SW 1280b Jensen Trafo proglems.

[Regalcustom73]

Hi! I’ve been modding my Sound Workshop 1280b, you know with the faster Op Amps,( I’m starting with TLO74’s) ,a complete re-cap, and to start, 1 Jensen Jt-115 k-e mic input tranny. Problem is I’m getting next to nothing in terms of level. Looking at the schematic it appears that the “hi” input actually goes to the negative side of the OpAmp. Could this really be? If so I think I have my red, brown input wires from the transformer switched as well as my yellow and orange output wires. Anybody work with these and can share some info? I will say I got all 12 channels worth of electrolytics soldered in correctly. At least that! BTW my console used to belong to Super Tramp, it was apparently what they used for demos at their practice spot. Thanks!

[winky dinglehoffer]

My first question: did it work fine before you put in the Jensen? You recapped & put in new opamps--was everything good with all that done & the original transformer in there (the 1280b has beyerdynamic input transformers, right? It's been a long time since I had one.)

2nd question: is the transformer backwards? The Jensen is a step-up transformer, & having it backwards would take levels down drastically.

Also, connecting to the negative input of an opamp is fine, if that's how it's designed. Here's a random site I found looking at basic opamp circuits. If you're unfamiliar or concerned, it might be helpful.
https://www.arrow.com/en/research-and-e ... p-circuits

[The Scum]

If the signal is present, but super low, you've probably got the transformer in backwards. Or maybe the new transformer is a lower step-up ratio than the old one?

I'm having trouble finding a legible copy of the schematic - the one I found also has the top cut off, so I'm not sure what that switch to the right of the transformer is doing. It looks like the input opamp might be set up as a differential amplifier - you'd get signals going to both the + and - inputs...but I can't clearly read those markings. Do you have a clearer copy to share?

[winky dinglehoffer]

And definitely verify that the TL074s aren't causing a problem. Depending on the circuit, it may not be as simple as just replacing the 4741s (that's the original IC, isn't it?) with 074s.

[Regalcustom73]

Thanks all. The Op Amp swap seemed to be okay, I had done some tracking with them before the recap and they seemed cool. It’s very possible I got my wires swapped, I will look at the schematic closely! I tried to share a large file but wasn’t able to. I do have a legible ( with reading glasses!) version. I will dig into this info about OpAmp circuits, I find that the more I understand the faster I make connections. Gracias!

[Nick Sevilla]

BTW my console used to belong to Super Tramp, it was apparently what they used for demos at their practice spot. Thanks!

I would have left it completely alone.

[Regalcustom73]

Yeah I could see not messing with the console but I’m not too precious about it, and I think their(Super Tramp’s of course) guy was going to mod it as he had extensive notes in a folder on different OpAmp chips. Turns out I had the hi and lo secondaries reversed coming out of the Jensen. I’ve got plenty of gain and a good looking signal. Sounds good too! With the TLO74’s and the stock Breyer I hear slightly more defined transients than with the 4741s.With the addition of the Jensen I hear that plus a bit more of a fuller range ( highs mods and lows), and way more dynamics, as in the bloom of my double headed kick drum, or even a really fast bloom that comes out of the snare after the initial attack. The 4741’s and Beyer sound more compressed and dirty like a finished record- LOL! Seriously I like the Jensen and I’m curious to hear what the 10k line input transformers and the Acopian beef supply I got will do for the sound.

[Nick Sevilla]

Yeah I could see not messing with the console but I’m not too precious about it, and I think their(Super Tramp’s of course) guy was going to mod it as he had extensive notes in a folder on different OpAmp chips. Turns out I had the hi and lo secondaries reversed coming out of the Jensen. I’ve got plenty of gain and a good looking signal. Sounds good too! With the TLO74’s and the stock Breyer I hear slightly more defined transients than with the 4741s.With the addition of the Jensen I hear that plus a bit more of a fuller range ( highs mods and lows), and way more dynamics, as in the bloom of my double headed kick drum, or even a really fast bloom that comes out of the snare after the initial attack. The 4741’s and Beyer sound more compressed and dirty like a finished record- LOL! Seriously I like the Jensen and I’m curious to hear what the 10k line input transformers and the Acopian beef supply I got will do for the sound.

Usually I use a scope to measure any differences, otherwise my ears will definitely trick me.

Glad to read you did sort out your tranny issue.