Couple of questions about a variant of the Fender tone stack

[aion]

I came across this tone stack in the schematic of a Lab Series L5 amp:



1. There's a capacitor missing - the 22k resistor is connected directly to the junction of the Treble and Bass controls. What's the effect of this? And if I'm running the stack simulation in the Duncan tone stack calculator, would I simulate this circuit by just using a giant capacitor here, like 10 or 100uF? (they don't let you jumper it, only change the value)

2. The 2.5k Mid pot... Why would the designers have chosen this value compared to the typical Fender 10k? I guess it is only a little "out of proportion" to the standard Fender tonestack (the Treble and Bass controls are 50k, which is 20% of Fender's values) but I suppose that prompts the question of why they'd be using 50k's here instead of the standard 250k for those. Is it possible to tweak some of the surrounding values to use either a 1k or a 5k pot here and have roughly the same sweep with the same tone?

3. And third question, I guess, while we are here. Looking at the 2.5k pot for the Volume control a little farther down... what would a guy have to do to adapt this to use a more common pot (either 1k or 5k), and what would be the effect? Never seen a volume or op-amp gain control quite like that.

Here is the full schematic for reference:
http://www.rru.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg

[Bill Mountain]

Try some other modeling software like falstad.com/afilter.  It's great for non standard tone stacks.

You could use a super large cap like 100u to mimic a short.

The 2.5k mid pot could have been chosen for availability but this choice will limit total mids in the tone stack.  Could have been intentional.  Could have not been.  Does it sound good?

The 2.5k level pot can be almost anything.  Just scale the bright cap to match.  Also scale the 6.8k input resistor and 10k feedback resistor of the next stage to match.

How much of the circuit do you plan to use?  Not sure yet what R156 (39k) is for.  That might need to be changed too.  From 1st glance it looks like it might be going to ground but I need to study the schematic more.

[Bill Mountain]

OK so R156 is coming off the 2nd channel with the Bax EQ.  If you scale the feedback resistor (R157) then you'll have to scale this too so the mix of the two channels doesn't change.

So...R165 and R166 are also in the mixer stage so you will need to pay attention to them as well.  Might be easier just to find a 2.5k pot for the volume.  Maybe a 1k or a 5k with a parallel resistor and then you shouldn't have to scale anything.

[aion]

Would the cutting of the pot values have anything to do with the difference in impedance between this circuit and a Fender? What difference would there be if I was to double all of the pot values and the 22k resistors and halve all the capacitors? Would the gain difference be noticeable enough that I should boost the next stage slightly (R111) to compensate?

Thanks for the insight so far.

[slacker]

Would the cutting of the pot values have anything to do with the difference in impedance between this circuit and a Fender?

Yeah that's it, opamps can provide more current than tubes so can drive lower impedances which means you can use use smaller pot values than the same circuit built with tubes. Lower pot values potentially means less noise. There would be no difference if you doubled the pot/resistor values and halved the caps. R109, R104 and C104 are also part of the tone stack so you should scale those as well.

[PRR]

> Lower pot values potentially means less noise.

Yes (with quibbles), but.... the designer totally blew that benefit by using huge resistors in the first stage. 220K in series with the input adds ~~5uV hiss, dwarfing the ~~2uV of the BiFET chip, ~~2uV of the '741, and ~~1uV of the 50K Treb pot in place of conventional 250K-500K treb value.

This may be an incremental "upgrade". The '741 sets the basic design at a certain place in time. The LF356 came later and was touted for hi-Z lo-hiss. If the 220K input series network really comes from a '741, it musta hissed like a snake, and indeed the '356 would be quieter in the same values. And showing that e-guitar does not have to have LOWest hiss, just not-high hiss.

The 2.5K VOL pot, you need to know what comes after (I'm too lazy to look). If there's a heavy load after, you want a low value. The '741 can drive a 2K load. But pot values are approximate; and the load counts too. 2.5K may have been the next-up value which was sure to be over 2K. Of course this forces a huge (costly) Bright-cap value.

[aion]

Paul, in your opinion why would the designers have chosen such high resistors for the input? Is there anything special about the LF356 that would warrant this setup as compared to another JFET-input op amp like the TL071/2? What would be the practical effect of dropping the resistor values by the same ratio, e.g. to 100k/470k for the "Hi" input and 22k/470k for the "Lo" input?

I built the whole preamp circuit (schematic) as a pedal, and it is indeed a bit noisy - no more than the original amp, but certainly something that could be improved on 40 years later. But I'm just wanting to understand a bit more about the design choices they made so I know what is best left alone.

[samhay]

>why would the designers have chosen such high resistors for the input?

The op-amp is inverting, so the 220k sets the input impedance of the LO channel.

[PBE6]

Further to that, it looks like the inverting stage could be replaced by a non-inverting stage to cut down on the noise a bit. It just needs a high-pass at 88kHz  to mimic the response. And I suppose a new voltage divider for the LO input.

[PRR]

> Is there anything special about the LF356  ... as compared to another JFET-input op amp like the TL071/2?

AFAIK: TL072 is just as good as LF356, and cheaper.

More time at the drawing board, more foundry improvement.

As I said: This may be an incremental "upgrade", and perhaps not totally optimized at every increment. The '356 was less hissy than a '741, which had seemed good-enough at the time, so everybody was happy. Or maybe the company head had a 3-drink lunch with the LF356 salesman and came back all hot to use LF356 for something.

A particular "feature" of the inverting plan is that with nothing plugged in, the amp is "silent" (because gain falls very low). This is of course practically pointless; but impresses some buyers (and occasionally helpful for many inputs being plugged and unplugged mid-show).

As Sam and PB say, a really-quiet (when working!) design would be non-inverting. Of course these hiss even with nothing plugged-in (unless you use added jack-contacts to kill signal further along). And the 2-level gain is harder to do without getting back into resistor hiss. (But with opamps at 19 cents you can hardwire each jack and opamp for design gain, mix at high level.)

[Rob Strand]

To a large degree it is a fender stack with the caps 5 times bigger and the resistors 5 times smaller.

The difference I see:
- the bass cap, which is kind of moved to the input stage by using the small input cap.
- there is low-pass filter, perhaps trying to emulate the roll-off of a tube stage (with input resistors)
  (keep in mind the overall frequency response needs to be taken into account ie. all stages)

The large input resistors are to prevent loading the pickups.  The inverting input stage has poorer noise than a non-inverting stage.   Most input stages use non-inverting stages for this reason.

The 2k5 volume pot may be trying to keep the impedance low to drive the next stage.  The link is broken so I couldn't check.