All things G1176 - the new "repost" thread.

[Janus Audio]

Hey. I have a problem with my nmats 1176. Posted it to a wrong thread though:

Hello erverybody.

After hours of trying to fix my mnats 1176 I'm again prone to give up; so I hope you guys have some ideas what to check, to find the problems.

The facts:
It's a mnats 1176 rotary switch version.
When I switch ratios, I only get small changes in compression.
It does compress though: Sine wave output changes when adjusting the q-bias pot.
I checked all the voltages in the input and GR control amps - nothing unusual.
The unit used to work fine, before I upgradet the input sections from IC to transformer input.

What seems strange:
The ouput pot affects the feedback AC (at point 22). Is this normal - the wiper only connected to the ouputsection?
Switching to "Bypass" doesn't affect the compression/ output level. I checked all the swiched for shorts etc. - no problem there.

So if you any ideas - would be great.
And are there any additional things I can ckeck (resitances, AC voltages maybe)?

Thanks, Sebastian

New facts:

AC voltage in the ratio switch chandes properly, but the generated DC voltage at the gate of the FET does not (stays around - 1,5V), so there must be a problem in the control amp section...

 

[Janus Audio]

Ok, I measured the AC voltages in the control amp section with a sine wave applied. The amplification is poor, so this is my problem. Probably one of the BC107s is blown. Any idea how to find out which one; the DC voltages are ok, as already wrote. Or is there another possible reason for the poor amplification; broken caps or so?

 

[ricothetroll]

Hi !

I'm calibrating my dual G1176. I use a quartett of 2N5457 I personnally matched. I used that circuit for the matching procedure :
http://www.wuala.com/ricothetroll/public/Fet_Matching.png/
I switched the values of R1 to have VGS = -3V, -2V, -1V, -0.75V, -0.5V, -0.25V and 0V (I actually duplicated the circuit 7 times on the breadboard so I only had to move the transistors from one circuit to the other for each measure). Here are the curves of the quartett I chose among 50 specimens :
http://www.wuala.com/ricothetroll/public/Fet_Matching_Curves.png/
I chose those 4 because the have the most progressive curve and are also pretty well matched (better than one dB on the whole range, Vds(dB)=20log(Vds/15) ).

While calibrating the unit, I encountered a few issues :

- Q Bias setting is VERY sensible ! Once the fet starts conducting, the marging between 0dB and -20dB is like on 1 degree turn... Is that normal ? The trimpot is also working reverse compared to the calibration procedure found here (he's using Mnat's PCB so that might be the reason). Should I replace it with a multiturn one ? Or maybe the fets I chose don't have a soft enough curve ?

- Meter tracking adj : I can't get the meter to reach the -6dB indication at a measured 6dB gain reduction (Gyraf's procedure). When the trimpot is totally CCW I'm reading -5dB one ch1 and -3dB on ch2. I though I could reduce R53 from 15k to 10k but I think the problem is somewhere else and I just don't know where. The meter I use is Hairball's HB 8037.

Best regards.

Eric

 

[Biasrocks]

Eric

All your trim pots should be multi-turn, makes it much easier to do a calibration. I've got a build the I inherited from someone that uses single turn pots and it drives me crazy.

Mark

 

[ricothetroll]

Hi Mark,
Thanx for your answer ! I'll definitely make the multiturn. By the way, does anyone know some multiturn pots that fit the 10mm Piher print that Gyraf used on his PCB ?
Does anyone has an idea concerning my meter calibration problem ?
Best regards.
Eric

 

[ricothetroll]

Well, I finally switched R53 from 15k to 5k6 and now I can calibrate the meter properly !
Then I did some measures of Vin/Vout and both compressors react pretty much the same way, excepted that i have a 1dB difference of threshold, but I think that could be solved by setting Q Bias more precisely with a multiturn pot.
Best regards.
Eric

 

[sr1200]

Im really confused about the whole revisions thing.  Perhaps someone can save me from pulling ALL my hair out lol.

I got a board from MNATS.  It says Rev J on the actual board. (green board)  I was under the impression that the A, D and F boards were different and the G1176 (J board) was for the G build. (hopefully i got that right lol...im litterally chuckling to myself as i write this)

So.. I ordered the G kit from hairball and ordered the BOM from the link on hairball for the G.

Only problem is that some of the components listed dont match or dont exist on the board. (ala R5... its listed as a pot  on the MNATS BOM and its listed as both a POT and a 270R on the hairball BOM. R87 is listed twice (and there are 2 R87s on the board one is a 150R and the other on one of the smaller boards is 120R.  And R84 is not listed at all on the Hairball BOM.)

