All things G1176 - the new "repost" thread.

[audiophreak]

Thank you both so much , but my newbiness is still shinning through  :-[

  what you both are saying makes sense , but unclear on how to connect the pot  if I put the pot ,  see attached to see if I have this right ..

 

[gswan]

That's right. The GND symbol near the lower Vgs measuring point means that it is connected to the circuit ground. Best to use a 25turn trimmer pot, so adjustment can be made fairly easily.

 

[audiophreak]

EXCELLENT  !!!!    I got it !!!! 

Thanks again !! 

 

[audiophreak]

Hi gswan,

  I built your test jig , and performed the procedure, not sure the results are correct , could you look at the attachment , its the spread sheet with my readings in the first four devices  , from my first post FETS 4 and 5  are devices 1&2  , and FETS 1 and 6  are devices 3&4
  .. additionally my FETs are the 245A's

  Thanks for all the help

EDIT : I cant attach an EXCEL file so here's a notepad file , and I'll try to PM or email the file if I can

 

[gemini86]

> what I'm matching for now is the G1176

What is a 1176? Is that one of those FET limiters that is a 27K series resistor, a shunt JFET, and a bunch of other junk?

Why are you "matching"? Are you trying to lash two together for Stereo? Are you expecting them to track so well that there will be "no" image-shift while limiting? I suspect that this is extremely difficult; that you are fighting the normal variation from one FET to the next. My inclination would be to find out if there are any dual monolithic JFETs still available, and base your stereo design around the one part. FET parameters are so variable that any precision limiter will have enough adjustment to use about any FET. But doubling-up the adjustments to match two separate FETs gets harder to build and real hard to calibrate.

Set up a test rig similar to the actual circuit:

R1 should be equal to the series resistor used in the actual limiter.

V4 is a DC or AC source. It must be 100mV peak or less to stay within the FET's triode range. A 1.5V battery and a voltage divider to give 100mV will work. Or use an audio signal generator set for about 70mV RMS (100mV peak) output. Read the voltage from ground to the "V" symbol. For the DC source, you need a DC meter that will read 100mV down to a clear indication of 10mV; a 199mV digital voltmeter is barely good enough. For AC source you can read the output with a high impedance ACVM, or a high impedance audio amplifier (direct-box into a mike input) monitored with a VU or PPM meter. The audio technique can read in dB, which is intuitively what we want.

V1 is a 9V battery and a pot so we can put 0V to -9V DC on the Gate. Put a good DC voltmeter on the Gate.

Start without the FET. Trim your source so you get 100mV DC or 70mV AC at the "V" symbol. If you are using a VU meter, first use an ACVM to confirm about 70mV AC, then trim the mike amp so the VU meter reads 0 VU.

Put in an FET and trim the Gate voltage to -9V. You should still have the same voltage at "V". If not, check your connections.

Now trim the Gate voltage toward zero, slowly. Watch point "V". At some point it will drop 10%: 90mV DC, 63mV AC, or -1VU. Write down the Gate voltage needed to make this happen.

Now find the Gate voltage to make point "V" 50% or -6dB.

Now find the Gate voltage to make point "V" 10% or -20dB.

Insert another FET. Repeat.

The 90% or -1dB point is the 1dB Gain Reduction point. If you are just trimming a couple dB off the top, this is the most important test.

The 10% or -20dB point is of course 20dB GR. If you expect zero image shift in sustained heavy GR, this has to match very well.

For general use, all three points must match well. Also all points in between, but JFETs are predictable enough that if you match at -1dB, -6dB, and -20dB, it will probably be close everywhere else.

I used this with good results.

 

[audiophreak]

Thanks gemini86,  ...  so now I have three Different ways to match my FETs     


Thank you all  .... off to make the third test jig  ... and buy more FETs    ;D

 

[gemini86]

the thing I like about PRR's simple approach is that it IS the 1176 circuit that the DUT (device under test) will be living in, so the results you get in the test jig are actual real world results. You can match fets at 1 point, but that doesn't make them matched at all if that isn't the constant operating point they'll be used at.

 

[audiophreak]

Thanks gemini86, that makes complete sense ..... now on to the build, let that matching begin    ;D

 

[gswan]

the thing I like about PRR's simple approach is that it IS the 1176 circuit that the DUT (device under test) will be living in, so the results you get in the test jig are actual real world results. You can match fets at 1 point, but that doesn't make them matched at all if that isn't the constant operating point they'll be used at.

