Blog Pages

12/20/13

UNIVERSAL AUDIO LA-610

LA 610 FREQUENCY SWEEP

While my PC is on hiatus from recording I figured it would be a great time to take a look inside my LA610. I absolutely love this thing. It's my "go-to" box for all things recording.











 LA 610 INSIDE VIEW




 LA 610 EQ

I set it up on the bench and ran some simple frequency sweeps to get an idea of how the EQ responded. I would guess it is similar to a Baxandall type circuit due to the gentle nature of the
response curves. Examples of boost and cut at 4.5kHz and 70Hz frequencies are
shown.

T4 MODULE

While I was inside I just had to take a look at the T4 module so I removed the black
cover and pulled it up from its header pins.

T4 PHOTO CELLS


T4 LUMINESCENT PANEL ASSEMBLY

T4 MODULE MOUNTED

I'm still working on getting my PC up and running again. The next install is BFD II. I hope to be back in business by the end of this week.

Thanks for reading,
Trenton

12/2/13

COLORED POWER CABLES

We've all been there.....
Crouched down under the table staring at the power strip in near darkness trying to guess which black power cable is the right one to unplug.

This can be a real problem if you don't want to accidentally unplug the computer that's running your recording session!

So, how do you prevent this? Some people like to label their cords. I like to use colored power cables. All critical items, like my computers, get red cables. 


I ordered mine directly from Stayonline.com at (12-11-14 updated link) www.stayonline.com .
The purchase process was straight forward and they were shipped in a reasonable time frame.

They have eight colors and many different plug types to choose from.
To replace most of my black U.S. power cables I ordered the NEMA 5-15 to IEC 320 C13.

Thanks for reading,
Trenton

11/3/13

1176 DC MEASUREMENTS

1176 & NIXIE TUBE DMM

I started to measure the DC potentials inside my 1176 kit when I remembered I had this old nixie tube digital multimeter. After digging it out from underneath a small mountain of gear I proceeded to dust it off and see if the little beast even worked. After probing around for a while I discovering that it wasn't very accurate when it measured only 9 volts for the negative 10 volt source. So, I decided to abandon ship on that idea.

 CLASSIC VACUUM TUBE SCOPE

But, while I was busy reminiscing into the technical past I decided that I should also use my old Tektronix vacuum tube oscilloscope at the same time. I ran a few different signals through the 1176 until it made a good signal on the CRT.


THE GLOW OF TUBES

This older gear is just too cool! The scope is an absolute engineering work of art! The glow inside the cover was so enticing that I just had to take a quick shot of the tubes.






INSIDE THE NIXIE DMM

The Nixie tube DMM is also very cool inside. It's made up of logic IC's, open frame rotary
switches, carbon composition resistors and a pair of Nixie tubes. How cool is that.....






PROGRESS

Finally, after an hour or so of being completely distracted by all of the old test gear, I did actually manage to make some useful DC measurements inside the 1176. I will post them soon.

Please stay tuned,
Trenton

8/5/13

CINEMAG 1176 OUTPUT TRANSFORMER

I just received my 1176 output transformer from Cinemag.

This little gem will be going into my next 1176 project build.


According to David Geren, this transformer is just like they built them for UREI back in the day.

I am definitely looking forward to hearing what it sounds like!

As always, stay tuned for more.
Thanks for reading,
Trenton

7/20/13

PULTEC PASSIVE EQ CLONE

PULTEC EQP-1A 

Just finished building my PULTEC 3U rack enclosure from http://collectivecases.com/pultec.html.
I'm very excited to get started on this project. This will be a clone build of a classic passive equalizer! My printed circuit board came from http://www.dripelectronics.com/.

COST
Rack cost was around $130 not including shipping. The transaction was a breeze. Also, it was shipped in a reasonable time frame considering that when I purchased it they still had to be painted.


FEATURES
1. (2) XLR and (1) IEC power inlet cutouts on back panel
2. Sides are vented
3. No holes on top or bottom panels (You will need to drill those yourself)

TWEAKS
I needed to enlarge the front 4 mounting holes a bit in order to get things to align.

