How to build amp measurement gear?

[DIYer]

Here are "Hanning" 4096.

Sound card.


Amp at 50W 8 Ohm.

 

[Flint]

Can you easily run the same test on a different amp? If the noise at 17k is still there, then it may have something to do with the test setup.

 

[soundhound]

Here are "Hanning" 4096.

Sound card.


Amp at 50W 8 Ohm.

Now place the top of the graph back at 0dB so the bottom has more resolution.....

BTW, what amp is this; that could have something to do with the noise at 17kHz.

 

[DIYer]

Can you easily run the same test on a different amp?

I can but I wanted to make sure I learn the set up and graphs right before moving on to next amp/s.

SH, it's a cheap transistor P-P amp. I'll post the revised graph later.

 

[DIYer]

If the noise at 17k is still there, then it may have something to do with the test setup.

Here is harmonics measurement of different amp (name will be disclosed later) using "Hanning" 4096 FFT at 20W 8 Ohm. No hump at 17KHz but different harmonics. How will this kind of harmonics affect the sound we hear? I'm not sure and I haven't compared it side by side at same volume.

 

[DIYer]

Now place the top of the graph back at 0dB so the bottom has more resolution.....

Here is the first amp with "Hanning" 4096 FFT at 20W 8 Ohm. That hump at around 15.9KHz is definitely from amp. Could 19KHz peak be the harmonics of that hump?

 

[soundhound]

Now place the top of the graph back at 0dB so the bottom has more resolution.....

Here is the first amp with "Hanning" 4096 FFT at 20W 8 Ohm. That hump at around 15.9KHz is definitely from amp. Could 19KHz peak be the harmonics of that hump?

The peak at 19kHz appears in both graphs with different amps, so I would imagine it is something (a sub-harmonic of some clock maybe) coming either from your interface or computer.

 

[soundhound]

How will this kind of harmonics affect the sound we hear? I'm not sure and I haven't compared it side by side at same volume.

It would appear to be a typical solid state amp which is only differential (push-pull) on its output stage. I would expect it to sound accordingly; not all that great under the right circumstances.

BTW, either both these amps, or your interface is pretty noisy.

 

[DIYer]

They are both transistor P-P amp. The second amp (blue) somehow shows more of second harmonics despite its harmonics out to high frequency. That second harmonics would mask the following odd harmonics I suppose.

 

[DIYer]

BTW, either both these amps, or your interface is pretty noisy.

How long should I run the measurement? The graph seems to flatten out between harmonics when it's run longer (>5 sec).

 

[soundhound]

They are both transistor P-P amp. The second amp (blue) somehow shows more of second harmonics despite its harmonics out to high frequency. That second harmonics would mask the following odd harmonics I suppose.

No they would not in this instance. The only time this would be true is if there was a very strong second harmonic and a relatively strong 4th harmonic relative to the 3rd and 5th etc. And the odd harmonics would have to be basically gone by the 7th or so. Just the presence of even order harmonics at approximately the same level (or slightly higher) of the odd harmonics will not mask the presence of the odd harmonic distortion products.

 

[DIYer]

Here are couple more. Can someone (soundhound excluded) guess which amp these 2 are?

2W 8 Ohm.


2W 8 Ohm.

 

[DIYer]

Here's the last amp I measured.

20W 8 Ohm

 

[DIYer]

Here are the names of these amps.

Blue plot: QSC RMX850 rated at 200W @ 8 Ohm (pro amp transistor P-P), MSRP $350. I use it for surround sound.
Green plot: Audio Source AMP100 rated at 50W @ 8 Ohm (consumer amp transistor P-P), MSRP $100
Yellow plot: Dynaco Mark IV rated at 40W @ 8 Ohm (tube P-P amp), MSRP $400 - 450.
White plot: Tubelab SE rated at 7W @ 8 Ohm (tube SET amp - diy), cost: $350 - 700.
Purple plot: Ashly SRA-120 rated at 45W @ 8 Ohm (pro amp transistor P-P), MSRP $550 - 700.

 

[soundhound]

Here are the names of these amps.

Blue plot: QSC RMX850 rated at 200W @ 8 Ohm (pro amp transistor P-P), MSRP $350. I use it for surround sound.
Green plot: Audio Source AMP100 rated at 50W @ 8 Ohm (consumer amp transistor P-P), MSRP $100
Yellow plot: Dynaco Mark IV rated at 40W @ 8 Ohm (tube P-P amp), MSRP $400 - 450.
White plot: Tubelab SE rated at 7W @ 8 Ohm (tube SET amp - diy), cost: $350 - 700.
Purple plot: Ashly SRA-120 rated at 45W @ 8 Ohm (pro amp transistor P-P), MSRP $550 - 700.

I think the high noise floor of your interface is obscuring most of the view of the distortion products. The Dynaco MkIV is a bit surprising because the sidebands on the sides of each distortion product is intermodulation distortion (IM). I know my Dynaco ST70 has very clean distortion products with no IM visible (the ST-70 is a stereo version of the MK IV). There might be something wrong with your measurement setup, the amp might have some problem, or you're overdriving it.

I'm not really surprised with the presence of higher distortion components on the TubeLab amp. I have been pretty critical about this amp from the first time you showed me the schematic, and was also hesitant to give it a thumbs up when Pauly was interested in it. It's just a too-complex design, and the solid state components have no reason to be in an SET amp.

The solid state amps....what can I say. The results are typical.

Do try another (better) interface though; the noise floor of input to ouput should be around -120dB or more.

 

[soundhound]

Here's a plot I just did going from the output to the input of my sound card, which is in this case the Asus Xonar Essence STX; The distortion and noise floor are very low.

 

[DIYer]

I did calibrate the input and output voltage of this program but didn't see a feature that calibrates the noise level of sound card (M-Audio Mobile Pre). Your sound card seems to have about 18db lower noise than mine (download/file.php?id=1345&mode=view).

I happened to have an extra sound card (M-Audio Fast Track) which I'll try and report back. This one has output level control which may allow me to do gain structure method (?).

As for Tubelab SE, here is measurement at 1W 8 Ohm (vs. 2W above).

 

[DIYer]

I think the high noise floor of your interface is obscuring most of the view of the distortion products. The Dynaco MkIV is a bit surprising because the sidebands on the sides of each distortion product is intermodulation distortion (IM).

Does it have something to do with bias voltage? I have it set at 1.1VDC instead of standard 1.56VDC. I've read it somewhere that the original design was made when typical residential wall outlet was supplying around 115VAC 40 years or so ago.

 

[DIYer]

Just tried the other sound card and it's no better. I wonder if being USB external sound card has something to do with it. I'll just have to live with it for now.

 

[soundhound]

Just tried the other sound card and it's no better. I wonder if being USB external sound card has something to do with it. I'll just have to live with it for now.

I don't know. I have a USB interface in my shop (M-Audio Audiophile USB), and its noise is almost as low as my ASUS interface. Could be noise coming from your computer over the USB power?