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”

When protection fails

“Don’t Stop Til you Burn Enough!”

Merry Christmas to you all! Christmas Eve ended up with some smoke and the party was over a bit early. We played music really loud last night and got carried away with some dancing around whilst playing “Don’t stop til you get enough” from Michael Jackson. And literally, the 814 SE didn’t get enough!

Let me explain briefly what happened. The 814 output stage has a crowbar protection circuit. It is configured to trigger around 200mA. The crowbar works brilliantly well, however, what is tricky on my design is the fact that you do get proper grid current in A2 operation. This grid current adds to the cathode current and flows through the crowbar sensing resistor. Well, volume was so loud so am sure that when the drum or bass kicked in, the crowbar was triggered. Interestingly enough, the shunt resistor is a 330Ω / 50W piece which should (in theory) blow the 500mA fast fuse. Well, it didn’t. The current peak wasn’t big enough before the output voltage of the 600V supply dropped significantly. Bear in mind that 330Ω should take serious current out of a 600V supply in theory!

The result was evident in a couple of seconds. The shunt resistor went madly hot, burned the plastic stand-off isolator quickly and fell over one of the current meters and burned the plastic cover badly as you can see on the image above. Luckily only one channel crowbar got activated.

When I rushed into the 600V supply mains switch I saw the secondary pair of dampening resistors (100Ω 7W wire-wound) melting and red hot. They were acting as a fuse and obviously preventing the supply to deliver further current.

Of course, the 814 was intact as it was the OT. At least the crowbar did it job, but far too early 🙂

Fixed it this morning after doing the Christmas present stuff. I couldn’t end up without music in Christmas!

Lesson learned here. Crowbar needs proper testing! I will buy smaller fuses – probably a pair of 200mA slow burn will work fine next time.

Happy Christmas!

Merry Christmas!

It is so great to have another Christmas. It gives us an opportunity to wind down and spend proper time with our loves ones. The ones who are physically with us, and remember the ones who are no longer here, but still very present in our memories.

In fact, we all look forward to the holiday period to crack on with our projects. We all have a pile of endless projects and ideas and no better time than Christmas to start working on them.

Continue reading “Merry Christmas!”

Custom Transformer Supplier in Europe

Custom transformer supplier

Given I had several emails asking about the output transformers I used in the recent upgrade of the 814 SE Amp, I asked Mr. Vincenzo to kindly provide an email address so you can reach directly to him. You can send your request via the form below which protects his email address from internet SPAM :).

Alternatively, if you don’t want to use the form below you can browse his website (in italian only sorry).

814 SE Amplifier: Custom Output Transformers

Improving the 814 SE Amplifier

After more than a year listening to this fantastic amplifier, it was time to do the first significant improvement to it despite I resisted to modify it after so much work and effort put into the design and build. The evident upgrade was the output transformer. When frequency response was measured, it was evident to see that the HF response was lower than expected. This is the result of the transformer and its configuration in this circuit. The LL9202 is a better OT for higher impedances and in this circuit, it is used in the 6KΩ /8Ω mode.

Continue reading “814 SE Amplifier: Custom Output Transformers”

Robustiano (V0.7)

Hacked a simple PCB to build the follower to drive the 4P1L as suggested by Rod. I had to play with the LND150 setting resistor (R4) to achieve the 2mA of idle current. I ended up biasing the 4P1L rather hot at about 11.5W which exceeds the specs. The Q2 VBE was not possible to measure as the Q2 would oscillate I guess when I place the tester lead on Q1 collector and the voltage seems to drop when I try to measure it. Should have added a ferrite bead:

When measuring distortion against frequency, I was keen to see that the follower provided some impact in reducing the HF distortion. For example at 20kHz, THD reduced from 0.96% to 0.59% @1W output power and from 7.84% to 3.52%, that is close to half the distortion I had before:

