The LM1875 is excellent as a middle/basic high current op-amp. The influence that you bring is probably what it will deliver. So long as you didn't need much over 20 watts (most people don't need that much indoors), then the LM1875 will deliver what you asked for. It probably will deliver what you asked for even if that is not what you wanted. It is easy to use, it is informative, and it performs very well indeed. Just be mindful what you ask of it, because it probably will do it with excellent accuracy. This why it is favored for education. One possibility is that you might ask it to do music entertainingly and that it can do.
The BLOK20 claims this: "the amplifier circuit is 1 chip and 3 resistors". It also claims this: "It is ultra minimalist. No Zobel network, no input capacitor".
I've attached the Build a Bare-Bones “Skeleton” Amplifier schematic and crossed off in red the components that the BLOK20 hasn't implemented or can argue that is not part of the amplification stage. In green circles [3 resistors!] are probably what is implemented as part of the amplification circuit but I'm confused about why/how C107 can be left out.
Attachments
The picture looks like there are 4 caps per channel, not counting the power supply.
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By managing the value of input bias resistor or the nfb resistor you can tame the output DC offset.
What I amaze is that they dare to remove output filter.
(maybe there is already a zobel network available in the speaker box)
😕
LM1875 Instability.
Best thread I could find:
For a composite amplifier experiment, I use two LM1875 power amplifiers. They sound fine and only when I put my oscilloscope and sine-wave generator on the amplifiers, I noticed a self-oscillation problem at higher power levels. I have tried whatever I can imagine with power decoupling and Zobel-element but without much result. Gain set at 27 times with purely resistive feedback.
The two amplifiers behave the same. They are stable until +/-29.5V supply voltage (my test limit) with low input signal levels. Only when I increase the input signal level, a self-oscillation (around 1 MHz) phenomenon starts always in the negative half-period. When the output current approaches 3A (peak), the output voltage “collapses” in a self-oscillation until the current level is reduced significantly on the return-slope of the sine-wave. Only for the negative half-wave. The positive half-wave remains fine until clipping.
With 16 Ohm loads, I can take the amplifiers to +/-29.5V without clipping. With 8 Ohm loads, the self-oscillation phenomenon is only observed for supply voltages from +/-25V and above. With 4 Ohm loads, self-oscillation is avoided with supply voltages below +/-18V.
I can imagine two characteristics being involved in creating the oscillation:
* the output current limiter (typically 4A, tested above 3A) that is datasheet specified to be lowered at higher levels of output transistor losses (no detailed explanation),
* a reduction of the loop stability for the LM1875 when the operating current is (dynamically) increased (mentioned as a general class B problem by Doug Self I believe).
I have found no way to tame this dynamic self-oscillation phenomenon. Perhaps it is simply an inherent characteristic of the LM1875. It irritates me not to have full control of the LM1875.
Does anyone know the LM1875 that well to be able to tell me exactly what is the cause?
Best thread I could find:
For a composite amplifier experiment, I use two LM1875 power amplifiers. They sound fine and only when I put my oscilloscope and sine-wave generator on the amplifiers, I noticed a self-oscillation problem at higher power levels. I have tried whatever I can imagine with power decoupling and Zobel-element but without much result. Gain set at 27 times with purely resistive feedback.
The two amplifiers behave the same. They are stable until +/-29.5V supply voltage (my test limit) with low input signal levels. Only when I increase the input signal level, a self-oscillation (around 1 MHz) phenomenon starts always in the negative half-period. When the output current approaches 3A (peak), the output voltage “collapses” in a self-oscillation until the current level is reduced significantly on the return-slope of the sine-wave. Only for the negative half-wave. The positive half-wave remains fine until clipping.
With 16 Ohm loads, I can take the amplifiers to +/-29.5V without clipping. With 8 Ohm loads, the self-oscillation phenomenon is only observed for supply voltages from +/-25V and above. With 4 Ohm loads, self-oscillation is avoided with supply voltages below +/-18V.
I can imagine two characteristics being involved in creating the oscillation:
* the output current limiter (typically 4A, tested above 3A) that is datasheet specified to be lowered at higher levels of output transistor losses (no detailed explanation),
* a reduction of the loop stability for the LM1875 when the operating current is (dynamically) increased (mentioned as a general class B problem by Doug Self I believe).
I have found no way to tame this dynamic self-oscillation phenomenon. Perhaps it is simply an inherent characteristic of the LM1875. It irritates me not to have full control of the LM1875.
