Electronics


3
Sep 11

ADK Vienna Tube Modification

I recently got an ADK Vienna large-diaphragm capacitor microphone from a friend. It actually sounded pretty ok as-is, but soon the switch for the built-it 18 dB pad started to crackle. Instead of repairing (or disabling) the switch, I opted to replacing the electronics with the so-called Royer tube modification, or actually, a circuit loosely based on Royer’s idea. I have tested my circuit already a couple of years ago in another mic, which uses a Neumann capsule and a Lundahl output transformer.

Since I don’t have any extra money at the moment, I decided to use the stock transformer. And since the original idea was to use an AKG C12 -style, i.e. edge-terminated capsule, I naturally left the original in place as well. Of course I may later swap both the transformer and the capsule, since – I’ll reveal it already – the mic sounds amazing, actually somehow “better” than the other mic!

This is the starting point.

And without the cover. A 2SK170 FET-based amplifier, not that bad actually, but nothing special either.

This is pretty much everything one needs to build a mic. A tube socket, tube (5654W), transformer, 7-pin XLR connector, couple of resistors (metal film to minimize noise) and two capacitors. The electrolytic capacitor and three resistors are used to filter the capsule polarisation voltage. The remaining two resistors are the plate (anode) and cathode resistors of the tube, which is by the way triode connected. The 2,2 uF plastic capacitor is the coupling capacitor for output. No ‘lytics on the signal path!

After taking the mic apart, I started with replacing the connector. The wiring is teflon insulated and includes some silver as well. Expensive, but good stuff. I nowadays only use this type of wire when I build equipment.

Transformer and connector are back in place. Now it’s time to build the amplifier circuit!

Since the circuit is so simple, I decided to build the mic point-to-point. No problem as long as you remember to insulate the live parts with heat shrink tubing. I used the center tap of the tube socket as the B+ connection and one end of the heater pin as ground.

Everything in place, ready to be tested! Btw. I use a slightly modified power supply from a t.bone SCT-700 tube mic, which also provided the body for my another tube mic.

And as I said, it sounds wonderful! Watch this space later for sound examples.


10
Jun 11

Hammond L-100 restoration

Almost an year ago a friend of mine gave me his father’s chopped Hammond L-100 wreck with the idea that I’d restore it to usable condition. Finally in the beginning of the week I decided to start the project. The organ was really dirty everywhere, a lot of tubes were missing and there was bunch of non-functional modifications here and there.

After almost two full days of work it was practically finished and I even played it on a band rehearsal same evening. Here’s a little documentation on what I did.

The AO-42 power amp was heavily customised. The reverb circuitry was partially removed and the reverb output RCA socket was used as a line output. I rewired the amp according to the original specs, except that as usual I disconnected the negative feedback loop to be able to enjoy the wonderful distortion of the EL84/6BQ5 output tubes. Also I wired couple of 10 watt power resistors to function as poor man’s dummy load, since the speakers weren’t installed.  The power amp coupling caps were looking really bad, so just to be sure I swapped them to some NOS Philips 47n/400V caps.

I had never before seen this dirty drawbar contacts. Half of them are already cleaned. The people in the States always praise Caig’s De-Oxit for this kind of cleaning, but that stuff is not available in Europe, at least not in Finland. However, CRC makes similar contact spray that really works – unlike basic isopropanol.

Here you can see the bottle. Really good stuff! I also cleaned the drawbars itselves. You can’t believe how much dust and dirt there was.

I tried to be lazy and didn’t unsolder these wires in the first place. However, I ended doing that anyway, since putting the drawbar assembly back together really wasn’t possible without it. That is probably the hardest part anyway.

The drawbar assy installed. The nuts and bolts are holding the assembly together. There’s holes on the cheek plate under the assembly for the nuts. I reversed the direction, since I think that the assembly is easier to put together this way. First insert the bolts from below and then just screw on the nuts. The four bigger screws are fastening the assembly to the cheek plate. The small screws only keep the busbars in place. I didn’t bother removing them.

