Topic: Korg MR-1

[mbgrace]

Hi all,
The pad design we made for scyue was based on three assumptions:

1) The reported difference in level between the V3 meters and the MR-1 meters is 17dB

2) The input impedance of the MR-1 is 10k Ohms (balanced)

3) The MR-1 maximum input level is +6dBV (for 0dBFS)

The last two assumption here are based on the specifications listed in the MR-1 manual. The last one was probably an erroneous assumption because upon further thinking it seems that this specification must indicate the highest voltage level the input electronics can handle and not the 0dBFS input level.
What is the Voltage level required at the input of the MR-1 to achieve a level of 0dBFS?  This is not clear in the manual, but we have a unit on its way here so we will be able to test this next week.

In the meantime I am including the formulas used to calculate the pad circuit.

To determine the Voltage ratio of this pad you must first know the input impedance of the device being fed by this circuit.  The parallel resistance of the device input impedance and R3 determines the actual resistance of R3  In the case of the MR-1, the specified input impedance (from the owners manual) is 10k Ohms.  The parallel resistance can be calculated with the following formula:
Rparallel=(R3*Rdevice)/(R3+Rdevice)

Use this resistance in the following equation to calculate the Voltage ratio of the pad:

Vratio=(Rparallel)/(R1+R2+Rparallel)

In the 17dB pad example circuit the 4.99k resistor is in parallel with a 10k resistor which makes a total of 3.40k Ohms
With the two 10k series resistors the voltage ratio is 0.145

You can then calculate the dB drop with:

dB=20*log(Vratio)

The example pad is -16.74dB

I hope this is helpful.  Once we get our hands on an MR-1 well make an exact measurement and post it here.
Cheers,
Michael 

[kskreider]

Mr. Grace just got his 100th ticket.  Very useful info.  Thank you very much.

And I will ask: How would setting a -20db balanced cable be different from putting the -20db pad on inside the V3 and running a set of regular balanced cables?  Or am I looking at this incorrectly?

[JasonSobel]

And I will ask: How would setting a -20db balanced cable be different from putting the -20db pad on inside the V3 and running a set of regular balanced cables?  Or am I looking at this incorrectly?

the -20dB pad on the V3 is on the input side.  if you use that pad, you'll just need to run the V3 20dB than you would have to otherwise, but the analog output level will be the same.  On the other hand, if you use a -20dB attenuator on the output of the V3, then you will be lowering the output level of the analog outs, the purpose being to allow whatever downstream gear you are using to handle the V3 output, as not too much gear can handle a +25 dBu analog signal.

[kskreider]

Thank you Jason.  I didn't realize the V3's pad was on the inputs.  Either that or I had forgotten since I used it in July.

[Nick's Picks]

one more reason the MMP tromps as a preamp.
that little output attenuator dial is *the shit*

[Digital Quality]

So your cable pad now gives a 19dB attenuation?

Yes - it's on the xlr end. I realized yours was just on the other end last night. Sorry for the confusion!

I made it unbalanced which gives -6db (from the V3 manual). I used 1.5k and 330 (1473 & 323 after some handmatching) to get another 13dB. Now, I'm going to build some nicer cables with final values 301 and 1.5k - done twiddling -20dB will nail it.

Here is Mikes old drawing again. I changed the Rs and tied pins 1 & 3.

BTW Mike, if I brought in my amp could you change the output drive by -20dB, or is that harder to do than say?? Also, it seems a lot easier to make the pad on the XLR end. Does that pose a problem?

[Nick's Picks]

or ..., an output POT on the back...a little screw aka MME to attenuate the level.
that would be slick.

[Chrysler]

another question (i guess this is rather for the cables section, but since its MR-1 related i hope someone can answer me the question here)

I want to connect my MR-1 to a soundboard - now the problem is that every mini phone cable that i did find is stereo.
Does it make no difference if two stereo mini phone cables are plugged into the MR-1?

E.g. two of these : http://www1.conrad.de/scripts/wgate/zcop_b2c/~flN0YXRlPTE3MjM0MjQ1NDE=?~template=PCAT_AREA_S_BROWSE&glb_user_js=Y&shop=B2C&zhmmh_lfo=&zhmmh_area_kz=&product_show_id=300901&p_init_ipc=X&p_page_to_display=fromoutside&~cookies=1&cookie_n[1]=b2c_insert&cookie_v[1]=F0&cookie_d[1]=&cookie_p[1]=%2f&cookie_e[1]=Thu%2c+24-May-2007+11%3a54%3a06+GMT&cookie_n[2]=b2c_hk_cookie&cookie_v[2]=WW2&cookie_d[2]=&cookie_p[2]=%2f&cookie_e[2]=Thu%2c+24-May-2007+11%3a54%3a06+GMT&scrwidth=1280

Also id need two Cinch > mini phone cables but have no idea where to get these in europe. Does anyone have suggestions or knows someone who can tailor such cables?

