All about electronic valves and hi-fi
I wanted to share a nice review provided by Lionel Borcelot on his journey in building the VT-25/10Y/801a preamp as shared previously. I think this is a good example on what to expect and how to adapt/reuse existing components at hand to get the most out of this design. Bravo Lionel, excellent job.
For more than 10 years I’ve been experimenting, designing and enjoying DHT pre-amps. Without over-dwelling on this subject, to me DHT preamps bring the unique sound transparency and detail we all seek for. They can make night or day difference in any system. And is not about gain, hence I’m focusing on sharing this blog entry.
How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.
However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.
With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.
I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.
This can be overcome in the following way: Continue reading “01a Low Gain DHT preamp”
My friend Mirek sent me from Czech Republic a few valves for testing, including a pair of precious EML300B Mesh valves. I managed to slot them into my system before departing for holidays. I only listened to them for a few hours, so these are only initial impressions.
I like the sound and was expecting the additional level of detail and sound of the mesh plate. Worth addition clearly. I wouldn’t say it’s a significant step improvement, just minor, subtle details are clearer. You won’t go wrong with the standard EML300B. The Mesh is a nice upgrade but you will need to think (as always) where it best to put the money on. I’d invest in the iron and overall circuit before you get to throw more money on the output valves. That’s my view.
Anyhow, great job Emission Labs for this superb valve.
As previously mentioned, I played around with the 46 driver. I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:
The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.
I made some progress on the LL7903 step-up transformer which is wired on 1:8.
One positive thing I can get out of this COVID-19 lockdown is that people are building more audio equipment. Here is a great story from Sridhar Ganti who has built a few DHT preamps over the years.
From left to right: 1) Standard Rev08 PCB with full flexibility of FET and TH components. 2) Rev 1.0s board with SMD except Rmu, protection drain resistor and LED as well as space for any nice big PIO capacitor. 3) the smallest version of all, all SMD except Rmu, film cap and standard TO-220 top FET and multiturn trimmer.
Very happy with the results in the board development. It does take more time and precision (you will need a microscope) to work with the MELF resistors and the SMD components in general. However, it’s worth the trouble if you’re looking to reduce the footprint.
A friend from Canada ordered a pair of boards and I used the opportunity to test the newly arrived hybrid mu-follower (aka as gyrator) Rev08 PCBs. He will be using these boards in the 300B design posted here.
It’s been far too long since I last posted on this blog. With the limited spare time I’ve got these days, I concentrated in setting up the new workshop and system since we moved back to our place. I’m nearly there, so now it’s time to get back to work
I made some updates to the “gyrator” PCB. I’ll stop referencing it gyrator from now on, since the name is misleading. However, it got popular that way. Nevertheless, it’s a hybrid mu-follower circuit but if only if you take the output from the anode, it behaves like a “gyrator” from a frequency response perspective. If you’re interested in this circuit in more detail, please read the lecture I gave last year at ETF.18. You can download it from here.
Back to the board, here are the few changes made:
Continue reading “Hybrid Mu-follower (aka Gyrator) Rev08 PCB Update”