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Phono Preamp Pt2: MC

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 10:12am
"The RIAA recording characteristic establishes a maximum recording velocity of 25cm/s in the range 800 to 2500Hz. Typically, good quality records are recorded at a velocity of 3 to 5cm/s."

Ref: Audio/Radio Handbook, National Semiconductor, 1980.

If a cartridge output is rated for 5cm/s (which many are), then the output can reach up to 5 times that (5 equals 14dB).

This suggests some headroom is built into vinyl to allow for signals above the average which music contains. Those familiar with recording meters having recorded vinyl onto analogue tape will have noticed considerable short duration peaks above the average output level. Often cassette recorder manufacturers would arrange for a LED to detect these peaks and thus average recording levels would have to be set quite low to accommodate such peaks.

The following, but relating to digital audio, tends to confirm +14dB  (5x) as being around the maximum recorded level:

"As a result, the common practice of mastering music involved matching the highest peak of a recording at, or close to, digital full scale, and referring to digital levels along the lines of more familiar analog VU meters. When using VU meters, a certain point (usually −14 dB below the disc's maximum amplitude) was used in the same way as the saturation point (signified as 0 dB) of analog recording, with several dB of the CD's recording level reserved for amplitude exceeding the saturation point (often referred to as the "red zone", signified by a red bar in the meter display), because digital media cannot exceed 0 decibels relative to full scale (dBFS). The average level of the average rock song during most of the decade was around −18 dBFS."

Ref: http://en.wikipedia.org/wiki/Loudness_war#1980s

Likewise, vinyl has a red zone past which groove cut over happens - an analogy of saturation. This having been recognised by RIAA by establishing the maximum recorded velocity of 25cm/s (+14dB) 800 - 2500Hz. All the information presented above suggests there is also headroom above and below 800 - 2500Hz but the actual limits are not stated.


Edited by Graham Slee - 28 Jun 2012 at 11:59am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 11:10am
Originally posted by franklin franklin wrote:

Everytime I see NE5534 I feel dramatic or on a "dark side", cursed.
Not that I don't like NE5534, but .....
Good to see we are getting away with something else Big smile


The NE5534 is not the easiest of op-amps to implement. The NE5532 (dual op-amp) is much better not only in that it is internally compensated for unity gain, but it is faster than the NE5534 when it is similarly externally compensated. This can only be so because of emitter degeneration in the NE5532 (a good thing), and the dead give away is right there in the data sheet, although totally absent from the internal schematic... the faster slew rate (9V/us), the higher noise (5nV x root hertz) and the lower open loop gain are all highly suggestive of emitter degeneration.

We can try the Genera MC with the NE5532 instead of the NE5534 - at least it doesn't need an adapter board.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg [RIP] Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 11:43am
OK, got it now, thank you for the explanation.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 12:58pm
Although I'm on familiar ground with the NE5532 I've decided to have a go first with the LM833, if only to please those anti-NE5534. I have not spent much time analysing the LM833 but the data sheets are here...

https://www.national.com/ds/LM/LM833.pdf (download and treasure this one - it won't be around much longer)

http://www.ti.com/lit/ds/symlink/lm833.pdf

http://www.onsemi.com/pub_link/Collateral/LM833-D.PDF

The National Semiconductor data sheet shows the internal schematic. If we take slew rate S.R. = Gain Bandwidth x 0.3 (derived for us in "The Art of Electronics"), and we note the Gain Bandwidth is stated as 15MHz, then S.R. is 4.5V/uS. However, the data sheet declares 7V/uS. Either it is slew rate enhanced or emitter degenerated but neither is declared so no surprises there then...

To work out the RIAA equalisation network we need to know the size of capacitors the op-amp will drive, derived from C= IT/V or, rearranged, C = I/(V/T) which is I/S.R. (current divided by slew rate).

What current? The devices undistorted output current.

The size of capacitors here not being the op-amp capacitive load by the way.

