Popping the Shunt Voltage Regulator’s clogs

Not a surprise

What I suspected it was going to happen, it did in the end. Although a bit premature and in a bad time. I’m expecting today a friend to come around for a listening session and having no amp wasn’t an option.

To cut a long story short, the Salas SSHV2 shunt regulator has been playing silly buggers for a long time. Since I upgraded the output transformers and readjusted the bias, it looks like I was operating it at the verge of its abilities. The CCS was running at 80-90mA and somehow the stability of the shunt regulator was compromised. Initially was a periodic lost of regulation during warm up, this created an annoying “pop” now and then, later I decided to replace it with a new SSHV2 and blew a pair of DN2540 after the regulator failed to set the output voltage randomly. It worked fine on the test bench, however there is something on my system which is disturbing / interfering with the regulator or the regulator isn’t stable enough at the hot operating conditions I was submitting it to. I have nothing against the SSHV2, in fact, I use it extensively in my preamps. However, I think I’ve found the limit at which it can safely operate. The additional drawback of the SSHV2 is its temperature stability. It’s not great as it drifts when temperature rises.

So the regulator went busted on Thursday evening and I was running out of time. Only Friday was available to fix the amp. Luckily, I was on holidays this week and had the time to fix this, but unfortunately this diverted my energies and time from the 300B amp 🙁 Continue reading “Popping the Shunt Voltage Regulator’s clogs”

4P1L Siberian DHT Preamp (Gen3)

Recently I finished the filament supply for the latest incarnation of my 4P1L pre-amplifier. Here is the next instalment of this project. The HT power supply was refined after builiding more than 7 stacked HT supplies for the 814 SE Amplifier.

The supply design is very simple. Perhaps the selection of components and the refinement of some aspects of it is what makes the difference to me: Continue reading “4P1L Siberian DHT Preamp (Gen3)”

26 Pre-amplifier Gen3

An updated DHT preamp

It’s been a while since I played with preamps. Here is my third iteration of the 26 preamp. I love the sound of this valve as many of you out there. I found my previous build a bit inflexible to modify bias points and play around so decided to go back to the workshop and update my preamp, again. The design is very simple as you can see in the figure below. I’m reusing an existing supply but you can get away with a 200V HT and probably a 15V raw DC for the filament circuit. Key characteristics are:

Filament bias as I’m a fan of it, no output or input caps sir!
Added a Salas SSHV2 regulator to provide HT voltage flexibility and vary the operating point of the 26. The SSHV2 is extremely quiet so will give a clean HT supply to the 26. Yes, the lovely colour of the glow valves are lost in this version 🙁
The Kelvin capacitor C1 (ala Morgan Jones) will help keeping Salas noise input low, specially HF.
R1 is a simple resistor to measure the anode current. It is located in a handy place to allow easy measurements
T1 is the Lundahl LL2745 in Alt R mode for 5.6:1 ratio. This provides the lowest gain and the lowest output impedance. In my new system, this preamp is more a line stage as don’t need further gain.
SW1 is helpful in case ground loops are to be broken
The Rod Coleman regulators are set to 800mA to starve the DHT and reduce microphonics and distortion. I found 800mA to be better sounding than my previous 760mA.
P1 is my stepped attenuator and R2 will help providing grid bias when breaking before making action of the attenuator.
RF is 2 pairs of 10Ω/20W parallel/series wirewound.

26 Preamp Gen3

The preamp is extremely quiet. The below measurement is with a noisy Sylvania ST valve which picks up significant 50Hz hum. Notice that 100Hz harmonic is very very low (can’t be seen at -100dB). Distortion at 5Vpp input (8Vpp output) is lower than 0.03% which is what you would expect from a 26. With a good selection of valves you can get this down to 0.02% for sure:

26 preamp gen3 THD test1

The most important part of the build work is grounding and avoiding ground loops. A combination of star grounding is recommended. I do the following star ground combination:

Input stage
Filament bias return
Output stage

You definitely want to avoid the filament current ground introducing hum in the output stage so you want to keep the three star ground points separate.

Listening to the 26 is a fantastic experience. If you haven’t built a DHT preamplifier then do it. The detail and colour of this stage is unique. I found ST valves being more detailed on the top end, a richer treble compared to the Globe ones. Globe ones are sweeter and mellow. Definitely prefer globes for classical and ST for rock and Jazz.

26 Preamp connected to the 4-65a SE Amplifier

The preamp is extremely quiet. Best build so far, I can’t hear hum on my high-efficiency FE167E full-range speakers and that is a real challenge.

I will try this new combination of 26 preamp and 4-65a SE for a while and report further impressions at a later stage.

Just need to fix my Lenco turntable and will be a happy man again 🙂

Hope you enjoy this post and encourage you to build one of this.

