Picked up a Technology Instruments Corp Dual Type 500 AR wideband voltage amplifier last year, because, well.. look at it.

Tired phrase, but: they don't make them like they used to.

Fairly no nonsense control set.

Yes, it glows in that logo.

Cracking the units open revealed a somewhat ugly bias battery assembly, comprised of two 625 mercury batteries bound together. This is only switched in at 1000 gain, and so I opted to remove it with the intent to replace in event of malfunction. My applications are confined to the audible spectrum of sound (as opposed to full 5Hz-2MHz claimed) so a little non-linearity may actually be welcome.

There's really only one way to find out.

Original circuit, gutshot topside.

Original circuit, gutshot from below. My intent is to replace the electrolytics and the selenium, and then give it a listen.

Modern parts make for a roomier experience.

A 100pf mica domino cap (visible on the original unit) has been installed on the wafer switch.

I've been going the budget route on electrolytic capacitors, building ugly little clusters costs less than a quarter of repop multisection can caps. These are not museum collection pieces, these are supposed to be used.

This was an easy fix that entailed shortening the power cable about a foot from the inside.

General purpose gutshot.

The earlier, original screw is quite evident compared to the newer screw, both of which exhibit a level of quality I don't see in a lot of current hardware.

So, after working one unit over I was slowly bringing it up on a variac and started to read voltage swings on output, one pulse every two seconds or so. Slow enough to confuse my multimeter on both AC & DC settings. The scope displayed a pulse that jumped toward the top of the scale, which indicated a frightful signal to be feeding gear downstream.

Plugging this 75 ohm terminator from my video junk into the input happens to snub the oscillation. It took me a little while to connect this particular set of dots.. after discovery of the motorboating, I shelved this for a while to clear my head. Then I observed that it didn't oscillate at 10, various resistors strapped on the output, loading the input with a signal source and listening to output, generally just fooling around with it brought about this observation.

So, the plan is to terminate the input with the largest resistor I can to prohibit runaway, which, I might add, only affects this unit (before and after recap). I dug into this one first because it was operationally impaired, so I could have a functioning unit to A/B while troubleshooting, should the need arise.

All in all, I would not describe this as a high fidelity preamp, it does however go deeply enough into the realm of character to justify having around. Plus, it's pleasing to the eye.

I'm not really one to get rid of stuff. A friend spied this in a clutter photo and voiced interest in it, so it's only fair that I go through it and assess the potential and value within.


The Industrial Research Products, Inc. Voicematic DE-4013A-0100; what amounts to a conferencing 10 into 1 microphone mixer that can duck inactive microphones. This unit contains one DE201 master level module, five DE202 dual microphone channel modules, a blank and one DE205 regulated power supply.


The DE201 contains an LM339 voltage comparator (presumably for the Voicematic level ducking circuit), a Harris HA3-4741-5 quad opamp and a variety of metal package transistors.


In addition to various discrete actives and Beyer TR/BV370 215 006 microphone step up transformers, the DE202 contains an MC14528B dual monostable multivibrator (presumably the respondent to a CV level message provided at the master LM339), a TL083 and one half of an HA3-4741-5 per channel. Component quality here looks pretty upper shelf. The only real questions I have is if the mic pres have usable gain and how they sound, the AGC circuit can be bypassed via front panel switch, so ducking artifacts are not a concern and could prove useful in certain circumstances.


Lead dress is often an indicator in regards to over-all attention to detail, which can factor into sound quality. I remain curious as to how this unit sounds, I'll have to cobble together a rough approximation of a studio session to check this thing out.


Looks like the analog circuitry is going to be seeing +/- 12 volts.


The orange/white twisted pairs are microphone feeds to the channel inputs. I believe all the channels are dumped onto a pair of these wires as a low impedance signal amplified by the HA3-4741-5 at the final level stage.

The aspect of this device that had thus far impeded my interest is the ten in, one out architecture, which isn't something I have a lot of use for. However, in speculation I realized I can switch in a bypass routing that will pull a card pair off the main output and route output to a jack on the rear of the case. The five switches can inhabit the face of the blank panel.

Examining the card also revealed a couple wire jumpers in prime positioning for a phase reversal switch, adding to usability of the preamps. Assuming of course this thing sounds good enough to justify the tinkering and 3U of rack space, which remains to be seen.

The past week and a half or so has been steeped in speculation and other mainly mental endeavours; so I'm digging up another long-standing project as opposed to some actual current progress shots, which is what I'd really prefer to populate these pages with. Elementary CAD screenshots and reinforced joint solutions are seem more boring than quips about blown woofers and asbestos bearing wire insulation, so without further ado I guess now is as good a time to unearth the RCA BC3C project as any.


I've plundered my binauralaboratories (out of date) write-up on these units for this shot taken in ~2001.


I'll upload some honest gutshot eventually. The mixer is essentially three pre-amplified microphone inputs, a transformer coupled input and a handful of low Z line level inputs feeding a two summed outputs, one tailored to line level feeds, the other bearing a PP 6V6 amplifier to drive in house monitoring systems.


I cut this card free a while back in order to scan the traces and facilitate tracing the signal path and recreating the schematic. This was right before I found a copy of the service manual, saving a lot of work.


I seem to have been suffering from focus related issues when I took this shot, but I'm sure the sense of a somewhat sparse printed circuit board is conveyed. Barring the modification of a printer to lay out diagrams and text, this is the level of construction I will be working with in the future.


This particular pre-amplifier module is in the summing recovery section that drives either the program output or monitor amplifier. The microphone level preamp card is similar, though it carries a few additional components to facilitate quiet switching. The first actual work on this will entail a power supply rebuild (squared, since there are two of these). Cost of a bunch of multisection can caps has been something of an impediment, but I can certainly dig into the rectifier and replace the selenium.