As usual with spinets, some keys were broken. Actually I was a little bit surprised that in total there was only eight of them, and only one of those in the lower manual! Getting there was quite a work, but it’s not difficult, just time-consuming. I should’ve had better tools, but I managed with the ones in the photos. When replacing a white key, one needs to remove the adjacent black keys as well.

When inserting the new key, make sure that the washer stays in the right place; between the key assembly and the screw. If the washer goes between the key and the manual assembly, you’re not able to secure the key in place.

When replacing a lot of keys, it is faster and easier to unscrew them all first and then remove and replace. Note the little pieces of metal that keep the key in right position. When tightening the screws, do that in two parts: first so that you’ll able to tilt the key a little so that it’ll hold in right position, hold it so that it stays in place when you finally tighten the screw.

As you can see, there really was a lot of dust and dirt in the manuals as well!

That was it, mostly. I still need to do some work on the swell pedal. For now I’m using an old Korg volume pedal between pre and power amplifiers. Works fine, but doesn’t do the real Hammond swell effect. I have a good plan on reworking the original pedal in my mind – more about that later.


30
May 11

What to do with a broken Behringer ADA8000 AD/DA converter?

Well, some may say, trash it, but I’ve heard from various sources that the converter chips are (were) one of the best available when Behringer started the production. Even the preamps are usable as is, albeit bit noisy, due to the cheap opamps on the signal path.

So when I noticed a broken ADA8000 for sale on Muusikoiden.net, I bought it. As I guessed, the power transformer was shot, so I ordered a replacement from PMS electronics. Also a couple of electrolytics and a regulator were in quite questionable condition, so just to be sure, I swapped them as well.

People at Gearslutz have noted that the power transformer outputs so high voltages that the regulators run really hot. Adding some resistors and capacitors help the situation and also reduce the ripple, i.e. improve the quality of the power supply. However, it is not really necessary to use 25-watt rated resistors; 10-watt ones are more than enough. The power dissipation is approximately 5 watts. Since I was lazy and I didn’t have the required 10 ohm / 10 watt resistor required for the 5V supply at hand, I decided just to mod the +/- 15 volt rails.

After I got the unit up and running and verified it was working fine, I noticed a horrible problem. Since all good-sounding units weigh a lot, like my Hammond C-3, I thought that the ADA wasn’t heavy enough. So it was time for fun. I had bought a bunch (~15) of old Sennheiser 1:5 microphone input transformers couple of months ago for just couple of euros a piece and now though that installing them on the ADA would make the preamps sound better, reduce noise without swapping the opamps and of course, make it heavier!

Unlike this guy (thanks for the inspiration!) my transformers did not have center taps, so I left the former circuitry as is, except that I removed the 47 uF / 50V input caps (check out the schematics) and soldered the transformers there.

Results are better than I ever expected. Now I’m really looking forward to make some real recordings to see if I still need to change the 1k3 that are now loading the transformers. Maybe, maybe not.

I’m going to get some velcro to fasten the transformers to the chassis. I originally planned to use a piece of aluminium profile, but I’m lazy and velcro will probably work well. And when it finally will be time for the Behringer to be trashed, it will be easy to recycle the transformers again.


14
May 11

Rhodes Suitcase Power Supply Repair

A friend of mine has a beautiful, late-70’s Mk. I Suitcase (“Janus”) Rhodes that had started to behave badly: it introduced very nasty-sounding distortion. I measured the voltages and soon found out that the PSU wasn’t putting out what it should.

At first it seemed to be an easy fix, since usually the problem are the regulators or rectifier diodes. However, this wasn’t the case this time, and actually I noticed quite weird behaviour: the voltages were exactly what they should when the preamp wasn’t connected, but just a couple of seconds after connecting it they dropped.

After quite a long research, I found out that there was something really weird happening on the power transformer: even while my voltmeter was showing exactly what it should, the PSU wasn’t keeping up. Finally I tried with another transformer and everything worked again. Too bad that the transformer used on Rhodes PSU isn’t standard stuff, so I had to order a replacement from EP-Service in the Netherlands.

While working on it, I decided to rebuild the preamp PSU: new rectifier diodes and electrolytic capacitors were installed.

Now, finally, the Rhodes is alive and kicking again. And sounds great!