All things G1176 - the new "repost" thread.

[gswan]

[quote author="kenrinc"]
Q1 has .005 at b, .375 at c and .001 at e. I haven't made the setup to match FETs yet so haven't gone through that process.
[/quote]

Q1 is a FET and so has a G, D and S terminal. Which terminal is at 0.375V?

It does not look as though the FET is biased correctly. It looks like it is fully on (ie full conduction) in which case it is attenuating your input signal.

The DC bias conditions look OK on the BJT's.

Disable the compression (or remove the Q1 FET) and perform a measurement of a 1kHz signal through the amplifier stages first, to verify the amplifier operation.

Then insert the FET and perform the calibration. Vgs should be somewhere between about -0.2 and -1V (this may vary a bit depending on the FET characteristics and type).

 

[gswan]

[quote author="ilikeit"]
i tested the PSU i have 30.6v & -9.83V DV
i checked all my number between the PCB it s ok
i checked my Q
q2 1.036 1.75 0.545
q3 1.755 12.56 1.166
q4 12.56 30.56 11.99
q5 4.943 28.79 4.507
q6 28.80 15.00 29.35
q7 29.35 15.00 29.95
q8 15.00 30.56 14.4
q9 13.68 0 14.27
q12 4.515 15.07 4
q13 15.06 30.56 14.50
q14 4.473 16.71 2.93
q15 16.7 30.56 16.2

and what is the E connections near Q8 & Q9?
[/quote]

A couple of readings look a bit out, probably due to the extra 0.6V on the 30V rail (eg Q6-c, Q14-b).

I'd suggest you now disconnect the FET (Q1) and perform an AC signal check through the amplifiers to ensure they are working correctly.

The 'E' connections near Q8 and Q9 are for alternative parts that have different pinout configurations.

 

[kenrinc]

I checked the BF245A FET and got S = .001 D = .001 For the G it started at -.125 and then moved up to around -.400 where it stayed. So remove Q1 and check signal through the unit?

ken

 

[gswan]

That looks kind of close to 0, however it will depend on your FET characteristics. Did you plot the FET characteristics (Vgs vs Id)? That will tell you where you are on the curve. Too close to zero and it means the FET is too far on and you are attenuating your signal too much.

It's a good idea, if you have the equipment, to check the signal through the amplifier stages without compression to make sure you hava a clean path and everything is operating satisfactorily before introducing compression.

 

[kenrinc]

gswan,

So I'm curious, is the idea to find two matched FETs for Q1 and Q10? I thought the Qbias adjustment was there to set the bias on Q1 ? I've got a bag of them so I just need to setup the matching fixture. I can get signal through the unit with GR off. I just need to come up with a better way to get signal in since right now I'm just using a little portable CD player and I don't think it's driving the input correctly which is why the level is so low. Trying to borrow a function generator from a friend.

Ken-

 

[gswan]

They don't have to be matched, since there is the separate tracking adjustment on the metering circuit. Qbias applies to both FETs. It helps in calibration if they are close though.

It's best to use the correct test equipment to align the unit and check it for problems. I'm surprised that the line level from your CD player is not sufficient to drive it. It should be more than ample.

 

[kenrinc]

gswan,

OK, worked on this a bit more. It looks like I can get some heavy signal in via the headphone jack. I hear output and level is effected by turning either output or input (both controls work). I can actually deflect the meter now in +4 out mode and can perform the Q-bias adjustment but does the reference signal have to be at exactly 0db? I'm not sure how to measure that coming out of the CD player. When I try to measure it I get nothing on the meter.

I can't complete the gain reduction meter tracking because my pot runs out of travel before the meter reaches 0 Pot is the one on Mnats bom (2.5k I believe).

Ken-

 

[gswan]

[quote author="kenrinc"]I can actually deflect the meter now in +4 out mode and can perform the Q-bias adjustment but does the reference signal have to be at exactly 0db?[/quote]

No, you are using the signal pre-amp section (which has 26dB gain) to amplify your input signal.

You need to wind out the Qbias so the FET is totally off, have the output pot fully up and inject a signal at the input.

Turn up the input pot so you are reading 0dB on the meter (switched to +4dBm).

Now you have a signal reference to work with so you can adjust the qbias slowly until the FET starts to conduct and you get a 1dB drop in output signal (shown on the meter).

I'm still curious as to why you are getting such a low signal with a line level output from the CD player. Something sounds fishy here. This unit is designed to work with line level signals, so if it does not work with your CD player, it's not going to work too well with your mixer.

