MixBuzz1 Build/Support thread

[Harpo]

For the first, I sent directly a signal trought the DBX to see if it's working or not.
This is the result.
Blue line is the signal sent and red the output signal.

Using a hammer might have been more fun to destroy the VCA, if you used a DBX202 or DBX202C for real ...

Here, Same thing but with the NE5534
Blue it's signal sent and red signal output.

would be normal for the factor 0.123 voltage drop to adapt for the 50mV/db to 6.1mV/dB differing VCA control laws.

(I'm using a DBX202c Black)

you could as well have said "(I'm using a rectangular circle)"
Report back what type of VCA you use
DBX202 (black can VCA, comming with a 6mV/dB control law),
DBX202C (gold can VCA, comming with a 50mV/dB control law),
Igors substitution circuit with 1...4 THAT2181 VCAs fitted and buffered control law adaption by voltage divider and NE5534 for a 50mV/dB control law
or another type of VCA, they are all different ...
and what is your fitted value for R3 and R6

 

[drask]

Using a hammer might have been more fun to destroy the VCA, if you used a DBX202 or DBX202C for real ...

Oh okay, I didn't know.. But It's working always.. In other case I'll change it and try to figure out on the second channel.

Report back what type of VCA you use

So I'm using a DBX202 (Black)
My R6 value is 3k74 and R3 4k99

Thank's for your answer Harpo.

 

[Harpo]

Using a hammer might have been more fun to destroy the VCA, if you used a DBX202 or DBX202C for real ...

Oh okay, I didn't know.. But It's working always.. In other case I'll change it and try to figure out on the second channel.

I hopefully might be wrong, but I would not expect your DBX202 to be still alife. You fed it with a +/-18V supply (instead of +/-15V +max.5%), VCAs are current-in/current-out devices (not voltage) and the input current limiting resistor needed will be about 22K with your -6dB THAT1246 signal feed in front that might not swing higher than +/-(18V-2V)/2=8Vpk or 5.66Vrms for recommended max.0.13mA input signal current. Your 3k74 would allow for 2.14mA instead and your 'I sent directly a signal trought the DBX' test only had signal source and trace/wire resistance in between, far exceeding max.allowed values if input level was high enough.
The MixBuzz1 pcb seems not to be designed for using a DBX202 type VCA. To make a DBX202 work in this circuit and using this pcb, you will have to change nearly everything (except the 50k trimmer for symetry) this type of VCA connects to, cut some traces to adapt for the different control port impedance and the different VCA control law, decrease supply voltage, ...

 

[drask]

I hopefully might be wrong, but I would not expect your DBX202 to be still alife. You fed it with a +/-18V supply (instead of +/-15V +max.5%), VCAs are current-in/current-out devices (not voltage) and the input current limiting resistor needed will be about 22K with your -6dB THAT1246 signal feed in front that might not swing higher than +/-SQRT((18V-2V)/2)=8Vpk or 2.83Vrms for recommended max.0.13mA input signal current. Your 3k74 would allow for 2.14mA instead and your 'I sent directly a signal trought the DBX' test only had signal source and trace/wire resistance in between, far exceeding max.allowed values if input level was high enough.
The MixBuzz1 pcb seems not to be designed for using a DBX202 type VCA. To make a DBX202 work in this circuit and using this pcb, you will have to change nearly everything (except the 50k trimmer for symetry) this type of VCA connects to, cut some traces to adapt for the different control port impedance and the different VCA control law, decrease supply voltage, ...

Okay.. But We can use a DBX202c (Gold) on the Mixbuzz but not a DBX202 (black) ?
Igor did it (here : http://www.groupdiy.com/index.php?topic=41654.msg587347#msg587347 )
So maybe you're right, and I haven't understood correctly.
In this case I'll build the replacement bord..

So you think my problem come from that, the DBX202 can't be implemented without PCB modification in the mixbuzz?

 

[Harpo]

Okay.. But We can use a DBX202c (Gold) on the Mixbuzz but not a DBX202 (black) ?

The gold can DBX202C will work (but R3 and R6 will be increased and same value), R2 will be 1M and R100 might need a little tweaking.

In this case I'll build the replacement bord..

