Unused Input - ideas?

[NateS]

I have a premade faceplate for my amp build, it's a two vertical input plate from ceriatone. I'm not currently planning to ever use both inputs. Kicking around for ideas for the 2nd hole. I could go ahead an make a 2nd input and never use it. Or I could do something else with it. I'd kinda sorta like to keep it more or less related to its functional grouping. (Ie, A knob in the "low sensitivity" hole is not where you'd expect to find the power scaling control)

My input jacks are on their on mini board, buffered right at the jack with a TL072 and it's making up the gain and pre-emphasis that would normally be done by V1a and it's tone shaping circuits. I'm already planning to use a few trimpots on the board for trimming out the gain, amount of preemphasis and the rolloff frequency. I've redesigned my input circuit a few times to make it more tweakable, and it's a lot better (at least in terms of logical separation) than the first passes, but the controls are still rather interactive, and don't lend themselves to frobbing, with the exception of the gain, but I already have a volume knob.



Posting the schematic in case it suggests features. Some kind of mid boost/fat control seems like an obvious suggestion. (I tried and tried, but couldn't match the rolloff in just the feedback loop using just one eq. The two get reasonably close.)
R2 controls gain
R4,R5,R8 control the amount of boost in the preemphasis, and the frequency, but are highly interactive. Changing frequency has major overall gain implications. If changing the rolloff point for preemphasis was a one knob deal, I almost think it'd be more useful to control the shift points on the pre-eq than another volume/boost. I mean, a master volume amp already has two knobs dedicated to "MORE DISTORTION" it's not like it needs another. I'm currently leaning towards trying to toss a notch into that feedback loop for a boost, but if anyone has suggestions on separating the frequency and gain of the 2nd stage, I'm all ears. (I'm thinking it needs to be inverting to do so.)

Here's how it behaves (Red is the original circuit. Blue is this one. Both are viewed at what would be the V1b grid)

[g1]

One thing I like about my Ampeg V-4 is the 3 way pad switch before the first gain stage. It allows you to run your guitar volume as high as you want and use the pad switch to prevent overdriving the input of the first stage. Typically the volume control is after the first stage which forces you to use your guitar volume to avoid overdriving the input.
Or, if you want the distortion (more for tubes) you can set the pad so you DO overdrive the first stage.

Edit: Sorry, I missed the part about not needing extra volume controls, never mind.

[NateS]

One other idea I had was to go ahead and make it an input, but use the switching to reconfigure something elsewhere in the preamp. Like paralleling or bypassing cathode resistors, modifying the NFB loop or something else that could be accomplished with a FET and a logic signal.

Using both/either inputs though requires mixing... and since each stage is buffering or boosting something or other, it probably warrants a dedicated mixing stage. Maybe I'll just throw a 3 way toggle in the unused jack, run two lines back to the micro and say it's "Reserved For Future Use". Unless it's boosting, cutting, or shaping in some way related to this input board, the actual circuit belongs elsewhere. The input board is the first board I'm cutting, so I think I like that anyway.

[madkatb]

Seems like a good spot to put a footswitch. Switching values for R8? Or any of your suggestions in your second post?

[Jazz P Bass]

Yeah, a footswitch.

How about switching in & out a different first stage bypass cap.

25uf is usually way too much.

Try a 10uf & switch a paralleled 10 (or 4.7) on top of it.

[NateS]

This is my "first stage". Since you will never ever overdrive V1a, I dropped V1a for the tl072 preamp, and v2b for a source follower with the series diode trick to mimic the dc coupled cf, voila. A 2204 w one 12ax7 and 2 el34's.

V1ak is mostly about preemphasis anyway, which is what I'm doing here. I've gotten out of the habit of stomping on things, but it is handy. In a single channel amp some sort of mild mid/gain boost or even a straight volume boost is handy.

[madkatb]

Pick any one of your variable resistors and use the second "input" via a volume pedal to defeat the panel mounted control and put it under foot control. That way, without the foot pedal in place, you're not losing what a switch might control.

[NateS]

Hmm, I REALLY like the idea of an analog control. I even have a few ADC channels left so it could really be almost anything under micro control. (power scaling volume pedal anyone?) Some sort of boost probably makes the most sense though.

[NateS]

The schematic and graph have been changed. Simpler, fixes DC offset headed into the first tube grid, and matches the stock 2204 eq curve MUCH closer.

[madkatb]

And using a stereo jack would give you two options...

[NateS]

Yeah, I think I'll design it to use either the Yamaha FC-5 or FC-7. I can easily get more buttons than I need on an analog input without rewiring my input board or micro.

[NateS]

Even though that EQ curve appears to be spot on... I'm leaning towards a shelving bass cut built as a 1 band EQ. That appears to get me better control independence for Gain, Bass Cut, Shelving Frequency controls. I also think I'm going to avoid throwing a mid boost or similar into the pre distortion tone shaping network. After the last gain stage, the voltage swing is too high to do anything too complicated (Although I did turn up a Burr Brown OPA445 or similar 100V opamp thats been in a drawer for 20 years.), but there are still plenty of low voltage switchable tricks (raising the mid control value, etc..)