QUALITY
Build quality is very good using heavy gauge steel with an aluminum front panel.
Fit-N-Finish was also good. Note: Make sure to bolt your front panel on first for good alignment.

As always, you will find my build information right here so please stay tuned!

7/5/13

1176 FREQUENCY RESPONSE

BOAT ANCHOR TUBE SCOPE
While making scope measurements I thought I it would be cool to fire up an old Navy vacuum tube oscilloscope and see if I could use it for my images.

But, I changed my mind pretty quickly.

After fighting to get a signal to show up after waiting 15 minutes to warm up, I decided to take the easy road.

But, it certainly makes for a better photo!


TRANSFORMER ABUSE IS NOT A CRIME
Ten hertz at 20 volts peak is absolute hell on small signal audio transformers.

The poor core has to start giving up somewhere! I felt very sad for it but now I'm over it.

Wow! It looks nasty but I'll bet it sounds really cool on some recorded Godzilla foot steps!


CLEANING UP YOUR ACT
This 1176 clone cleans up really fast above 20Hz. This is still at about 20Vpk output!

Seriously though, 20Vpk at 20Hz, this is crazy but fun. That's why we do this stuff !


FROM DC TO LIGHT
O.k., it doesn't do that. But, up to 20kHz and beyond looked really flat.

It started to roll off around the upper end of the audio spectrum.

The peaks are now just under the 20V division mark on the grid.

Again, no one in their right mind is ever going to run it this hot. At least I wouldn't think so...


COMING SOON
Next up will be some DC measurements around the circuit. Stay tuned!

6/29/13

1176 CLONE COMPLETE

FIRST POWER UP

Meter glowing with no smoke showing, looks like we are good to go!

With no obvious way to connect power to the meter lamp I used a large value power resistor to drop the voltage down from the unregulated voltage to 12V.

The resistor gets a bit warm but I don't care. The heat reminds me of vacuum tubes so that's o.k....


REFERENCE SCHEMATIC

When you build any type of kit you should always have an accurate schematic to reference.

I used a copy of the original UREI Rev D that I downloaded from the JBL Pro Audio site.

It's a huge reference manual filled with 1176 information. Thank you JBL!

 




CIRCUIT BOARD CARNAGE

I cut and I jumped and I blew this board apart. I dug into this circuit board so much that it looked like a hurricane hit it after I was done!
 
My goal was to make real AC and DC measurements during the build process.
 
Everything will be posted here eventually so keep checking back. I will be documenting my next 1176 build as well. I am planning that build for 2014.
 
 

GAIN REDUCTION RECTIFIERS

Measuring the output of the rectifier stage definitely surprised me.
 
It is not even close to what SPICE had predicted.
 
I need to go back and see what's going on. Either my model is crap or my circuit build is crap but most likely not both.
 
 

MAXIMUM OUTPUT

Measuring the output showed about a 20 volt peak output before clipping.
 
That's 40Vpk-pk ! You're not going to be slamming that into your DAW.
 
We'll, not without extreme converter clipping....
 
 

1176 KIT FIT & FINISH

Overall the Hairball Audio Revision D kit totally rocks. It is definitely not a kit for absolute beginners but those with some build experience should be able to push through it.
 
There were a couple of issues that could have been better though: For example, the output transformer wires didn't reach the XLR jack and the power transformer wires wouldn't reach the PCB if it was located at the other side of the enclosure.
 
Not huge issues, but rather final touches that would have made the build go a bit smoother.
 
Again, this kit is a great deal. I think the total build cost for me was around $600.
Remember though, these kits aren't going to perform like the real deal. So don't expect it to!
 
 

TEST CONNECTORS

Both inputs and output connections on an 1176 are balanced. This is the way Pro Audio gear is designed!
 
However, balanced XLR connectors make it very difficult to connect to standard electronics test equipment which is typically un-balanced.
 
