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Rob's Amp tests


peniku8
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Pmillett Sound Card Interface loopback measurements:

Measurement chain is Fireface UC → SCI (Generator input) → Internal loopback → Fireface UC
The SCI converts the balanced signal of the Fireface internally and outputs either balanaced or unbalanced, which can be switched. It doubles the output voltage, so if I input 2V balanced, it outputs 4V balanced or 4V unbalanced. Voltages seen are after conversion (means the Fireface outputs half the voltage displayed).

Measurements are sorted by voltage ascending.

91wVcNH.jpg

 

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2YsvVSP.jpg

The 1V measurement looks similar to the Fireface loopback, so I guess the pmillett isn't adding too much distortion.
The pmillett started distorting at about 8.6V output (unbalanced). When I switched it to balanced, the distortion increased a bit, but there was more headroom (presumably twice the voltage).
Here is a measurement of the Fireface at 0dbFS, which produced 13,25V at the Pmillett's generator output, which is about 24,6dbU:

XEKh1SO.jpg

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The level seems quite low.  I would expect the noise to be constant with level so SNR should improve if a larger signal is used.  The Fireface UC is spec: THD+N AD: < -98 dB (< 0.0012 %), THD+N DA: -96 dB (0.0015 %) so you're getting pretty close.*

 

*didn't see your last post I think optimal level for the Fireface will be around 4V from your results.

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11 minutes ago, kipman725 said:

The level seems quite low.  I would expect the noise to be constant with level so SNR should improve if a larger signal is used.  The Fireface UC is spec: THD+N AD: < -98 dB (< 0.0012 %), THD+N DA: -96 dB (0.0015 %) so you're getting pretty close.

What do you mean by low level? The last measurement was taken at 0dbFS, which produced 6,6V at the Fireface's output.

Here is a direct measurement of the FF without the pmillett in the loop (max output 6,6V as before):

iBiOQEj.jpg

Best THD is achieved at 0.5V output, best noise floor at maximum.

Here is the THD/Noise vs output plot:

gAlahVs.jpg

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  • 4 weeks later...

And the loadbank causes another casualty.. this time it's a Yamaha CR-600. Fuses are done for, but I hope everything else is still good. Measurements of my Hypex FA123 coming soon. Also ordered dedicated measurement equipment, since I already had trouble measuring the true performance of the Hypex amps... Fireface not clean enough. Getting the Cosmos ADC to use with a Topping D10b. Probably also going to get the Cosmos APU once it releases, which includes a 1khz notch and a preamp capable to delivering phantom power with lots of gain.

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Which of the two styles would you guys prefer? Currently it shows values for 1khz for full range reference and 30Hz for deep bass reference, but I'm not even sure if that's needed. The data table clutters the graph a lot imo

grafik.png.e0fa4538382fc06eb373a975b05f0b02.png


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Evaluations will additionally have these:
grafik.png.149e8aaadf5e042a811dc725beb054e3.png


Which will react like this when they're presented with differing values for bust and peak:
grafik.png.eeeb6cfaf9a831de43138bf6530d55c7.png

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Wow, these are nice!

If I may make a suggestion for the comparisons of power at different frequencies: plot these in units of dB instead of watts!  Use 1000 Hz as a reference, so it's always "0 dB" at 1000 Hz and will typically be small or negative at the other frequencies.  (So maybe use "+1 dB" for the top of the Y-axis.)  I think this will convey information that is much more useful to the reader as it clarifies exactly out much output you lose at some lower frequency.

The formula to calculate this is as follows:  dB(f) = 10 * log10( P(f) / P(1000) ) where dB(f) is the decibels at frequency 'f', and P(f) is the power at that frequency.  dB(1000) should always be 0.

Other graphs may also be more useful in decibel units, but decibel units only make sense when you have a meaningful reference point as above.  When comparing different amps to one another, there isn't a good reference unless you choose one of them to compare everything else to.

What do you think?

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13 hours ago, SME said:

Wow, these are nice!

If I may make a suggestion for the comparisons of power at different frequencies: plot these in units of dB instead of watts!  Use 1000 Hz as a reference, so it's always "0 dB" at 1000 Hz and will typically be small or negative at the other frequencies.  (So maybe use "+1 dB" for the top of the Y-axis.)  I think this will convey information that is much more useful to the reader as it clarifies exactly out much output you lose at some lower frequency.

The formula to calculate this is as follows:  dB(f) = 10 * log10( P(f) / P(1000) ) where dB(f) is the decibels at frequency 'f', and P(f) is the power at that frequency.  dB(1000) should always be 0.

Other graphs may also be more useful in decibel units, but decibel units only make sense when you have a meaningful reference point as above.  When comparing different amps to one another, there isn't a good reference unless you choose one of them to compare everything else to.

What do you think?

I record the amp's output as WAV, so I'm logging the db values in the first place, which I then use to derive the powers from. Changing out the values with db values shouldn't be a problem at all, but like you said, if people wanna compare amps at certain frequencies (say you're looking for an amp for your tactile system, so you look at the 8Hz results), you might be better off comparing absolute powers over relative db loss, which you then have to backtrack to compare.

For me it's easier to make out the db changes in my head (like when I'm seeing 1000W at 1khz and then I see 800W at 30Hz that's somewhere around 1db loss), than to reverse the process and I like dealing in absolutes, because we're comparing different amps in the big picture. I could maybe also display both, but I already had enough trouble with Excel to make this work as I wanted it to (it's a universal file and the graph needs to adapt to mono blocks, 4 channel amps etc...), and I think even more info would clutter the graphs a bit too much.

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On 11/28/2021 at 10:09 PM, peniku8 said:

And the loadbank causes another casualty.. this time it's a Yamaha CR-600. Fuses are done for, but I hope everything else is still good. Measurements of my Hypex FA123 coming soon. Also ordered dedicated measurement equipment, since I already had trouble measuring the true performance of the Hypex amps... Fireface not clean enough. Getting the Cosmos ADC to use with a Topping D10b. Probably also going to get the Cosmos APU once it releases, which includes a 1khz notch and a preamp capable to delivering phantom power with lots of gain.

Amp is up and running again, were just the fuses. 45 year old amp takes accidental 2ohm clipping like a champ and only causes 1$ of damage. I love this thing! And it's actually good THD, even by today's standards (0.007% 4V into 8R).

I fixed some other things and mondernized the amp while I was at it, fitting it with a C14 socket (replacing the fixed cable) and clamp style speaker connectors. There are LED-sized light bulbs (!) inside the panel that lights up the tuner's signal strength, of which one still works. I put it in the middle socket and it lights up both meters now, it's great.

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