Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

Swenson+ tests with SMPS

Here is a test you might want to take into account. I own an HGC-320-700A SMPS which only allows for maximum current trimming, but output voltage is fixed. In order to avoid unnecessary heatsink, I will suggest using an RC before the Swenson where the R is a big Alu clamped wirewound resistor bolted to the chassis. Just leave 50V for the regulator which may dissipate a lot anyway depending on the output current.
 
There is no caps below and was stable enough. I could have added an output cap but I didn’t on this test. You can see how all the HF noise is trimmed out! About 40 to 60dB 🙂
 
The 44mA and 88mA traces are the SMPS feeding the power resistor directly. 
 
 
This can be improved further with a (mandatory) output cap. A nice 30uF oil cap should do well.
 
I have offered some pairs built and tested to builders using the prototype boards. I may do a couple more if you are interested
 

Another DHT Preamp

Another DHT Preamp on the way with Rod Coleman regs. I will do a A-B comparison with some Tentlabs modules, it’s an interesting experiment to undertake. The preamp has the classic gyrator Rev07 and Source Follower boards…

NP Acoustic Transformers

This is a long overdue post which I never had the time to write about. I was hoping to get my measurement gear down to where my system is to take a final FR sweep analysis of my 4P1L PSE amplifier, but never got around to do it.  However, after the recent posts in DIY Audio, it was time I shared the measurements made and my listening impressions of the NP Acoustic Transformers.

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Valve test rig with fixed bias

My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.

Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:

For the curious builders, here is the rig mounted on a piece of floor board:

 

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…

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Swenson+ HT regulator test

So the developing is progressing. This time I tested the first PCB prototype of my enhanced “Swenson” regulator which I baptised Swenson+. It’s a great circuit and performs really well. Although component selection and track cree page and clearance is for about 1kV, I think it should safely operate at least to 800V. Not really thinking to use it at those voltage levels, but rather as phono stage supply instead, which requires much lower voltages.

As usual the focus of the PCB is to provide as much flexibility in component selection (SMD and TH) as well as use cases.


Still some layout refining and further testing but so far so good! Here is a taster which shows at least 60dB rejection of the incoming power supply noise which wipes out all rectified-related harmonics (100Hz etc). The 100dB 50Hz and byproducts is likely to come from my workbench:

More to come soon (hopefully)

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

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