The 1073 front end, for dummies

[merlin]

A question came up on another forum about building a cut-down 1073 in a guitar stomp box, or something inspired by the 1073 front end. Anyway, there was confusion about reading the original 1073 schematic, which is kinda complicated. As we all know, the 1073 has a pesky 22-way multi-gang gain switch. So I thought I would draw a clear diagram of what is actually going on in the front end, for reference.

Mostly the gain switch works as three stepped attenuators, for line, mic (less gain), and mic (more gain). If we re-draw those attenuators as three simple pots, things look much clearer.
Line goes through one pot and one BA284 section. Super simple.
Mic (less gain) goes through one pot and one BA284*
Mic (more gain) goes through both sections with an attenuator in between.
The complicating factor is the gain-boost resistor, which is also switched to different values at various mic settings, to distrubute the gain in a way that optimises the SNR.

*OK, purists will know there is a minor discrepency for the MIC 40 & 45dB setting. Stop being pedantic.


This got me thinking you could build a poor man's 1073 front end using pots and a 3P3T switch, if you're not fussy about reproducing the exact original gain range (guitarists aren't). Maybe this will be useful to someone, some day?

 

[moamps]

Nice exercise, I applaud thinking that isn't boring copying of famous designs. I would add a couple of remarks, if you don't mind: the microphone load is not the same for both MIC gain modes, I would use potentiometers with a log characteristic, I am not sure that the indicated areas of regulation can be obtained with the shown values of the potentiometers and associated resistors, and it seems to me that toggle switches 3P3T cannot be used to switch the operating modes. BBM-type rotary switches are a more likely solution.

 

[ruffrecords]

I have never quite grasped the purpose of the 'other' ac NFB feedback loop from the emitter of the second transistor via the 22uF capacitor.

Cheers

Ian

 

[merlin]

ac NFB feedback loop from the emitter of the second transistor via the 22uF capacitor.

Me too. My guess is that it results in a slightly improved SNR for whatever reason. I suppose I could test it, if I could be bothered

 

[ruffrecords]

Me too. My guess is that it results in a slightly improved SNR for whatever reason. I suppose I could test it, if I could be bothered

Thank heaven it is not just me! Edit: might have to do a sim.

When I was at Neve back in the 70s I built my second mixer. It used smoke damaged 10468s I bought at staff sale we had and I used a fixed gain version of the same circuit but without the extra feedback loop set for 30dB gain. 6 input plus four mix amps(L,R, foldback and echo) built on one large piece of Veroboard. I sold the transformers years ago for silly money but I still have the rest of the mixer.

Cheers

Ian