Line out

[Leevi]

I should add a line out for the 5F2-A (without NFB).

http://www.el34world.com/charts/Schematics/files/fender/PRINCETON_5F2A.pdf

I thought to add it on the secondary side of the OT.
Any ideas what resistor values R1 and R2 I should use?

/Leevi

[HotBluePlates]

How big does the "line out" signal need to be?

[Leevi]

How big does the "line out" signal need to be?

I think it should have a "standard" signal level used by computers and today's audio devices.
/Leevi

[HotBluePlates]

How big does the "line out" signal need to be?

I think it should have a "standard" signal level used by computers and today's audio devices.
/Leevi

Okay. We'll assume your amp is 4w. What is the speaker impedance? The standard 4Ω or ...?

[Leevi]

We'll assume your amp is 4w. What is the speaker impedance? The standard 4Ω or ...?

I have two outputs 16Ω and 8Ω. I take the line out signal from 16Ω tap.

/Leevi

[sluckey]

I'd probably just look thru my parts and find a pot with a value between 1K and 10K. Then I'd pick out a series resistor that was five times that value. And if that was too hot or too cold, I'd adjust the series resistor up/down accordingly.

[HotBluePlates]

So Sluckey is right, but I asked my silly questions to show how to figure a starting point from scratch.

Power = Voltage²/Impedance
4w = Voltage²/16Ω

Voltage = 8v RMS (for 16Ω tap)

According to Crown's website, Professional +4dBu line level is 1.23v RMS and -10dBV consumer line level is 0.316v RMS.

We don't know the input impedance of the gear you'll be using, but Crown says 10kΩ is typical. We want a source impedance 1/10th of this, because we want to have good voltage transfer not maximum power transfer (in which case we'd have source impedance equal input impedance).

So we need an output impedance of 1kΩ; let's use a 1kΩ audio pot. That will be your R2.

Figure R1 based on the most-loss situation of dropping 8v RMS to 0.316v RMS for consumer-level gear. Use the formula for a voltage divider, times our known source voltage (8v RMS) and equate it to our needed line level voltage (0.316v RMS). We know R2 will be 1kΩ, so substituting that, we get:

8v * [R2/(R1+R2)] = 0.316v
(8v*R2)/(R1+R2) = 0.316v
(8v*R2)/0.316v = R1+R2
(8v * 1kΩ)/0.316v = R1 + 1kΩ
(8v * 1kΩ)/0.316v - 1kΩ = R1

R1 = 24.3kΩ

24kΩ is a standard value in the E24 5% tolerance series, and is close enough. See the Standard EIA Resistance Values table.

For +4dBu pro line level, we need to raise the output signal to 1.23v RMS. Using the same process as above and substituting our new output voltage, we find R1 = 5.5kΩ.

We could get very close to that value in the E96 series, using a 5.49kΩ 1% resistor. But let's figure out what resistor to add in parallel with the existing 24kΩ resistor.

Performing some algebra on a common formula for 2 resistances in parallel, if R2 and Rtotal are known, we can find R1 using:

R1 = (R2*Rtotal)/(R2-Rtotal)
R1 = (24k * 5.5k)/(24k - 5.5k)

R1 = 7.135kΩ

7.15kΩ is a standard value in the 1% tolerance, but you can get reasonably close to accurate in a 6.8kΩ resistor in 10% or 5% tolerance ranges.

Unfortunately, I can't draw this right now (chasing a toddler), but use a 1kΩ audio pot, with a 24kΩ resistor in series (your -10dBV setting) and a SPST switch which can add a 6.8kΩ resistor in parallel with the 24kΩ resistor (your +4dBu setting). Power dissipation even with the speaker unplugged will only be a couple milliwatts, so 1/2w parts are fine.

There's a little slop, but that may help you get a strong enough signal when you're playing very cleanly and well below the full 4w output; if the signal is too big or the output really tends towards 5w you're able to trim the signal down with the pot.

[Leevi]

HotBluePlates, Thank you for your deep analysis.
I'll try this.
/Leevi

[kagliostro]

Very good explanations

Thanks

K

[llama]

That's a One, Two punch from 2 Level 5 Masters!  Highya!

[Leevi]

but use a 1kΩ audio pot, with a 24kΩ resistor in series (your -10dBV setting) and a SPST switch which can add a 6.8kΩ resistor in parallel with the 24kΩ resistor (your +4dBu setting).

22k was the closest value I found and used that and 1k pot. Everything worked well even the range of the pot.
I didn't use the SPST switch and 6.8k resistor.

Thanks
/Leevi

[HotBluePlates]

I recommend including the switch and the 6.8kΩ resistor.

Sooner or later, you (or a customer) will find a case where you really need that +4dBu signal level.

With your 22k resistor, 7.3kΩ is the "correct" value to land on 5.5kΩ total resistance for R1; 7.5kΩ is a standard value in a 5% resistor. Using this arrangement, the +4dBu setting will only be about 1% higher than intended.

[PRR]

> We don't know the input impedance of the gear

We must also consider the OUTput we are adding load to.

(In this case it turns out it does not matter; but for completeness....)

The speaker output is (say) 16 ohms. Knowing what we know about speakers, they vary +/-10%, so we assume the amp is fine with a 14.4 ohm load. This puts a minimum added load of about 160 ohms in parallel with the 16 ohm speaker.

We also want to ensure that if the part exposed to the outside world gets shorted (stuff happens), it will not blow-up the amp and preferably not hurt our tone. So we want to be well above 16 ohms. Again we aim above 160. (The one-6V6 Princeton won't blow-up with a dead short, but some amps do; anyway if the PA is shorting-out we'd like the Princeton to still carry the room as best it can.)

Solid-state amps usually do not sag with load, but tube amps often do. In particular the speaker's bass resonance of 50-100 ohms un-sags the tube amp somewhat, the amp/speaker pair has been tuned to allow for this, we don't want to mess it up. We aim well above 100 ohms.

As we see in Sluckey's and HBP's figurings, the first resistor will be  several K or more, so this minimum-load figuring does not crimp our design at all.

> 1.23v RMS ... 0.316v RMS ... ..a standard value in the 1% tolerance

2-digit precision is not warranted. Line Inputs have knobs to tolerate some range of input levels. They will often take a signal 1/10th of nominal and be able to pot-up to reference level.

OTOH, you are figuring on a Princeton's MAX output. Leevi may be playing very soft, below 1 Watt. OTOH he may be using a steel pick and an over-drive, banging "10 Watts" of grinding distortion out of the poor 4W amp.

And you seem committed to a pot.

I'd go low on the voltage ratio (higher voltage), and always start with the pot down toward "3". Noodle at your intended playing level, then ask the engineer if he wants more.

I'd also favor output impedance well below 2K so long lines are not an issue; or "Y"-ing a second 22K recorder onto the line does not cause major level sag.

I call it a 1K pot and 3K (2.2K, 4.7K, whatever is handy) before it.