FET "Grinder"

[Gustav]

If you are using a different transistor, Google the data sheet and check the pinout.

I might have reversed the viper on the Q-bias pot compared to the original. Just turn it to the position that doesn't affect the signal and adjust from there. (wether thats CCW or CW doesn't really make a difference, and its not something I even notice when working on the the unit)

Dirt pot - I thought I answered that, so I am not sure what you are looking for!?  Don't bother with it, just put it about 2/3 up or drop in the trimmer (footprint on the PCB included) and trim it.

Hope that helps!

Gustav

 

[Gustav]

Latest build...new PCB

 

[Mazieresantoine]

Hi

I am doing the fet grinder
I am a begginer

I buy this meter but i don't know who to make it workshop
https://pcbgrinder.com/index.php?route=product/product&path=94_95&product_id=101

To have it in vu i need à diode bridge but which one?

And when i want to see g.r. nothing happens

Any help?

Thanks

 

[gyraf]

Yes, you will want to use a diode bridge with that meter.

You will want four diode with low forward drop.

I use schottky diodes - either BAT81 or 1N5819 - works fine with that meter

Jakob E.

 

[Mazieresantoine]

Thanks for your quick response.

It's working, but now i have a problem with the sound : no bass, like i have a high pass filter ON.
it the same if the comp is bypass or not

 

[Gustav]

Thanks for your quick response.

It's working, but now i have a problem with the sound : no bass, like i have a high pass filter ON.
it the same if the comp is bypass or not

Compression working? Unit seems to be working?

For most problems, I would start out by verifying voltages, but in this case, I would start looking at how and what you are connecting the unit to.

So how and what? And are you using the Lundahl output transformer that drops into the PCB or something else?

Gustav

 

[Mazieresantoine]

Yes the compression is working, everything seems to work, it's just a sound problem, no bass.

I have my PSU board with 2x30VA In and +30V/-10V out, i plugged this to main board, and yes I use the lundhall for audio transformer

i can send you picture if you want.

thanks for your help

 

[kante1603]

Yes the compression is working, everything seems to work, it's just a sound problem, no bass.

I have my PSU board with 2x30VA In and +30V/-10V out, i plugged this to main board, and yes I use the lundhall for audio transformer

i can send you picture if you want.

thanks for your help

Hello and welcome to the forum.

I agree with Gustav to look at the transformer first,maybe the input stage next.
Thin sounding signals often lead to a missing in- or outphase signal wether at the input debalancing circuit or the output,in this case the transformer.
I'd check the transformer first by measuring the windings for dc resistance.

Best regards,

Udo.

 

[Mazieresantoine]

How can I check the Lundhal?

sorry i'm starting the DIY...thanks for your help

 

[kante1603]

How can I check the Lundhal?

sorry i'm starting the DIY...thanks for your help

At least the output side going to the xlr connector.
Set your dmm the reading in ohms,measure at xlr output between pins 2 & 3.
You should get a lowish reading there,not zero.

If all is good then you could measure levels from in to out.
But let's check the transformer because it's easy at the pins as described.
From the schematic I can see a 1k resistor in parallel to the windings.

You said the behaviour is the same both in active and bypass mode,right?
So it should be an issue in the i/o hopefully.

Tell us what you get in ohms first,

Udo.

 

[Mazieresantoine]

thanks for your help,

i will check it tonight or tomorrow, and i come back to you

 

[Mazieresantoine]

So i check the resistance at XLR output and i get 54 ohm

 

[Mazieresantoine]

And yes it's the same sound if i bypass or not the comp.
But the comp is working when i put it on, we can hear the compression

 

[kante1603]

So i check the resistance at XLR output and i get 54 ohm

O.K.,that is about correct,2 x 30 Ohms in series=60 Ohms,these in parallel with the 1kOhm give you 56,6 Ohms after the math.The slight difference is coming from tolerances of the transformer and your DMM loading it.All good so far.
For sorting out a transformer issue completely you can do a quick measurement on the opposite side of it at pins 7 & 10,should be arround 13,8 Ohms.
Here the spec sheet for your reference:

https://pcbgrinder.com/download/Data_Sheets/LL5402.pdf

I bet it's o.k.,

After that we can do a level check from input to output.

Udo.

 

[Mazieresantoine]

between 7 and 10 i've got 13,8/14 ohm

 

[kante1603]

between 7 and 10 i've got 13,8/14 ohm

Excellent.
Although your issue is not sorted yet it means that the most expensive part is healthy!
Btw.,I haven't built this piece,I'm just thinking about it.
So next step can be a level check from in to out.
E.g. a level drop of 6 dB would lead to either a faulty chip or a missing/bad connection in the debalancing stage.
Hopefully you have checked all solder points in that area,so cold joints or solder bridges?
Are the correct parts in the right places?
Oh,and did you do the calibrations so far?
For level check you need a generator to get a sine wave and your DMM for looking if you have unity gain.As a generator you can use your daw/interface.The signal must be balanced.
Check your DMM up to which frequency it can read ac voltage(in the manual,technical specs).
Check your software if it has a generator,most have one is an insert.
Check your interface what 0 dB refers to.
Then let us know.

Best,
Udo.

 

[Mazieresantoine]

i will do it tomorrow with my sound card.

On my DMM tech specs, it say : Alternative voltage measurement 40hz to 400Hz

So i generate a 200Hz and i see what happen?

I do the calibration, but it's for the VU?
I check the BOM and everythings is at the good place.

 

[kante1603]

i will do it tomorrow with my sound card.

On my DMM tech specs, it say : Alternative voltage measurement 40hz to 400Hz

So i generate a 200Hz and i see what happen?

O.K.,generate a sine wave lower than 400Hz and bring the output of your soundcard to a defined level,say +4dBu as it´s the studio standard.
Before pluging in the xlr to the compressor you can verify it by measuring between pins 2 & 3 again,it should give you a reading of 1,226 VAC (DMM set to AC reading of course).
Now plug it to the compressor,set it to bypass mode and measure the output xlr.You should get roughly the same voltage as on the input.If it´s in the range of 0,625 VAC you have a -6dB drop leading to a faulty input/debalancing stage.
If this is o.k. compared to the input then Gustav or anybody else (Jacob?) might chime in.As said I haven´t built this compressor version (yet),so I´m not too familiar with it.

I do the calibration, but it's for the VU?

Not only but also for the fet start ("Q-Bias") which is very important.

I check the BOM and everythings is at the good place.

Cool,tell us what you get and good luck,

Udo.

 

[tata]

gents,

the GR meter started going up to the max these days. it starts at 0 when switching the unit on, but in a few hours it goes up to +3 and above.
any idea what should i check, please?

 

[gyraf]

A general reminder:

Note that the FET-Grinder is a special variation over a classic theme - the 1176 - that has it's own giant help thread: http://groupdiy.com/index.php?topic=646.0

All circuit-specific questions has probably been answered there more than a couple of times, so you should search there too before asking in this thread.

Jakob E.