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Old 2nd April 2015, 02:54 AM   #1
jinja is offline jinja  Brazil
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Unhappy please help me to calculate/guess Vrms input?

Hi, clever bods! these are probably dumb questions, but I need to know the answer(s)...

I have a Yamaha AX-490 that has no listed Vrms spec for inputs. The specs sheet says 150mV/47 K-ohms 'sensitivity'. I am guessing this is not Vrms, as at 0.9Vrms from my MiniDSP it sounds quiet and flat (compared to input not via the MiniDSP).

Is there any way to calculate/guesstimate the appropriate Vrms level for input to this unit?

Also, I am getting a Marantz PM5005, which says it has an input sensitivity of 200mV/20 K-ohms, but also says it has an S/N of 103db (2V input, Rated output). I am guessing this means it is best at 2Vrms. Am I correct?

The Marantz specs seem to hint at Vrms being a different animal to input sensitivity expressed as mV/k-ohms, but I don't really understand this stuff very well. Help!
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Old 2nd April 2015, 09:12 AM   #2
DF96 is offline DF96  England
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Quote:
Originally Posted by jinja
I have a Yamaha AX-490 that has no listed Vrms spec for inputs. The specs sheet says 150mV/47 K-ohms 'sensitivity'. I am guessing this is not Vrms, as at 0.9Vrms from my MiniDSP it sounds quiet and flat (compared to input not via the MiniDSP).
The voltage will almost certainly be RMS, as that is the normal way to specify AC voltages. What you may be confusing is 'input voltage for maximum output power' and 'input voltage for some standard (smaller) output power (possibly 1W?)'.

Quote:
Also, I am getting a Marantz PM5005, which says it has an input sensitivity of 200mV/20 K-ohms, but also says it has an S/N of 103db (2V input, Rated output). I am guessing this means it is best at 2Vrms. Am I correct?
No. It means at 2V input it will have s/n of 103dB. At lower inputs it may have worse s/n; at higher inputs it may have better s/n. The details depend on the exact gain distribution through the amp.

Quote:
The Marantz specs seem to hint at Vrms being a different animal to input sensitivity expressed as mV/k-ohms, but I don't really understand this stuff very well. Help!
You may have misunderstood, or they may have misunderstood.

Specifying 'sensitivity' as mV/K-ohms is misleading - it seems to imply some sort of division is taking place! Sensitivity should be specified in mV or V (as appropriate) - in both cases an RMS figure would be normal. As a separate matter, the input impedance can be specified as something like '20k' or '47 000'. "K-ohms" are not an internationally recognised unit, although most people will recognise what they are trying to say.
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Old 2nd April 2015, 02:19 PM   #3
jinja is offline jinja  Brazil
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Ah, OK. I understand that it is a measurement of 'sensitivity'

Then how do I know what would be a good input range for the Yamaha, for instance? What specs. should I use to calculate the max. or a 'good' Vrms level to send to it from, for example a miniDSP?

Or can I ONLY calculate this using test equipment? (which I don't have)

Thanks!
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Old 2nd April 2015, 05:52 PM   #4
DF96 is offline DF96  England
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It may specify a maximum input.

If not, you either have to measure or calculate from the circuit details.
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Old 2nd April 2015, 07:59 PM   #5
jinja is offline jinja  Brazil
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It doesn't specify a maximum input (unless I'm missing something), that's why I am asking. The Yamaha AX-490 specs can be seen at the end of the pdf here. The Marantz PM5005 specs are on page 41 of the pdf here.

How to calculate, and what details I would use/need for this calculation is also what I was asking.

Any ideas would be appreciated. Thanks!
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Old 2nd April 2015, 09:01 PM   #6
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Input sensitivity (like 150 mV) denotes the minimum input level required to achieve full rated output power with the volume cranked all the way up. So it's a good idea if nominal input level were at least as high as that. The nominal consumer audio level used to be considered 300 mV.

I would suggest trying to obtain input levels such that a comfortable listening volume ends up in about the 10 o'clock position of the amplifier's volume pot. Channel imbalance should be low by this point, and there should be enough volume to spare if need be.

While I cannot find any schematics for the AX-490 anywhere, I assume it'll be similar to the AX-590. In this one, any input but CD connects to the volume pot, so the overload limit would be given by the pot's power handling (>10 Vrms, typ). In practice, you would be bothered by irritating channel imbalance long before this point.

