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Phono Preamp Project

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Jan 2010 at 7:38pm
Get the paracetamol out! Here comes the RIAA bit!

Here is a simplified active RIAA filter courtesy of one of the best guys in the industry: Walt Jung, from an Audio Engineering Society pre-print.



Now guess what we're going to discuss first? Yes, slew rate! (What? Again?) Wink

We need to know if the slew rate is sufficient for this RIAA phono preamp, and how the op-amp slew rate affects the choice of the RIAA replay filter component values.

Just look at what's hanging off the op-amp output! A large value of capacitance in series with a small value of resistance (C1 in series with C2, and R3 in series with that). The op-amp has to drive that at 12V per micro-second. If it cannot drive it, it will become unstable and ring, if not oscillate. We could over compensate the op-amp, but that would be a waste.

To answer if and what the LF356 output can drive we need a formula...

And here again it's Q = CV = IT

The first answer we need is to know if 12V/uS is sufficient, and a derivation of the formula gives

SR (slew rate dv/dt) = 2pi x V(peak) x (1/T)

(derived from from "Audio/Radio Handbook", National Semiconductor, 1980)

We need to know the maximum output voltage (V), the desired frequency response to derive time (T), and later, the op-amp's output current (I), to find what capacitance it can drive.

Now, the datasheet tells us it can drive some real heavy capacitive loads, but it won't be performing all that well while doing that: we want the op-amp to be lightly loaded to give off its best.

So, we now need to gather the terms V, T and I.

Voltage

"V" is going to depend on a couple of considerations: 1] the power supply voltage, and 2] more importantly, how much the LF356 can take without its input leaving its linear (or near linear region). This will, in turn, determine the overload margin or headroom. When an input leaves its input linear region it still works, and it'll probably measure quite well on THD (total harmonic distortion), but on the inside it is under undesirable stresses if quality audio is what we're after.

We found from the previous post, that the LF356 is 8 times more linear than a transconductance input op-amp. The maximum (peak) linear input for a transconductance input is about 50mV (before things get forced), so here it's going to be 50 x 8 = 400mV.

That should be more than plenty, but hold on! The RIAA curve tells us that the output of a magnetic cartridge rises with frequency - how high?

Musical instrument's fundamental frequencies don't go as high as you'd imagine, say 8kHz (often much less with traditional instruments). But perhaps the early harmonics have good output, so let's allow 20kHz. At that frequency the cartridge output is ten times higher than it is at 1kHz. Taking a healthy output of 5mV, used on a "hot pressing" (because the young are into vinyl too...), its output is going to be the same as a 10mV output cartridge (ref 1kHz), and so the input should be able to handle 100mV. Now that's RMS, so peak will be 140mV, which means we have headroom of nearly 3 (our 400mV divided by 140mV). Studio PPM meters redline at +8dB which is about 2.5. So on most high energy peaks this input will not distort as badly as a transconductance input (this is pereceived distortion, not steady state THD as based on a sine wave). That covers the high frequency requirements.

Now, the rest (the lower frequencies which are lower output from the magnetic cartridge) are going to depend on what the preamp output can swing.

Some hi-fi magazines state maximum input as being where the output of the phono preamp hits 1% THD.

To determine what the output can swing before clipping (hitting the supply rail), we need to set out the gain we're looking for and determine the supply voltage...

What about gain? I don't think it reasonable to expect a high performance phono preamp to directly drive a power amp. I'd be happy if it did a gain of 100 (40dB) which would give an output of 500mV for a 5mV sensitivity moving magnet cartridge.

What is going to be the largest signal the phono cartridge is going to output? Consider something few others consider - the test tracks on a HFS75 test record (or similar) used to optimise cartridge set-up. Band C tracking test is +18dB and 18dB is times 8. So with a 5mV output cartridge its (normalized) output is going to be 40mV giving rise to a 4 volt output - can we accomodate that with a voltage supply suitable for the novice DIY enthusiast?

One of the highest and easiest to obtain power supply voltages is the CPC 24 volt plug top "wall-wart", which also suggests this design needs to be single supply (and why not keep things simple?).

However, that 24 volt is not regulated, so we need to choose a regulator that can do a slightly lower voltage, say 18V.

