Amp In A Box input & output impedances

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Amp In A Box input & output impedances

Postby POTL » 28 Jul 2017, 01:01

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 50 kilo ohms
c25844Marvel cctedb238.png


Just as interesting about these schemes
kejina.png

Image

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
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Re: Amp In A Box input & output impedances

Postby bmxguitarsbmx » 28 Jul 2017, 06:00

Hi POTL,

I've been wondering the same thing. So many stompboxes with really high output impedances. Anything over 10k will be noticeable driving a 20 foot cable (~1nF capacitance). For effects loops, I prefer to go below 5kOhm output impedance as I'm assuming 40 feet of instrument cable (~2nF).

Love to have someone with experience address this :)

Cheers,
BMX
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Re: Amp In A Box input & output impedances

Postby POTL » 28 Jul 2017, 07:23

That's why I want to know if it makes sense to refine these schemes.
Is it worthwhile to install input and output buffers from Marvel Drive
Output buffer from Zvex Distortron
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Re: Amp In A Box input & output impedances

Postby juice2 » 28 Jul 2017, 07:28

POTL wrote:Here, on the input and output cascades, the sound goes through the drain
How does this affect resistance?

I suggest you reading about JFET mu-amp and input/output resistance calculation. And usually simulator apps have a feature to calculate these parameters.
POTL wrote: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

No, it won't. What I can say for sure is that it is lower than 50K :D
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Re: Amp In A Box input & output impedances

Postby juice2 » 28 Jul 2017, 07:38

POTL wrote:That's why I want to know if it makes sense to refine these schemes.
Is it worthwhile to install input and output buffers from Marvel Drive

The question is why you need this. The good practice is to keep the rule "Rin > 10 Rout", as long as your effects in chain meet this rule you are ok. On the other hand nobody can stop you from experimenting.

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Re: Amp In A Box input & output impedances

Postby POTL » 28 Jul 2017, 15:35

This is necessary for normal operation with different types of pickups and other pedals.
So that there are no problems like fuzz face and wah or wampler sovereign and powerful guitar pickups
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Re: Amp In A Box input & output impedances

Postby bmxguitarsbmx » 30 Jul 2017, 20:41

juice2 wrote:The question is why you need this. The good practice is to keep the rule "Rin > 10 Rout", as long as your effects in chain meet this rule you are ok. On the other hand nobody can stop you from experimenting.


Don't forget that your instrument cable is a load too. While your amp may have a 1M input impedance implying that a pedal with an output impedance of 100k is acceptable, a 20 foot cable has 1nF of capacitance creating a first order low pass filter with a -3dB corner at 1.6kHz. Food for thought.

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Re: Amp In A Box input & output impedances

Postby toneman » 30 Jul 2017, 22:26

An LPB-1 just AFTER the guitar but BEFORE the guitar cord can drive 20ft cords with ease! :D

The LPB-1 was originally designed to actually plug INTO the guitar.

Long ago, I modified an LPB-1 for an old steel guitar with old, weak lipstick pickups.
The guy loved it!! and used it constantly on stage.

Don't overthink this too too much :slap:

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Re: Amp In A Box input & output impedances

Postby POTL » 30 Jul 2017, 22:31

I built it with a switchable output buffer, in principle the effect on the sound is almost not noticeable, I will leave the final version without buffer, I'll make the buffer separately =)
Thank you for helping everyone who responded!
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Re: Amp In A Box input & output impedances

Postby POTL » 09 Aug 2017, 06:36

Image
OK one more question
Input impedans on this schematic is 500K(R3//R4) or 330K (R2//R3//R4).
Or R2 is not taken into account?
Or R2 it input impedance?
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Re: Amp In A Box input & output impedances

Postby POTL » 09 Aug 2017, 07:03

I found the answer
On electrosmash website
The value will be 330K
Correct if wrong =)
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Re: Amp In A Box input & output impedances

Postby toneman » 09 Aug 2017, 07:44

DC impedence is the 1M to gnd. That's what the guitar drives/sees.

The AC impedence goes thru the input cap, C2, and sees another 1M to gnd.

I would say the overall impedence is > 300Kohms.

The MOSFET is superhi Z so i'm not counting it.
U think it is 1M/3M?

There's another unused MOSFET buffer at Q4.
Use that to drive an NPN or MOSFET emitter follower for a low-Z out.
time for bed......
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Re: Amp In A Box input & output impedances

Postby POTL » 09 Aug 2017, 15:19

Thanks Toneman :thumbsup
In my country morning 8)

Stupid question
Can you explain to me the difference between DC impedance and Overall impedance
If DC Impedance is associated with the effect of the guitar (or effects standing before the circuit) on the sound of the current circuit.
Then why is Overall Impedance needed?

Yeach and one more question& About output buffer.

Image
I think that setting the buffer would be a good idea if you set the buffer after the potentiometer of the volume, to avoid its effect on the output impedance
I like the scheme of Jack Orman
It has an output resistance of several hundred ohm
But whether it is necessary to put a shunting resistor (since the volume knob will be up to the buffer,
Or a resistor at the output of the circuit is optional?
Or I can put a resistor (say 1K) in series with the capacitor without connecting it to ground (like in BOSS buffers)
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Re: Amp In A Box input & output impedances

Postby toneman » 09 Aug 2017, 17:54

basicly:
a capacitor BLOCKS DC.
a capacitor allows AC
AC impedance is in direct relation to frequency

I think it would be easier if U googled "Impedance dc ac" for more professional explanation.

2nd Q:
1M resistors at input(guitar side) and output(guitar side) are usually there to eliminate any residual DC charge.
residual charge will cause popping when circuit is switched in/out.
for the output, a 100K(even a 50K) pot to gnd with the output coming off the wiper is done a lot
this is the way the LPB-1 works.
Build it and see/hear if it works :thumbsup
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Re: Amp In A Box input & output impedances

Postby POTL » 09 Aug 2017, 18:29

I can hear it only with a long cable :horsey:
By itself, the difference will be invisible, I have already tried to play with the buffers.
I collected LPB-1 for a long time, did not appreciate it.
Simply if the circuit is terminated with a potentiometer of volume, then there will not be a fixed output impedance (especially in average values).
So I'm thinking about adding a buffer like BOSS / Ibanez / MXR to get a constant output value
Jfet MOSfet Jack Orman buffers look good, they are better than buffers on BJT or OpAmp (in the sound plan), have high input impedance and low output (a few hundred ohms)
But I'm not sure whether it's worth adding a resistor (shunt or in a series with a capacitor) to the output or the circuit can be ended simply with a capacitor?

If I add a 1K resistor to a series with a capacitor will this make sense (it seems to me that I will get a fixed output impedance of 1K)
Is it necessary to shunt the resistor to the ground at the end of the circuit (the BOSS and Ibanez buffers are structurally more complicated than the Jack Orman buffers and have both shunt resistors and resistors that go in series with the capacitor)
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