Tube amp speaker out -> tap/splitter -> instrument/line level

Thread Starter

jjredfish

Joined Apr 25, 2019
30
Hi all,

First post here. I'm a guitarist, but I know some electronics, can solder, etc.

I'm wanting to make a simple splitter box with one 1/4-inch input jack on one side, and two 1/4-inch output jacks on the other side. I want to plug my 50-watt tube amp speaker-out cord into the "input" side of this box, and have that speaker output continue straight through the box (unaffected) to a speaker output jack on the "output" side of the box (this will always have a speaker load plugged into it so I don't blow up my tube amp). In between, I would like to tap into this speaker output line to create a instrument level/line level that goes to the other output jack. It might be cool to also include a switch for instrument level/line level if it doesn't complicate things too much... I know this should be a fairly simple project, but I am being overly cautious and asking the experts here so as not to blow up my amp.

Can anyone help with a circuit diagram/resistor values, etc? Thanks!
 

AnalogKid

Joined Aug 1, 2013
9,096
You need to show the ground connections in your schematic. First, it completes the picture and reduces mistakes. Second, R2 should connect between the right side of R1 and ground. R1 and R2 form an L-pad, a simple resistive voltage divider. The switch can be SPST between the "bottom" end of R2 and ground, but this leaves R1 in the signal line in both instrument and line level modes. If you want to bypass the attenuator completely in instrument mode, the the switch becomes SPDT (as shown) but the center of the switch goes to the output and the two poles go to each side of R1.

ak
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
Hi Analog Kid, thanks so much for the advice!

My original thought was that R1 would set the level needed for Line level, while R1 and R2 would combine to set the level needed for Instrument level. I don't think this drawing would accomplish that, but I tried to draw it as you described:

Screen Shot 96.png
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
You need to show the ground connections in your schematic. First, it completes the picture and reduces mistakes. Second, R2 should connect between the right side of R1 and ground. R1 and R2 form an L-pad, a simple resistive voltage divider. The switch can be SPST between the "bottom" end of R2 and ground, but this leaves R1 in the signal line in both instrument and line level modes. If you want to bypass the attenuator completely in instrument mode, the the switch becomes SPDT (as shown) but the center of the switch goes to the output and the two poles go to each side of R1.

ak
(making sure AK gets notified about the post above, since I didn't hit the "reply" button)
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
You need to show the ground connections in your schematic. First, it completes the picture and reduces mistakes. Second, R2 should connect between the right side of R1 and ground. R1 and R2 form an L-pad, a simple resistive voltage divider. The switch can be SPST between the "bottom" end of R2 and ground, but this leaves R1 in the signal line in both instrument and line level modes. If you want to bypass the attenuator completely in instrument mode, the the switch becomes SPDT (as shown) but the center of the switch goes to the output and the two poles go to each side of R1.

ak
Or maybe this? :

Screen Shot 98.png
 

AnalogKid

Joined Aug 1, 2013
9,096
#4 looks good to me.

So, treating this as a learning experience, what is *your* plan for determining the resistor sizes? Hint - try to determine the input impedance value of whatever is going to plug into the I/L output. If it might be multiple things, determine (or guess) the input impedance values for all of them, and select the lowest one. It acts an another resistor in your circuit.

Where are you located?

ak
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
#4 looks good to me.

So, treating this as a learning experience, what is *your* plan for determining the resistor sizes? Hint - try to determine the input impedance value of whatever is going to plug into the I/L output. If it might be multiple things, determine (or guess) the input impedance values for all of them, and select the lowest one. It acts an another resistor in your circuit.

Where are you located?

ak
Great! Thanks AK. Does R1 become redundant with R2 and R3 where they are?

Well I, er, um... didn't really have a plan... I figured there must be standards for "line level" and "instrument level", as that's about all they ever tell us guitar-folk... I would like to plug it into the effects loop of a multi-effects unit. Not sure if that loop is line or instrument level, so I added the switch. I'll see if I can find that info online (I don't own the unit yet)

I'm in the countryside between Austin and San Antonio.
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
The online manual for the effects unit simply says you may select either line level or instrument level in the Global Settings menu.
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
#4 looks good to me.

So, treating this as a learning experience, what is *your* plan for determining the resistor sizes? Hint - try to determine the input impedance value of whatever is going to plug into the I/L output. If it might be multiple things, determine (or guess) the input impedance values for all of them, and select the lowest one. It acts an another resistor in your circuit.

Where are you located?

ak

I just noticed the messages have numbers on the far right, so we are talking about the 2-resistor drawing on message 4. I will be calling the manufacturer when they open this morning (pacific time) to try to get actual ohm values for both line level and instrument level of their device.
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
#4 looks good to me.

So, treating this as a learning experience, what is *your* plan for determining the resistor sizes? Hint - try to determine the input impedance value of whatever is going to plug into the I/L output. If it might be multiple things, determine (or guess) the input impedance values for all of them, and select the lowest one. It acts an another resistor in your circuit.

