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12/21/14

Patchbay Neutrik NYS-SPP-L1

Patchbays can help minimize headaches when hooking up your studio. One unfortunate thing about patchbays is that there isn't any industry standard for defining how each connection is described.

PATCHBAY 101 - PART 1

trenton blizzard patchbay neutrik nys-spp-L1
In this post I will be demonstrating the basic connections of the Neutrik NYS-SPP-L1. The NYS-SPP-L1 is a relatively inexpensive 19" rack-mount patchbay that requires 1/4" TRS (Tip-Ring-Sleeve) plugs.

 WHAT IS A PATCHBAY?

A patchbay is a box that is made of jacks and switches. Depending upon how the "Panel Jacks" are internally connected determines how the signals will be routed when you insert your plugs.

WHAT ARE THE DIFFERENT CONNECTIONS?

There are many terms to describe the internal connections of patchbays. Each manufacturer interprets these terms a bit differently. So, it pays to ask a few questions before you buy one. The most common connection term is "NORMAL". Normal describes how the front panel jacks interrupt the rear panel connections when a plug is inserted.

"NORMALLED"

Full-Normal - Both front jacks interrupt the rear connection
Half-Normal - Only one front jack interrupts the rear connection (but you don't which one until told)
Half-Normal (Top) - The "top" front jack interrupts the rear connection
Half-Normal (Bottom) = NYS-SPP-L1 = The "bottom" front jack interrupts the rear connection

trenton blizzard NYS-SPP-L1 half normal

Neutrik identifies the "Half-Normalled" jack with grey plastic. See the first image for connection reference.

REAR PANEL CONNECTIONS

The rear panel connections are historically used for routing signals from the output of your mixer to the input of your tape deck or DAW these days. These rear panel connections are maintained until you insert a plug into the front of the panel (for "normalled" configuration).

MIXER OUTPUT SIGNAL


trenton blizzard NYS-SPP-L1 rear input signal

This oscilloscope image "represents" a signal that could be coming out of your mixer and into the "top-rear" jack on the patchbay. This signal is automatically connected, through front jack switches, to the "bottom-rear" jack as will be seen in the next image.

MIXER OUTPUT ROUTED TO YOUR DAW INPUT


trenton blizzard NYS-SPP-L1 rear panel connection

The pink trace represents the signal coming out of the rear-bottom jack. This is the same signal as the top-rear jack. Keep an eye on the pink trace as I insert plugs into the front panel jacks.

MIXER OUTPUT ROUTED TO "FRONT-TOP" JACK

trenton blizzard NYS-SPP-L1 mixer output routing to front plug

The green trace represents the output signal available at the "top-front" jack. Because the NYS-SPP-L1 is "half-normal (bottom)", the top-front jack does not interrupt the rear signal connections.

MIXER OUTPUT SIGNAL "INTERRUPTED"

trenton blizzard NYS-SPP-L1 mixer output interrupted

Notice that the "pink" signal is gone. Because the NYS-SPP-L1 is "Half-Normal (Bottom)", only the bottom front plug interrupts the connection on the rear jacks.

Note: The mixer output signal is still present at the front-top jack. So, to complete the connection we need to route the signal from the "front-top" jack into the "front-bottom" jack.

MIXER OUTPUT ROUTED THROUGH THE PATCHBAY

Assuming we plugged the "top-front" cable into the input of your compressor and the output of the compressor back into the front-bottom jack, this is how you would "route" the output of your mixer through a compressor, before you go into your DAW.

Patchbays can be very handy.
In a future post I will demonstrate the "SPLIT" patchbay configuration.

Thanks for reading,
Trenton

12/4/14

Vacuum Tube Preamp Stage

Calculating the "AC gain" of a 12AX7 common cathode stage is a fairly straight forward process. Follow along as I go through hand calculations, SPICE simulation and real world testing.

12AX7 GAIN STAGE SET-UP

The signal generator is on the left, the high voltage power supply is in the center, and the oscilloscope is on the right. The circuit was built on an electronics breadboard using a 12AX7 vacuum tube breadboard adapter.


THE CIRCUIT

This is the common cathode 12AX7 gain stage we will be building. It is a common design found in some popular amplifiers. The plate resistor is 100k ohms and the cathode resistor is 1k ohms. The predicted SPICE simulation is show as well.


This stage is "inverting" which means the output will be 180 degrees out-of-phase with the input at mid-band frequencies. The SPICE simulation plot shows the out-of-phase signals. SPICE predicts the output will be about 3.6 Vpk for a 100 mVpk input signal. This is a gain of 36 V/V.

HAND CALCULATIONS

You should always begin your designs with hand calculations. It is a good way to check your understanding of the circuit you are trying to build.


The equation in the top left is what you will use to calculate the gain of an "unbypassed" common cathode gain stage. If your cathode resistor is bypassed with a capacitor, you can remove the Rk(u+1) from the calculation and get much higher stage gain. Note: I have omitted the "minus" sign that indicates "phase inversion".

 ASSUMPTIONS

 The gain equation we are using is based on the following assumptions:
1. All tube parameters are held constant
2. The signals being amplified are small
3. We somehow know the value of internal AC plate resistance "rp"
4. We somehow know the value of the tubes amplification factor "u" mu

I will show you how to get the values for "rp", "u" and "gm" in a future post.
For now, let's assume u = 82 and rp = 40k ohms. We don't need "gm" for this exercise.

 BUILDING THE CIRCUIT

The circuit was built on a breadboard using the resistor values shown in the schematic. The resistors are simply connected to the appropriate tube section with the header pins that are identified on the PCB. Heater connections 4, 5, and 9 are the center three pins. These adapters are designed for dual triodes so the second tube section is connected to the three pins on the right.


REAL WORLD RESULTS

Simulation is one thing. But, reality is what really matters. According to my hand calculations the voltage gain should be about 36.7 volts per volt. That means if we put a 0.1 Vpk signal into this stage we should get an output signal around 3.67 Vpk.


 The blue trace in the image is the 0.1 Vpk input signal. The yellow trace is the output signal which measures right around 3.6 Vpk. Not bad! Notice that the signals are out-of-phase.

DC PLATE VOLTAGE

SPICE predicted the DC plate voltage would be around 151.43 VDC. The acutal Plate votlage measured was 150.5 VDC. Again, not bad at all.

 

DC CATHODE VOLTAGE

SPICE predicted the DC cathode voltage would be around 0.985 VDC. The actual cathode votlage measured was 1.112 VDC. Again, pretty close.


WHERE TO LEARN MORE

If you are really interested in tube audio design here are some books I recommend.

1. Radiotron Designer's handbook by RCA - Totally old and totally awesome.
2. Audio Cyclopedia - Expensive but worth it.
3. Beginner's Guide to Tube Audio Design - Not for "beginners" but a handy reference for equations



Thanks for reading,
Trenton