The positive feedback encouraged me to record this second video. Unrehearsed rambling on the CX-112a preamplifier. Hope this is useful to many of you and would love to hear your feedback.
Tag: dht
DHTs and cathode bias degeneration
A few years back I went down the path of exploring different DHT in pre-amp stages for sonic improvements on detail and dynamics. I explored all sorts of different DHTs as have posted in the blog and many more I never got around to write up about.
300B SE Amp: build part IV (Board 1 finished)
Wiring job is done. The fixed bias supply delivers from +50V to -300V. It has more voltage capability as am planning to use this same supply for future builds (e.g. 845 SE). The filament supply set to minimise power dissipation on the filament regulators for the 300B. All working fine, so am happy with this board. Filament noise is 0.3mV.
300B SE Amp: build part III (Layout)
Playing with the layout a bit, here is a view of potential layout of the amplifier board. The PCBs for mounting sockets, turrets and switches are very handy. It accelerates the build process and provides full flexibility.
You may not like the open look and feel, but who cares! I love the aesthetics and those OPTs are enormous!
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):
EML-20A DHT Preamp
300B Un-bypassed Rk Bias Line Stage
Many times I get emails from DIY Audio builders who embark on building a DHT preamp when they don’t need gain, but instead what they need is a simple line stage to drive their amplifiers and interconnect cables effectively. Then they come back asking: “can I reduce the gain of the 01a or 4P1L preamps?”
For those who don’t need the gain, here is an interesting idea which brings together several design decisions which makes the DHT sound to its best. The challenge with many of the best sounding DHTs of low-mu is that is very hard to implement with filament bias. I’ve done a driver with a 46 in filament bias which was a crazy idea. I could turn of the heating with the amp running! It was a nice experiment though. With exception of the 71a and some other few DHTs, if you’re looking for good anode current and low ra, you’re in trouble. The 300B, 45/46, 50 and some other variations can’t be used in filament bias.
Subject to your religious beliefs in audio, you may not want to add a capacitor in the cathode, like me. I won’t dive into this discussion which is pointless as is a personal decision. If you continue reading this is simply because you value the sound difference in the DHT without a capacitor bypass in the cathode. Keep reading then…
EML 20-AM DHT curves and model
There is one DHT which attracted me from Emission Labs which is the EML20. If you’re looking for a mid-mu DHT valve these days, this one is the way to go. In order to meet with the two key requirements of linearity and low microphonic noise, EML made a great effort in producing this valve.
For me it’s a great candidate as a driver or for a Spud amp for headphones. I will likely use them in several places, but will start with a preamp stage, as you would expect from me anyway.
First step: tracing the EML20AM
Ba German DHT Preamp, here we go…
Flying around
Travelling around Europe on business is paying its toll. I’m away from home every week and pretty exhausted now. I don’t have much time free and whatever is available I spend with my family. Hence, the lack of posts recently. I hope this will change in the future.
Anyway, what’s up in the DHT world? I listened the Aa/Ba valves long time ago but never played with them. Mainly due to their higher anode resistance. With the gyrator load and the source follower output, things take a different dimension.
German precision
I have a nice stash of Aa from Valvo (globe) and Ba from Siemens. Interesting to see that curves are not easy to find, so I submit them both to the mercy of the uTracer. Nice to see the linear curves with high mu about 14 on the Ba to 30 in the Aa.
Here is an example of the Ba loadline:
2K2M DHP Test
The 2K2M is an unknown directly heated pentode from Russia with has variable mu. I have several pieces of these, and was wondering how useful they could be. This is part of my on-going saga of the “Mule“.