... the only difference between men and boys
is the price of their toys ...

back in the day

Heath 4550
Our first piece of test gear was a moving coil VOM acquired about age 12. Next a digital hand held VOM. Sometime in the 70's a Heath 4550 10MHz Dual Channel Oscilloscope was built. Over time, the collection kept growing with meters, signal generators, LCR meters, etc. Techcron IMD Analyzer, Leader portable scope, HP 330x Analog Test Sets, Philips DSO @ ≈$20,000, Audio Precision Portable One...

Upon retirement, we gave away what remainded. We were DONE!

if only mom had intervened when I rebuilt the telephone... at age 7!

new tinker toys

To overcome the M-125 flaws as detailed in teaching the Tubes4HiFi / VTA M-125 to sing, we needed some test gear! The digital revolution has given us incredible power for pocket change. One can purchase a DSO for under $100 that does almost everything as well and in some cases better than a $20,000 product of 30 years ago. It fits in a pocket and doesn't require two small men and a boy to shift.
LOTO Instruments makes a nice range of PC based DSO. The OSC802, complete with probes, is around $100. It includes FFT, Trace Math, Recorder, etc. Software is continually improved with new features, bugs are fixed timely and the company is responsive to suggestions and comments. Well recommended.

AkitikA_1v_0 TDM-1901_mod
Casting about for a distortion set, the LongWei TDM-1901 seemed to have possibilities. It's a Chinese rip of a Leader and under $200. Like a lot of Chinese product, it's sow's ear in a silk purse. As can be seen in the first image, it capable of impressive performance, accurately measuring the 0.002% distortion of the superb AkitikA 2PPM Sine Generator.
The gotcha is it is only capable after modification to remove 120V noise. This involves rotating the power transformer, removing the untwisted 120VAC power switch wiring which runs full length along the distortion board and adding a transformer shield. This unit also had a AC power connection that opened when the transformer was rotated.

Solo Gen3 TRSInputPAD Solo Gen3 noise FFT
Impressive specs. So-So performance. Founded in 1985 by Rupert Neve, Focusrite was a sine qua non for top quality recording. Dead quiet, fabulous sonics, meticulous design and construction fairly oozed quality. An absolute pleasure to use. Sadly, the Scarlett line trades on the reputation. The specs are post an A-weighting filter. Usable. Just. Tolerable for the tariff.

The SOLO Maximum input is +22dBu or ≈9.5V. With a 20dB pad, safe input levels are about 60W. Over that and the smoke may escape.

the Pudding

My dear old Nan was oft heard to say “The Proof of the Pudding is in the Eating.” So, how do our Tinker Toys hold up in real world testing?
Rt THD pre Rt THD post Rt Noise pre Rt Noise post
Solo REW Rt THD pre
Modified amplifier has slightly lower THD and much less 60Hz in the noise floor. IMD is rather more than expected.
The Solo has steadily increasing noise that peaks around 100kHz. Good Enough for the task at hand.

Test Load: 8Ω  16nH --- Amps drive Kimber Monocle XL 2.5m & LFT8b mid panel: 10.7Ω  37nH

Measurements above made with Room Eq Wizard aka REW. A truly excellent product no self-respecting audiophoole should be without. And for pity sake, if you use it, DONATE!

stock 16Ω feedback

Dist Pre 3V Dist Pre 10V
Neither before nor after modification do the amps come anywhere near close to claimed distortion specifications.

AkitikA source is 1kHz 0.002% @ 1V, 0.003% @ 100mV.

modified 8Ω feedback

Dist Post 1V Dist Post 3V Dist Post 10V Dist Post 125W
In all cases distortion is lower from the 8Ω tap.