IC boost and cable /length capacitance interacting !?

[oliphaunt]

Your "low cut" performs the same function as an output pull-down.  A really quick test would be to see if it still pops with the "low cut" engaged.  If you hear any pop at all, a leaky cap is not the culprit.  Well, maybe a "short circuited" cap, but not a normally leaky one.  It would be good to check the DC voltage on the output jack in any case.

Earthtones, thanks for the suggestions.  My schematic is still not exactly correct.  The low cut is on a switch, and is only engaged when the buffer is switched to boost mode.  So it doesn't act as a pulldown when in buffer mode, which is where the voltage builds up.  

The way you've implemented the low-cut filter is not ideal for a TL071 op-amp.  They don't like to drive much lower than 2k (I think) of resistive load, let alone 300 ohms through a huge cap.

I am not following you here.  If I remove the low cut resistance will that raise the resistive load to acceptible levels somehow?  Sorry, I'm not familiar with what you are seeing happening here at all.

A "nicer" way to do the low cut would be in the negative feedback portion of the circuit.  There, your components could be high impedance and let the output focus on driving the long cable and nothing else.

I would love to know more about this, can you be more specific about how I would implement the low cut in the feedback portion of the circuit?

[earthtonesaudio]

It appears that you want a low output impedance (to drive long cables), and so you have a large value output cap (which has a low impedance at most signal frequencies).  The trouble comes from the low-value resistance to ground.  The op-amp's output can be thought of as a pretty decent voltage source, so it will put out any current necessary to make the requested output voltage.  So if you ask it to make a fast +7V output swing, the cap will be basically a short circuit (for a fast signal) and then the voltage is mostly across the 300 ohm resistor.  7V/300ohms=23mA.  Now if you look at the datasheet you'll notice that the output resistance of the TL071 is about 30-odd ohms.  So trying to drive 300 ohms will automatically take your output voltage down by 10%. 
Contrast this with the performance into 10k, and it's almost ideal.

More simply, the 300 ohm value on the output is too low.  It will degrade the performance of the op-amp.


A low-cut in the feedback loop would be something like putting a small value (10nF maybe) cap in parallel with the 10uF to ground, then adding a 10k resistor in series with just the 10uF cap.  Or take a look at the Timmy, I think it uses that sort of technique.  Basically you're presenting the inverting input with either a small cap or a large cap, which means either a little bit of low end (small cap) or a lot of low end (large cap) is boosted.


The point of putting the tone controls at this location (feedback loop) is that they're relatively decoupled from the input and output impedances, and therefore don't mess with the buffering action.

You could alternatively implement a low-cut at the circuit's input, just like the one you have at the output, but this would mean the input impedance would vary based on the low-cut setting.  Similarly, placing it on the output means the output impedance varies based on the setting.  But in the feedback loop, neither the input or output impedance varies all that much as you make adjustments to the control.  This leads to a more predictable behavior from the circuit, which is generally (but not always) desirable.

[Gus]

http://www.analogzone.com/acqt1211.pdf

http://www.analog.com/library/analogDialogue/archives/31-2/appleng.html

http://www.national.com/appbriefs/files/AppBrief108.pdf

http://www.analogzone.com/acqt0704.pdf


Now for the bass cut with gain.  Look for the Dist + schematic note the RC  leg and that the resistance goes down as the gain goes up this can be adjusted in a noninverting amp for the break from a gain of one frequency.

Look at this for another idea for a movable high pass.  I am surprised it has not been used by the "newtweekers" but maybe because it is hand drawn and opens big, people miss it.  It should open in windows picture and fax viewer
http://www.aronnelson.com/gallery/main.php/v/gus/plus.GIF.html?g2_imageViewsIndex=1

Stick you circuit in the middle.

IIRC GFR posted a TS mod years ago to have the frequency change as the gain was increased.

Do people look at my schematics?  I post some good stuff and people miss it or others get credit.

[merlinb]

  Your poor TL071 is trying to drive only 400R resistance, or less if the cut control is turned down! That's way too much for a TL, even running at 9V. Get rid of the cut control and sub in a 100k pull-down resistor or similar.

Bass cut should always be performed at the input (or somewhere other than the output). If you do it at the output then you completely ruin the circuit's output impedance, which means you lose the ability to shunt any hum picked up on the cable.

EDIT: Oops, didn't see there was a 2nd page to this thread, looks like I'm preaching to the converted!

[therecordingart]

Thanks guys!

Would this fix it?   Is 1K a good value to keep the output impedance as low as possible?

If you remove the switch won't the pot take you between unity (buffer) and a gain of 6? I'm working on something similar and it works on the breadboard.

[Gus]

Found the Plate to Plate article
http://web.archive.org/web/20091021025317/http://geocities.com/SunsetStrip/Studio/2987/fatts.html
Does anyone remember when that was first posted?

[PRR]

> won't the pot take you between unity (buffer)

The diagram you quote: He's back to two 47K resistors. Pot full-down gives gain of 2.

The added 47K (at the pot) was an early attempt to avoid oscillation when feeding loooong lines with huge capacitance. 100+ ohms at the output jack is a better fix for that. Now he can eliminate (short) the 47K and let the pot dial-down to unity.