Tascam 388 Master Send Return Insert Mod

[ArnoP]

I wondered if somebody modified a Tascam 388 by adding a master Send-Return so that a master mix compressor could be looped in

 

[sweetbeats]

Not here, but something you could try is when you are mixing down, rather than assigning your mixer channels to the L-R buss, assign to PGM 1-2. Connect the PGM 1-2 OUT jacks to the inputs of your buss compressor, and the outputs to the STEREO BUSS IN L-R jacks. That, in effect, accomplishes the same thing as insert jacks…dumps the processed signal right to the L-R buss…Correct me if I’m wrong but you should be able to monitor that by selecting the STEREO buss in the monitor section by the MONITOR/PHONES level control, and then connect the STEREO OUT jacks to your master recorder.

 

[ArnoP]

Thanks for the very creative approach! Could use such an alternative if the real mod wouldn't work....but it seems do-able and doing this trick at the master sliderconnections. Looking at the schematic this seems te best spot and the signal will be well buffered for the sending side as well as receiver side. Figuring out where to put these extra TRS connectors

 

[sweetbeats]

Good luck. You’ll have to see how well it works if you’re going to go that route. Typically, if there are unbalanced insert points in a circuit like that there are two additional amplifier stages. What you have in the 388 on the master buss is a summing stage ahead of the fader and a fader booster following. If you add insert points you would have the summing stage, an output booster to the send jack with gain, an input stage following the receive jack, and fader, and then the output booster with gain. You’ll likely have signal loss for which you won’t have enough range on the fader to counter, and, at the very least you’re going to want to add an output coupling cap and output series resistor, and an input coupling cap and series input resistor, depending on where you splice in the insert jack(s).

 

[ArnoP]

 

[ArnoP]

The U202 is a low impedance driver...so it should be suited for driving the external connection to the compressor input whereas U103 is the buffer for the return signal which is driven by the 600 Ohm output of the compressor output. Good shielding included of course...then this looks hopefull ;-)

 

[sweetbeats]

Good luck. I would still add output and input series resistors. Let us know how it turns out.

 

[ArnoP]

Agree about the resistors, especially on the output of an opamp as the design also does it elsewhere to avoid any bold short circuit or too strong currents getting it or out. Let's see.

 

[ArnoP]

Done!

 

[sweetbeats]

What values are those resistors? I can’t quite tell what the colors are on the bands. And aren’t the resistors supposed to be in series? Maybe I’m missing something but it looks like you have R1 strapped from the tip to the ring, and R2 strapped from the ring to the shield, but they are all strapped together, so the tip is also strapped to the shield through R1+R2. I am missing something.

 

[ArnoP]

Sorry for the confusion. 100 Ohms in series when the insert jacks receive a plug and this resistor is sorted without plugs. 100k permanently connected after the seriescapacitor as a pull down to ground...that's it and works well ;-)

 

[sweetbeats]

The 100K pull-down resistor is already part of the circuit…the resistor to ground ahead of the non-inverting input of what is now your input opamp sourcing the return…no need to have that on the jack. It’s already there on the PCB.

The 100R resistor…so that’s supposed to be an output resistor? How do you have the wiring from the PCB connected to the tip conductor of the jack then if the 100R resistor is strapped across both PCB pins of that jack? Is the wire from the PCB connected to one of those pins? If so, then it is bypassing the resistor and you have, essentially, no output resistor. And am I seeing that correctly the 100R resistor is strapped across both the tip and ring pins?

And do you have no series input resistor then? How is your outboard gear handling the low impedance input?

If it was me I’d get rid of the superfluous 100K resistor, and implement proper series output and input resistors, that, ideally, would be out-of-circuit when there is nothing plugged into the jack…not sure exactly how that would be implemented without drawing it out, but I’m sure it could be accomplished without drawing the right jack and normalling contacts.

Did you put the jacks under the front armrest?

 

[ArnoP]

A lot of questions! Part of the story is the use of switching jacks (3 contacts) Are you planning to implement it yourself on your 388?

 

[sweetbeats]

I don’t own a 388. No wasn’t planning on implementing, at least not in the same way, just curious to understand what you implemented, why the duplicate 100K resistor, and how the load sees a high impedance input at the return, and how the 100R series input resistor is not bypassed. If it’s too complicated to explain that’s fine. I’m just curious.

 

[ArnoP]

Ah I see.
Perhaps the initial schematic with the two red arrows was confusing since this is different than the actual implementation.
Adding the overview for clarification (hope this helps) and maybe others can also have the benefit of it

 

[sweetbeats]

Yes that does help.

So, yes, for sure, you do not need the 100K resistors. They are totally redundant. Their function is already installed on the PCB post fader and ahead of the next amp stage; R129 and R229.

And you have no series input resistor. This is okay if your outboard gear that is interfacing your custom insert points is designed to drive low impedance loads. I bet it’s not. And what happens then is the output driver of the outboard gear won’t necessarily perform to spec with the impedance mismatch…output stage may have higher distortion. What outboard gear are you typically connecting to the insert points? If it expects high impedance loads on its outputs, there should be series input resistors, maybe 10K. And here’s where we come full circle: if you install those, it will ensure your outboard gear (if it is indeed designed for high-impedance loads) performs within spec…signal stays clean, etc. But the series resistor will also reduce the signal level, which is why the correct implementation would include additional amp stages, to provide counter-gain to offset the series resistor that provides for a high-impedance path for your outboard devices. So you can absolutely do what you’re doing, and certainly not worry about the input impedance on the return side, *especially* if your outboard gear is old-school and designed for low impedance operation (like the classic 600 ohm environment), but if you leave the series input resistor out of the circuit and your outboard gear wants it, then expect you might experience some crunch and munch especially on transient peaks, depending on how hard you’re driving it. And if you have to go conservative on the level as a result, you can just do that too, but then of course you’re self-limiting your dynamic range (higher noise floor).

 

[ArnoP]

Hi, thanks for the reflections on this subject. Allow me to briefly respond per item.
* 100k: I prefer that the circuit has a local pull-down right after a coupling cap instead of going out, getting back and then get the 100k better S/N.
* The signal from the output of the outboard gear will get back to the max/10 end of the 10k Stereo master potentiometer and its wiper continuous the signal to U103.
* Outboard output impedance is 600 ohm (Compressor)
Measured all and there is no impact of the F-range