EBFA-565 versus BFA-565 (Validating a new design)

Launching a new product is a lot of work! I’ve been working on these new boards for the Adcom GFA-565 for about six months now. (Now available here.)

This is the last step before I consider it fit for sale—design validation. It has to perform at least as good as the previous version, and it has to deliver on the usability improvements. I need to install it into an amp, and not only test its electrical operation, but the user experience of installing it.

A little background on my boards for the Adcom GFA-565

In 2016, I had a customer who wanted a pair of Adcom GFA-565’s restored. The input boards were so ruined from capacitor leakage, that it motivated me to learn PCB design in Eagle CAD, and make new boards. This was the product that launched Hoppe’s Brain circuit boards!

Adcom fans know about the issues these amps have with exploding capacitors.

Long story short: Elna—a good quality brand—made a bad batch of these little brown capacitors, ironically labeled “Long-Life”, LOL. Adcom put them in the GFA-565, and they destroyed nearly every one of these beautiful amplifiers, and destroyed many speakers with them. This random component failure put an undeserved dent in Adcom’s reputation. They were using a quality supplier, and they used very nice quality parts throughout the amp.

The boards get ruined. The electrolyte soaks into the fiberglass and causes it to become slightly conductive. This all happens around the sensitive high-impedance circuitry of the input section, where a tiny leakage can cause a DC offset to form, which is amplified by the circuit.

To fix the original board, one must fly leads above the board, or run bodge wires beneath the board.

Eww.

I could fix it but I don’t want to. It’s not up to my quality standards. I’d be worried about it breaking sometime in the future, and once I fix something… I set it free, and I never want to see it again. :^}

The new board was called the BFA-565. (Better F–cking Amplifier) It’s gone through a number of improvements over the years, as I learn more and more about PCB design. (Thanks to authors and YouTubers Eric Bogatin, Henry Ott, Dave Jones, Susy Webb, Robert Feranec, Rick Hartley and Zach Peterson)

This has been a best-selling product for me. Many GFA-565’s have been saved from the landfill and are playing music right now. So I thought it was time for a re-design, using what I’ve learned about PCB design to improve it’s electrical performance, and implementing features that make it easier for customers to install and troubleshoot.

The layout is an almost complete departure from OEM, so of course everything needs to be thoroughly tested.

I don’t have a working original GFA-565 board to compare this new EBFA-565 to, so I will instead compare it to my current BFA-565 board, which has been field-proven since 2016.

I want to do a before-and-after comparison, so the test subject is this GFA-565 with one of my BFA-565 boards installed.

I ran this amp through a battery of tests to collect as much data as I could, before swapping in the new EBFA-565 board. Then I ran it through the exact same tests with the new board.

Testing:

I love my HP 8903B Audio Analyzer, however being from the late 1980’s, its lower measurement limit is around 0.005% THD+N. (Published spec is <0.01%) Differences in performance between the BFA-565 and the EBFA-565 are subtle, and close to the lower limit of this instrument. Still, I do believe there are statistically significant improvements, as observed by the HP. A more modern analyzer like an Audio Precision system would reveal more.

Pasted below are my notes from the process. The most notable improvement, was lower distortion at high power levels, below 1KHz.

Overall I’m delighted! The EBFA-565 board shows some real improvements. Stability is excellent as before.

Graphs generated automatically with Pete Millet’s HP 803 Software.

BFA-565
Frequency Response, 0.12V input, 2.8V output 8 ohms

EBFA-565
Frequency Response, 0.12V input, 2.8V output 8 ohms

BFA-565
Frequency Response, 2.0V input, 46.2V output 8 ohms

EBFA-565
Frequency Response, 2.0V input, 46.2V output 8 ohms

No change in frequency response.

BFA-565
THD+N v power 20Hz, 8ohm

EBFA-565
THD+N v power 20Hz, 8ohm

BFA-565
THD+N v power 1KHz, 8ohm

EBFA-565
THD+N v power 1KHz, 8ohm

BFA-565
THD+N v power 10KHz, 8ohm (oops forgot to scale to 100mV)

EBFA-565
THD+N v power 10KHz, 8ohm

BFA-565
THD+N v frequency 4 ohms, 0.12V input 2.8V output, 20Hz- 60KHz (Ignore spike at 13KHz, it shows up in all my measurements and I haven’t figured out why yet.)

EBFA-565
THD+N v frequency 4 ohms, 0.12V input 2.8V output, 20Hz- 60KHz

Slight distortion reduction above 30KHz, nice!

BFA-565
THD+N v frequency 4 ohms, 1V input, 23.1V output, 20Hz- 60KHz

EBFA-565
THD+N v frequency 4 ohms, 1V input, 23.1V output, 20Hz- 60KHz
Very slightly less distortion above 20KHz

BFA-565
THD+N v frequency 4 ohms, 2V input, 46.2V output, 20Hz- 20KHz

EBFA-565
THD+N v frequency 4 ohms, 2V input, 46.2V output, 20Hz- 20KHz

Less distortion below 1KHz! Old board was about 0.025% below 1KHz near full power, new EBFA-565 board is 0.004% throughout this band! Again I should point out the caveat that this is down in the weeds of the resolution of the HP 8903B, and day-to-day sample variations might explain some of these results. But I think this is statistically significant. Even though a human being couldn’t reliably discern 0.025% THD from 0.004% THD, this result suggests something is right with the amplifier’s signal integrity.

BFA-565
Input voltage vs THD+N 20Hz 4 ohm

EBFA-565
Input voltage vs THD+N 20Hz 4 ohm

Slightly better overall

BFA-565
Input voltage vs THD+N 1KHz 4 ohm

EBFA-565
Input voltage vs THD+N 1KHz 4 ohm

Slightly better overall

BFA-565
Input voltage vs THD+N 10KHz 4 ohm

EBFA-565
Input voltage vs THD+N 10KHz 4 ohm

About the same

SQUARE WAVE TESTS

Open-circuit vs difficult load consisting of 1uF MKP cap in series with 2 ohm dummy load. (This my “pickled weiner dummy load”, which consists of four ceramic power resistors, suspended in a Ball jar filled with mineral oil.)

BFA-565

Input: Square wave generated by Rigol DG1022Z, 25MHz Function Generator, 1KHz, 830mV P-P.

GFA-565 Output: 20V P-P

50uS Horizontal

5V/div

Open circuit

With load 1uF in series with 2R

Zoomed to 1V/div

Open Circuit

With 1uF/2R load

About 0.4V P-P ringing, well-damped.

EBFA-565

(Spoiler: no difference)

Input: Square wave generated by Rigol DG1022Z, 25MHz Function Generator, 1KHz, 830mV P-P.

GFA-565 Output: 20V P-P

50uS Horizontal

5V/div

Open circuit

With load 1uF/2R

Zoomed to 1V/div

Open Circuit

With 1uF/2R load

Conclusion: No significant difference in square wave response determined.

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