Ale Moglia

Aa DHT Preamp (Part I)

Obviously it was time to test the little brother of the “Ba” DHT. In particular, as I have such a low DAC in place now, I need the gain. With nearly a gain of 30, it’s an attractive fellow to work with.

First good sign is that it doesn’t pick up as much hum as the Ba. That’s good, nevertheless I placed a back copper plate which acts as enough earth shielding to keep the Ba quiet.

I worked with my breadboard to find a good “sweetspot”( at least electrically) for this valve. Given the low signal source level, I aimed for a low bias. I played with my fixed bias setup before replacing it with the SiC bias board. I found that 2V/160V was very good in terms of keeping the distortion profile to minimum. Here is what I ended up with:

Only 2 SiC diodes are enough. The source follower PCB is mandatory given the low anode current. I run it at 20mA (hot) to get best results of the stage. Rest of the circuit is very simple, achieving a gain of about 30. Here is the distortion profile:

There is no shielding and you can see some minor IM distortion with mains hum. Harmonic decay is nice with H2 being strongest. THD is very low at 4Vrms which is good sign. Let’s see the frequency response now:

Nearly 140kHz of bandwidth which is plenty for the stage. Great response. I just need to plug this one in and listen to this promising German DHT.

01a with filament SiC bias

I tried the below configuration and the results are very promising. Sound is as good as previously implemented SiC cathode bias.

Now looking at the TH and frequency response:

The stage performs as expected. I built some prototype PCB for the SiC filament bias. One board per channel. If there is any interest, I will run a bigger batch

Sounds great, as always.

Cheers

Ale

Ba DHT Preamp Build

Just in the last days before the arrival of our second child, I had a spot of luck and managed to get some free time to work on my DHT preamp experiments.

A pair of NOS Siemens “Ba” valves under initial testing

Originally I was put off by the hum pickup susceptibility of the Ba German DHT. However, I decided it was time to listen to the design I worked and experimented in my workshop time ago. I remember listening to this valve and was very pleased with its sound.

Django Reinhardt

Whilst listening to the master of Django

ER801a Driver Stage

Received the ER801a today back from Thomas. They were working fine, it was a faulty UX-4 socket pin of my tracer which annoyingly made the grid to appear faulty and open at certain voltage level. I need to replace it.

Either way, they are now in my system and I can only say how much I was surprised with the sound of these. They take the detail and timbre of the VT-25 to a next level. Amazing result, happy to have these as part of my driver stage. The preamp is the 201a feeding the ER801a into the 4P1L PSE.

Highly recommended valves, worth the price indeed.

Morgan Jones’ visit

I had the pleasure of Morgan Jones visiting me today. We’d discussed for some time a meeting and I was looking for the right time. August looked to be it as I was taking some days off work before the arrival of our next child.

Morgan had planned to ride in his bike into London. What could have gone wrong? Well, after more than two months of hot weather and no rain, we had a classical British rainy day. He arrived completely drenched in his leather bike gear.

Oh well, it all went uphill after that. We listen to Tom Waits on the new setup after the DHT shootout held yesterday.

Playing “Heart Attack and Vine” from Tom Waits

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DHT Audio Shootout @ London

Here’s the summary of the shootout day we had yesterday at my place with Tony Rees and Andy Evans

Test Music

For testing the pre-amps Andy suggested the following 2 tracks:

Ravel Feria (Cleveland/Boulez)
Janacek The Cunning Little Vixen Act 3 Scene 7 “A vixen’s running through the woods” Prague Theatre/Gregor

The Ravel is orchestrated for a large orchestra and percussion, and listening was particularly for the timbre of strings, brass and woodwind and the clarity of the percussion. The Janacek is for several different voices and chorus, and listening was for the tone and character of the voices and how well they were differentiated from each other. In both cases the overall clarity, life and crispness was considered.
Andy Evans

Although I wasn’t familiarised enough with these 2 tracks myself, I found them to be an excellent choice and a reflection of Andy’s musical experience and knowledge as musician.

Crazy DHT shootout

We compared 10 different DHT preamplifiers today with Tony Rees and Andy Evans. Very insightful findings which I will report shortly

For now, a picture of the laboriously day we had!

Spud Push-Pull Amplifier

I devoted the majority of my audio life to single-ended DHT amplifiers. I don’t regret the path, however, I do not want a polarised perspective on amplifiers. I do like very much a good push-pull amp, though. Over time, I stumble across poor implementations or excessive use of gNFB. Local feedback (lFB) designs well implemented are music to my ears.

I played around with SPUD amps over the years, with happy results. One of them I loved over the breadboard is the 6e5p in push-pull. I will definitely build a definitive version of it. Here is the concept overall:

The input transformers is step up. It will need 1:4+4 ideally. My friend Dorin from DVB Transformers has a great device which I reviewed before and would be ideal for this amplifier. To simplify the power supplies, I used only one, which provides a bit of a strain into the source followers SF1 and SF2. If you have a good driver, you can omit them, I wouldn’t. They take care of the miller capacitance which is worsened by the step up transformer. It’s just a pair of PCBs, not big deal. RK1 is an arrangement with a pot to balance the bias. C1 is in ultrapath. I used an Oil 40uF one. T1 was a Lundahl 8K8:8Ω PP OPT.

The CCS provides a stable bias current into RBIAS1 which sets the bias voltage.

Here is the operating point I used:

The 6e5p is biased at about 40-43mA and 240V. It needs 5V bias so a cathode resistor of 58Ω should do. For 4W output in A1, you need 4Vrms, which only with a 1:2+2 step up should do. You can get about 5W in A2 or a tad more.

I loved the sound of the 6e5p as output stage (as much as I do as driver).

There are multiple variances to explore indeed

I have a pair of Toroidy PP OPTs with CFB, so would be interested in running them in tetrode instead.

ELROG ER801a Update

It’s been really hectic for me over the last months. You will notice that I’ve not posted anything on this blog. Moving house and running a house renovation project whilst doing your day job is tough. Despite all this, I managed to set up a temporary lab in my new place. Yes. The system is running and I have also a workshop (or kind of).

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