Mojotone Effects Loop V2 sounds terrible

[66 Kicks]

Yet another approach is to use a Cathodyne style configuration with radically unbalanced resistors. Again, it allows the use of larger resistors for degenerative feedback. The Mojo Return circuit could probably be modified somewhat easily to look like this one.

These approaches are all taken from old tube technology.

 

[Bruce_W]

Yet another approach is to use a Cathodyne style configuration with radically unbalanced resistors. Again, it allows the use of larger resistors for degenerative feedback. The Mojo Return circuit could probably be modified somewhat easily to look like this one.

These approaches are all taken from old tube technology.

View attachment 72475

I think the Metro loop uses the same transistors? I thought I read that somewhere

 

[66 Kicks]

I think the Metro loop uses the same transistors? I thought I read that somewhere

LND150's

 

[Gene Ballzz]

While the Metro Zero Loss loops get really good reviews, I've not yet used one. I have had good results with the units from Granger Amps. Besides working well/sounding good, I liked the format/feature of have both the send and return level pots mounted on the board with access for a small screw driver through the jack hole! Similar price to the Metro & MOJO.

https://grangeramp.com/product/ultimate-fx-loop-2/



Just My $.02,
Gene

 

[ampmadscientist]

Here is a schematic for version 1.5. The picture they have of version 2 looks like the same circuit to me.

View attachment 72421

Yeah I see it's a design error.
The input stage is boosting the signal, when it should be reducing the signal.

On the metro Loop, the first stage is negative feedback which reduces the signal. Not a gain stage.

And so the Mojo loop is really a botch job.
If we had test equipment we could probably re-design it.

 

[ampmadscientist]

While the Metro Zero Loss loops get really good reviews, I've not yet used one. I have had good results with the units from Granger Amps. Besides working well/sounding good, I liked the format/feature of have both the send and return level pots mounted on the board with access for a small screw driver through the jack hole! Similar price to the Metro & MOJO.

https://grangeramp.com/product/ultimate-fx-loop-2/

View attachment 72572

Just My $.02,
Gene

The Metro loop is OK.
I would have used real Cliff Jacks, but instead Metro uses cheap knockoff jacks which I really don't like.
The level matching is so-so. I would usually change some of the resistors on the board to correct the level matching.
I don't use the bypass switch at all (another cheap part from China).
But besides the level matching, it works alright.

I added a mix control for adjustable series - parallel - bypass. I did this with a dual Alpha 1 meg pot.
After I tweaked it out it sounded pretty good.

You will also notice:
The Metro board has protection diodes for the FETs.
The Mojo board has no protection.
And so you could probably conclude that the Metro board is designed better.

 

[Matt_Krush]

ampmad, correct me if I am wrong...
Looking at the Mojo loop, the first stage is a unity gain source follower circuit, with a voltage divider for the output.
Running my numbers at 25vrms from the treble pot, it should be about .730vrms at the send....near line level

I did a quick model of this circuit in multisim, and it jives with my calculations.
I suspect that the circuit (while not the best design) should have worked, I thing that the traces or components are not correct on the board.

Just the 'send circuit'




Even at 310-320vdc for B+, the send voltage is where I calculated at line level (Channel B -Blue Trace is output)

When I had the board in my amp, the send level was only attenuated a couple volts and was more squared off than the amplifier was doing naturally.
I can only surmise that the Fet is installed backwards or the trace for the send output is coming off the drain instead of the Source.

Someday...I will actually check the circuit on the board.

 

[66 Kicks]

While the Metro Zero Loss loops get really good reviews, I've not yet used one. I have had good results with the units from Granger Amps. Besides working well/sounding good, I liked the format/feature of have both the send and return level pots mounted on the board with access for a small screw driver through the jack hole! Similar price to the Metro & MOJO.

Granger has a newer version that is really small. I don't know what the circuit is on the new one, but the one in the picture you attached is like this:



The trim pots are a pretty good idea, but the design makes poor use of them. I like their T-shirts.

 

[66 Kicks]

ampmad, correct me if I am wrong...
Looking at the Mojo loop, the first stage is a unity gain source follower circuit, with a voltage divider for the output.
Running my numbers at 25vrms from the treble pot, it should be about .730vrms at the send....near line level

A couple of strokes on a calculator will tell you what the Send circuit will do.

With 250Vdc on the Drain, there will be about 125Vdc on the Source because the 1M/1M voltage divider puts 125Vdc on the Gate.

125Vdc divided by 47K equals the idle current of 2.7mA.

The AC load is the 47K Source resistor in parallel with (10K + 470) which equals 8.6K.

The peak AC voltage when the Source hits 0.00mA is 2.7mA times 8.6K which equals 23Vp.

In actual tests the thing clipped at 25Vp, which is pretty close to the predicted 23Vp. This puts 1.1Vp at the Send which is far in excess of what it takes to clip the return circuit.

 

[66 Kicks]

Another neat feature of the Mojo loop is that it loads your tone stack with 500K when there isn't an optional trim pot. The loading is 333K to 1M with a 1M trim pot that has one leg grounded and the wiper attached to the 0.1uF input capacitor.

 

[1199RS]

LND150's

View attachment 72479

Hi can someone help me, am i correct in saying that there is a mistake in the schematic for the metro ZL loop? The 1.5K resistor in series with the 1M on the return circuit should have lower leg moved to the node on the gate?

Also can you see any issue with using a 47K pot instead of 200K bypassed with 33K if i wanted to remove the switching option.

 

[Kelia]

Old thread but curious if Mojotone solved their loop issue ?
Need a loop and was wondering if I should go with the Granger instead .

 

[adew1]

Need a loop and was wondering if I should go with the Granger instead .

I've used both the Metro and the Granger loops in recent builds and wouldn't hesitate to use either in future. Perhaps the Granger is slightly better thought out thanks to the arrangement of trim pots (send and return trim) that can be accessed via the jacks, without having to remove the chassis from the shell. However, this is a set and forget adjustment, so I wouldn't base my decision on this aspect.

 

[Kelia]

I've used both the Metro and the Granger loops in recent builds and wouldn't hesitate to use either in future. Perhaps the Granger is slightly better thought out thanks to the arrangement of trim pots (send and return trim) that can be accessed via the jacks, without having to remove the chassis from the shell. However, this is a set and forget adjustment, so I wouldn't base my decision on this aspect.

Thank you !

 

[ThesoundofUkraine]

I installed one on a bandmaster for a friend a few years ago, worked fine. They come with trimpots for input and output. I replaced them with chassis mounted pots. I assume it's before the phase inverter, getting what comes out of the treble pot.
If you have a scope, see where is this clipping action happening. Is the LND150 transistors on the FX loop or is the FX loop pushing the phase inverter into overdrive?
If it's the FX loop clipping, try a 1M pot on its input (from treble pot to the centre log, one log goes to ground, the other to the FX loop). See if lowering the input fixes it. You'd want those pots on the back permanently. You'll be adjusting them often.
If I remember correctly, they also had a resistor in series with the signal right at the front, try increasing the value of that too if it's there.
I used shielded wire all around (ground only one end of each wire's shield, heat shrink the other end so it doesn't touch anything) otherwise it picks up AC noise from the filament supply.

I hope it helps

My bandmaster reverb reads 411 volts on B+4 the last filter stage. Will the loop take the voltage down enough for this to be a viable install for me? Or should I use a step down circuit? Also you ent out on the mix, and in on the PI?