What makes testing a breeze are XLR to 1/4" adapters. I picked these up at a local San Diego electronics supply store.
 
They cost about $10 ea. But, they make testing so much easier that it is totally worth the investment. You will need one XLR-M and one XLR-F type adapters.
 

 
That's it for now. As I go through my images of the build I will post them here.
 
There's a lot more to come so please stay tuned.
 
Thanks for reading!
Trenton
 

6/10/13

1176 Gain Reduction Stage

The gain reduction stage of an 1176 is made up of two phase inverting amplifiers followed by rectifying diodes. The circuit is shown here as simulated. The 47k resistor in the feedback loop, near the bottom of the schematic, helps keep the overall circuit gain within usable limits.

The first stage is made up of a common emitter stage followed by a buffer. This stage feeds the bottom side rectifying diode through the 6.8uF low leakage capacitor.


Another common emitter stage follows the first one. This stage feeds the upper rectifying diode through the other 6.8uF cap.


Notice that the second stage is padded down with a182k resistor. Not only does this provide DC coupling and bias for the next stage, but it also reduces the signal level to the second stage. Without this pad, the second stage would over amplify the signal beyond what the feedback could correct.

Gain reduction in an 1176 happens when the positive voltage that is created by rectifying the signal is added to the negative JFET bias. This positive voltage turns the JFET "ON". The output rectifier section shown next. The diodes are low leakage types and the capacitors should be low leakage as well to help maintain a stable DC bias point for the JFET. The two 47k resistors create an AC ground through the 100uF capacitor for the rectified signal. They also provide a means of connecting the negative bias voltage (not shown) to the Anode side of the two diodes for proper circuit operation.


The rectified output signal calculated by SPICE is shown next. This signal is for both Attack and Release controls at their fastest settings. Notice that the signal starts at -4VDC and goes up towards 0V. I added a negative bias (not shown in the schematic) to help model real world results. During the time when the rectified output is equal to 0V the JFET would be fully "ON" in this example and maximum signal reduction would be happening. This of course is assuming that -4V is the required bias voltage for the JFET during normal operation. We shall test that really soon....


I will measure my clone 1176 and take oscilloscope pictures to compare real world gain reduction stage performance with simulation very soon. Stay tuned!








6/8/13

1176 JFET DIVIDER STAGE

I was going over the schematic the other night and was wondering to my self what the total signal reduction is at the JFET divider stage. What we have is a basic voltage divider circuit with a JFET as part of the variable resistance.

When the JFET is totally "ON" there will be a very small Drain-Source resistance (RDS On) between the 27k resistor and the 10k resistor. For simplicity, let's assume it's ideal and RDS On = 0 ohms. Then we have:
At this point we can calculate the total signal reduction for the JFET Divider stage:

Total signal loss is almost -11.5db. A SPICE simulation calculates the loss to be:

This is the total signal reduction that can be expected from the JFET Divider stage.
Remember, this is only the loss of the JFET stage. This does not include any loss from the input transformer or the input attenuator.

I will run some actual tests and compare the results soon. Stay tuned !

6/5/13

1176 PREAMP RESPONSE SIMULATION

Here is a plot of what SPICE predicts the frequency response will be for the preamplifier stage in the 1176. The low frequency rise is most likely due to my inaccurate modeling of the source impedance by not taking the transformer secondary impedance into consideration.

It could also be the feedback network not having enough feedback at low frequencies. Anyways, I don't really care because the only thing that matters is how it responds in the real world. I will compare this with real test results after I run it on an Audio Precision. Stay tuned!

6/3/13

1176 Preamp Stage SPICE DC Voltages

Update: New Schematic that includes DC values at node locations. Real circuit values to follow very soon.

5/24/13

PULTEC EQ

Next up after finishing my 1176 clone will be a Pultec clone based on a PCB from DRIP Electronics . I am looking forward to building this classic EQ. Build details will follow. Also, the 1176 is in its final build stages so details will follow very soon!