Ale

Adjusting the shunt regulator

One of the problems I found when using the 4-65a SE amp was that my version of the Salas SSHV2 drifted significantly with temperature. This was due to the 01N100D characteristics (see extract from datasheet below). We can see that @ -1.5V VGS the temperature dependency is as much as 50mA/100°C. With the smaller heatsink I had before and the significant voltage drop given raw supply voltage levels this caused a problem:

01N100D temperature dependency — IXYS IXTP01N100D datasheet extract

I then reduced the input raw voltage level to 300V to ease the power dissipation across this FET. This was not sufficient so I proceeded to look at various things on the regulator as described below:

Change log

Replaced M1 for DN2540. The DN2540 has a better temperature response. The voltage levels now allowed me safely use the DN2540 and no need for a 01N100D.
Added bigger heatsinks for M1 and M3. Even 2.5W across M1 would need a bigger heatsink if we are looking to run the CCS as high as 90mA.
Removed TP resistor to allow higher CCS current
Set CCS current to 85mA
Load will be now 30mA per channel (30mA each 46 driver valve)
Set output votage to 270V

All tested well in the workbench. I used an external CCS set at 60mA and the output voltage stabilisation was rock solid at 270V. Temperature of the heat-sink was around 40-45°C after 30min of continuous use.

Back in the 4-65a SE Amp, the raw supply level at full load was 297V. The regulator played nicelly at 270V and the CCS current drifted only 5mA from 90mA down to 85mA after 2 hours of continuous play. The temperature of the CCS increased only to 47°C whereas the shunt FET got up to 54°C given the continuous 7W dissipated on it. I may try to reduce the CCS current a tad to ease here.

The regulator is fitted back into the 4-65a SE amp

Played the amp for about 2 hours and slowly started to appreciate more and more the sweet sound of this beast. Bass has increased significantly in my systems and previously haven’t been able to extract the deep bass out of the SE amp through my Fostex FE167E full range drivers.

I’m really going to enjoy this amp for a while until I start looking at changes. Oh yes, many come to mind, but easy for now. Other projects are awaiting for a long time so far.

Ale

4-65a SE Amp: Fitting the HV regulators

A bit of further progress today as managed to build two Salas Shunt HV regulators (SSSHV2) with the slight tweaks I tried recently. Both regulators will provide a very stable voltage reference (+280V) for stacked supplies. Now time for completing the wiring of the 46 drivers and do some further testing…

Salas SSHV2 Regulators

4-65a SE Amp: Shunt regulator

Thought it was going to be an easy task as I’ve done it before many times and building a Shunt regulator seems to be not the challenging part of this amplifier build. We all know that life brings surprises and specially when we are not expecting them. My 4-65a SE amplifier requires a very stable DC as part of the DC-coupling design. The Salas Shunt Regulator version 2 (a.k.a. SSHV2) is a good choice for this task.

New pass FET

Fitting new pass MOSFET and CCS

Salas SSHV2 shunt regulator

After building it very quickly I struggled to get it to work. To cut a long story short which involved some IRF840, PNP and JFET replacements, I discovered that the stabilising RC wasn’t connected as the 330nF MKP capacitor was not properly soldered to the right holes. The PCB has multiple holes to accomodate capacitor sizes, however only the top two correspond to one capacitor pin and the remaining bottom ones are for the other. My logic of placing the capacitor in the centre clearly didn’t work and the capacitor was disconnected in the end. Finally, when hooking the regulator to the raw supply and switching it on, the whole thing produced the unwanted smoke particular of sand devices getting blasted. What happened? The maximum input voltage to the regulator evidently exceeded the CCS voltage and the top FET (M1) blowed away and therefore the regulator cascode CCS (J1) and the pass FET (M3) as well. My PCB was already suffering from multiple solder work and was reaching to its usable life. I looked at using HV parts as hand to increase the robustness of the regulator. The pass-FET was replaced by a 1kV part (STE5NK100Z) and the Mosfet CCS DN2540 pair for an IXTP01N100D which is also 1kV part:

All worked well until I realised that the differences between DN2540 and 01N100D’s VGS(th) and gm made the CCS maximum to be limited to about 40mA given the test point resistor value. As M2 can be a simple DN2540, I replaced it back and all worked well to get 60mA and deliver about 280V @ 40mA rock-solid!

Salas HT shunt regulator SSHV2

It took me probably half an hour to build this brilliant shunt regulator. I waited for a long time until picked up this board and stuffed all components. I will use it in my next generation of DHT preamps.

Output ripple is below 5mV, can’t even measure it given the noise in my workbench. The test gig included my variable power supply (600V) feeding the shunt regulator which was set for 40mA. output load is three 3K3 power resistor clads (50W each).

I adjusted the regulator to provide about 160V, so current is about 16mA. Interesting to seer the harmonic profile to have a higher peak at 200Hz compared to the 100Hz harmonic….