On an 18 volt supply the maximum peak to peak swing is 18V if the op-amp can output rail to rail. Most cannot, and can only get within +/- 1.5 volts of the rail. So the maximum peak to peak is 15V. We are interested in the RMS voltage which is 10/28th of the peak to peak output, or 5.35V. So with a gain of 100, the maximum input is going to be around 53mV. That's +14.5dB for a 5mV cartridge playing a "hot pressing" and +20.5dB for the same cartridge playing band C of HFS75's worst tracking test.... good enough?

So, as far as voltage is concerned, it's 5.35V RMS (7.5V peak).

Time

1/T is frequency in the case of the first question we need to answer. It's the highest frequency we want to reproduce accurately. Often, in the past, designers simply plicked 20kHz out of the air, but is 20kHz really the highest frequency we want to reproduce accurately?

Often, older phono preamps emphasised clicks and pops - well that was my earliest experience of phono preamps. Read about clicks and pops and you'll find that vinyl clicks and pops are all in the audible range. True, because they can be heard, so I guess that's how they justify that, but what about the leading edge of a scratch? Could a scratch be chissel shaped? I reckon so. It will have steep sides. So if we imagine the stylus is moving across the record (that's an easy one - for years man thought the Sun revolved around the Earth) at speed, then it finds it has taken off across this ravine, and then hits the cliff on the other side - ouch! It is not trying to reproduce the full width of the scratch, but the "cliff-edge" it just hit. That could be just ten percent of the width?

So will 20kHz cover that? I don't think so. Maybe 200kHz? So let's see if 200kHz will fit our 12V/uS slew rate...

Is the slew rate sufficient?

From SR (slew rate dv/dt) = 2pi x V(peak) x (1/T)

SR should equal or be higher than 2pi x 7.5V x 200kHz... is it?

The answer is 9,424,777.961 V/S which is 9.4 V/uS, so 12 V/uS is sufficient!

Can 12V/uS drive the RIAA filter?

Here we need the op-amp's output current (I). When we find that we can use this derivation C = IT/V or I x 1/(SR x 1,000,000) (as you can see, this is derived from Q = CV = IT) to find what capacitance the op-amp will drive.

So let's take a look at the data sheet (pdf)

And guess what? It doesn't say!

Often, output current is not specified in op-amp data sheets, so how are we going to find it out? One way is to look at the output voltage swing which is usually referenced to a particular resistive load. Here two loads are shown: 10 kOhms and 2 kOhms. Into 10 k it swings +/-13V and into 2 k it swings +/-12V. By the looks of it, 2 kOhms seems to be the limit (or corner) where its output swing starts to drop with increasing load (maximum power theorem), so I think we can assume that Ohms law will tell us its output current.

I = V/R

12V/2,000 Ohms gives 6mA. Huh!

That's where the output stage similarity with the NE5534 (which will do 16mA) ends!

But still, the LF356 has got all the other features desirable for a phono preamp, so let's carry on.

So by dividing 6mA by the inverse of and normalized slew rate we will see just what capacitance its output can drive whilst maintaining both stability and slew rate.

from C = IT/V we get 0.006 x 1/12,000,000 = 0.000,000,000,5 Farads or 500pF!

Now, luckily, that 500pF is the smaller of the two capacitors required (C2 in the above diagram) - the one that dominates at higher frequencies. But will 500pF (470pF being the nearest "prefered" or available value) result in unusable values for the rest of the filter? The only way to find out is to try it! This is where phono preamp design starts to become an empirical excercise.

I'm going to finish this posting here to allow a page break (hopefully), and continue it in the next reply.



Edited by Graham Slee - 04 Jan 2010 at 12:18pm
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 04 Jan 2010 at 10:11pm
Looking at Walt Jung's diagram again...



We see the dominant capacitor C2 is 10nF (0.01uF), and we worked out, that by not compromising the LF356 slew rate, C2 needed to be 500pF. What if we traded a little compromise which could mean having to over-compensate (or the op-amp's capacitive load drive compensating for us) such that slew rate is halved? 6V/uS is only 60% of what we said we'd need to go as high as 200kHz (to accomodate record surface damage)?

If we could make C2, in the case of the LF356, 1nF (1/10th of the Jung value), we could simply scale the component values shown and we should get the required RIAA replay curve. The Walt Jung diagram results in 40dB gain (ref 1kHz), which is the gain we want.

It would mean R3 being 1kOhm, and that will contribute extra noise, but maybe not that much to resort to a complicated solution, or scrapping the use of the LF356 altogether. The LF356 has so many good points, it would be a shame to have to drop it.

At this point we could build a trial circuit and measure it, or simulate it. Simulations never tell the full story - a real circuit can be measured - but it may save us time by picking out unforeseen problems.

