why don't people use capacitance multipliers in pedals?

[R.G.]

I believe it is a grounded inductor (disagreeing with R.G., at my risk; but agree about the likely faults).

It also has one more resistor than it needs for most audio work; and higher hiss therefrom.

I didn't go compare it part for part against the standard gyrator circuit. I (at my own risk!  ) just took their word that it's a capacitance multiplier. I've seen similar so I let it go with what it said it was.

vaugely on topic - what effect does a cap mult have on the cap's ESR? do the benefits of the low ESR cap also multiply?

I think it varies tremendously with the cap multiplier circuit. The one circuit referred to in the link (if it is a cap multiplier, not an inductor sim; I guess I gotta to look that up) makes no bones about it - it inserts R3 in series with the cap, whatever the capacitance's ESR is. I think that will be the case for all cap multipliers, and probably gyrators.

If you think about it, there has to be a current-sensing part in the multiplier circuit. A capacitance differs from a resistor in that the voltage and current are not directly related, they're time-related, as in I = C* dv/dt. So to fake both the voltage and current, you have to sense the current through some element in the circuit, and a resistor is the right way to do that. That sensing resistor is likely to appear in series with the "multiplied" capacitance no matter what you do.

Since a current sense resistor is almost certain to be bigger than even a normal cap's ESR, the effect of the multiplier on the cap's own ESR is immaterial - the inserted resistance is many, many times bigger and unrelated to the actual cap's ESR.  So no, the benefits of a low-ESR cap are not multiplied; in fact, they're completely lost. 

[amptramp]

This schematic of the RCA VRA 141 amplifier and Magic Monitor shows the use of the Magic Monitor with its reactance tube capacitance multiplier to reduce treble frequencies for low level signals.  This is a preferred alternative to a noise gate:



There is no amplification of the signal from the phono input to the Magic Monitor output - the circuit acts as an attenuator with a capacitance that increases as signal level decreases to keep noise and hiss out of the output.  The input is amplified and rectified and drives a reactance tube to increase the shunt capacitance to ground as the signal gets quieter.

[nognow]

I found this:


if it is not ok, could you please help me find a schematic that fits?...

[amptramp]

The National Semiconductor app note capacitance multiplier is good, but it shows obsolete devices - the LM102 is not an op amp, it is a unity-gain buffer and the LM101A requires 150 pF for C2 whereas more recent amplifiers would not require C2 at all.  The thing you have to note about this design is that (1+(Rb/Ra)) input signal has to remain within the rails so for this circuit operating at ±15 volt rails, the input has to remain within 1/11 of this or 1.36 volts and this does not take into account the fact that the op amp input range is not rail-to-rail, so this is reduced even further.

Let's look at how this works: the signal is applied at C-> and with R2 adjusted to the right, the output of the LM101A is effectively grounded and the value of C looks like 0.1 µF, as you would expect.  As you move the slider of the pot to the left, you get inverting gain out of the LM101A so the capacitor has the input on the left and an inverted proportional signal on the right, causing more current to flow.  But more current through a capacitor at a given frequency looks like an increased value of capacitor.  That is the capacitance multiplier effect.

[R.G.]

I found this:
...
if it is not ok, could you please help me find a schematic that fits?...

(1) The value of "OK" has to be defined. A horse is OK for a single person transportation device if and only if the person has the skills to ride it and means to keep it fed and watered. A horse is not OK as a means for flying. Well, at least not for flying more than a few meters.   
If your value of "OK" includes a capacitance which has one lead grounded, that circuit is OK. If your definition of "OK" has to have the multiplied capacitance with both leads not connected to ground, that circuit is "not OK".
(2) A good way to figure out whether a capacitance multiplier is grounded or floating is to count the output leads. If there is only one output lead (as in arrow pointing to "C" in this one), then a grounded second lead is implied, and it's not a floating capacitance multiplier.

So, let's write down the objectives: do you or do you not need a floating capacitance for your multiplied capacitance? It's really a big deal in figuring out what will be OK.

[nognow]

has any of you guys found a good floating multiplier?

[R.G.]

has any of you guys found a good floating multiplier?

So, let's write down the objectives: do you or do you not need a floating capacitance for your multiplied capacitance? It's really a big deal in figuring out what will be OK.

