet:
"Although higher gain amplifiers provide
greater output power and dynamic headroom capabilities,
there are certain shortcomings that go along with the so
called “gain”. The input referred noise floor is increased and
hence the SNR is worse. With the increase in gain, there is
also a reduction of the power bandwidth which results in a
decrease in feedback thus not allowing the amplifier to respond
as quickly to nonlinearities. This decreased ability to
respond to nonlinearities increases the THD + N specification."
I'm wondering because my chip amp for the last several years has had a gain of ~32 and for my next build I was planning for a gain ~18, to see if there was much difference.
Scott
This from the LM3875 datasheet:
"Although higher gain amplifiers provide
greater output power and dynamic headroom capabilities,
there are certain shortcomings that go along with the so
called “gain”. The input referred noise floor is increased and
hence the SNR is worse. With the increase in gain, there is
also a reduction of the power bandwidth which results in a
decrease in feedback thus not allowing the amplifier to respond
as quickly to nonlinearities. This decreased ability to
respond to nonlinearities increases the THD + N specification."
I'm wondering because my chip amp for the last several years has had a gain of ~32 and for my next build I was planning for a gain ~18, to see if there was much difference.
Scott
adason said:
You should be carefull here. Many chip amps have a MINIMUM gain below which they will oscillate. The reason is that with the increased feedback, the amp will have a wider bandwidth. That in turn means that the high frequencies are still amplified, even when they approach 180 degrees phase shift. If you feedback a 180 degrees phase shifted signal it turns into POS feedback rather than neg feedback -> oscillator instead of amp.
The datasheet should detail this.
Even getting close to the min gain should be done with care, because external circuitry, pcb layout etc can add phase shift to that of the amp itself.
Bottom line, stay away from min gain plus a healthy safety margin.
Jan Didden
PS For minimum DC offset (assuming your feedback network is DC coupled, no cap to ground) make the parallel value of the two feedback resistors equal to the DC resistance seen by the + input.
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