The CD input is different since it adds about 10 dB of voltage gain - another 10 dB gain block after the volume pot is then bypassed in CD-Direct mode, which results in almost as much reduction in output noise, making even sensitive horn speakers happy. (The articles section of this site contains something on gain staging if you're interested.) In fact, the '590 makes use of two NJM2068s in parallel for this, with each having to drive about 1k5 worth of feedback network up to 6 Vrms for a 2 Vrms input. (I presume the '490 will only have one opamp.) Maximum permitted input level is less than 3 Vrms before things start to clip.
This input stage is a good example for misguided overdesign because:
1. the noise even of one opamp stage would be entirely swamped by source noise in practice (or have you ever seen a CD player remotely approaching 130 dB SNR at 2.0 Vrms out? Input noise sims at ~0.55 V!), and both are reduced to inaudible levels by turning down volume, and
2. the '2068 won't be too happy (distortion wise) having to drive that kind of load at that kind of levels, which should be measurable. (The "CD" input should better be called "anything but CD", which is a bit ironic - it would be a good match for up to ~1 Vrms from a soundcard or portable device.)
IOW, they designed for low noise where it was not necessary and only puts an undue strain on the opamp output. I would resize the feedback network from 1k/470 to more like 1k8/1k or 2k2/2k2 (some reduction in gain isn't a bad idea, since CDs have become so much hotter in level since the early '90s that people often have to turn the volume way down and then run into channel imbalance issues).

As for the problem in question - is the MiniDSP output 0.9 V nominal, or 0.9 V full-scale? In a DSP setup, you always tend to leave some processing headroom (staying anywhere from 3-10 dB away from full-scale), and overall gain from input to output may vary according to how the DSP is set up. Next step would thus be finding out what input-to-output gain of the MiniDSP is, what sort of input levels it'll handle and what its maximum output is.

Last edited by sgrossklass; 2nd April 2015 at 09:04 PM.
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Old 2nd April 2015, 10:17 PM   #7
sreten is online now sreten  United Kingdom
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Hi,

The description of the CD input makes no sense, it does not need
+10dB gain. The +10dB stage is most likely applicable to all inputs
but only bypassable for the CD input which has more output.

rgds, sreten.

Nominal audio line level is 300mV rms.
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Last edited by sreten; 2nd April 2015 at 10:46 PM.
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Old 2nd April 2015, 11:30 PM   #8
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Quote:
Originally Posted by sreten View Post
Hi,

The description of the CD input makes no sense, it does not need
+10dB gain. The +10dB stage is most likely applicable to all inputs
but only bypassable for the CD input which has more output.
No, that really is how CD-Direct works with these Yamaha amps (up to AX-596 and A-S700) - take away 10 dB after the volume pot, add 10 dB before the volume pot (only at CD input). Result, overall gain is the same, but output noise floor goes down by almost 10 dB. The CD input amp has to handle pretty high levels though, as described.

It does get a bit dumb in non-CD-Direct mode, where the amplified CD input is immediately attenuated again in order to be compatible with the other inputs - worst of both worlds, so to speak.

Of course a CD player is easily the loudest source these days, but remember this series came out in '95, when average CD levels were a fair bit more reasonable still. And with CDs being the best medium available to the general public, having a dedicated higher-performance CD input made sense. They also had entirely dropped 4-gang volume pots by this series (AX-1070 still had one, as did AX-1050 and AX-750) and probably wanted to offer something to make up for it.
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Old 2nd April 2015, 11:50 PM   #9
sreten is online now sreten  United Kingdom
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Hi,

Well i'm bemused. I used to make 7dB attenuator leads for non
CD amplifiers and your description doesn't follow all line inputs
being 150mV sensistivity, at all, I severely doubt your idea
10dB is added to CD before the volume pot, very pointless.

Or your assumption the noise floor is improved.

rgds, sreten.
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Last edited by sreten; 2nd April 2015 at 11:53 PM.
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Old 2nd April 2015, 11:56 PM   #10
jinja is offline jinja  Brazil
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sgrossklass...thanks...interesting stuff...unfortunately, I do indeed have the 490, not the 590, so most of your hypothesizing is moot in my case... re. the minidsp...it likes to be set at the higher 1.9Vrms input setting for my input source (a mixer unit which is showing good levels at this input...i.e. entering 'orange' on the VU fits well with a -3 to -6 db channel in on the MiniDSP VU). The other available input setting is 0.9V which is just way to low for my source. I'm thinking I probably need the minidsp balanced 2x4 which also gives a 1.9Vrms output (instead of ONLY a 0.9Vrms), but I would like to know how to calculate a 'safe' Vrms output for both amps I mentioned (which I can then set on the MiniDsp by doing a Vrms to -db calculation and adjusting the line out level of the miniDSP accordingly, I guess)...though going by these discussions, my gut feeling is tending towards 'set it at what sounds right' and don't worry too much about the amp (but probably worry more about the 20 watt speakers I'm going to connect to the Marantz)...cheers!

Last edited by jinja; 3rd April 2015 at 12:22 AM.
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