So researching further, i went to the MNATS site and printed out the BOM there.  this one stated for the G and H build! (yay another letter!)

So my question is: did I buy the right board for a G build. 

I appreciate any light that can be shined on this... I think im going to order a rev D next just to avoid confusion LOL.

 

[Biasrocks]

So my question is: did I buy the right board for a G build. 

I appreciate any light that can be shined on this... I think im going to order a rev D next just to avoid confusion LOL.

You're good, MNat's "REV J" can be built to a number of different versions (revs F,G) of an 1176.

The main difference is that REV F uses a transformer front end and the REV G used an IC front end.

I imagine the Hairball BOM is for the REV F w/transformer front end.

Stick to the REV G bom and you'll be good.

Mark

 

[kepeb]

hello people.
i will be attempting anytime soon to build my dual unit.
have a question regarding inputs.

was considering mounting a switch ready for a selectable transformer/balanced ic input.
are there any problems just mounting ALL the components onto the (j)boards and just using a dpdt's to direct the input signal accordingly? or will i need to alter any traces?
anyone done this?

 

[gyraf]

both input-options can be installed at the same time, no problem

 

[ArnauTS]

Hey all,
i build a revision J and read a lot of pages but not found and answer for my problem.
After the calibration it's done i get hum and distortion. I'm sure it's something around the compression circuit, if i turn the bias trimmer off the unit pass the audio/tone.
After a long time checking voltages and all it's seems that i not see the obvious error.
If anyone can give me a pointer to look at, i will apreciate it.

 

[gswan]

After the calibration it's done i get hum and distortion. I'm sure it's something around the compression circuit, if i turn the bias trimmer off the unit pass the audio/tone.

What do you mean by 'hum'? You may need to look at the output with a scope to see what is happening to your signal. Do you perform the entire calibration successfully? Have you over-biassed the jfet and attempt to compensate with too much gain?

 

[ArnauTS]

After the calibration it's done i get hum and distortion. I'm sure it's something around the compression circuit, if i turn the bias trimmer off the unit pass the audio/tone.

What do you mean by 'hum'? You may need to look at the output with a scope to see what is happening to your signal. Do you perform the entire calibration successfully? Have you over-biassed the jfet and attempt to compensate with too much gain?

The entire calibration performed just fine but the audio out it distorts and i have hum too 60Hz and it increases obiously if i turn up the output pot.
If i turn off the bias of the fet i not have any problem but i have no compression sure.
Hope you guys understand me..

 

[gswan]

If all the DC voltages checked out OK and your PSU is ripple and noise free, and the calibration performed OK then check the gains of each of the stages with compression bypassed.
You may find more info here:
http://www.axtsystems.com/index.php?option=com_content&view=category&id=34:1176ln&Itemid=62&layout=default

The AC hum can be caused by proximity to stray fields, wither through wiring or the transformer. Try moving the AC wiring and transformer away from the audio modules and wiring and see if it makes a difference. Also make sure that your chassis is properly grounded (including the front panel) and that your audo signal ground connects to the chassis ground at one point only (usually at the output socket or PSU ground).

 

[ArnauTS]

If all the DC voltages checked out OK and your PSU is ripple and noise free, and the calibration performed OK then check the gains of each of the stages with compression bypassed.
You may find more info here:
http://www.axtsystems.com/index.php?option=com_content&view=category&id=34:1176ln&Itemid=62&layout=default

The AC hum can be caused by proximity to stray fields, wither through wiring or the transformer. Try moving the AC wiring and transformer away from the audio modules and wiring and see if it makes a difference. Also make sure that your chassis is properly grounded (including the front panel) and that your audo signal ground connects to the chassis ground at one point only (usually at the output socket or PSU ground).

Ok will try to check the gains stages and if i not find the problem i will ask more questions.
Thanks! ;D

 

[beatnik]

Hi there,

One channel of my dual mono 1176 (mnats rev J boards) stopped working.

My guess is that I could have done something bad in the calibration procedure, like feeding it with a too high level test signal. Because in the beginning was all ok.

I've made a lot of search in the forum, and tried to resolving it by myself. The other channel is working, so this is a great help because i can compare the two units. But, I have not enough electronics knowledge, so I am asking your help to figure out where the fault is.

In fact the unit has a very low and distorted output. Can't perform calibration because can't reach +1 on the vu meter when turning the input pot up.