Would be my descision as well and jumper R6 (this 5K input resistor =4x 20K in parallel is already on the quad-VCA replacement board).
If you fit the quad-VCA pcb with only one THAT2181, R3 would increase from 4K99 to 20K and C1 might decrease to 33pF.
Your R6 with 3K74 is ment for a DBX202X (again another type of VCA).

 

[drask]

Would be my descision as well and jumper R6 (this 5K input resistor =4x 20K in parallel is already on the quad-VCA replacement board).
If you fit the quad-VCA pcb with only one THAT2181, R3 would increase from 4K99 to 20K and C1 might decrease to 33pF.
Your R6 with 3K74 is ment for a DBX202X (again another type of VCA).

Okay Thank's Harpo.
I'll try that. Hope it's going to fix my problem.

Thank's

 

[mrcase]

Hey all!

Finally finishingmy Mixbuzz! it compresses nicely but I have 2 problems:

1. If I switch the Turbo switch I get 14V on the LED and it doesn´t lit up (should be about1.8V, no?)
also I don´t hear the relay clicking..

2. the LED meter isn´t doing anything at all. neither in OUT GR or IN mode.

Thank you for your help!

martin

 

[Harpo]

1. If I switch the Turbo switch I get 14V on the LED and it doesn´t lit up (should be about1.8V, no?)
also I don´t hear the relay clicking..

Martin, have a look at the schematic. Turbo is default mode (switch open). IC2 Pin15/10 is set for Low thru R2. LED is connected between 0V and -15V thru current limiting resistors R2+R3 = 13K. Current thru LED is (15V-1.8V)/13000=0.001A=1mA=not enough to show up.
Closing the switch, IC2 Pin15/10 is set for High and LED is connected between +15V and -15V thru current limiting resistor R3=3K. Current thru LED is (+15V--15V-1.8V)/3000=0.0094A=9.4mA=LED showing up.
IC2 (DG 403) isn't a relay.
Got the LED fitted in the right orientation?

 

[mrcase]

Thanks Harpo!
got this issue sorted. I was irritated by the 14V but it works now (swapped the LED)
even my LED meter is kind of working.
It works in the IN & OUT mode,although it always shows 2 led lit up even with NO sound. OFFSET doesn´t do anything.
I have both wipers of the OFFSET trimpots shorted to the bottom pin of the trimpot as Igor says in the meter calib procedure.
but this kind of sets them to 5K and it doesnt change with the position of the screw...???
the other problem I have is in GR mode. I can set the GR-OF SL trimpot to show no GR but the GR-MTR trimpot doesn´t do anything aswell.
So it doesnt show me any GR happening...
any hints? I´m confuuuused
thanks a lot
martin

 

[Harpo]

It works in the IN & OUT mode,although it always shows 2 led lit up even with NO sound. OFFSET doesn´t do anything.
I have both wipers of the OFFSET trimpots shorted to the bottom pin of the trimpot as Igor says in the meter calib procedure.
but this kind of sets them to 5K and it doesnt change with the position of the screw...???

Measure voltage at the 50K (22 turn) trimmers wiper. Should vary between 0V and +14V from schematic. If not, trimmer is overturned/broken and no offset current gets injected to opamps inverting input thru RM19 or RM20.

the other problem I have is in GR mode. I can set the GR-OF SL trimpot to show no GR but the GR-MTR trimpot doesn´t do anything aswell.
So it doesnt show me any GR happening...

What is the control-voltage (when audio-signal compression is happening) at TL072 IC4A output (pin1), TL074 IC202A input (pin3) and output (pin1) ? Nada volt - nada GR-meter.

 

[mrcase]

Thank you Harpo!

I was checking pins while massive gr happening:

TL072 (the underneath the meter driver board, right?)
Pin 1: -1.36V

TL074
PIN1: -0.85V
PIN3: -0.15V

 

[mrcase]

OK. in the meantime I swapped the 50k offset trimpot but while calibrating the 10r R44R resistor burned.
I tried to locate any shorts on the meter board but didn´t find any. what elese could it be?
I am a bit clueless here...

thanks a lot

 

[Luax]

There is one question I have regarding meter calibration.
There are several options to choose such as:
-DBFS
-K-20,12,14....
....
All setting give different output readouts so I'm wondering what is best to use?