Where are you located?

ak

Figure 2: Simple resistive voltage divider

A resistive divider is the case where both impedances, Z1 and Z2, are purely resistive:

V out = R2 / (R1+R2) x V in
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30

An L pad used to match a source to a load of a different impedance.
If a source and load are both resistive (i.e. Z1 and Z2 have zero or very small imaginary part) then a resistive L pad can be used to match them to each other. As shown, either side of the L pad can be the source or load, but the Z1 side must be the side with the higher impedance.

Rb = Z2 / sqr (1-Z2/Z1)

Ra = (Z1Z2) / Rb
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
#4 looks good to me.

So, treating this as a learning experience, what is *your* plan for determining the resistor sizes? Hint - try to determine the input impedance value of whatever is going to plug into the I/L output. If it might be multiple things, determine (or guess) the input impedance values for all of them, and select the lowest one. It acts an another resistor in your circuit.

Where are you located?

ak
Alright, here we go - "the input impedance value of whatever is going to plug into the I/L output" - straight from the engineers:
"Instrument level" impedance = 1 megohm
"Line level" impedance = 10k ohm
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
Over 150 views... Anyone out there know the equations and/or values for R1 and R2? The ones I posted above don't seem to apply as 8 ohms is not greater than 10k ohms.

Here is a simplified version of the circuit (that may or may not be correct...) to hopefully make it easier for someone to figure out. I got rid of the switch, so it's just line level now. That should help. I also lowered the amp to 20w, and assumed 8 ohms for the amp and speaker.

Screen Shot 99.png
 

AnalogKid

Joined Aug 1, 2013
9,096
The goal is for the output impedance of the attenuator network (basically R1 and R2 in parallel) to be so much less than the 10 K input impedance that the output voltage does not change (much) when the output is connected to the device. The engineering rule of thumb is for the two to be at least 10:1 - for a 10 K load you want the source impedance to be less than 1 K. Not a problem in your case, so let's go to 100:1. Fix R2 at 100 ohms, and calculate R1 for the desired attenuation. Whatever R1 is, it will lower the output impedance even more, not a bad thing.

Now comes the fun part - there are several definitions of "line level": https://en.wikipedia.org/wiki/Line_level

Can you get back to the engineers with this question: "What is the value of "line level" that they design to?" An answer in dB doesn't tell us much because a dB is a ratio, and without knowing which standard or reference value they use, we cannot calculate anything. We prefer an answer in volts RMS, volts peak, or volts peak-to-peak.

ak
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
Thanks for the reply. I sent the engineers another text with your question, but won't hear back until tomorrow.

They had previously said they measure in dBu, which (from your Wikipedia link) implies +4 dBu "Professional Audio" (vs -10dBV "Consumer Audio"), which corresponds to Nominal level VRMS of 1.228, Peak amplitude VPK of 1.736, and Peak-to-peak amplitude VPP of 3.472
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
The goal is for the output impedance of the attenuator network (basically R1 and R2 in parallel) to be so much less than the 10 K input impedance that the output voltage does not change (much) when the output is connected to the device. The engineering rule of thumb is for the two to be at least 10:1 - for a 10 K load you want the source impedance to be less than 1 K. Not a problem in your case, so let's go to 100:1. Fix R2 at 100 ohms, and calculate R1 for the desired attenuation. Whatever R1 is, it will lower the output impedance even more, not a bad thing.

Now comes the fun part - there are several definitions of "line level": https://en.wikipedia.org/wiki/Line_level

Can you get back to the engineers with this question: "What is the value of "line level" that they design to?" An answer in dB doesn't tell us much because a dB is a ratio, and without knowing which standard or reference value they use, we cannot calculate anything. We prefer an answer in volts RMS, volts peak, or volts peak-to-peak.

ak
OK, so it took them a while to get back to me, but they confirmed: +4 dBu "Professional Audio", VRMS of 1.228, Peak amplitude VPK of 1.736.
 

Thread Starter

jjredfish

Joined Apr 25, 2019
30
The goal is for the output impedance of the attenuator network (basically R1 and R2 in parallel) to be so much less than the 10 K input impedance that the output voltage does not change (much) when the output is connected to the device. The engineering rule of thumb is for the two to be at least 10:1 - for a 10 K load you want the source impedance to be less than 1 K. Not a problem in your case, so let's go to 100:1. Fix R2 at 100 ohms, and calculate R1 for the desired attenuation. Whatever R1 is, it will lower the output impedance even more, not a bad thing.

Now comes the fun part - there are several definitions of "line level": https://en.wikipedia.org/wiki/Line_level

Can you get back to the engineers with this question: "What is the value of "line level" that they design to?" An answer in dB doesn't tell us much because a dB is a ratio, and without knowing which standard or reference value they use, we cannot calculate anything. We prefer an answer in volts RMS, volts peak, or volts peak-to-peak.

ak
Can we calculate the resistor values now?
 
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