I was trying to establish the pecking order of nc400, nc500oem and ncxxxmp from the data sheets. THD+N are difficult because you get a mixed bag of HD, noise and EMI.
I thought IMD might be a nice way to establish whether the amp has sufficient loop gain at near 20 kHz but it turns out the plots are a bit of a mystery and highly inconsistent.
Let's take the nc250mp for starters. IMD at 18.5 and 19.5 kHz at P_R/2 at 4R load.
- the two signal peaks sit at about -3dB (whereas other hypex data sheets have them at -6 dB, and Amir has his at 0 dB - this is all convention, but you have to pay attention when comparing)
- the noise level is at about -134 dB or -131 dB relative to those signal peaks
- both 2nd order products (17.5 and 20.5 kHz) sit at -105 dB (relative to peaks from now on) -- usually the 20.5 kHz would be higher because of degrading loop gain
- 3rd order (16.5 kHz) at -120 dB
- 4th order (15.5 kHz) at -128 dB, higher orders not discernable
- there is a -115 dB peak at 8.25 kHz - what is this?
- there is a DC peak at -91 dB - does this have any meaning, such as in CMRR?
- the first order differenc tone at 1 kHz sits at -104 dB
- 2nd at -119
- 3rd at -126
Now something is very strange in the nc252mp data sheet, same PR/2 at 4R:
- the two peaks sit at - 6dB, so this is a different level that I will have to use as a reference
- noise is at -132 dB
- instead of the expected peak at 17.5 peak there is a doublet of two peaks at 17.75 and 18.25 kHz at about -106 dB
- instead of the expectd 20.5 kHz peak there is a doublet at 19.75 and 20.25 kHz at -104 dB
- there are three more peaks at 17.25, 16.75 and 16.25 kHz in decending magnitude
- the spectrum between 4 and 16 kHz looks very clean, just like the nc250mp except that it had a peak at 8.25 kHz
- the DC peak and some stuff that might be the difference tone and its 2nd and 3rd oder are there but at the wrong frequencies
--> This is the only Hypex data sheet that has the products at the wrong frequencies. Did they misuse their analyser or is this a spurious mode that the module can get into?
Taking Amir's review of the IOP with an nc252mp module, this looks more like the nc250mp data sheet.
His IMD is at 5 W (into 4 R?) and with 19 and 20 kHz tones.
- peaks sit at 0 dB
- noise at roughly -138 dB, i.e. 3-4 dB lower than hypex at 200 W (why?) but with a jitter-like "podestal" around the test tones
- 2nd order at 18 and 21 kHz at -98 dB (strangely this is 7 dB higher than the nc250mp data sheet says - how can the amp be more nonlinear at 5 W than it is at 200 W?)
- 3rd order at 17 kHz at -109 dB (11 dB higher than nc250mp)
- 4th order at 16 kHz at -116 dB (12 dB higher than nc250mp)
- no higher orders, same as nc250mp data sheet
- 2nd to 7th order from 21 kHz up pretty uniformly at -106 dB (this is the declining loop gain - I like Amirs wideband measurement better than hypex's)
- no 8.25 kHz peak!
- no DC!
- 1 kHz at -91 (13 dB higher than nc250mp data sheet)
- 2nd at -108 (11 db higher)
- 3rd at -117 (9 dB more)
IMD according to data sheet for nc100hf for PR/2 at 8R is very similar to nc250mp in that it has mostly 2nd and 3rd order distortion and a tiny bit of 4th. However, the 8.25 kHz peak is missing, and 2nd order seems to be higher.
IMD according to data sheet for nc122mp at PR/2 at 4R is very similar to nc250mp. The 8.25 kHz peak is present. Third and fourth order products seem a lot lower than nc250mp.
In summary:
- The nc252mp data sheet is the only outlier with strange frequencies of the distortion products.
- This behavior is not replicated in Amir's test, but Amir measures lower noise and higher low order products at much lower power.
- The 8.25 kHz is present in most hypex data sheets.
- The DC peak is present in all hypex data sheets but not in Amirs.
