Inrush Revisited

A while back I reported the problems I was having with a SMPSU I planned using to power the heaters and how it would not power up with a full compliment of boards. I suspected this was due to inrush current which occurs because the heater resistance is much lower when cold.

I have now investigated this further. First I purchased an International Power linear 12 volt power supply rated at 5 amps. Another EZTubeMixer builder had reported that even this power supply would not power up with six boards (check out Pierre's blog here):

http://theworldin35mm.com/espressojazzdiy/category/diy/).

I suspected this might be due to a fold back current limit circuit in the power supply. The manufacturer does not supply a schematic so the first thing I did was take mine apart to try to trace out the current limit part of the circuit. The main regulator element of this power supply turns out to be the ancient LM723 which was introduced in the 1970s!!! Together with three equally ancient 2N3055 power transistors wired as darlington series  pass transistors this forms a linear regulator capable of at least 5 amps output current. So why would it not power up six boards? Further investigation showed it includes a current limit pot which, when somewhere in the middle of its range, implements a foldback current limit, but if turned fully clockwise should revert to a regular current limit. So in theory, turning this pot clockwise should fix things.

First I tried it on a six board load with the pot untouched. It did power up but very slowly. It was at least 15 seconds before you could see the glow from the 6922 tubes. Of course, once the tubes have been powered up, if you turn off and then turn it on again, it powers up straight away because the heaters are still warm. I found I had to leave it for at least 15 minutes before you could repeat the test. Next I hooked up a DVM, set to its 10 amp range, in series with the supply to the mixer. On powering up, the current started out at about 0.8 amps then slowly rose to about 3 amps when it suddenly peaked at over 6 amps and then settled slowly down to about 2.75 amps as the tubes lit up. I repeated this several times at 15 minute intervals with the same result. This seems to clearly demonstrate the foldback current limit operating but allowing enough current through to to slowly heat the tubes until a point is reached when the foldback limit shuts off and the regular current limit takes over.

The odd thing was that my supply worked on 6 boards but it did not work for Pierre. This could be differences in the tubes and also the tolerances in the power supply. Either way it seemed marginal. So next I tried with the pot fully clockwise and I was very pleased to discover that the foldback current limit was no longer operational. Instead the current rose rapidly to over 6 amps then slowly fell back as the tubes lit up. The time from switch on to seeing the heaters glow was less than 5 seconds. I told Pierre who tried this with the same result. So it looks like rotating the pot clockwise does turn the supply into a regular current limit and allow you to power 6 boards.

The international Power 12V, 5 amp supply is large, heavy and expensive so I wondered if I could come up with something smaller, lighter and cheaper. After looking at a range of regulators I finally realised that the LM317 regulator in its TO220 package, has the same pin out as the TL783 I use in my phantom power supply PCB. Using this chip, changing a couple of resistor values and changing the smoothing capacitor would allow the same PCB to be built as a 12V heater supply capable of supplying one and perhaps 2 amps of current. Quite handy for a couple of tube mic pres or a tube gain make up in a passive summer, but not enough for this mixer. To provide in excess of 3 amps really needs the TO3 version of the LM317 and a bigger heat sink. Perhaps this part with is chunky heat sink could be mounted on the rear of the PCB and wired by flying leads to the PCB? The TO220 heat sink I use on the PCB has two mounting holes. Not surprisingly, these are not the same spacing as a TO3. However, by chance, the heat sink is off centre and if you drill a hole a few mm past the off centre fixing hole you can fit a pair of pillars to the rear of the PCB that mate with a TO3 and it sits almost central to the PCB.

So I built one. I had a spare 12V 5A toroid transformer, not ideal but good enough for an experiment, I built the PCB and mounted the TO3 version of the LM317 regulator  and its heat sink from the rear of the PCB, like this:

Then I connected it up to the mixer via an ammeter as before and switched it on. And it worked! The meter flipped up to just over 6 amps then slowly settled down to just under 3 amps as the tubes lit up. I ran it for half an hour and the heat sink got quite warm but no warmer than the International Power supply did. I will probably use the International Power supply for this mixer because it works and I would need to get a higher voltage toroid to ensure the LM317 has enough operating voltage but at least I know an LM317 can be used.

The PCB will need to be updated though for two reasons. First, the IN5400 series rectifiers it uses are not really up to supplying more than 3 amps continuously so I plan to modify the board to accommodate readily available 4 amp and 6 amp bridge rectifiers. The other problem is the power tracks were never designed for 4 amps; they were designed for 100mA of phantom power current so they need to be replaced by some copper areas. The good thing is the new PCB can still be used for phantom power as well as heater supplies.

REDD EQ Added

I have just completed tweaking and testing a new EQ based on the EQ used in the famous EMI REDD desks. I have duplicated the standard per channel +- 10dB range stepped bass and treble controls (both 'classic' and 'pop' versions) and also included the brilliance boost/cut control from the EMI RS127 plug-in module which provided boost/cut at frequencies of 2.7KHz, 3.5KHz and 10KHz in addition to the 4.7KHz of the 'pop' plug-in. This all fits on a small PCB just 100mm by 100mm.

