Matching Ins and Outs

dBm and dBV–kinda lost here…

Hi guys,

I have a Yamaha S08 keyboard synth going into an Audiophile 2496–analog L/R line outs to L/R line ins. Problem is I’m getting distortion even with all VUs looking OK. One person on the 'net claimed this is a design flaw in the S08, which, after lots of observations, I’m starting to believe–either that or it needs some repairs. But I decided to look at the in/out specs for both devices anyway to calculate if they are compatible or not, at least on paper.

Slight problem though: I don’t know what the heck I’m doing!

I only know enough about electronics to inadvertently destroy them, so I thought I’d ask here first before I proceed to uh…destroy them. :D The specs for the S08 outs and the line-ins on the Audiophile 2496 are:

S08:
MAXIMUM OUTPUT LEVELS:
Line(s) Out: +9±2dBm (10k ohms)
Phones: +0±2dBm (33 ohms)

Audiophile 2496 Sound Card:
Peak Analog Input Signal: +2dBV.
Input Impedance: 10k ohms minimum.

The specs are quoted from the manuals. For starters, I don’t know how to relate the dBm figures to the dBV figures. I’ve been googling a while and have learned some things, but only enough to be dangerous. Are there any glaring issues here?

Another related questions is, in a pinch, does it appear reasonable to split the headphone out to go into the Audiophile L/R ins, or would I lose dynamic range, fry something, etc.? I’ve already tried this as quick a test, and I don’t hear the distortion or a noticeable amount of added noise, at least; I was just worried about any possible long-term effects or risks. (Yes, I’m clueless!)

Thanks in advance for any help!

[EDIT: Oh, and should probably also mention that I record to N-track through these devices, just to make this topic technically N-track-related. :p ]

Tony

Just a little more info which might explain what interests my electronics-ignorant mind about the original figures. I’ve looked at a couple of conversion calculators (e.g., this one) and have plugged in the numbers. When I put in the Audiophile input specs of 10k ohms minumum and +2dBV, I get -8 dBm. Does this mean that if the S08 is putting out -7 dBm at 10k ohms, I am already overloading the Audiophile inputs? Could this explain the distortion even when the S08 (capable of over 9 dBm on the line-outs) is at low volume, or would I see clipping in the Audiophile’s VUs in that case (which I’m not)? Has a poor, defenseless little kitten died because of my reckless misuse of the conversion calculator, electronics terminology, and this forum? In any case, you may see why I need help here. Stop me, EEs, before any more kittens die! :p

Tony

bump Sorry, just wanted to bump this once before conceding defeat… :D TIA for any help on this!
[EDIT: Actually, I may post once more if I find the answer, just in case anyone has a similar issue.]

Tony

Thank you, Ali, for your insightful input! I truly know next to nothing about this stuff, so I appreciate your help. Yes, I had read some things about the difference between dBm and dBV and understood it only as far as knowing that there is a difference, and I had read that dBv was different from dbV, yet effectively the same as dBm, which is somewhat “obsolete”, etc., but I hadn’t pieced together what all this stuff means in a any practical sense, which is the important thing.

Quote (nergle @ Mar. 21 2005,13:03)
And the other possible difference is that the headphone o/p goes through some sort of variable attenuator I’d guess, whereas your line o/p doesn’t.

Hmmm. I think I understand what you’re saying here. I guess this kind of thing would fall under the category of “S08 design issue”, and I would have to deal with it from that point of view.
Quote (nergle @ Mar. 21 2005,13:03)
So, by all means use your headphone o/p. You won’t fry anything, nor kill your cat (regrettably :D).

Thanks for the reassuring words, Ali; I think this’ll be the way to go. Oh, and the cat is safe, since I don’t have one (anymore) :p .
I did some tracking tests using the headphone out this past weekend without problems.
Quote (nergle @ Mar. 21 2005,13:03)
But, if you are distorting as badly as you say you are, and the i/p VU’s aren’t indicating clips, then that’s a wee bitty puzzling.

Yes, clipping without VU indication is odd, and I’m leaning toward the opinion that it’s something amiss inside the S08 either by design, defect, or damage, but I just wanted work the numbers on paper before jumping to conclusions, to see if it’s really spelled out there.
Quote (nergle @ Mar. 21 2005,13:03)
If you plug your keyboard straight into an amplifier, does it sound distorted on that?

