Measurements of vintage Kenwood KA-5010 amplifier clone - Now with SMPS!

[pma]

I made some improvements in power supply section and in wiring. As a result, the amplifier is completely free from mains spuriae lines and other mess. High order harmonics have been also considerably reduced.

More THD vs. frequency plots at 2W - 40W/4ohm and @45kHz BW

 

[pma]

So, where is the key to the improved spectrum cleanliness and disappearing of mains spectral line and its multiples? The attached photo gives the answer.

 

[pma]

This is THD vs. frequency with 2.4 ohm load. Many speakers go near 2 ohm in complex impedance magnitude and many more in EPDR! So this is not any unrealistic load.

It is measured at 30W and 60W output power. We can see that the amp distortion does not depend much on power, but at 60W there is a sudden rise of distortion below 35 Hz. The reason is the SMPS use, it is unable to feed the amp with enough current at very low frequencies, limited to 4.5A DC.

It is still very good at 50W/2.4 ohm, measured with 2Hz - 45kHz BW. The rise of noise floor below 100Hz is the soundcard characteristic.

 

[AnalogSteph]

So, where is the key to the improved spectrum cleanliness and disappearing of mains spectral line and its multiples? The attached photo gives the answer.

View attachment 194455

Now that makes a bunch of question marks appear over my head:
Is this an IEC Class I device entirely (if so, how do you avoid ground loops with the unbalanced inputs?) or else how are you tackling mains leakage? These look like Meanwell LRS series supplies, so you could in principle go for "IEC Class II with benefits" (even if it would require isolating the PSUs from the chassis, which instead would connect to -V/+V).
What is the rectifier diode over the output doing?
I see your L and R inputs are quite far apart, how do you avoid creating a ground loop when a stereo source is connected?

 

[pma]

The chassis is grounded and signal ground is coupled via 47nF capacitor to the chassis. For all measurements, signal source has been Topping D10s DAC that is connected through USB-ISO isolator to kill the loop, ADC is an USB soundcard. The signal from the amp output goes to the resistive voltage divider and then to the soundcard RCA input. This path completely wired with RG-59 cables. Look, I am very good in killing interferences and interference shield currents . The reasons is that I was designing wideband measuring instruments that were asked to work with small signals in the hostile environment of High Voltage testing labs during lightning and surge impulse tests and in high power testing labs. This is unbelievably similar to small signal audio issues.
I will post some ultrasound spectra resulting from these SMPS operations. They start at about 70kHz, nothing in audio band. Still less issues than with class D switching that masks SMPS residuals.

If you are interested in the 2nd half of the story - the amp with SMPSs is supplied from 1:1 isolation transformer from the 230V/50Hz room mains. This is how the SMPS leakage currents are killed almost totally. But even without this transformer, the 50Hz leakage is completely acceptable, you will see it somewhere at -110dBr in those plots.

 

[pma]

I will post some ultrasound spectra resulting from these SMPS operations. They start at about 70kHz, nothing in audio band.

This is the amplifier output wideband noise. Please note that the spectrum is clean up to 60kHz and then we can see SMPS switching frequency residuals and its multiples.

I think that it is a good idea to make a mix of linear amplifier and SMPS power supplies, but I would not recommend this to someone without necessary instrumentation, skills and experience. Still a lot of caveats.

 

[pma]

Some RMAA measurements with 4ohm load. Should be noted that the ADC chip CS5361 in the soundcard used has weighted DR 114dBA and unweighted 111dB.

and THD vs. output power measured at 20Hz, 1kHz and 6kHz into 4ohm load

 

[pma]

I have set the class AB output bias current to the optimum value (24-25 mV across Re resistor) and made new set of measurements THD vs. power with @45kHz bandwidth, at 20Hz, 1kHz, 5kHz, 10kHz, 15kHz

 

[maty]

A design evolution to double the power would be interesting, I say.

 

[pma]

A design evolution to double the power would be interesting, I say.

It would be no problem, if I'll get interested enough .

I have an amplitude/phase response for you. -1°/@20kHz .

 

[pma]

10kHz square response into 6.8 ohm load. Blue trace = input, red trace = output.

 

[pma]

I have replaced the SMPS power supplies with a linear power supply 2x25V and made a set of distortion measurements. The distortion is same as with the SMPS's, there is a small rise of mains related spuriae, and also disappearing of HF noise above 70kHz.
Idle power consumption has dropped from 30W with SMPS to 13W with linear power supply. The reason is simple - idle power of 2 SMPS's has been removed! One can see that "green" label given to SMPS supplies which is not supported by deeper thinking is wrong. This linear amplifier has in fact lower consumption than my NC252MP or UCD180 based amplifier.

Below are some distortion measurements + ASR beloved 5W/1kHz/4R SINAD.

And now the 5W/1kHz/4ohm SINAD

(SINAD = 91.7dB)

.... and much more useful IMD 19kHz+20kHz