 

[kenrinc]

Part of it is probably the wiring from the headphone jack to the XLR in on the 1176. The cable is stereo 1/8" mini to XLR male. I just checked the wiring and I see L and R wires are going to pin 2 and 3 respectively. Not sure why I wired it this way. Can I just take the Left or Right signal only for mono? Or is there some way to sum them?

Just did the Qbias calibration and it worked. Now that I've done this, when I switch the unit to GR mode the meter pegs hard to the right.

Ken

 

[gswan]

[quote author="kenrinc"]Part of it is probably the wiring from the headphone jack to the XLR in on the 1176. The cable is stereo 1/8" mini to XLR male. I just checked the wiring and I see L and R wires are going to pin 2 and 3 respectively. [/quote]

OK, that's why you don't get much signal. You are cancelling all the common mode signal between the L and R. Try using one channel and using the GND wire as well (hook GND and L to the transformer input pair). I assume you are using a 1kHz sine wave test CD to perform the calibration.

[quote author="kenrinc"]Just did the Qbias calibration and it worked. Now that I've done this, when I switch the unit to GR mode the meter pegs hard to the right.[/quote]

Better go back to the Qbias calibration procedure and repeat it using a proper input signal wired correctly.

 

[kenrinc]

Jeesh, I don't know what's going on. If I take just the Left signal and GND at the line out of the CD player and the input all the way up on the 1176, I get -5 on the meter. I tried both L and GND and R and GND but either way the level was the same. Yes I'm using a 1khz tone off a test CD. I guess I'm going to need to stop and get this part figured out because I need to be able to test other DIY gear I may build in the future. This is my first build so I appreciate the help.

I thought about if the input transformer might be wired wrong but it's installed correctly (2:1, with traces cut and jumper soldered).

Ken-

 

[gswan]

[quote author="kenrinc"]I thought about if the input transformer might be wired wrong but it's installed correctly (2:1, with traces cut and jumper soldered). [/quote]

You can always check this, but you'd need either a scope or a good audio mV meter. Measure the signal on each side of the transformer and the voltage ratio will give you the turns ratio directly.

 

[kenrinc]

gswan,

I was working on this again tonight. The trannie looks OK wired as 2:1 but I decided to try something else. I rewired the cable coming from the line out of the CD player (1/8" stereo to XLR) and wired L channel to pin 2 and GND to pin 3. In otherwords I did not use shield pin on 1176. Now I have a huge amount of clean output. So much so that keeping the input knob off and turning the output knob just to about 2 is off the charts pegged! :shock: Does that sound right?

ken-

 

[gswan]

[quote author="kenrinc"]I rewired the cable coming from the line out of the CD player (1/8" stereo to XLR) and wired L channel to pin 2 and GND to pin 3. In otherwords I did not use shield pin on 1176. [/quote]

That's what I was telling you to do. Wire your signal across the transformer input.

[quote author="kenrinc"]Now I have a huge amount of clean output. So much so that keeping the input knob off and turning the output knob just to about 2 is off the charts pegged! Does that sound right? [/quote]

Nope. The input knob fully CCW should give no output, regardless of the position of the output knob. If it does then your amp stages are either full of noise or they are oscillating.

You will need to check through the stages with a scope and see whether you have too much gain or other problems. The first stage should be about 26dB and the second stage about 10dB.

-Geoff

 

[3nity]

guys help me out i cant seem to find R50 on mnats Rev J. anyone?

 

[s2udio]

[quote author="3nity"]guys help me out i cant seem to find R50 on mnats Rev J. anyone?[/quote]
mounted on the meter switch 8.2k :?

 

[3nity]

thats R51 not R50

 

[s2udio]

[quote author="3nity"]thats R51 not R50[/quote]
Not on this schemo m8
G Swans pdf
R50 8.2K +8 METER SW
R51 3.6K +4 METER SW
http://www.axtsystems.com/index.php/projects/34-1176ln/45-1176lncircuits

 

[3nity]

i dont get it R50 is 8k2 and R51-R59 are 3k6.???

 

[gswan]

[quote author="3nity"]i dont get it R50 is 8k2 and R51-R59 are 3k6.???[/quote]

R50 is 8.2k, which sets the meter range to +8dBm and 3.6k for the +4dBm meter range.
Why would the other resistors be 3.6k as well, when they are clearly marked as something else (in the GR metering circuit)?