Thanks,
Jan

[Nick's Picks]

you can't use a stereo cable.  the inputs are balanced.  a stereo cable is wired differently, and carrying two unbalanced signals.

[richardl]

I want to connect my MR-1 to a soundboard - now the problem is that every mini phone cable that i did find is stereo.
Does it make no difference if two stereo mini phone cables are plugged into the MR-1?

I have been using a balanced cable with a 1/4" male plug on the end and then using a stereo 1/4" female to stereo mini male adapter (i.e. a headphone adapter). I think this is ok. It's a little clunky and puts a bit of strain on the MR-1 input jack though.

[Nick's Picks]

hmmm....

a stereo mini has 3 contacts.  when wired for "stereo", the tip is left, the ring is right and the rest is ground.
when wired as "balanced", the tip is + the ring is - and the rest is ground.
using a plug wired for stereo is not a balanced input.  it would be wired backwards and out of phase.

[Roving Sign]

hmmm....

a stereo mini has 3 contacts.  when wired for "stereo", the tip is left, the ring is right and the rest is ground.
when wired as "balanced", the tip is + the ring is - and the rest is ground.
using a plug wired for stereo is not a balanced input.  it would be wired backwards and out of phase.

?

I would think any 3 conductor wire would be fine...the purpose might differ, but the wires should be the same...

[mbgrace]

So your cable pad now gives a 19dB attenuation?

Yes - it's on the xlr end. I realized yours was just on the other end last night. Sorry for the confusion!

I made it unbalanced which gives -6db (from the V3 manual). I used 1.5k and 330 (1473 & 323 after some handmatching) to get another 13dB. Now, I'm going to build some nicer cables with final values 301 and 1.5k - done twiddling -20dB will nail it.

Here is Mikes old drawing again. I changed the Rs and tied pins 1 & 3.

BTW Mike, if I brought in my amp could you change the output drive by -20dB, or is that harder to do than say?? Also, it seems a lot easier to make the pad on the XLR end. Does that pose a problem?

I looked at the pcb and it would not be very difficult to put the attenuator on the outputs of the V3.  the only real drawback here is that it is relatively permanent and if you needed the outputs at full line level then you would be stuck.  If this is not an issue for you fell free to bring the unit by and we can make the modification.
Building the pad circuit in an XLR cable connector is fine also.  While the ultimate way to do it would be to have the resistors at the destination end of the cable as long as we are talking about short cables (under a few meters) it shouldn't be an issue to have them in the xlr.  Also, it makes sense to take advantage of the balanced input on the MR-1 and use a balanced attenuator and cable.  I don't think there will be any measurable noise contribution from resistor thermal noise in this circuit.
Buy the way, I assume that the main reason for needing the pad in the first place is so that you can run the V3 A/D converter at the same time you are feeding the MR-1 and have matching recording levels on both.  If you are simply running the MR-1 it seems you should be able to turn the gain down enough on the V3 to provide a proper level for the MR-1.  Is this the case?  (We have not been able to actually determine the MR-1 input sensitivity yet)
Mike

[Digital Quality]

Buy the way, I assume that the main reason for needing the pad in the first place is so that you can run the V3 A/D converter at the same time you are feeding the MR-1 and have matching recording levels on both.

Yes, that's our mission.

Thanks for all the help!!

[mbgrace]

Here's what I have found regarding the input alignment of the MR-1.
In DFF mode the top of the MR-1 meter scale is +3.  However, it is probably best practice to not use a DSD level higher than 0dBFS.
To align the V3 and MR-1 levels we need a pad of approximately -23.8dB
In the pad circuit shown earlier in the thread we need a total shunt resistance of 1380 Ohms.
I measured the input impedance of the MR-1 to be 54k Ohms, not the 10k as indicated in the manual.
With a 54k input impedance R3 should be 1416k Ohms.  I had a 1.4k Ohm 1% resistor here and the pad came out just right.


Korg MR-1 input levels with record level set to 0dB
DSD Mode

0dBFS              -0.8dBV          1.4dBu          0.94V
Top meter bar(+3)   1.9dBV      4.2dBu      1.25V
0.6% THD+N(clip)   4.9dBV      7.2dBu      1.76V   


PCM Mode

0dBFS                    -0.8dBV    1.4dBu    0.94V
0.6% THD+N(clip)   -0.6dBV      1.6dBu      0.93V

Maximum input level with record level set to anything less than -8.4dB is 8.4dBV.  This is the analog input circuit clip level which means that any record level setting below -8.4dB
will only serve to limit the dynamic range of the MR-1 as the input stage will clip before the ADC.

I hope this info is helpful.
Michael