The only data sheet which goes anywhere near explaining its maximum undistorted output current here is the Texas Instruments one. It is disguised as VOM in the electrical characteristics table. It shows it can typically swing 10.7 volts positive into 600 Ohms, but note the op-amp is driven in differential mode at +/-1V input. The gain being around 5-6 here and the differential input combining to produce very little distortion. With all that negative feedback I guess it would drive 600 Ohms, but I'm sure this op-amp isn't a fully specified 600 Ohm driver - the other data sheets show 2k Ohms. Texas claim 10.7 V/600 Ohms which is nearly 18mA. On-Semi say 13.7 V / 2k Ohms which is nearly 7mA, and National, 13.4 V / 2k Ohms which is also nearly 7mA.

We'll need about 6 volts peak into the load - a preamp input - let's say 10k Ohms, so that uses 0.6mA, and an output filter to keep the RIAA roll-off going where the non-inverting characteristic lets it flatten out. That filter is a simple RC job and the R sets the output or driving impedance and its maximum should be around 1k Ohm. This is where calculations get really complicated. The output will have fallen to unity gain where the filter takes effect, but as discussed earlier, that can be as high as 280mV, so at the highest frequencies (not music but stimulus noise) this 280mV sees a 1k load (because the cap to ground will be a virtual short), and 0.28 V / 1k Ohm uses 0.28 mA.

So if we can get 7mA out of the op-amp and we've used 0.88mA satisfying the output, we've about 6mA left.

So from I / S.R. we can now do the simple math: 6 / 7 giving 857 pF. This is the dominant capacitance of the RIAA network and will let the op-amp deliver its full 7V/uS slew rate.

The typical RIAA feedback network comprises 2 capacitor stages in series so the 857 pF is the sum of the two capacitances in series, so we know the actual value of the smallest capacitance will be higher than 857 pF. 1nF (1,000 pF) is a easily obtainable value in the high pulse rating (dv/dt) type capacitors we need for good RIAA sound.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 3:53pm
At this point I thought it a good idea to upload the schematic with as yet some undecided component values, so we can follow what's going on.




Edited by Graham Slee - 28 Jun 2012 at 4:05pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 6:31pm
Thanks Graham, I have learnt some new things in the first page of this thread. The MC/MM difference is much clearer to me now. Thumbs Up
Jon
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 7:43pm
If we take a look at R8A we can see that the voltage to the inverting input of the op-amp is developed across it. Therefore the source impedance this input sees is the value of R8A with the RIAA network in parallel with it. R8A is domineering it all, so we can simply call the source impedance the value of R8A whatever that will be.

If we use the LM833 and knowing we'll have to use the value 1nF for the high frequency RIAA equalisation staging, I predict the value of R8A is going to be around 100 Ohms. Luckily, the input noise of the op-amp is referred to a source resistance of 100 Ohms! Going below this value isn't going to improve noise matters. It can in some applications where the noise of a resistor dominates, but 100 Ohms was obviously found to be optimum by the op-amp designers.

I'm sure there will be a question on resistor noise being different between different resistor types, but this isn't the case. Noise increases with increasing resistor value.

The main input (non-inverting input) should also represent 100 Ohms or less, otherwise there will be a possible noise differential. If we take the MC internal resistance to be around 30 Ohms maximum, the value of R3A, in series with the input, should be around 70 Ohms. Because there is a loading resistor R1A in parallel with the cartridge and this will usually be a small value, we can go to the nearest preferred value above, which is 75 Ohms.

We need this 75 Ohms to try and roll off high frequency energy along with C2A. As the total source resistance is going to be around 100 Ohms we could use a value of 4.7nF to bring down the input voltage above 300kHz. This would make sense from a RF interference standpoint. However, if we used a Moving Iron cartridge this would represent a heavy capacitive load. I think we'll have to forget making this Moving Iron compatible. For a MC the 4.7nF capacitor isn't capacitive loading because it is "isolated" by the 75 Ohm resistor. If anything, the cartridge sees a heavier load but at frequencies far higher than the audio range.
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