I measured the power output of one sample of an LM1875 IC at various supply voltages and non inductive loads. Supply voltage is the voltage measured directly on pins 3 & 5 while driving the load at measured output. I used a split supply, so 30v is +/- 15v, for example. Vout rms is measured right at the load resistor which is soldered to the amp board to minimize losses. Iout is the calculated rms output current.
Signal used was a low distortion sine wave at 1KHz. I used a spectrum analyzer to set the output at the point just before clipping (no clipping harmonics showing).
My recommendations: Since loudspeakers present a reactive load to the IC's output transistors and the transistors are current limited to about 4 amps across much of the output voltage range, I would not exceed the supply voltages that gave a 2.5 amp output. For example with a 4 ohm speaker, I would not use a supply voltage beyond 36v.
Signal used was a low distortion sine wave at 1KHz. I used a spectrum analyzer to set the output at the point just before clipping (no clipping harmonics showing).
My recommendations: Since loudspeakers present a reactive load to the IC's output transistors and the transistors are current limited to about 4 amps across much of the output voltage range, I would not exceed the supply voltages that gave a 2.5 amp output. For example with a 4 ohm speaker, I would not use a supply voltage beyond 36v.
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Some of your results indicate very good performance as regards current capability.
eg.
at 24Vdc (+-12Vdc) the maximum outputs are
6.54Vac into 8r0
6.32Vac into 4r0
5.75Vac into 2r67
The 4r0 value is -0.3dB ref 8r0, this is superb (better than excellent).
The 2r67 value is -1.1dB ref 8r0, this is a very good result (2 grades below superb).
To me this indicates that the 1875 on +-12Vdc makes a very good 5W, 8ohms capable amplifier for current capability (upto 3.46Apk).
There is only one comparable supply voltage above that, 30Vdc (+-15Vdc)
8.54Vac into 8r0
7.15Vac into 2r67
The 2r67 value is -1.5dB, this is a good result (3 grades below superb).
To me this indicates that the 1875 on +-15Vac makes a good 9W, 8ohms capable amplifier for current capability (upto 4.52Apk).
There are no results for 2r0 to allow any assessment for 6ohms capability, nor for 4ohms capability. From experience of the 3886 chipamp, I would estimate that the 1875 is probably not suitable for 4ohms duty, but may give adequate performance into 6ohms speaker.
Discrete output stages can generally better chipamps due to the massively larger die area of the output devices. I have described all the National Chipamps as "current crippled".
These results do not change my opinion.
Thank you for taking the time to report all those results.
eg.
at 24Vdc (+-12Vdc) the maximum outputs are
6.54Vac into 8r0
6.32Vac into 4r0
5.75Vac into 2r67
The 4r0 value is -0.3dB ref 8r0, this is superb (better than excellent).
The 2r67 value is -1.1dB ref 8r0, this is a very good result (2 grades below superb).
To me this indicates that the 1875 on +-12Vdc makes a very good 5W, 8ohms capable amplifier for current capability (upto 3.46Apk).
There is only one comparable supply voltage above that, 30Vdc (+-15Vdc)
8.54Vac into 8r0
7.15Vac into 2r67
The 2r67 value is -1.5dB, this is a good result (3 grades below superb).
To me this indicates that the 1875 on +-15Vac makes a good 9W, 8ohms capable amplifier for current capability (upto 4.52Apk).
There are no results for 2r0 to allow any assessment for 6ohms capability, nor for 4ohms capability. From experience of the 3886 chipamp, I would estimate that the 1875 is probably not suitable for 4ohms duty, but may give adequate performance into 6ohms speaker.
Discrete output stages can generally better chipamps due to the massively larger die area of the output devices. I have described all the National Chipamps as "current crippled".
These results do not change my opinion.
Thank you for taking the time to report all those results.
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@ johnr66 : thanks a lot for actual testing and posting 
Very useful.
I was amazed at the excellent performance with such a high voltage as 55V
@ Mark Whitney : in theory all amps same class, same voltage and load impedance are the same ... in practice they are not.
SOAR, current capability, thermal dissipation, internal protection, be it plain current limiting or complex SPIKE say they are not.
So this kind of real world tests are invaluable.
Even more so in such a popular chip amp as 1875
Doubly so now that TDA2050 disappeared from the market.
Very useful.
I was amazed at the excellent performance with such a high voltage as 55V
@ Mark Whitney : in theory all amps same class, same voltage and load impedance are the same ... in practice they are not.
SOAR, current capability, thermal dissipation, internal protection, be it plain current limiting or complex SPIKE say they are not.
So this kind of real world tests are invaluable.
Even more so in such a popular chip amp as 1875
Doubly so now that TDA2050 disappeared from the market.
I used different power supplies. I used a regulated dual rail supply with 6,800uf caps added. For higher voltage /heavier loads I had to switch over to a 350 VA toroid on a variac. I monitored the supply voltage at the IC pins during output voltage readings to be sure I was getting a good reading and monitoring the output wave form / distortion on my oscilloscope / spectrum analyzer.
I will set up with a 2 ohm load and take some measurements when I get the chance.
John is stating the total voltage across the supply rails...............
I was amazed at the excellent performance with such a high voltage as 55V
55.2Vdc is equivalent to +-27.6Vdc
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Note the drop from 2r67 to 2r0 30Vdc
-1.46dB with that small change in current demand.
This confirms that the chip is incapable of performing properly into 4ohms speakers with +-15Vdc supply rails.
the 28V, 26V & 24V results 4r0 vs 2r0 are only a little better.
By the time the supply is down to 22Vdc the 2r0 is only -0.81dB ref 4r0.
This would indicate that the amp can just about manage a 4ohms speaker on +-11Vdc supply rails. It becomes an 8W into 4ohms capable amplifier.