 

[mrcase]

ok. I can´t figure this out.  I am still having a problem with my led meter not showing any gainreduction.
I traced the Gr signal and from the meter board I get -0.725V with full GR at PIN 1 + 4 at IC204.
on the front meter board the signal at Pin 5 at IC2 is -0.637 and at IC8 -0.640
Are these values ok? where could the problem be - I am out of ideas and totally cluesless by now... 

thankful for any help
cheers
Martin

 

[Harpo]

Martin, the short might just be a mechanical fault. Double check your mounting screws/nuts not shorting out a trace or component.

ok. I can´t figure this out.  I am still having a problem with my led meter not showing any gainreduction.
I traced the Gr signal and from the meter board I get -0.725V with full GR at PIN 1 + 4 at IC204.

with IC204 pin 16 same value as pin 1 (or pin 4,5,8,9) ? If IC204 pin 10+15 doesn't change from 0V to high when switching to showing GR, your measured voltages would still be VU mode voltages for the chips default setting.

BTW revisiting Igors schematic, the 10nF timing caps value for CM7 and CM8 seem to be a typo and more likely be 10uF, caps positive side going to THAT2252 RMS-level detectors pin6.

 

[mrcase]

hey harpo!
I have checked for shorts, I use plastic standoffs for mounting. Maybe the problem is on the mainboard? what voltage for GR should be going to the meterboard?
how high should that voltage signal for GR be? are my values totally off? unfortunately I don´t have a working unit to compare... 
and are you sure about the cap? It´s supposed to be a ceramic cap and it says 10nf everywhere.
also I get +14V at pin 10 & 15 when switching to GR mode. so this seems to be ok.
how high should the voltage signal for GR be? are my values totally off? unfortunately I don´t have a working unit to compare.

thanks for your help
martin

 

[Harpo]

Ooops, I missed the sign 'I traced the Gr signal and from the meter board I get -0.725V with full GR at PIN 1 + 4 at IC204'.
You want a positive voltage reading here. Adjust the 10K GR_OFSL. Wiper will be set at about -2V (~15% rotation).

-0.725V will be about -0.415V at the DBX202X VCA control port. -0.415V/(-0.05V/dB) giving about 8.3dB gain reduction.

and are you sure about the cap? It´s supposed to be a ceramic cap and it says 10nf everywhere.

At least the THAT2252 datasheet says so (at least 1uF, suggested value 10uF) with RM11 and RM12 being 2M2. The supposed ceramic is a followup from the 10nF typo. This is no audio thing in this spot. Only your LED meter sees this timing.

 

[mrcase]

Ooops, I missed the sign 'I traced the Gr signal and from the meter board I get -0.725V with full GR at PIN 1 + 4 at IC204'.
You want a positive voltage reading here. Adjust the 10K GR_OFSL. Wiper will be set at about -2V (~15% rotation).

so I need a positive voltage here? but it won´t become positive by turning the 10K trimmer, no? anyway I tried, but nothing.
but I´ve put a hairballs 1ma meter to the meter connector and this works fine. only the led meter does nada.
this is starting to freak me out 

 

[Harpo]

but it won´t become positive by turning the 10K trimmer, no? anyway I tried, but nothing.

With a not overturned/broken trimmer, injecting a negative voltage (between 0...-14V from schematic) into the inverting input of a not broken opamp IC202A, it will.
With RM34 being only 10K you could push opamps output to +66V if there wouldn't be the rail voltage limits.

only the led meter does nada.

Have a look at Igors led_front_sch.gif schematic and the LM3914 datasheet. The 'inputs operate down to ground' from the chips datasheet already tells you it doesn't go below ground. Ground is a reference voltage that could be anywhere, but the chips connection and surrounding parts on pcb from Igors schematic tells you, 'ground' isn't shifted down to a negative voltage in respect to the common 0V reference voltage, so the input voltage needed for the LED meter to show up will be a positive voltage reading. The 'Input withstands +/-35V without damage or false outputs' from datasheet only says you won't blow up this LED driver when you feed it with a negative voltage, but for showing up a LED a pos.voltage is needed.

 

[MrBlomski]

There is one question I have regarding meter calibration.
There are several options to choose such as:
-DBFS
-K-20,12,14....
....
All setting give different output readouts so I'm wondering what is best to use?

DBFS = Decibel Full Scale
the K metering is from Bob Katz his mastering scale K-system
So DBFS is the next best thing
Cheers.