Blog Pages

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.