Of couse, this is all ...
The device transfer curve can be measured and matched quite simply using other DC transfer methods, which are probably more accurate. The device in circuit operates over the linear part of the transfer curve from Vbias down to pinchoff, which is why it is a good idea to plot this and see how close the devices are.

 

[audiophreak]

well ....  I've build all three jigs , so I'll check them all on all three , plan on doing the stereo link , so I'll need 4 as closely matched as possible anyway  ;D

  I'll keep you posted , I;m sure I'll have more questions ,
                                                                                                Thank you all !

 

[Chrome Heart]

A quick question about wiring the attack pot with the SPDT switch on the pot. Do I just wire the switch to short pad 22 to ground or is there more to it than that? This is a Rev. J PCB
TIA
CH

 

[JW]

You know, I love the sound of my dual G1176, but I wish that the input wasn't so hot. I generally have to pad down whatever is feeding it, or I can barely turn up the input knob. I have MNats' reccomended OEP transformers wired as the inputs in stepdown fashion. Should I just put an H pad or something at the input or is there a better way to remedy this?

 

[Chrome Heart]

So, does anyone see a problem with using the SPDT switch on the attack pot to short pad 22 to ground? Rev J board

 

[Chrome Heart]

So, does anyone see a problem with using the SPDT switch on the attack pot to short pad 22 to ground? Rev J board

seems to work fine.

 

[audiophreak]

Hi All,
        I'm building a Dual 1176 mnants Rev J  - R54 is listed as a 100k Trimmer , PCB has EDIT :  500k  trimmer and Gyraf has 500k trimmer . I did read on mnats 1176 page that Gyraf may have used / listed a 470r  for my adjustment , I have 1M multi-turn trimmers - would that work ? 

  Thanks in advance , I'm sure I'll have more questions  ;D

EDIT : answered my own question   

 

[sedit1]

Hi!

I'm having a problem with my g1176 rev#7, the unit is working fine but i have a little hum cuntrolled by output gain, so i measured my signal line amp. All the voltages are correct but not in q9

In q9 I have E:14.1v B:13.7 and C:14.1, so the C should be 0.

I'm littlebit confused, because when I took bd136 off all the voltages to earlier components (47u and 4r3) were correct but when I place my bd136 it almost seems like emmiter is leaking 14.1v to collector, I have replaced bd136 two times and the diode measurements are right so bd136 should be ok. I have also checked all the earlier components and they seem to be ok and as I said earlier all the other transistor voltages are like they should be.

Do you have any idea what could be wrong? I'm out of ideas  :-\

Thanks in advance! 

-Matias

 

[audiophreak]

Exactly. The Vgs can be adjusted using a 25-turn trimmer between 0V and -5V, which is more than enough for these FETs. Measure Vgs at the points shown.
After measuring Vds, you can calculate Id using Id = ((+V - Vds)/10) mA
You can use a couple of 9V batteries instead of +/-10V to do this test, just measure the V+ value.

They look fairly close, close enough to be easily trimmed.
Here's what I do for matching. Look at the graph of 10 devices, you can see a fair spread. Particularly look at the drain current when Vgs=0 and where the linear parts of the curve are.

http://www.axtsystems.com/index.php?option=com_content&view=article&id=46:1176lnfets&catid=34:1176ln&Itemid=62

Hi gswan,
              I've been testing the FETs to match , and something just wasn't jiving , now I suck at math and even worse with Ohms Law , but given that equation, and looking at the readings at 0.0Vgs , if using a +/- 10v ..  using 10v - Vds  divided by 10 ... theres no way the answer could be over 1.0 ... yet in the graph on your page all of your tests at 0.0Vgs are 1.4  /  1.36.. 1.42  ??????
  what am I missing ????

  the info I am getting does give me a curve / slope characteristics ,  and I am going to use the other test set up sugested by gemini86  just to try it , hopefully gain some knowledge , and have the most closely matched FETS anywhere .. LOL    ;D 

 

[audiophreak]

Hi All,
    .. O.K. ... this is what I have so far ... I used the nrgrecording test circuit and method to test 40 BF245A's  ,  took the  closest matching FETs , then did the same test , but dropped Uds 100mv at ten intervals and plotted these charts by by hand ... there pretty dang close.

  Then I used gswan's circuit / method on those five FETs and used his handy Spread sheet and got these results ... again .. pretty dang close.

  I am going to build the test procedure suggested by gemini86 on these same five FETs  just to see and compare .. but I think I'm good ...  ;D

  Thanks gswan, gemini86, nrgrecording , and All, for taking the time

 

[audiophreak]

.. and another

 

[audiophreak]

and another