What is nice to see now is the effect of the follower providing sufficient source current to the 4P1L grid. Above 2.5W, the grid current kicks in and we can see how “Robustiano” can deliver 3W at less than 1% until starts to clip about 3.5W:

I found that if I were to reduce the Rf further and therefore increasing the collector current but obviously exceeding the 4P1L power dissipation too much as collector current was about 45-48mA, the distortion at 20kHz falls significantly. I suspect I should increase the collector current to enable better drive of Q2 due to its Cib (30pF). To keep the current feedback arrangement this could be done by reducing the negative emitter voltage source (V1). Should try this I guess…

Cheers

Ale

Robustiano (Version 0.4)

Finally back home after a long trip and had the opportunity to put the DN2540 at test and try the topologies discussed for the “Schade” feedback 4P1L SE amplifier. So I re-build my test rig and tried the DN2540 and LND150 at various drain currents. It was clearly to see that in order to keep distortion to a minimum, the VDS needs to be greater than 60V to keep the output capacitance of the FET low. Here are the results of the frequency response at nearly maximum output power (Po=2W):

It is interesting to see that the LND150 which has Coss (max) of 3.5pF doesn’t perform much better than the DN2540 which has Coss (max) of about 30pF. Operating points are different for both FETs but the 4P1L is running about the same operating conditions. What is also interesting to verify with this test is that the higher the drain current, the more capability the FET has to drive the 4P1L input (and Coss) capacitance at higher frequencies as the slew rate of the FET is higher.

We can see an interesting improvement from my initial tests at 5mA when drain current was just about 1.5mA. The yellow trace (Id=5mA) shows the best performance of the DN2540. Surely higher drain current will perform better but at a cost as the drain current is part of the OT primary current.

So how do we keep the gain of the FET when increasing the drain current? The natural approach will be to reduce Rf, but this affects the FET gain and the feedback. The alternative is to increase the supply voltage respect to ground. The price we pay here is to increase the cathode resistor and burning the power on it. With -4V as the negative source supply voltage, I had to only reduce RF to 51.5K to set 5mA on the DN2540. The supply power was increased to 350V, the screen (Vg2k) to about 140V (240-98.6V) which is lower than the 150V used before. There is a tad of extra power to extract on the 4P1L but here is close to its maximum dissipation. The Rk is a pair of wirewound 4K7 in parallel.

I think it is now time to try the BJT driver. I suspect that it will need at least 5mA of collector current to get on with the task of the input capacitance of the 4P1L when anode to grid feedback is in place.

cheers

Ale

814 SE Amplifier: measurements

It was time to take the 814 SE Class A2 amplifier measurements. The challenge though, is that the amp is so heavy that I will never take it up to the workshop. Therefore, I decided to take my workshop PC down from the loft this time to see how the 814 really responded.

First test was to do a THD analysis as a function of the total output power. As you don’t want to do this with your speakers, and also the classic wire wound resistors (Alu-clad) are inductive, you want to use non-inductive resistors like this test jig:

I bolted on to a large heat-sink an array of resistors to form 8Ω in value by using a pair of 10Ω in parallel and three 1Ω in series. I added a set of binding posts and connectors for the speaker cables. this way you can easily wire your speakers and connect your audio test set to take the measurements.

Continue reading “814 SE Amplifier: measurements”

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)”

Siberian DHT Preamp Gen3: filament supply

Time to upgrade my pre-amplifier (again) and is perfect timing to go back to 4P1L. The Siberian preamp had a fantastic bass response and detail.

Here is the new filament raw supply. It has split bobbin transformers, schottky rectifier bridge and input choke LC filter stage. It also has a CM choke and EPCOS electrolytic capacitors:

Power supply ready

Input chokes

Chokes and capacitor bank

Schottky Rectifier Bridge plus snubber

Building the power supply

Dead quiet at 16V output and 550mA which is the load used by 4P1L starved filaments in parallel with filament bias.

Soon to build the preamp!