Amp In A Box input & output impedances

[POTL]

Hi
A question
This scheme has no input and output buffers, which are desirable for preserving the tone.
Tell me, how are things at this pedal with input and output impedances?
What are their sizes
Does it make sense to add buffers or input impedance is 1 megohm and the output does not exceed 10 kilo ohms
Modify message

[POTL]

Just as interesting about these schemes


In the effect pedals it is recommended to create circuits with a high input resistance around megohms
And a low output resistance of not more than 10 kilo ohm
Usually buffers are used for these purposes, in the buffers the signal leaves the emitter or source
Here, on the input and output cascades, the sound goes through the drain
How does this affect resistance?
The filter at the end of Marvel Drive has 2 resistors with a nominal value of 50 kilo ohms, will the output resistance be 50 kilo ohms

[PRR]

> it is recommended

By who?

These are EFFECTS. If the sound is right (or "wrong" in a GOOD way!), that is more important than impedances.

[POTL]

In most cases this has become a kind of standard in many pedals like BOSS or MXR
High input impedance allows to not lose frequency, low output does not load subsequent devices
For example, the Wampler Sovereign / Pinnacle has problems with sensors with a strong output signal (as stated on the Wampler website),
So I heard complaints about the pedals that go after this
Fuzz Face has a low input impedance and is not friendly with Wah pedals facing it, the problem is solved by installing a buffer between them
If in the Zvex Distortron circuit the input impedance is 1 megohm and there are no problems with the effects in front of it, then its output resistance is unknown for me, will it be 10 kilo ohms due to the resistors in the filter before output
In Marvel Drive with output, the situation is similar, and the input impedance is also incomprehensible.

My task is to understand whether it makes sense to add buffers to the pedals at the output and input (for Marvel Drive and Mu Amp) or they meet the necessary criteria.
In addition to the sound of the effect itself, I want him to be friends with other effects.

[POTL]

Addition
For boosters as the first cascade of Zvex Distortron and Marvel Drive, I found the information that the input impedance will be equal to the gate resistor (Zvex 1M and Marvel Drive is 33K or 1M)?
The output impedance will be (or is very close) to the value of the resistor in the drain. It turns out that in Zvex this value is 5K1, but will the filter of the two resistors 10K and the two capacitors 2n2 affect this value?
Update I found that Marvel Drive Have input impedance 1M im right
http://www.runoffgroove.com/fetzervalve.html

[antonis]

Usually buffers are used for these purposes, in the buffers the signal leaves the emitter or source
Here, on the input and output cascades, the sound goes through the drain
How does this affect resistance?

I'm confused a bit about your query..

Buffers are designs who work as current (power) amplifiers with gain of near unity..

It can be obtained either via Emitter/Source output or via Collector/Drain output (of an amp with unity gain & phase reversal)

In both cases, impedance of signal is set by its specific out-coming point total resistance (as seen from whatever comes next..)

Generally, the "hint" for impedance calculation is to set a specific point (separate stages) and then "see" left (from what it's driven) & right (what it drives)..

e.g.
<The output impedance will be (or is very close) to the value of the resistor in the drain. It turns out that in Zvex this value is 5K1, but will the filter of the two resistors 10K and the two capacitors 2n2 affect this value?>

Drain drives 5k1 and filter impedance (in parallel) but the filter is driven by the 5k1 impedance and it drives R16/C8 // R17/C9 // Vol pot

[POTL]

Antonis,I need to help calculate the output and input impedances of the Marvel Drive and tell if the filter is wiring to the final impedance value.
In fact, for me it is important to know how to calculate or equal the value of the input and output impedance in the amplifier part of the marvel drive (1 and 3 cascade or in the circuit as a whole).
It's also interesting how to calculate the output impedance of the first Zvex cascade and the output impedance of the whole Zvex circuit

[reddesert]

What problem are you trying to solve? Does the circuit sound bad? Does it sound different if you plug a buffered pedal in front of or behind it?

Amps need to have a high input impedance, close to 1 megohm, because they are designed to have an electric guitar that has a high output impedance pickup and a 250K or 500K volume pot plugged into them. If the guitar is plugged into a low input impedance device it can (sometimes) suffer treble loss - this is what "tone suck" originally meant. On the other hand, sometimes, as in the case of the Fuzz Face, it sounds good.

In the first schematic, the input has a 1M pulldown resistor to ground, and then a 33K resistor to a 180 pf cap to ground and to the gate of a JFET. The 180pf cap has a high impedance at audio frequencies (reactance = 1/(2*pi*f*C), so at 100 Hz, it's 8.8 Megohms).  And the gate of a JFET draws essentially no current. So there is no low-impedance path to ground for the input signal. Putting a buffer in front of this seems superfluous.

[POTL]

Hello
The scheme sounds great
I want to solve the problem of matching effects.
thanks for the answer
I correctly understand that in the first scheme the input impedance is 1M
Or should I remove the 33K resistor in order to get the desired 1M?
Tell me by the schemes zvex and Marvel about the output resistance.
What is it after the last cascade of amplification (3 transistor zvex and 4 transistor marvel), what is the output resistance after the filters.
The only question is whether the input and output impedances are close to the recommended standards

[POTL]

Can someone else tell me how to calculate the output impedance? 

