Bottlehead Crack Measurements

Test Setup:
Arta 1.8.5 -> ASIO USB -> Hilo Line-Out 0dbV @ -5dBFS -> DUT -> Hilo Line-In 24dBU @ -3dBFS

For reference, that puts the Crack's unloaded gain at ~24dB (15x). This figure did vary slightly based on driver tubes. In just about every plot that means the Crack was swinging 8.7 Vrms (24.5 Vpp).

When loaded, gain drops to 20dB. Loaded with 6080 tubes dropped to 17dB. FWIW I did correct for level when AB. I think 5998 tube might sound best because it has the lowest Zout and best drive characteristics.

#1: 1k THD
#2: 1k (below)
#3: Power supply noise floor*
#4: HF 19k & 20k IMD
#5 Hi-low 250 & 7k IMD
#6: 20Hz THD**
#7: 1k THD (333 load)
#8: Reality Check***

* My build of the amp is surprisingly quiet for a "cheap" DIY tube amp. With the choke in place, I hear no hum or hiss at full pull on the volume knob. CCS and choke combine to do their job quite well. If you're getting hum, also check that input wiring.

** Not shown, but distortion across audioband was constant. OTL's don't have issues with LF linearity.

*** I'm not aware of many headphones that require 24Vpp for normal listening levels. This plot shows a more realistic level (0.6 Vrms w/ 333 load) of how much distortion is there in a usage scenario. Even then, keep in mind 0.6Vrms is about 98dBA for something like an HD800. Realistically, you would only see that distortion on large transient peaks.

**** Slight differences in distortion and gain were seen with different tubes, but nothing that really correlates with perceived SQ.

 

Yikes! I'll be sure to only listen at reasonable volumes where distortion is much lower. Do you have any frequency response curves?

 

Feel free to listen as loud as you like. Unless you're driving something ridiculous you're not going to get any hard clipping. I should know; I can listen a bit louder than even guys like Marv.

Response is really flat, didn't bother posting it. So long as you don't load it down with a 25 ohm headphone (HPF bass roll-off w/ output capacitor)

 

The noise floor on your crack is amazing ~ -135db noise floor! Did the choke lower the noise by a significant amount?

 

It did slightly but quoting a 135dB SNR figure would be misleading. It's partially a trick related to how ARTA does its FFT processing.

You can see in the 2nd and 3rd images that there actually are some power supply components (60Hz and its multiples) hanging out at -102dB which would be a more relevant spec if you wanted to quote the amplifier's SNR at full output. In normal usage with the recommended headphones (HD800, HD600, etc.) you will hear no noise, hum, hiss, etc.

Crack's great noise performance can be mostly attributed to the CCS loading. Choke just helps a wee bit. But yes, very low noise floor not considering the slight ripple components.

 

ARTA readout says 0.37%, but yeah, about that.

 

I love the second harmonic, it shouldn't even be called distortion.

My crack is very quite as well at max volume I can't hear a thing but have a feeling a 60hz harmonic is there.

I want to measure my crack. Is there a low priced and good enough measurement gear to measure my Crack with? I want to test out my mods I do to it.

 

What about the QA400? since it's $200.

edit, I see the QA401 has a lower noise floor.