Old vintage radio mixer (flea market find)

[jacobsteel]

I bought this yesterday at a Gothenburg Radio-Museum Flea market (a very good Radio Museum BTW). Link to the Radio Museum (in Swedish)



Besides having a very handsome enclosure, there are several large connectors and four very large stepped attenuators (all measure appr. 81 ohm (!)).



There might have been several transformers? Yet, now only the above remains.

Apart from the dB ratings, front and back have markings of 1-6, so likely this may have been a 6 channel passive mixer with no master gain?


The attenuators are amazingly engineered. I would guess a date around 1930? It's marked Telegrafstyrelsens Radiobyrå MX 17.


Does anyone here have more information about this mixer? Is it a vintage RCA (rebranded as "Telegrafstyrelsens Radiobyrå Typ MX Nr 17"?




A schematic, more info about the transformers would be very interesting. For one, how could 81 ohm attenuators have been practical?

Also, suggestions of how best to put this very nice piece to good use are welcome!
(regrettably, I cannot afford to build a Fairchild 670... )

External links: Tekniska museet in Stockholm have two very similar mixers in their collections (image of complete unit borrowed from this source):
6 channel microphone mixer
6 channel microphone mixer (second unit)
https://digitaltmuseum.se/021026312081/mikrofonmixer/media?slide=0

 

[rock soderstrom]

I bought this yesterday at a Gothenburg Radio-Museum Flea market (a very good Radio Museum BTW).

Very nice! Beautiful desktop enclosure.

Also, suggestions of how best to put this very nice piece to good use are welcome!

REDD47 (or GATES SA70) and a RS124 in one box! Recording front end deluxe...

 

[ruffrecords]

81 ohms seems a strange pot value. Maybe the inputs were all 50 ohm or 25 ohm mics.

Cheers

Ian

 

[Tubetec]

Passive low -z mixers were often used before a pre-amp stage , thats my guess what this thing is .

 

[jacobsteel]

Here are the insides of one attenuator (they all measure ≈ 80-82 Ohms). Resistance appears to be built very precisely with turns of wire.

Also, both left and right sides have identical connectors for some form of parallel arrangement. Possibly several similar mixers could interconnect to form a larger system?

 

[xazrules]

Amazing piece! I’d go with a 4x1 tube preamp ala altec, if there is space.

 

[PermO]

Spend a couple of $ on alieexpress and fill that big hole with a fitting VU.

Resistorwire wound stepped attenuators, wow ! that's cool

 

[xazrules]

I’ve ordered a vu meter like that from aliexpress and arrived broken! So, beware

 

[Gavin Pursinger]

COOL FIND !
Top and bottom snare.mix.
Mono leslie
Bass mic & bass di mix
Channel patching during mixdown for color.
Tap outs of individual channels opens up many possibilities...
COOL KNOBS..
.

 

[emrr]

The #1 problem is the attenuators aren’t really good for anything one would want to build. Not many other (inexpensive) things will retrofit those holes.

 

[Disco Volante]

I believe I have the meter that's missing;-)
Very nice enclosure and the knobs are fab...

Cheers, V!

 

[jacobsteel]

The #1 problem is the attenuators aren’t really good for anything one would want to build. Not many other (inexpensive) things will retrofit those holes.

- yes! exactly. I did some experimentation, some calculation and... really??? Like 80 Ohm stepped attenuators? How could this possibly have worked for mixing?
Of course, it HAS worked, but I don't know anything on what was removed (but probably transformers).
Still, the stepped attenuators are gorgeous! And even after just a little cleaning the switching was very silent.


Thank you for all excellent suggestions!

What would you say about experiments with a passive Low Pass Filter?

I searched the forums for some good schematics, found some color pictures, and older (regrettably now dead) links to schematics for e.g the Altec 9069B.

On the same flea market I also got some nice variable caps (but for radio, so mainly pF, nF ranges) and some large rotary switches.

I guess inductors in the single digit H range together with capacitors in the nF range could perchance produce a variable low pass filter for audio frequencies. I have to do the math on this again.

All passive filters I've managed to study so far (i.e. the Bruel & Kjaer 1613) have several Resistor-Inductor-Capacitor combinations per octave...

Link to filter Calculator page

 

[emrr]

- yes! exactly. I did some experimentation, some calculation and... really??? Like 80 Ohm stepped attenuators? How could this possibly have worked for mixing?
Of course, it HAS worked, but I don't know anything on what was removed (but probably transformers).

It was common to mix mics passively with matching Z attenuators way back when. Be great if you have nothing but Western Electric 50 ohm mics, which are designed to EQ correctly at matching Z.

The majority of American multi-input remote mixers are passive mic level mixers with matching Z values that feed a single amplifier. A thing we mostly don't want to do today.