Discussion: Noisy Mewtron Micro V

[Prehistoricman]

I got a Mewtron Micro V (auto wah) for Christmas from jedspeds but I had two issues with the build:

• Typical jedspeds service: not the right components. I have 3 spare resistors and the kit was also missing one! I've had similar experiences before.
• Significantly noisy when engaged. I have cleaner overdrive pedals!

The schematic (build doc) requires an LM1458. I kinda doubted it would be anything to do with the noise given how many high-value resistors are in this build, but I was shocked to find it was 100% responsible. I swapped it with a TL072 and situation was far better.

Is this the opamp's fault? The LM1458 doesn't list a noise characteristic (figures!) and is basically the same price on Farnell as the TL072.

[Fancy Lime]

Hi Prehistoricman,

well, parts that don't list noise specs are typically quite variable in terms of noise. An individual part may be ok or it may be very noisy. So yes, the LM1458 is probably not a good choice today. I have recently "discovered" the NJM2068, which is very low noise. But the NE5532 is probably easier to get and almost as good noise wise. On the other hand, if the thing still has more than very slight noise with a TL072, then the opamp is probably not the only problem. The 100k input resistor (R1) certainly does not improve things. The CA3080 is also notoriously noisy and I am unaware of a better drop-in replacement, although there probably is one. Generally speaking, OTA-based filters are relatively noisy compared to other solutions, so don't expect it to get dead quiet. The whole design is clearly not optimized for minimal noise, but that is true for many "classic" designs.

Cheers,
Andy

[Kipper4]

Whats the voltage at collector Q2?
This schematic (build doc)
Half of the 1458 is used as the rectifier Ic2 A (same doc from jed)
the other half is part of the filter Ic2 B

lm3080 might be a good drop in but............


heres a similar build with voltages reply 7

https://www.diystompboxes.com/smfforum/index.php?topic=108342.0

[bean]

Huh. My favorite part of this is how he's using my schematic drawings without permission or even an acknowledgment.

http://www.madbeanpedals.com/projects/_folders/FilterMod/schematics/Kraken.gif

Anyway, as far as noise, etc: IMO, the 1458 works better as an envelope detector. Whether that comes down the the BJT input of the 1458 vs. JFET input of the TL072 I honestly don't know. Probably a "better" design would be for to eliminated Q3, used the second half of the 1458 for the Vb buffer and used the TL072 just for the filter part. But, the Kraken (which his project is based on) was designed for 1590B with everything PCB mounted so space is tight. Still, I never had a problem with the 1458. We even use it in the Function F(x) Accufunkture which is based on the Kraken (although it uses an LM13700 instead).   

[Prehistoricman]

Generally speaking, OTA-based filters are relatively noisy compared to other solutions, so don't expect it to get dead quiet. The whole design is clearly not optimized for minimal noise, but that is true for many "classic" designs.

Cheers,
Andy

Thank you, I will listen to the hiss with nostalgia and snobbery

Whats the voltage at collector Q2?

0.7V, basically the same as the post you linked.

Huh. My favorite part of this is how he's using my schematic drawings without permission or even an acknowledgment.

And it's not modified for this kit build because the circuit board doesn't have a space for R19.

Anyway, as far as noise, etc: IMO, the 1458 works better as an envelope detector. Whether that comes down the the BJT input of the 1458 vs. JFET input of the TL072 I honestly don't know. Probably a "better" design would be for to eliminated Q3, used the second half of the 1458 for the Vb buffer and used the TL072 just for the filter part. But, the Kraken (which his project is based on) was designed for 1590B with everything PCB mounted so space is tight. Still, I never had a problem with the 1458. We even use it in the Function F(x) Accufunkture which is based on the Kraken (although it uses an LM13700 instead).   

The box I'm using is a bit bigger than a 1590B in all directions.
So you're saying that you never came across noise issues in the past with 1458s?

[PRR]

You got a bad LM1458.

Back in the day, we used them all the time, no great hiss, at least for 99.44% of samples; processing *was* a black-art then, and monday morning batch might be jinxed. But it is same-as a '741 (twice), and those were 99.44% low-hiss.

But '1458 has been out of style many years (except in pedal-land). What you buy today may be leftovers. Yes there may be no hiss spec, but wise foundries set-aside hissy batches because they were potential trouble. (Excess hiss relates to reduced reliability in hot environment.) As supply of prime '1458 faded, set-aside batches tend to come to market. (There's a complex ecology of chip-brokers hoarding and hawking obsolete chips for long-run uses like the B-52.)

[Kipper4]

Maybe try a 4558 as a drop in if you have one.
It's a while since I did this.............