The three level control switches are mounted directly onto the PCB and the brilliance frequency select switch is connected by a short length of ribbon cable. The schematic is a little more complex than some of the other EQ circuits I have designed because the RS127 appears to have the same Q in both cut and boost. This necessitates using separate inductors for brilliance boost and cut as shown in the protootype schematic below:

I am going to fit this EQ to one of the channels of the demonstrator mixer. I plan to have two channels with Sowter input transformers and Pultec EQ. As well as being suitable for recording, these two channels can also be used for mastering as they have identical transformers and EQ. The Cinemag input transformer will be paired with the Helios 69 EQ and the Jensen with the new REDD EQ. Prototype test results can be found here:

Test Graphs

I also made a short demo recording showing the REDD EQ in action on a pre-recorded track. You can hear the result here:

Just Bob

The left channel is the track with EQ applied and the right channel is me commentating on the EQ settings at each point in the track. The track was recorded by by good friend Bob Wright:

The Song Factory

Building and Testing

I have been busy soldering the last couple of weeks. All six amplifier boards are complete apart from a small number of components that have not yet arrived and I have two EQ boards yet to build. As I have a full complement of amplifier boards I thought this would be a good opportunity to check that the little 12V 3A SMPSU, that I got to power the heaters, was going to work. I hooked up the PSU output to the motherboard, plugged in one board and turned on the power supply. The tubes lit up. So far so good. So I plugged in a second board and turned on again. Again the tubes lit up. So I carried on adding boards. This is how far I got:

The SMPSU refused to power up more than four boards which is a nominal consumption of just over 2 amps. Four it will do, five it will not. It seems to go into some sort of short circuit shut down mode because with five boards in the voltage across the heaters is zero volts. It seems to be completely unharmed by this as you can go back to four boards, turn it on again and it is fine.

I suspect this is an inrush current problem. Cold heaters have a much lower  resistance than when they are powered up and hot, just like any incandescent bulb. It appears the SMPSU can cope with the inrush current from four boards but no more. I shall next try a SMPSU with a much higher reating, say 6 amps to see if that works.

Four Channel Panels

Today I received two more front panels from Frank. These are my own layout for the Pultec styple EQ. I just laid them all in place on the sub-rack to see what they look like.

The text around the gain control is definitely too big on my layout. The 54dB gain legend is too close to the left hand edge. The font would definitely benefit from being smaller. This problem is made worse by the fact I am using a quarter inch diameter shaft on the gain switch which sets the minimum knob size you can use. In future it would make sense to use the switch with the one eighth inch diameter shaft as this would allow a smaller diameter knob to be used. My horizontal lines look thinner as well but they do line up nicely. I am not sure which I prefer at the moment. Overall not bad. Now I just need to finish all the PCBs so I can complete all four modules.

First Helios Channel Module

Using Frank's front panels, I finally got all the parts together to start building one of the channel modules with Helios 69 type passive EQ. I need to shorten the shafts on some of the pots so the knobs seat properly (why do pot manufacturers have so many different shaft lengths???) and the knob colour scheme is not finalised but still the module looks pretty good. Next job is to wire all the pots and switches.

First Real Channel Panels

The first panels arrived the other day from Frank Roller. The finish is absolutely first class. The white in-fill on the blue anodised background looks very professional and they only cost about 50 Euros each. Well done Frank!

The change in colour from top to bottom is caused by the camera flash. The colour at the top is closest to its real appearance.

Professional Looking Panels

Having decided not to make my own front panels for this mixer after stabbing myself with a needle file, I need to find a route to fabricating front panels of a professional appearance.The obvious route is to use the excellent Font Panel Express program provided by Schaeffer and have them make the panels.

Unfortunately there are two stumbling blocks to overcome. The first is that I have zero artistic skills  This means that, although I can draw a perfectly functional front panel layout it will at best  not offend the eye. I can tell a good, eye pleasing layout when I see one but there is no way I can create one. Fortunately, in the last few days Braeden, a fellow member of groupdiy.com, has begun to publish his versions of the layouts of my Helios and Pultec channels as well as ones for other other popular DIY audio projects and it is clear his artistic skills are streets ahead of mine Here is a composite picture of Braeden's Helios 69 channel layout (left) side by side with mine (right).

Check out Braeden's other designs.

The second stumbling block is that although panels made by Schaeffer look really good, they are rather expensive and the company does not provide any feedback on your proposed layout. Once again, someone on groupdiy.com knows of an alternative supplier and in this case it is NRG in Germany. NRG is run by Frank Röllen who can accept panel layouts drawn in Front Panel Express but will make them for 30% less than Schaeffer. His range of materials covers various colours of anodised aluminium as well as some plastic materials but the best part about using Frank is that when you send him a panel design he emails you back  to check you have thought of everything from a mechanical engineering point or view. Things like clearances for pots and switches and adding radii to slots. When you are both happy he makes the panels. I was so impressed by Frank's approach I have already ordered a pair of Braeden's Helios 69 front panels and, as soon as Braeden has finished it, I plan to order a pair of EQP1A versions too.