Power rails are ok

I feed the input with a 1kHZ 0dB sine and traced the signal starting at the ic input stage, with FETs out of the circuit

- compared to the other channel the first strange thing is no signal on pins 2,3,5 and 6 on the NE5532. Of course I have replaced the IC, with no results

- input pot works (can vary the signal amplitude on pin2 of the pot with turning the pot)

- i have signal on Q1 gate, but on the source signal is very low, compared to the other channel (both without the fets)

- the signal on Q3 base and Q4 emitter is very low too. signal at output pot very very low.

My first guess was some transistor is fried, so i replaced all BC107 (i had BC109 on hand) with no results

Here are the voltages on the transistors. Please let me know if this is enough for localize the error, or what additional measurements i should do. Many thanks in advance

                B            C            E
Q2          1,53          2,46      2,46
Q3          2,46          1,94      1,85
Q4          1,93        30,3        2,46
Q5          4,94          28,6        4,51
Q6          28,6          14,6        29,1
Q7          29,1          14,6        29,7
Q8          14,2        30,3          14,8
Q9          14,1          0            13,5
Q12        4,6            15,1        4,8
Q13        7,2            30,3        6,7
Q14        2,54          28,1        2,04
Q15        25              30,3      24,5

              G                S            D
Q1          0                0            -1,6
Q10        -1,39        -9,41        21

 

[gswan]

One channel of my dual mono 1176 (mnats rev J boards) stopped working.

My guess is that I could have done something bad in the calibration procedure, like feeding it with a too high level test signal. Because in the beginning was all ok.

Highly unlikely

I feed the input with a 1kHZ 0dB sine and traced the signal starting at the ic input stage, with FETs out of the circuit

- compared to the other channel the first strange thing is no signal on pins 2,3,5 and 6 on the NE5532. Of course I have replaced the IC, with no results

That's where the problem is then. Try looking at the cable and connector. Is it shorted?
Can you get any signal on the input pins of the board?

 

[beatnik]

That's where the problem is then. Try looking at the cable and connector. Is it shorted?
Can you get any signal on the input pins of the board?

Thanks for pointing this. I've checked the cable and connector. No shorts between xlr pins and the signal goes into the board, connecting to the 100pF and C30/C31 capacitors.

Considering that i have removed the fets, the device should act as a line amp, with no compression. But the signal in the pre-amp is very low, so i suspect that something is wrong in the gr control amp, permanently lowering the signal. (Please correct me if i'm wrong, i'm trying to understand the circuit)

Even more, my voltages look very bad in those sections, signal pre amp and gr control amp. The others look almost right. Voltages on NE5532 looks right too, compared to my working unit.

However, I don't have a clear idea of what's happening, I hope in more suggestions. Thanks

 

[gswan]

Thanks for pointing this. I've checked the cable and connector. No shorts between xlr pins and the signal goes into the board, connecting to the 100pF and C30/C31 capacitors.

Considering that i have removed the fets, the device should act as a line amp, with no compression. But the signal in the pre-amp is very low, so i suspect that something is wrong in the gr control amp, permanently lowering the signal. (Please correct me if i'm wrong, i'm trying to understand the circuit)

Even more, my voltages look very bad in those sections, signal pre amp and gr control amp. The others look almost right. Voltages on NE5532 looks right too, compared to my working unit.

However, I don't have a clear idea of what's happening, I hope in more suggestions. Thanks

If you've taken the FETs out, then the GR amp can have no effect whatsoever on the input signal.

Apply a signal at the input and measure the signal at the top of the INPUT pot (1).
If you don't get a clean signal there then you need to examine the input balanced amp (NE5532). Check PSU volts on the device (+30V on pin 8 ) and the half-rail bias on pin 3 (+15V). Check the output of the inverting stage (pin 1) for signal. This is summed with the inverted signal from the XLR and the buffered signal can be seen on pin 7.

 

[beatnik]

thanks, this makes sense, and helps me to learn the circuit a bit more.

i've done the tests you suggested

The signal comes to the input pot pin 3, and goes out to pin 2. Turning the pot makes the signal on pin 2 louder, and it's distorted only when pot is at max, but i think is normal behavior.

voltages on ne5532 look right:

pin1  15,26
pin2  15,26
pin3  15,24
pin4  0
pin5  15,21
pin6  15,24
pin7  15,26
pin8  30,3

I have sound on pins 1 and 7.

Going through the pre-amp section, signal stops at Q2 base. I have no signal on Q3, Q4, Q5. The signal re-comes around Q8/Q9 in the line amp, but it's very low.

I guess my problem is in the pre amp section, but i have replaced the transistor two times with no results. of course i even checked the solderings and the wiring, all is ok. Should i try replaceing the electrolytics too? What you suggest?