[nognow]

has any of you guys found a good floating multiplier?

So, let's write down the objectives: do you or do you not need a floating capacitance for your multiplied capacitance? It's really a big deal in figuring out what will be OK.

I'm not sure...the stock caps that I want to replace are obviously floating...
how can I decide?

[bool]

The decision should be simple: If it isn't grounded, you can't use it.

[nognow]

The decision should be simple: If it isn't grounded, you can't use it.

has any of you guys found a good floating multiplier?

So, let's write down the objectives: do you or do you not need a floating capacitance for your multiplied capacitance? It's really a big deal in figuring out what will be OK.

I'll go with floating I guess...

[R.G.]

OK then, floating it is.

Google gives over 150,000 results for "floating capacitance multiplier". A quick look at the top results and the images show that really floating capacitance multipliers are possible, but require lots of active elements and suffer from various ills. Many of the results recount the use of differential voltage current conveyors (DVCCs) and current controlled current conveyors (CCCs) as well as multiple OTAs, and list this as problems in making floating capacitance multipliers. Here's an excerpt from one of the papers:

Unfortunately, the circuits in [19] [21] consist of many different active elements while the reported circuit in [20] comprises 4 OTAs and the reported circuit in [22] comprises 2 CCCCTAs. There can be found that the recently proposed topologies need to employ a lot of elements, which is not convenient to either further fabricate in IC or practically implement and providing higher power consumption.

From this I make:
(1) the current state of the art for floating capacitance multipliers is that they're complex (4 x OTA!)
(2) they're not easily amenable to reduction to an IC

There is also a fair amount of discussion on temperature sensitivity and the need for matching components and so on - the bane of precision analog work.

I have no delusion that my knowledge is comprehensive, but as far as I can tell - as I said earlier - the good ones aren't simple, and the simple ones aren't good. And in circuit design for pedals, that's often enough to rule them out. Pedal circuit design is inherently limited in how much circuitry can be used, just the nature of the beast.

If the real question is can we provide you with a simple, easy capacitance multiplier for you to use in some unstated circuit you're working on, I can't. Maybe someone else has run into a simple, easy, economic circuit that does it.

[nognow]

OK then, floating it is.

Google gives over 150,000 results for "floating capacitance multiplier". A quick look at the top results and the images show that really floating capacitance multipliers are possible, but require lots of active elements and suffer from various ills. Many of the results recount the use of differential voltage current conveyors (DVCCs) and current controlled current conveyors (CCCs) as well as multiple OTAs, and list this as problems in making floating capacitance multipliers. Here's an excerpt from one of the papers:

Unfortunately, the circuits in [19] [21] consist of many different active elements while the reported circuit in [20] comprises 4 OTAs and the reported circuit in [22] comprises 2 CCCCTAs. There can be found that the recently proposed topologies need to employ a lot of elements, which is not convenient to either further fabricate in IC or practically implement and providing higher power consumption.

From this I make:
(1) the current state of the art for floating capacitance multipliers is that they're complex (4 x OTA!)
(2) they're not easily amenable to reduction to an IC

There is also a fair amount of discussion on temperature sensitivity and the need for matching components and so on - the bane of precision analog work.

I have no delusion that my knowledge is comprehensive, but as far as I can tell - as I said earlier - the good ones aren't simple, and the simple ones aren't good. And in circuit design for pedals, that's often enough to rule them out. Pedal circuit design is inherently limited in how much circuitry can be used, just the nature of the beast.

If the real question is can we provide you with a simple, easy capacitance multiplier for you to use in some unstated circuit you're working on, I can't. Maybe someone else has run into a simple, easy, economic circuit that does it.

any other options to adjust capacitance perhaps?...

[R.G.]

any other options to adjust capacitance perhaps?...

Switches, including high frequency switching where the duty cycle makes a capacitor appear duty-cycle-times smaller than it really is.
There are adjustable capacitors, where a set of capacitor plates is inserted or removed from between another set of plates. Big, mechanically clumsy, fragile, etc.

Here's an idea - tell us what you're trying to do, and we can hypothesize some alternate ways to do it that may be simpler and easier.

Unless the idea is just to use adjustable capacitors and/or capacitor multipliers. We're close to exhausting that one, I think.