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Krell FPB 350 mcx

RayDunzl

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A quickee look and see...

Bought 9 years ago used, no rebuild, no repairs, no faults. Serial number indicates 2005 build date.



Left monoblock, approximately 5W/4Ohm tone, loaded by the speakers:

1573686481295.png


Not great...

Left Channel.

695 more rated watts available, though.



Let's see:

THD 0.012% = -78dB
Noise 0.091% = -61dB

Maybe adjust by 2.5dB since the signal wasn't full-scale, if you like.

Should investigate the inaudible power fundamental and harmonics.

At least the harmonics don't look like some of the very low priced stuff*.



Then again, the speakers distort more than the amp, and the room is noisier...

1573688059704.png



Yup.

THD 0.21% = -53.5dB
Noise 3.66% = -28dB

---

*recent modern low price example
1573688669797.png
 
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RayDunzl

RayDunzl

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The only thing looking off is the high level of 60 hz and harmonics in your results.

Could be the "test" wiring.

A 30ft XLR to a naked XLR plug with three 18" alligator jumpers to the amp terminals and, maybe, ground.

The whole area over there is a rats nest.
 
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RayDunzl

RayDunzl

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Hmm...

Amplifier powered but on Standby....

And I see this - black
With microphonics - peak red

And I'm reading the EMF of the woofer of the speaker... or possibly the panel too...

1573694304333.png


Looks the same as above with the amp completely powered off.

---

Power on to standby bump (inaudible).

1573694538632.png





Amp fully powered on, no signal, preamp off, with inaudible standby -> ready turn-on bump:

1573694673132.png





Amplifier ready idle, add idle preamp - preamp power bump (inaudible) and ah-ha.

The source of additional noise and high frequency slope.

1573694913609.png



With the amplifier fully on, no microphonics visible from the speaker. Making noises looks same as above.
 
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Blumlein 88

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So if I didn't misunderstand, you are getting hum from the preamp?
 
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RayDunzl

RayDunzl

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So if I didn't misunderstand, you are getting hum from the preamp?

Measurable, inaudible.

With preamp full volume, about the same measure, no audible hum, ear to woofer.

1573695544396.png




With the pre volume at minimum:

1573695731028.png



I'll have to try straight from the DAC sometime.

Not that I'll use it that way, but...
 
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MediumRare

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Looks to me like the stats in the first chart are 0 to 45,500 hz. Can you run 20-20k to see what the distortion and noise % are?

Maybe with laptop to DAC direct to amp, no preamp?
 
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RayDunzl

RayDunzl

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And microphonics from the speakers?

When the amplifier is off, yes.

No amplifier feedback trying to maintain zero volts, to absorb/counteract the back EMF from the speaker.



Looks to me like the stats in the first chart are 0 to 45,500 hz. Can you run 20-20k to see what the distortion and noise % are?

I can run it as far to the right as you like. The amp spec says

1573696673006.png


I can only measure to 96k, though.



But later.

Dinnertime.

I made enough pocket money on YMZ19 this evening to pay for a couple of steak dinners. Short at 27747, close at 27732.


Wow. Volatile tonight,

1573697129458.png


Oh well. Don't be greedy, they say.
 
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RayDunzl

RayDunzl

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Back by popular demand!
One Night Only!



The usual REW measurements at an calculated estimated measured 4.72V across the ouput for 5W into the 4 ohm speaker load.

Using PC REW Measurement Sweep @ -25.9dBfs -> USB -> Focusrite Clarett 4pre ch7
-> internal routing -> coax left channel ->Benchmark DAC2 -> XLR -> Krell FPB 350 mcx -> jumper clips -> XLR
-> Focusrite ch 2 ADC -> Ch2 USB out -> REW analyzer



Frequency Response and Phase:

1573966135098.png



Distortion:

1573966618365.png



Impulse Response:

1573966727418.png



Group Delay

1573966840635.png


Step Response

1573967166021.png



---


Idle noise signature

1573968349586.png



And 5W tone

1573968454347.png
 
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RayDunzl

RayDunzl

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I guess at least it didn't embarrass itself too badly, so, still good to go.

Estimated 5W SINAD at 80dB, based on the third harmonic.

1574019169242.png
 
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RayDunzl

RayDunzl

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So if I didn't misunderstand, you are getting hum from the preamp?

Uh...

The preamp sits on top of the Equitech, and the amplifiers flank it.

Just maybe that isn't the best place for it to reside...
 
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RayDunzl

RayDunzl

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So if I didn't misunderstand, you are getting hum from the preamp?

No audible noise of any kind at all from the speakers at any time or volume setting.

My room isn't totally silent, but the system seems to be within that constraint.
 
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