LM1875 - Best Components and values

matherp

Member
2009-10-26 11:52 am
Hi all

I've read all the threads I can find on the LM1875 and seen all the great layouts that have been produced but there seem to be some big variances in component values which I don't understand. I'd really appreciate views and advice.

The layout attached is as per the datasheet with the exception of RX which some posters have included. So :

RX - some have included 1K - why?
R1 - standard at 1M - some omit - why?
C1 - ranges between 1u and 2.2u - what are the pros and cons?
R2 - standard at 22K
C4, C3 - standard at 0.1uF
C6, C7 - ranges between 100 and 220uF. Use lower values with a higher VA transformer assuming it has adequate smoothing?
R4 and R3 - set the gain but some use high values (180K,22K) whereas the datasheet shows 20K and 1K - pros and cons?
C2 - most seem to stay around 22uF, I assume a bipolar is best? If I went to say 47uF what would be the effect?
R5, C5 - the Zobel network to stop capacitive loads creating high frequency oscillations - any reason to change 1R and .22uF?

Thanks in anticipation

Peter
 

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If you want to know the motivations why people skip or include certain components, you will have to ask them directly. Many components have more than one function. You will find them also explained in datasheets.

RX
- limits the incoming current to the amplifier's input.
- forms an RF filter with a follwing capacitor to ground that is not shown in your schematic. You can find such a capacitor e. g. in the LM3886 datasheet on page 5 labeled Cc.

R1
- terminates the cable coming from the source.
- holds C1 close to gnd to avoid or reduce pops when the source is switched, plugged or unplugged.

C1
- determines the roll-off at low frequency.
- protects amp and speaker from DC coming from the source.
- relieves amp and speaker from working in a range where you cannot hear or where the speaker cannot reproduce any meaningful content.
Pro is protection, con is influence on the sound due to capacitor parasitics.

R2
- references the non-inverting input to ground. Can be used to adjust the DC offset.
- forms a high-pass filter with C1.

C3, 4
- decouple the amplifier from lead inductance and filter out high-frequency disturbances on the rails. The standard value gives good results. In some cases smaller values can improve their effect.

C6, 7
- depends on the application. Higher output power or lower impedance (both mean higher current demand) require higher capacitance. Bigger values are also good for the bass performance. 100-220 µF may be a bit small. The datasheet shows PCBs with 1000 µF per rail, which is still small, but may be sufficient for this power range.

R3, R4
- some people prefer low values to keep Johnson noise low. Others prefer high values to reduce thermal drift that may come from heat dissipation (low resistance -> high current -> heat). The datasheet values seem to be a good compromise. The values also influence the DC offset. They should be in a range where R2 values of 10-100k (practical values for amplifier input resistance) can bring the output DC offset down to almost zero. Good amps have less than 2-3 mV, mediocre amps have up to 10 mV. 100 mV are often accepted by chip amp enthusiasts (especially the DC-coupling aficionados), but would be considered crap in almost all other amps.

C2
- protects the non-inverting input from DC.
- reduces the DC gain to 0 so that the DC offset is not amplified by the same gain as the music signal and does not endanger the speaker.
- is a high-pass filter that should have a roll-off lower than or equal to the input filter (formed by C1 and R2).

R5, C5
- It is usually good to stick to the datasheet values, if you don't own at least an oscilloscope to check what happens when you use others.
 
If you want to know ......................

C2
- protects the non-inverting input from DC.
- reduces the DC gain to 0 so that the DC offset is not amplified by the same gain as the music signal and does not endanger the speaker.............
good explanation, except.
adding the DC blocking capacitor to the Negative FeedBack route reduces the DC gain to 1 (=+0dB).
 

matherp

Member
2009-10-26 11:52 am
pacificblue

Thanks for the detailed reply and also Andrew for the clarification. I thought to experiment I'd start by making up a stripboard version as a first try.

I used standard components except that C6 and C7 are 470uF, C2 is a 47uF bipolar, and R5 is 1.5ohm as I had those available. The power supply is a 12-0-12 100VA torroidal with full bridge rectification and 4 off 2200uF caps. The result is remarkably good. Absolutely stable, zero DC offset and a clean sound that seems more musical sound and has better bass than the TP2050 amp I have been using.

I laid out the stripboard using Veecad and am pleased with the clean signal lines I was able to achieve. Perfectly usable for experimenting without the effort of making a PCB.

Best regards

Peter
 

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Passband.
What range of frequencies do you expect the amplifier to pass to the load?

If you want to drive a speaker from 20Hz to 20kHz then expect an amplifier with a frequency response of 4Hz to 100kHz +0dB -1dB to work fairly well at letting you hear to the extremes of the audio range.

To achieve those F-1dB frequencies adopt passive filters at the input of the amplifier that have single pole roll-off F-3dB about one octave wider,
i.e. 1Hz to 200kHz F-3dB
Ensure that the amplifier can pass a signal that is outside the range of the passive filters without misbehaving (stability).
 
Last edited:
Sorry guys Im activating an old thread.My problem is that my tda 2040 is sounding good with tone control.but for this I used Tl072 which sounds quite noisy .Tda 2040 alone doesn't give me musical sound.So I have decided to go for standalone Lm1875 .I want mid range n treble frequencies .I will be very thankfull to u if u pl z provide any change from original circuit.
 
Dear Pacific Blue (or whoever can provide insight),
Back in post number 2 of this thread you wrote that the input resister R2 influences the DC offset and that R3 NFB resistor and R4 influence the DC offset. In another thread



Understanding and redesigning the input stage of LM1875



you wrote that R2 and R3 should be the same in theory to create the lowest DC offset. My question is does R4 come into the equation?



In a website called "op amp practical considerations"



Op-Amp Practical Considerations | Operational Amplifiers | Electronics Textbook


The author would seem to be suggesting that the resistor that goes to ground, R4 in Texas Instrument's LM1875 data sheet must be added in parallel to R3, which would be 22K added in parallel to 1K = 0.96K, which is 20 times less than the R2 value of 20 K. The author's reasoning is quoted below:



"For both inverting and noninverting amplifier circuits, the bias current compensating resistor is placed in series with the noninverting (+) input to compensate for bias current voltage drops in the divider network:


wFCRUcKRRz6GQAAAABJRU5ErkJggg==

In either case, the compensating resistor value is determined by calculating the parallel resistance value of R1 and R2. Why is the value equal to the parallel equivalent of R1 and R2? When using the Superposition Theorem to figure how much voltage drop will be produced by the inverting (-) input’s bias current, we treat the bias current as though it were coming from a current source inside the op-amp and short-circuit all voltage sources (Vin and Vout). This gives two parallel paths for bias current (through R1 and through R2, both to ground). We want to duplicate the bias current’s effect on the noninverting (+) input, so the resistor value we choose to insert in series with that input needs to be equal to R1 in parallel with R2"


It should be noted that the numbering of the resistors is different to the LM1875 data sheet. So is this author talking about something else entirely? Does the "grounded" resister come into the equation when determining theoretical DC offset? Or is is it just the input resistor NFB resistor and it is best to have them equal?


Kind Regards,
Ben Gaffney