2-channel passive mixer-independent volume control for each pickup

[bluebb]

If the metal case of a pot picks up electrical hum then it will feed the hum into the circuit. But if the metal case is connected to the circuit ground then there will be no hum produced by the pot.

Since the output impedance of the preamp is much higher than all other preamps then the output level will be less and maybe distortion will be increased with the pot values we show.
I think the spec's for the K&K preamp means its output can feed 10k.

I thought that Alec_t in his diagram had supposed that I knew that the lug 1 must be welded on the metal box, and for this he did not put it.
I know that it must be welded on the box but I asked for be sure 100%.
I need that you tell me exactly everything, even where the gnd must be welded.
Now, after only 6 hours, Fishman's support has answered me, the output impedance of the Fishman 501 T is 3.5kOhm.
For the K & K pure preamp wr do not understand what they mean with 10kohms, it seems too much, but I downloaded it from the official website.
At this point what do I do? I can not tell you the input impedence of the P.A.s, I can only tell you the output impedence of the Fishman 501T.
It's not important for me that it's perfect, the important thing is that it is safe.
If you did not tell me you I would have directly connected the two outputs on the endpin directly. Fortunately i wrote here.
If for safety I put 2 resistors a bit higher for each preamp what can happen?
What do I lose a bit of volume ??
The important thing is that is safe.
Thx
regards

 

[Alec_t]

If the pots have metal cases then those should both be connected electrically (by soldering or with tags) to the circuit ground, which pin 1 of the log pot (NOT the lin pot) also connects to.
A 10k blend pot will be safe, but with preamp output impedances of ~3.5k the blend adjustment range won't be large. A 25k pot would give a bigger range, but at the expense of reduced volume.

 

[bluebb]

If the pots have metal cases then those should both be connected electrically (by soldering or with tags) to the circuit ground, which pin 1 of the log pot (NOT the lin pot) also connects to.
A 10k blend pot will be safe, but with preamp output impedances of ~3.5k the blend adjustment range won't be large. A 25k pot would give a bigger range, but at the expense of reduced volume.

Ok,I understand,
And this pot is no needed to be adjustable everytime? One time for always?i can leave it inside the body of the guitar?
How i will understand at which value i will set it?
Somewhere in the middle?
Thx

 

[bluebb]

If the pots have metal cases then those should both be connected electrically (by soldering or with tags) to the circuit ground, which pin 1 of the log pot (NOT the lin pot) also connects to.
A 10k blend pot will be safe, but with preamp output impedances of ~3.5k the blend adjustment range won't be large. A 25k pot would give a bigger range, but at the expense of reduced volume.

is ok the wiring on the photo i attached?

 

[bluebb]

I think the spec's for the K&K preamp means its output can feed 10k.

also the KKsound replied me,i told them that 10k is very high,maybe impossible :-D
and their answer:

10K ohms is certainly not impossible. The Pure Preamp uses a very simple potentiometer on the output to act as the volume control. This results in the output impedance being a variable amount from around 470 ohms to about 10K ohms depending on the volume control setting. This impedance also changes with frequency as there is capacitance in the circuit as well.

Best regards,

 

[Audioguru]

A preamp with an output impedance that is very high at 3.5k and another preamp with a variable output impedance as high as 10k? VERY weird.

 

[bluebb]

A preamp with an output impedance that is very high at 3.5k and another preamp with a variable output impedance as high as 10k? VERY weird.

K & K's technical support continues to say that the output impedence of the K & K pure preamp goes from 470ohms to 10kohms.
While I found this page where it is said 3k ohms.

https://www.gollihurmusic.com/product/1304-KANDK_SOUND_PURE_PREAMP.html

From fishman they confirmed that for the Fishman 501T the output impedance idms 3.5kohms.

I will put this lin pot of 10k and that's it!what you say?

How i will understand at which position will be set the knob ot this lin pot of 10k?
Thx

 

[Alec_t]

How i will understand at which position will be set the knob ot this lin pot of 10k?

It's wherever the sound blend suits you best.
Since the preamps each seem to include a level control pot, set those for maximum level (volume).

 

[bluebb]

It's wherever the sound blend suits you best.
Since the preamps each seem to include a level control pot, set those for maximum level (volume).

For me is important that is safe.
There is a knob position that can lead to some failure with a 10k pot?
Or if I turn the knob to the maximum or minimum,and one preamp has zero ohms resistance ,does this can failure/burn one preamp?

 

[Alec_t]

Any setting of the blend pot would be safe.
I'm thinking that IF each preamp has a level setting control built in, and you don't want to change the blend setting during a gig, then the blend pot is redundant and you could simply mix the two preamp outputs as shown in Audioguru's third circuit in post #23. Perhaps the two fixed resistors in that circuit could be increased to 4k7 Ohms to be extra safe.

