Low sound level sammichFM

4 posts in this topic

I have a MBFM and a sammichFM.

I have built the MBFM with a OPL3 module that I have modified with a 5V option I have described here http://wiki.midibox.org/doku.php?id=opl3_module_modified

It uses a MCP6400 op-amp http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf

Lately I have been using my sammichFM, and that sounds much quieter. The sammichFM uses these op-amps OPA4348 http://www.ti.com/lit/ds/symlink/opa2348.pdf

Attached is an image of a recording with the MBFM on the top and the sammichFM on the bottom. Same note, velocity and patch.


Can anyone see a reason why the sammichFM is so much quieter? Are the op-amps that different?

Cheers Hal


Edited by eptheca

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In this post by @Sauraen, @nILS explains the op-amp circuit schematic on the sammichFM 

He used a voltage divider to lower the signal to prevent clipping. I don't have that in the 5V mod on the MBFM.

I haven't noticed any clipping, but I haven't really listen for it either. Maybe it's there, and I just haven't heard it.

Other people also comment on the sammichFM being quiet, but it sounds like it is a conscious decision.


Has anybody compared the sammichFM and the original MBFM (TL074  +/- 12V op-amps)? Output level and clipping?

Should I add a voltage divider to the 5V mod?

How can I measure the potential clipping? 

Can the sammichFM be modified to sound louder? Change resistor values in the voltage divider, or remove it? Change op-amp?

What are the pros and cons of the two op-amps? Differences? 

MPC6400 op-amp http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf

OPA4348 http://www.ti.com/lit/ds/symlink/opa2348.pdf


Cheers, Hal 


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I never did extensive research of this--I designed the circuit that I thought was best, built it, it worked, and I moved on. I'm not surprised there's differences. I'd suggest you try bypassing the voltage divider on the sammichFM and see what happens. Clipping won't break things. It should be pretty obvious if things are clipping--play sine waves on every channel at max volume, and see if the output sounds distorted (not sine waves). The two op-amps you listed look pretty similar to me at a glance.

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Thanks for the suggestion.

I removed the 12 resistors for the voltage dividers, and replaced R52,53,54,55 with jumpers.

Now it's louder, same level as my MBFM with your mod.

I do notice some distortion with all 4 operators on full volume though, so nILS design make sense and is probably "cleaner".

I guess I just got used to the sound and level of my MBFM. At least my patches now sound the same :)

Is this maybe why the original design has amplification in two stages with the 3 TL074, to prevent the clipping and still get a high output level? 





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