Help with modifying a Circuit (Fostex MN-50)

[Kizzer]

Hi guys

I recently watched a video on this fun little compressor and thought it would be a fun little project to build on stripboard for smashing drums through. I want to cut down the circuit as it has a bunch of things I don’t need. These are:

Line input only. (Get rid of Mic inputs and Instrument inputs)
Change power switch to ON or TRUE BYPASS instead of comp in/on/off
Get rid of all indicator lights

Could any of you guys help me with what can be emitted from the schematic? I’ve marked parts I think can be cut, but I’m not understanding that Comp in/on/off switch.

Any help would be gladly received.

Cheers

 

[Kizzer]

So here's an update of where I'm at:


I think I've cleaned up the input, cutting out the 4 tape inputs and mic input. I've also cut out the power switch and everything in the bottom right around U4 as this is just for the compression indicator lights and I don't need the power on switch.

I'm still really confused with the Comp in/on/off switch. I want it to get rid of the switch all together so the compression is on all the time and just set up a passive true bypass at input.

Does anyone have any ideas on how the comp in to comp in should be connected, and if so what other components could be left out?

Cheers

 

[NOON]

There's two parts to the switch with 3 positions, indicated by the dotted line joining them. You're trying to replicate position 3. Part one is in the power supply and disconnects the power in position 1 and connects it in positions 2 and 3. Part two is in the compressor sidechain and disconnects it from the FET in positions 1 and 2. So for position 3, just join across where the switch is in the power supply and again in the sidechain.
Be aware that this circuit is designed for 'consumer' -10db levels when you're sending signals to it. You will easily overdrive and distort the input as well as compress it with the FET. That may be desirable in some cases, not in others.

 

[NOON]

You could give it more headroom by replacing the single rail 9-15v power supply with a +/-15v power supply and removing the bias supplies that shift the signal to half vcc. This should also involve some tweaking of signal and gain levels to take advantage of the headroom so becomes a bit of an overall redesign and hence a different unit. Probably not worth the effort if what you want is an ugly little compressor for doing nasty things to signals anyway.

 

[Kizzer]

So for position 3, just join across where the switch is in the power supply and again in the sidechain.

Thanks very much for the reply! I get what your saying about the levels, but it'll be for trashing audio, so no worries.



Does this look right to you?

 

[NOON]

No! The dotted line means the two switches are physically joined so they move together but are electrically independent. Treat them separately. Look at what each switch is doing when it's in the top position.

 

[Kizzer]

No! The dotted line means the two switches are physically joined so they move together but are electrically independent. Treat them separately. Look at what each switch is doing when it's in the top position.

I see it now! Cheers buddy, took me a while to get the switch, but thanks for the help.

 

[NOON]

That looks correct now. I'm pretty sure changing R20 to a pot would vary the attack in this circuit. Start with a 5K, 1K may be enough of a variation.

 

[Kizzer]

That looks correct now. I'm pretty sure changing R20 to a pot would vary the attack in this circuit. Start with a 5K, 1K may be enough of a variation.

That's awesome man, thanks very much for all the help! Defiantly worth a go!

 

[Kizzer]

So, this project isn’t going that well. I have tried a Vero layout which didn’t work, got a low level pass through of signal even when the circuit wasn’t powered which is a head scratcher! I have traced pictures of the circuit included below and found a few discrepancies with the schematic.

The schematic shows lug 2 of the input pot going to R42 only, where as the real circuit also links to C5. Should I be getting rid of R42/C19/C10/R30/R12/R43/C18/R7/R6 and U1 altogether?

Also the schematic shows R18 linking to C8, but again, this is different to the real circuit.

Am I right in thinking that the emitter of Q2 along with C15/C16/C17 and R35 are creating the VCC for this circuit? I have VCC going to pin 8 and GND to pin 4 of every opamp.

Voltage wise from a 9.3V supply I am getting 6.75V on the emitter of Q2.

Cheers for any help or pointers you guys can give me.

 

[Kizzer]

All sorted and working now!

Fun little unit, thanks to NOON for the help.

 

[Bo Deadly]

All sorted and working now!

I was going to say there's a mistake in the original schematic but it seems you found it on your own.



Note that this circuit is clearly a direct rip of the Urei 1176. Aside from the obvious circuit topology, the 2.2u cap and 500k release is a direct scaling of the 220n cap and 5M release of the 1176. The 220R is equivalent to about 1/10th of the 25k attack pot of the 1176. So it's a fairly fast setting.

My guess would be that a stock MN-50 would be pretty great for mixing and leveling synths.

 

[Marssound]

R107 can be another control I think?
it's controlling the bias for the FET and can change the sound character of the compressor. around 11 o'clock is kind of factory setting.
I am busy with this compressor too. building two units in a 1U 19 inch rack.

 

[Marssound]

Does the attack control really work? has it ever been tested?

 

[Marssound]

https://www.diystompboxes.com/smfforum/index.php?topic=124036.0

 

[bowanderror]

I was going to say there's a mistake in the original schematic but it seems you found it on your own.

View attachment 105152

Note that this circuit is clearly a direct rip of the Urei 1176. Aside from the obvious circuit topology, the 2.2u cap and 500k release is a direct scaling of the 220n cap and 5M release of the 1176. The 220R is equivalent to about 1/10th of the 25k attack pot of the 1176. So it's a fairly fast setting.

My guess would be that a stock MN-50 would be pretty great for mixing and leveling synths.

Can you let me know which schematic is correct: the original schematic (where the above green crossing is a connected node) or the edited one (where the crossed lines are not connected)?

I've sim'd this in LTSpice, but am getting some odd behaviors that I'm trying to run down.

 

[Kizzer]

Can you let me know which schematic is correct: the original schematic (where the above green crossing is a connected node) or the edited one (where the crossed lines are not connected)?

I've sim'd this in LTSpice, but am getting some odd behaviors that I'm trying to run down.

Unconnected is the only way to bias the Fet. Built and works great.

 

[bowanderror]

Unconnected is the only way to bias the Fet. Built and works great.

Gotcha, good to know!

Is the original MN-50 schematic incorrect in connecting the 1M resistor to lugs 2 & 3 of the Release knob? Basically, is this schematic the correct one?

 

[Marssound]

I bought two FOSTEX MMN-50 mixers for building in a 1U rack.
my plan was to get rit of the mic and instrument inputs and only use the line input..like you do.
but I tested the compressor carefully and you will get a lot more extra squashing and (nice) dirty sounds
when using the mic/instr input.
My suggestion is (and that's what I'm gonna do): keep the mic, instr and line input and use a switch to have a choice of input gain/attenuation.
the tape inputs with the sliders can be omitted.

 

[Kizzer]

Gotcha, good to know!

Is the original MN-50 schematic incorrect in connecting the 1M resistor to lugs 2 & 3 of the Release knob? Basically, is this schematic the correct one?

The 1M doesn't connect to the release knob, just to the trimmer (R107) D5 and R35. If you take a look at the digital scan it confirms this. The only place which I thought was a little strange is that it looks like Input Lug 2 connects to C5 cathode as well as R42.