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.
Author: Ale Moglia
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
01a Low Gain DHT preamp (finished)
It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:
I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.
The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.
This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.
300B/845 Amp: Driver Options
For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:
- Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :
- Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:
01a Low Gain DHT preamp
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”
Mono Amp: EF37a driver (part III)
After a few exchanges with different people, it was time to make a few tweaks to the EF37a driver. Increasing the G3 voltage helps in squaring up the pentode curves as shown by few before. By increasing G3 to +12V or up to 20-25V can be beneficial in the long voltage swing of the pentode. There are a couple of interesting threads in DIYaudio to check out if you’re interested.
I should have fired up the eTracer and play with G3 biasing before modding the driver, but couldn’t help myself from doing this mod as it was very simple. The Screen voltage circuit I use has 2 identical versions of it in the PCB as it was designed for a stereo setup in mind. As I reused a PCB I built for other tests, I had a readily spare screen voltage regulator which could be easily tweaked to supply G3 at the levels needed: Continue reading “Mono Amp: EF37a driver (part III)”
Mono Amp: EF37a driver (part II)
On the last post, I shared this great circuit. Now, with the Holiday season and being locked down I somehow find the time to build it. A very quick process as it’s all modular. I’ve got all PCBs that can be interconnected like LEGO, that’s what I have always in mind when I design a new PCB.
Mono Amp: EF37a driver (part I)
It’s been a while since I last share some of the experiments. I’ve done a lot with different drivers topologies and pentodes lately, yet not much time to sit down and do a proper write-up.
SRS551 Pentode Curves and Model
It’s been a long and weird year with COVID-19. This evidently has kept me a bit distracted and changed priorities (as probably has done to you as well). Anyhow, here are the belated curves I was asked to publish.
The SRS-551 is a lovely powerful transmitting pentode from RTF. Similar (or close enough) to RS1003 and F3a. Much more accessible in price is the SRS-551. I have a nice stash of NOS valves somewhere lost in my attic storage. Either way, they sounded nice and could definitely live in my setup. The valve was intended for audio modulation in FM transmitters so it’s very reliable and linear device. Distortion was extremely low when I measured it but most importantly it could work really well with local feedback to squeeze out 14W in Single-Ended at nearly 490V/100mA bias point with an LL1623 OPT. The distortion was below 0.7% predominantly H2.
EL152 Pentode Spice Model
Some time ago I got hold of a nice stash of Telefunken EL152. These German pentodes are amazing. After playing with the RL12P35P and then obviously GU-50 (which is a copy of the LS-50), the EL152 was a nice valve to have at hand as it’s actually the LS-50 in a different bottle.
The B-10V socket is quite tricky as it seems like it was designed for the EL/FL-152 and similar Telefunken valves. Anyhow, managed to get some new ceramic ones to trace the curves and generate a Spice model. Hope you find this useful.
Fixed Bias Board
Long time ago I built a series variable voltage regulator for 600V. It worked flawlessly and survived all sorts of abuses as is on my bench HT supply.
With the same circuit design, I developed the final stackable PCB (see previous post here) with this regulator:
Looks more complicated than it is. The single-supply Op Amp (LM358) needs a low voltage supply. I derived this from a simple CCS (DN2540) and a pair of 12V Zener diodes. The voltage reference is the famous TL431A and with P1 you can adjust the output voltage. The feedback resistor pair (R14 and R15) senses the output. C6 is for frequency compensation. The MOSFETs used are ST3LN80K5 which have built in protection Zener diodes, so none of the ones shown in the diagram are actually needed. T4 provides current protection to the pass device T2.