Does anyone know the LM1875 that well to be able to tell me exactly what is the cause?
Maybe you are using a supply that's higher than usual. In most applications ( kits etc) I see a +/-25 V limit for 8 ohms. If the load is low ( 4 ohms) then the supply typically is not above +/- 22V. These are unloaded supply voltages not the regulated dc you get from a lab dc supply !
I've built several 1875 amps but always limited the supply to just under 25 V and never had a problem of oscillation ( as seen on a scope !).
You said " Perhaps it is simply an inherent characteristic of the LM1875 " . In that case you need to write to National Semi for an answer ? Most circuits in application manuals of the inside of chips are incomplete to understand such behaviour....I think!
You did check page 7 of the Ti data sheet ? It talks about oscillation. But I guess you have all those details already sorted out !
I've built several 1875 amps but always limited the supply to just under 25 V and never had a problem of oscillation ( as seen on a scope !).
You said " Perhaps it is simply an inherent characteristic of the LM1875 " . In that case you need to write to National Semi for an answer ? Most circuits in application manuals of the inside of chips are incomplete to understand such behaviour....I think!
You did check page 7 of the Ti data sheet ? It talks about oscillation. But I guess you have all those details already sorted out !
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Thanks a lot for the swift advise. I know, LM1875 should not be used with 4 Ohm loads and, though it is specified for operation up to +/-30V, perhaps not be used above +/-25V. +/-25V is where trouble starts with an 8 Ohm load.
My reason for investigation is that I like the LM1875 a lot for its simplicity and good sound (within right limits). I do not need more power but I like to have the operation under full control and not with a clearly unintentional effect.
LM1875 is an "old" design and finding someone at NS who still remembers such details may be difficult. Perhaps their advise will be like yours: not less than 8 Ohm and max. +/-25V.
Anyway, I believe some NS/TI employees keep a look at the experiences on this forum.
Thanks for your reply, FF
My reason for investigation is that I like the LM1875 a lot for its simplicity and good sound (within right limits). I do not need more power but I like to have the operation under full control and not with a clearly unintentional effect.
LM1875 is an "old" design and finding someone at NS who still remembers such details may be difficult. Perhaps their advise will be like yours: not less than 8 Ohm and max. +/-25V.
Anyway, I believe some NS/TI employees keep a look at the experiences on this forum.
Thanks for your reply, FF
I use mine with 4 ohm speakers too without any problems. Supply is about +/-22 volts with a 16-0-16 volt transformer. It isn't exactly 16 volts but dc is about +/- 22V after rectification. The chip amp sounds best with no input cap ( C101)(ensure the source is dc free). If there is no dc at the input you can try removing the feed back capacitor ( C107) as well. Check output offset without connecting a speaker. If its below 50mV you can work it this way and the sound improves significantly. So the input cap and feedback cap contribute to the overall sound. If you need them try various types till you find an acceptable one. It's a very good chip amp. Beats a lot of large amps if kept within it's power limits. 20 watts per channel can be quite loud with reasonably efficient speakers. The ones I used with this are rated 88db/watt SPL and has a 4 ohm dip in its impedance curve. Plays very well.
With an unregulated supply of +/-25 volts and a 4 ohm load it's still OK as the supply line will drop with high volume.
With an unregulated supply of +/-25 volts and a 4 ohm load it's still OK as the supply line will drop with high volume.
I will keep my upcoming “version 2” DC-coupled as you suggest. In order to avoid crossing the 3A output current limit, I will use three LM1875 in parallel for each channel. That should leave me margins for running the amplifier in BTL configuration with 8 Ohm loads or normal 4 Ohm operation at full supply voltage. In both cases, more than 60W per channel should be reached.
The summing resistors should reduce the sensitivity to capacitive loads.
Sound-wise I have absolutely no complains, only irritation from this self-oscillation collapse at high output levels.
Thanks a lot for your suggestions.
The summing resistors should reduce the sensitivity to capacitive loads.
Sound-wise I have absolutely no complains, only irritation from this self-oscillation collapse at high output levels.
Thanks a lot for your suggestions.
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If you plan on using the direct coupled scheme or the dc coupled feedback, better use a relay protection for the speakers. I do. That way in case something wrong happens and you get a large dc offset, it will switch out the speakers and protect hem. Especially important if you are using good speakers with them. Can't afford to blow your speakers due to accidents.
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