I really, really wanted to try this, but I don’t have a real amp on hand, unfortunately. :( I’m trying to borrow one from a friend right now for this purpose. The only other amplifier I have is on my “entertainment” system, which I’m going to try just to see what happens on something other than the Audiophile 2496.

In any case, I think I’ll be good to go with the headphone out for DI recording as I work at this problem piecemeal. I may even contact Yamaha, for what it’s worth (not expecting much there). If I do find out exactly what’s going on, though, I will post it.

Thanks again for all your help, Ali!
:cool:
Tony

Got it sorted by now?

As Ali says, dBV is relative to 1Vrms. dBm is relative to 0.775Vrms (1mW into 600ohms).

Put simply, dBV = dBm + 2.2dB

ie, 0dBm = -2.2dBV, and 0dBV = 2.2dBm

The decibel is just a ratio between two figures (used for voltage and power)

So the S08 gives +9dBm (ie +7.8dBV) max, which is presumably when you bash a bunch of keys using a loud patch.

Audiophile handles max +2dBV input (ie +4.2dBm)

Theoretically, a 10k output driving a 10k input results in 6dB signal loss (Ohm’s law stuff) but it depends how Yamaha specified the output…

So it’s quite possible the Audiophile inputs are being slightly overloaded (unlikely to actually fry anything though). If the Audiophile input gain sliders (in the control panel) are pulled down slightly, you won’t see 0dB clips on the VU meters, even though the analog inputs are being overdriven, because the converters are after the gain control.

Could be that the S08 outputs are a bit rough though.

If you don’t need to use headphone on the S08, just plug the phones out into the Audiophile. The signal level will be about right if you have the phones volume control fairly high. The actual voltage coming out is what counts in this case, and the phones level is about a third of the line output level.

It’s likely that the headphone output has its own amplifier to drive the low impedance. That way you get a volume control on the phones without affecting the line out.

Matt.

Thanks for shedding more light on this issue, Matt! I had reluctantly resolved to put this away for now until I had time to look at it more (e.g., do more reading, try the S08 on an amp as I mentioned, etc.), but I really wanted to get to the bottom of this and determine if the specs for both devices at least partially explained things. If I’m understanding your analysis, it seems this is the case.
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So the S08 gives +9dBm (ie +7.8dBV) max, which is presumably when you bash a bunch of keys using a loud patch.

Also with the volume being cranked to the max, I assume, since it affects the line-out levels along with the headphone out (one volume slider for both).
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Theoretically, a 10k output driving a 10k input results in 6dB signal loss (Ohm’s law stuff) but it depends how Yamaha specified the output…

Ahhh…
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So it’s quite possible the Audiophile inputs are being slightly overloaded (unlikely to actually fry anything though). If the Audiophile input gain sliders (in the control panel) are pulled down slightly, you won’t see 0dB clips on the VU meters, even though the analog inputs are being overdriven, because the converters are after the gain control.

Ah, so this explains that mystery. Actually, though, I can push the Audiophile gain sliders to max and still see no indication of overloading on the VUs but still hear the clipping/distortion. But I assume the same principle holds true either way.
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If you don’t need to use headphone on the S08, just plug the phones out into the Audiophile. The signal level will be about right if you have the phones volume control fairly high. The actual voltage coming out is what counts in this case, and the phones level is about a third of the line output level.

This is exactly the case. With the headphone out, I seem to be getting very good levels for most patches with the volume at or near max on the S08.
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It’s likely that the headphone output has its own amplifier to drive the low impedance. That way you get a volume control on the phones without affecting the line out.

Ok, so in this case the single volume slider on the S08 would control two separate amplifiers, i.e., one for the line-out and the other for the phones. Gotcha (I think).

Along with Ali’s info, I feel very confident now recording with the headphone out, and I have a much better understanding of what’s going on with the line-outs.

Again, Matt, thanks for your response. At this point, I’m satisfied with the immediate situation. I may look at this a little more just for learning purposes, but my tracking situation looks quite good now.

:cool:

Tony