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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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. 

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ER801a Driver Stage

ER801a as Driver in my system

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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