Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

ETF.18 DHT Preamps Lecture

ETF.18 has been an emotional journey. I moved house literarily when I got back from ETF so my life has been more than hectic over the past few weeks.

I promise I will do a write-up of this amazing experience. There’s a lot of people out there who would love to attend so is my duty to reflect and share as much as I can.

In the meantime, I wanted to share the lecture I gave at ETF on DHT preamps. It was a challenge on its own but went really well. This was my first ETF and without knowing the audience I had to guess the level of detail, entertainment and expectations of an unknown audience. I knew a fair bit of the ETF folks, but audience was big and wide.

I struggled to find the time to prepare this lecture I have to confess. Between moving house, house building works, my second daughter’s arrival , weekly work travel and everything else, I seemed not to find the time to get this done. Thank you Morgan Jones and Rod Coleman for proof-reading and making this an easier task.

I hope you enjoy it. There are some notes on the slides I put together for the people who didn’t attend ETF. Otherwise the slides aren’t of much use on their own.

I’m writing this blog entry whilst enjoying the lovely Bourbon that Pete Millett gave me on the way back. Thanks Pete!

ETF Lecture on DHT Preamps (with notes):

DHT-Preamps-ETF2018-final-notes

ETF Lecture on DHT Preamps (slides):

DHT-Preamps-ETF2018-final

Tuning the system for ETF

This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage

I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.

It’s going to be interesting!

Cap Multiplier test

I managed to find the time this weekend and do a quick test on the Cap Multiplier PCB. It turned out that I missed a thermal pad on the PCB so had to add a short jumper. Nothing major, but the PCB needs adjustment.

Below is the diagram of one of the ways the PCB can be used. This is the most complex circuit, a basic one can be wired instead. The CCS (M1 and M3) provides better PSRR as well as regulation. A stable current is fed to R4 and P1 to set the voltage. C2 is the cap multiplier and the M5 used can be any suitable MOSFET. T4 provides current surge protection to the MOSFET as well as short circuit. R7 sets the current limit.

The boards fits various film caps, I have some WIMA DC Link which are great and fit perfectly:


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300B Amps: an early breadboard

Browsing my schematic archive I found this early implementation from some years ago on the 300B:

The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.

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