[PRR]

[POTL]

Yes
I understand that there is a variable resistor, but usually the value in such circuits varies in the range from 8K to 16K(100K in original schematic)
Volume Pot is 100K
I found some formulas to calculate, but I hid in the notation
For example, I understand the meaning of the symbol "//" (what is this double division?)
I will be grateful for the answer.
If you can tell, it would be interesting to get similar information about Zvex Distortron
Thank you!

[POTL]

.

[reddesert]

Hello
The scheme sounds great
I want to solve the problem of matching effects.
thanks for the answer
I correctly understand that in the first scheme the input impedance is 1M
Or should I remove the 33K resistor in order to get the desired 1M?
Tell me by the schemes zvex and Marvel about the output resistance.
What is it after the last cascade of amplification (3 transistor zvex and 4 transistor marvel), what is the output resistance after the filters.
The only question is whether the input and output impedances are close to the recommended standards

What problem of matching effects?  There isn't usually a problem. You are potentially inventing a problem by taking a circuit that has input running to a JFET gate (extremely high impedance), and talking about throwing something else in front of it that might be worse.

The best suggestion that I have is that you could construct your own buffers as separate circuits: for example, you can copy the tube screamer input and output buffers shown in the first schematic on Jack's page, http://www.muzique.com/lab/tsbuff.htm  Then try wiring these up before and after the circuits you are interested in.  My guess is that for most modern circuits, it won't make a difference, but for some older circuits (Fuzz Face, the circuit with a 4049 output that someone asked about in the Red Llama tone control thread), maybe it will.

[duck_arse]

For example, I understand the meaning of the symbol "//" (what is this double division?)

the " // " is generally taken to mean "in parallel with". [I often mis-use it to between component numbers to indicate a common/connecting point.]

[POTL]

Hello
The scheme sounds great
I want to solve the problem of matching effects.
thanks for the answer
I correctly understand that in the first scheme the input impedance is 1M
Or should I remove the 33K resistor in order to get the desired 1M?
Tell me by the schemes zvex and Marvel about the output resistance.
What is it after the last cascade of amplification (3 transistor zvex and 4 transistor marvel), what is the output resistance after the filters.
The only question is whether the input and output impedances are close to the recommended standards

What problem of matching effects?  There isn't usually a problem. You are potentially inventing a problem by taking a circuit that has input running to a JFET gate (extremely high impedance), and talking about throwing something else in front of it that might be worse.

The best suggestion that I have is that you could construct your own buffers as separate circuits: for example, you can copy the tube screamer input and output buffers shown in the first schematic on Jack's page, http://www.muzique.com/lab/tsbuff.htm  Then try wiring these up before and after the circuits you are interested in.  My guess is that for most modern circuits, it won't make a difference, but for some older circuits (Fuzz Face, the circuit with a 4049 output that someone asked about in the Red Llama tone control thread), maybe it will.

I agree
But buffers are needed if the pedal has a low input or high output impedance.
As we managed to find out together, I asked about the circuits about which the input impedance is high enough and there is no need to stop the buffer before the pedal.
The question remained with the buffer after the pedal.
If the output impedance of the effect is within the standard resistance (no more than 10K), then the buffer is also not required.
I just want to understand whether it makes sense to install a buffer in these cases or they have a suitable impedance.

[PRR]

> from 8K to 16K

For easy math, take Drain || drain resistor as equivalent 13K.

The resistances are bad enough, but there are three capacitors in there also.

A cap is open (infinite impedance) at low-low frequency, short (zero impedance) at hi-high freq. In between it is in between. And that may matter.

Here we suspect we have two sorts of caps.

C13 is a coupling cap, "low impedance" (relative to related parts) over the entire audio band.

C14 C15 act as treble-cut caps. They probably do not act low-impedance until the top of the audio band.

Right away we call C13 "zero", turn Vol to the top, and see we have (13K+50K+50K)||100K, which is a little over 50K, which exceeds your arbitrary 10K spec. So "fail".

However if Vol is turned way-low the output impedance is low. At 10% electrical volume it is just 10K impedance (||203K) and passes your spec.

But where C14 C15 are cutting severely, Vol full-up "sees" C15, which is cutting against R15 50K, and output impedance is under 33K.

This is typical of all RC networks. Impedance high in bass and lower in treble.

But to put better numbers on this requires solving a 9-leg network, for all useful Vol-pot positions, and I do not have a matchbook big enough to figure that out.

[PRR]

> whether it makes sense to install a buffer

The less-brain-pain plan would be to build a buffer in a box, try it against unknown pedals, see if it makes a difference.

[PRR]

me> solving a 9-leg network, for all useful Vol-pot positions, and I do not have a matchbook big enough

I have an Idiot Assistant for that. Here is a SPICE plot.


Over the guitar band, my quick estimates are OK. I said "a little over 50K, ... under 33K ... turned way-low...low" and this is seen over the guitar band. C13 is indeed picked to pass guitar bass but no deeper, and by 1Hz all we have is a 100K pot.

[POTL]

Thank you very much!
I'll try to add a buffer after the potentiometer of loudness (by analogy with classical distortion pedals).