-1.46dB with that small change in current demand.
This confirms that the chip is incapable of performing properly into 4ohms speakers with +-15Vdc supply rails.
the 28V, 26V & 24V results 4r0 vs 2r0 are only a little better.
By the time the supply is down to 22Vdc the 2r0 is only -0.81dB ref 4r0.
This would indicate that the amp can just about manage a 4ohms speaker on +-11Vdc supply rails. It becomes an 8W into 4ohms capable amplifier.
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The LM1875 passed tests fairly with 12+12vac transformer and 4 ohm speakers; however, I suspect that, during such observations, the transformer was in 1a~2a (24va~50va) range, which is not cruelly excessive. The voltage would have fallen somewhat, during high load.
But, even in such case, it did manage more than 10W to 4R. Please check?
I've actually been using 27+27vdc (18+18vac transformer) all this time, regardless of 8 ohm or 4 ohm loads, but that was with just 1a (36va) transformer for a monolbock. This performs practically identically (22W) to either 8 ohm or 4 ohm loads. Also, that is since 2007. So, at least we know it is durable.
The 8W figure seems a bit shortfall. Why would this be?
But, even in such case, it did manage more than 10W to 4R. Please check?
I've actually been using 27+27vdc (18+18vac transformer) all this time, regardless of 8 ohm or 4 ohm loads, but that was with just 1a (36va) transformer for a monolbock. This performs practically identically (22W) to either 8 ohm or 4 ohm loads. Also, that is since 2007. So, at least we know it is durable.
The 8W figure seems a bit shortfall. Why would this be?
What we are seeing is that the LM1875 is current limited to about 4 Amp. (2.9 x 1.4)
I think the AndrewT approach is to build a 4 ohm amp for a 8 ohm load and a 2 ohm amp for a 4 amp load, this will give you an amp that is theoretically capable of supplying half of the output power. Theoretically because we all know that a speaker is a complex combination of multiple L R C and never just a simple R.
I think the AndrewT approach is to build a 4 ohm amp for a 8 ohm load and a 2 ohm amp for a 4 amp load, this will give you an amp that is theoretically capable of supplying half of the output power. Theoretically because we all know that a speaker is a complex combination of multiple L R C and never just a simple R.
Not quite.
The 4r0 TEST is to check whether the amplifier can pass sufficient current for a reactive 8ohms load.
Similarly the 2r0 TEST is for assessment of a 4ohms load.
Those two TESTS only ask the amplifier to pass twice the nominal load current.
I now design (and have done for several years) for three times the nominal load current.
i.e. I test into 2r66 for a 8ohms capable amplifier.
An amplifier is just a modulator of the CURRENT passing from PSU to Speaker and back again.
The current capability starts with the PSU and continues through the amplifier.
Whatever the speaker demands must come from the PSU and local decoupling.
The amplifier output voltage should not collapse just because the speaker demanded a bit more current than expected.
A test for peak current into 0r1 is not useful for current capability of the PSU nor the amplifier. This peak current test can be used to validate IV limiting circuits.
The 4r0 TEST is to check whether the amplifier can pass sufficient current for a reactive 8ohms load.
Similarly the 2r0 TEST is for assessment of a 4ohms load.
Those two TESTS only ask the amplifier to pass twice the nominal load current.
I now design (and have done for several years) for three times the nominal load current.
i.e. I test into 2r66 for a 8ohms capable amplifier.
An amplifier is just a modulator of the CURRENT passing from PSU to Speaker and back again.
The current capability starts with the PSU and continues through the amplifier.
Whatever the speaker demands must come from the PSU and local decoupling.
The amplifier output voltage should not collapse just because the speaker demanded a bit more current than expected.
A test for peak current into 0r1 is not useful for current capability of the PSU nor the amplifier. This peak current test can be used to validate IV limiting circuits.
Of course.
Did I say otherwise?
Are we in the middle of the nitpicking season or something?
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I my case the supply voltage was measured right at the pins of the IC while under the measured output. Because the supply voltage will decrease/increase a bit as I increased/decreased amplitude of the input signal, I had to fine tune the supply voltage and retune the output to max signal before clipping a couple times to get the correct measurement.
AndrewT is designing for worst case reactive loudspeaker loads. This is good practice, but remember that worst case is not going to be the typical load. I do have a set of 4 ohm ported speakers that makes a TDA2050 pop and click on drum beats with only a 12.6-0-12.6 2A transformer as a supply. With the typical music signal, the supply voltage will be higher than with a continuous load and the output swing can reach higher. This can get you into trouble with the SOA protection even though the power transformer is not that "beefy".
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MORE MEASUREMENTS
Example with a 12.6-0-12.6 2A transformer, single bridge rectifier and two 6,800 uf filter caps.
Set at 120v AC on my variac, the quiescent output voltage was 35.5 volts.
The below measurements are at the maximum non clipping output
RL Vs Vout Pout
None 35.50 0.00 0.00 (no signal quiescent current ~60ma)
8 ohm 31.79 9.44 11.14
4 ohm 30.42 8.47 17.94
2 ohm 28.60 6.40 20.48
I'll have to double check the 2 ohm measurement as it is higher output than I was getting in the other test.
Example with a 12.6-0-12.6 2A transformer, single bridge rectifier and two 6,800 uf filter caps.
Set at 120v AC on my variac, the quiescent output voltage was 35.5 volts.
The below measurements are at the maximum non clipping output
RL Vs Vout Pout
None 35.50 0.00 0.00 (no signal quiescent current ~60ma)
8 ohm 31.79 9.44 11.14
4 ohm 30.42 8.47 17.94
2 ohm 28.60 6.40 20.48
I'll have to double check the 2 ohm measurement as it is higher output than I was getting in the other test.
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