 

[bluebb]

Any setting of the blend pot would be safe.
I'm thinking that IF each preamp has a level setting control built in, and you don't want to change the blend setting during a gig, then the blend pot is redundant and you could simply mix the two preamp outputs as shown in Audioguru's third circuit in post #23. Perhaps the two fixed resistors in that circuit could be increased to 4k7 Ohms to be extra safe.

I prefer the solution with two resistors,I will make the blend with the volume controls of the two preamps.
So, ascertained that the two preamps have an output impedence of 3.5k the fishman, and from 470 to 10k the K & K, put two resistors of 4.7k for me should be fine.
If I lose a bit of volume it is not a problem, I increase the gain on the active mixer of P.A.
The important thing is that with 4.7k for each preamp will be safe to avoid failure.

 

[BobK]

I don’t think the K & K guy knows what he is talking about. If the output is from an op amp driving a 10K pot, the impedance would be low at both ends and 5K in the middle.

Bob

 

[bluebb]

I don’t think the K & K guy knows what he is talking about. If the output is from an op amp driving a 10K pot, the impedance would be low at both ends and 5K in the middle.

Bob

K&K owner manual and k&k support - 10kohm

This seller - 3kohm

https://www.gollihurmusic.com/product/1304-KANDK_SOUND_PURE_PREAMP.html

Thomann - just "low output impedance"

https://m.thomann.de/gb/kk_pure_preamp.htm

I will put a 4.7k resistor for each preamp and that's it.

 

[Audioguru]

If the output impedance of each preamp is crazy at 3.5k and you add un-needed 4.7k resistors in series with them and connect them to a 10k blend pot then you will have a reduced output level and not much blending action.

I suspect the preamp designs are old school using germanium transistors, not modern opamps.
Replace the old preamps with simple circuits using audio opamps. Then their input impedance and gain can be as high as you want and their output impedance can be low to drive a 10k blend pot with lots of blending action.

 

[bluebb]

If the output impedance of each preamp is crazy at 3.5k and you add un-needed 4.7k resistors in series with them and connect them to a 10k blend pot then you will have a reduced output level and not much blending action.

I suspect the preamp designs are old school using germanium transistors, not modern opamps.
Replace the old preamps with simple circuits using audio opamps. Then their input impedance and gain can be as high as you want and their output impedance can be low to drive a 10k blend pot with lots of blending action.

The preamp fishman is factory installed in the body of the acoustic guitar,and phisically perfect, I can not replace it, and is designed for the piezo fishman, and only the piezo costs 100 euros.
The same goes for the piezo K & K with his preamp.
I need only a general volume for these two summed signals.
The amount of volume for each piezo I do with their preamp's control.
I had just designed this project, and now I read your answer

 

[Alec_t]

Give the post #55 arrangement a try. Use screened connecting cable to minimise noise pick-up.

 

[bluebb]

Give the post #55 arrangement a try. Use screened connecting cable to minimise noise pick-up.

Sure, but I was asking here to be sure if this thing can be done or not?
And if yes, how.
Now I can proceed,
I would have already tried but I have to order the second preamp, the K & K.
Now I have the two piezos and the Fishman preamp.

I was thinking,especially
1.to have a box less in the body of the guitar 2.not to have the problems of having to mix two outputs of two preamps connected in parallel
3.to spend less money
4.avoid laborious changes on the K & K, which is also heavy,and with it I would make only one hole on the body for the volume pot and the thin wood of the guitar body would suffer (i need at least 2 pots).

to buy this:

https://schatten-pickups.myshopify....series-miniature-preamps/products/micro-pre-2

And make this circuit:



which is calculated for it,and connect the two piezo pickups on it.

The output impedence is 3.5k, but then together with the 3 pots I do not know what the output impedance would be.
But i think that two preamps in series is less complicated.

The question is, if I put the output of this micropre 2 at minimum at the input of my fishman preamp should it work?
The fishman is calculated to receive a voltage from the piezo of μV, but putting this buffer to a minimum would be too high for Fishman?
Thx


Thank you

 

[Audioguru]

The volume controls for the pickup preamps are connected backwards. They short circuit the preamp output and also the other preamp output to reduce their levels instead of using each volume control as a separate voltage divider. The master volume control is wired correctly as a voltage divider.

I am surprised to see that the micro-pre device has absolutely no audio spec's like distortion, frequency response and noise.

 

[Audioguru]

Looking again, it looks like guitar preamp outputs have a high output resistance so that their volume control can be connected backwards. When one preamp volume control is turned down to short its output level then the other preamp output level is not reduced. I call it Mickey Mouse mixing.

 

[bluebb]

Looking again, it looks like guitar preamp outputs have a high output resistance so that their volume control can be connected backwards. When one preamp volume control is turned down to short its output level then the other preamp output level is not reduced. I call it Mickey Mouse mixing.

So is it possible to connect this little preamp in the chain first and then my Fishman, but is it just a matter of correcting the connections?
I downloaded the diagram I put here from the official website schtten.
It would be nice if you put the right diagram here.
Thx