I needed a power amp to replace a failed HSU power amp on my home theater subwoofer. I bought the B&O Icepower 1000ASP at Parts Express and the Ghent Audio chassis for it. After assembling I powered up the amplifier, measured a few AC signal levels with a DVM and an audio signal generator.
At first I couldn't believe it but my RMS reading DVM showed 300mV or so on the output terminals without any input, or input shorted to ground. Since the outputs are both floating at +65VDC above ground, I bought a differential high voltage probe for my oscilloscope so that I could "see" the output waveforms. Imagine my surprise when I verified that I was seeing a 5V pk-pk sine wave with a period of about 2.5uS, again inputs shorted or open no difference. The DVM's lack of high frequency response hid the truth. The amplifier does indeed amplify but the waveform looks horrible with all that high frequency "grass" riding on top of it. I realize it is at 400KHz and well above the audio band but is this normal for these modules?
Has anyone who has used a 1000ASP made any output level measurements and what have they found? I don't see how this gets to the 30-50uV level of output noised spec'd by the data sheet for the amplifier. Is there some specsmanship involved, maybe limiting the bandwidth of the noise measurement to something like 40KHz to get that low a noise number?
Any help is appreciated. Thanks!
At first I couldn't believe it but my RMS reading DVM showed 300mV or so on the output terminals without any input, or input shorted to ground. Since the outputs are both floating at +65VDC above ground, I bought a differential high voltage probe for my oscilloscope so that I could "see" the output waveforms. Imagine my surprise when I verified that I was seeing a 5V pk-pk sine wave with a period of about 2.5uS, again inputs shorted or open no difference. The DVM's lack of high frequency response hid the truth. The amplifier does indeed amplify but the waveform looks horrible with all that high frequency "grass" riding on top of it. I realize it is at 400KHz and well above the audio band but is this normal for these modules?
Has anyone who has used a 1000ASP made any output level measurements and what have they found? I don't see how this gets to the 30-50uV level of output noised spec'd by the data sheet for the amplifier. Is there some specsmanship involved, maybe limiting the bandwidth of the noise measurement to something like 40KHz to get that low a noise number?
Any help is appreciated. Thanks!
ICE power
Hello
The answer is yes.
the 400 Khz is called switching noise, and I don't like it.
I use the ICEpower 125ASX2
2 V ptp output noise 485 Khz.
I put some LC filter at the output and now +/- 250 mv
cap 470 nF (conrad art nr 001578609) L ? recup from a broken powerline adapter.
i try to send some of my own pictures but the system ask me a URL.
anyway sound is ok.
guy
Hello
The answer is yes.
the 400 Khz is called switching noise, and I don't like it.
I use the ICEpower 125ASX2
2 V ptp output noise 485 Khz.
I put some LC filter at the output and now +/- 250 mv
cap 470 nF (conrad art nr 001578609) L ? recup from a broken powerline adapter.
i try to send some of my own pictures but the system ask me a URL.
anyway sound is ok.
guy
That’s normal switching noise for Class D amps. You can’t hear it and the speaker transducers can’t react to it as it is too fast. So for all intents and purposes, in the human audio band, it is noiseless. If you don’t like that, then Class A or Class AB amps might be more to your liking. But beware, they can actually sound audibly noisier, especially near 50/60Hz mains frequency. So called “hum”. Class D amps can avoid this by using SMPS that changes the audible 50/60Hz line frequency up to inaudible 400kHz range. You can use SMPS for analog amps to reduce the line hum. But you may end up with switch noise artifacts if not enough filtering is applied.
Thanks for your responses. So I guess I will have to live with it. I understand that it is above audibility and won't affect the woofer. I didn't expect to see that much noise, given my experience with A/AB amp designs. Live and learn.
I've been raising this issue since the first time i laid my hands on a class D amp. You may not hear it but a powered up class D amp with long speaker cables at output affects the sound of my system in a rather negative way and especially the phono reproduction, even when not playing.
For those who want to reduce the output noise from the 1000ASP, I performed some tests with various inductors and capacitors and found what appears to be a solution, though not a total reduction of noise at the output. I used a 50 uH series inductor in each lead, with a 1 uF capacitor from each output terminal to earth ground. I was able to reduce the pk-pk level to about 150mV with this filter network at the output of my amplifier. Still not totally "quiet" but much better than my original 5V pk-pk measurement. The inductors I use are:
Hammond 1538M24, available from Digi-Key. Rated for 15A and with only 13 mOhms resistance, it should handle the output load at power.
Note that this rolls off the amplifier's frequency response starting at about 5Khz or so which is fine for me as I am using it for a subwoofer in the range of 10-100Hz. For wider bandwidth, reduce the size of the output capacitors as needed.
Thanks to all for the good advice!
Hammond 1538M24, available from Digi-Key. Rated for 15A and with only 13 mOhms resistance, it should handle the output load at power.
Note that this rolls off the amplifier's frequency response starting at about 5Khz or so which is fine for me as I am using it for a subwoofer in the range of 10-100Hz. For wider bandwidth, reduce the size of the output capacitors as needed.
Thanks to all for the good advice!
By being radiated from speaker cables and getting into equipment inputs. A phono pre is perfectly suited as a recipient. Anyone with experience into building PC transports knows how important for good sound is to reduce, if not eliminate all sources of RF and how much better PC transports work with linear power supplies. Once a powerful RF source as a class D amp is introduced in a well tuned system, all that tuning is for nothing. If an SMPS is also used a secondary pathway to noise will be open through power supplies. Apparently a lot of people are not bothered at all, yet some are.
Its indeed not ideal to have a powerful class D output right next to a low level analog input - its an argument for active speakers so the class D noise is confined to the speaker, and for placing a phono-preamp on the deck itself, reducing the chance for noise pickup and for all cabling to be line-level or digital signalling.
Hi hkgee,
Are you sure the 1 uF capacitor you're using is up to the task for this power level? It looks like it might be a fairly ordinary 50V unit.
If it were a Zobel across the output (in series with a 10 ohm or so resistor), for example, we would often specify a 100V or even a 250V part -- not because that voltage might occur -- but for the higher ripple current it would tolerate.
Just my 2¢. 🙂
Cheers
Are you sure the 1 uF capacitor you're using is up to the task for this power level? It looks like it might be a fairly ordinary 50V unit.
If it were a Zobel across the output (in series with a 10 ohm or so resistor), for example, we would often specify a 100V or even a 250V part -- not because that voltage might occur -- but for the higher ripple current it would tolerate.
Just my 2¢. 🙂
Cheers