- All IMD measurements so far are consistent in that these modules have mainly 2nd and 3rd order distortion and a tiny bit of 4th.
(more to follow)
I thought IMD might be a nice way to establish whether the amp has sufficient loop gain at near 20 kHz but it turns out the plots are a bit of a mystery and highly inconsistent.
Let's take the nc250mp for starters. IMD at 18.5 and 19.5 kHz at P_R/2 at 4R load.
- the two signal peaks sit at about -3dB (whereas other hypex data sheets have them at -6 dB, and Amir has his at 0 dB - this is all convention, but you have to pay attention when comparing)
- the noise level is at about -134 dB or -131 dB relative to those signal peaks
- both 2nd order products (17.5 and 20.5 kHz) sit at -105 dB (relative to peaks from now on) -- usually the 20.5 kHz would be higher because of degrading loop gain
- 3rd order (16.5 kHz) at -120 dB
- 4th order (15.5 kHz) at -128 dB, higher orders not discernable
- there is a -115 dB peak at 8.25 kHz - what is this?
- there is a DC peak at -91 dB - does this have any meaning, such as in CMRR?
- the first order differenc tone at 1 kHz sits at -104 dB
- 2nd at -119
- 3rd at -126
Now something is very strange in the nc252mp data sheet, same PR/2 at 4R:
- the two peaks sit at - 6dB, so this is a different level that I will have to use as a reference
- noise is at -132 dB
- instead of the expected peak at 17.5 peak there is a doublet of two peaks at 17.75 and 18.25 kHz at about -106 dB
- instead of the expectd 20.5 kHz peak there is a doublet at 19.75 and 20.25 kHz at -104 dB
- there are three more peaks at 17.25, 16.75 and 16.25 kHz in decending magnitude
- the spectrum between 4 and 16 kHz looks very clean, just like the nc250mp except that it had a peak at 8.25 kHz
- the DC peak and some stuff that might be the difference tone and its 2nd and 3rd oder are there but at the wrong frequencies
--> This is the only Hypex data sheet that has the products at the wrong frequencies. Did they misuse their analyser or is this a spurious mode that the module can get into?
Taking Amir's review of the IOP with an nc252mp module, this looks more like the nc250mp data sheet.
His IMD is at 5 W (into 4 R?) and with 19 and 20 kHz tones.
- peaks sit at 0 dB
- noise at roughly -138 dB, i.e. 3-4 dB lower than hypex at 200 W (why?) but with a jitter-like "podestal" around the test tones
- 2nd order at 18 and 21 kHz at -98 dB (strangely this is 7 dB higher than the nc250mp data sheet says - how can the amp be more nonlinear at 5 W than it is at 200 W?)
- 3rd order at 17 kHz at -109 dB (11 dB higher than nc250mp)
- 4th order at 16 kHz at -116 dB (12 dB higher than nc250mp)
- no higher orders, same as nc250mp data sheet
- 2nd to 7th order from 21 kHz up pretty uniformly at -106 dB (this is the declining loop gain - I like Amirs wideband measurement better than hypex's)
- no 8.25 kHz peak!
- no DC!
- 1 kHz at -91 (13 dB higher than nc250mp data sheet)
- 2nd at -108 (11 db higher)
- 3rd at -117 (9 dB more)
IMD according to data sheet for nc100hf for PR/2 at 8R is very similar to nc250mp in that it has mostly 2nd and 3rd order distortion and a tiny bit of 4th. However, the 8.25 kHz peak is missing, and 2nd order seems to be higher.
IMD according to data sheet for nc122mp at PR/2 at 4R is very similar to nc250mp. The 8.25 kHz peak is present. Third and fourth order products seem a lot lower than nc250mp.
In summary:
- The nc252mp data sheet is the only outlier with strange frequencies of the distortion products.
- This behavior is not replicated in Amir's test, but Amir measures lower noise and higher low order products at much lower power.
- The 8.25 kHz is present in most hypex data sheets.
- The DC peak is present in all hypex data sheets but not in Amirs.
- All IMD measurements so far are consistent in that these modules have mainly 2nd and 3rd order distortion and a tiny bit of 4th.
(more to follow)