I think at this point it would be a good idea to simulate and discuss the results. Stay tuned!

PS. page break didn't work...


Edited by Graham Slee - 04 Jan 2010 at 10:12pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 05 Jan 2010 at 12:30am
This is fascinating, thank you very much for sharing your knowledge on this project.
Jon

Open mind and ears, whilst owning GSP Genera, Accession, 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: 05 Jan 2010 at 6:11pm
A PCB, case, and components kit will be offered at diy-audio-kits.com, plus we offer to build and test the Genera for all those not blessed with DIY skills - watch out for updates here.

Here is the schematic for the "sanity-check" simulation...



C test is where the capacitive load went.

The phase margin with a 500pF load is...



...(where gain cuts through 0dB) 180-120 = 60 degrees, which will not lead to excessive ringing.

The gain margin with a 500pF load is...



... around 25dB, so just better than our rule of thumb ten.

The phase margin with a 2,200pF (2.2nF) load is...



...180-145 = 35 degrees which indicates execessive ringing, but we may be able to compensate for this in a real design.

The gain margin with a 2,200pF (2.2nF) load is...



... around 15dB which is less than the rule of thumb ten (20dB = 10)


Edited by Graham Slee - 17 Jan 2010 at 4:27pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 2010 at 6:25am
By now most should realise phono preamp design is a bit complicated...

Do you think there are lots of op-amps well suited to phono preamp use?

Here, we have found one of the best in the LF356, but even then it's not perfect or easy to implement. Ever wondered why there are so few? Is it a fact that vinyl almost became extinct in the eighties? What suitable op-amps existed prior to then?

I have seen one since and many rave about it. In fact there are 2 or 3, but they are in fact one and the same, just variations of the same design (one works on +/-18V, another +/-22V). In fact, they are 3, 4, or 5, because there are single and dual types. What should have "clicked" by now is that even with a high slew rate, which is condusive to good audio, without the current, high slew rate is useless. It is little wonder then, that these modern op-amps, although everything sounds different, lead to the eventual subjective conclusion (once the hype has worn off), that there has to be better.

Therefore, although it could look like the LF356 is not the best choice, it has the input linearity needed, and it has a trick up its sleeve that the masses do not see - two additional inputs - and that may just clinch it.

For the time being, we have to deal with the value of dominant RIAA filter capacitor, such that the Genera phono preamp sounds fast - gives a realistic tempo - and relegates vinyl surface noise to last place.

We could simulate this design for the next 20 pages, but I'm sure that will bore the pants off everybody. So I'll launch straight into the wrinkles that will make this a viable design...

We see that with a 2.2nF dominant capacitor we have little phase margin, but place a small value resistor in series with that, and it takes the impedance up to beyond the zero dB crossing point, and therefore provides more phase margin. This begs the question of what is the best phase margin? One thing for certain is that you can't get a full 180 degrees (which ensures absolute stability) because the op-amp (and it's the same for discrete including valves) is rolling-off - it can go no further - high frequency bandwidth is not infinite.

As we have seen, all op-amps or amplifiers in general (when designed properly), when open loop - before the application of negative feedback to reduce the gain to what is required - roll-off from some frequency, and that rate is (or should be) 6dB per octave. That correlates to a phase angle of 90 degrees, which means there is 90 degrees left, and that's the phase margin.

By assuming that using a 2.2nF dominant capacitor, that R3 (in the Jung circuit) is going to be scaled by the same factor, we arrive at 454 Ohms. The nearest prefered value is 470 Ohms. As this is a non-inverting amp, and we make the 2.2nF series resistor the same value, we see we have 6dB gain (x 2) at that frequency and all the way to where the output goes through zero dB. Therefore the phase will be near to 90 degrees (simulation suggest 94 degrees), and so it's going to be highly stable.

But what about gain margin? Is that going to be our rule of thumb ten (-20dB)? No, it stays the same. How can the gain margin be improved? This is done by inserting some resistance between the op-amp output before the negative feedback take-off point.

Coincidentally, the value here that takes gain margin to just better than 20dB is 470 Ohms also. But this has taken the bandwidth down to just 2MHz. It has slightly improved the phase margin to around 100 degrees.

The reduced bandwidth of 2MHz means it will not slew as fast. From our earlier (important) exercise where we found the constant of 8 to go into the slew-rate formula (SR = 0.3mft) we can work out that slew rate is going to be about 5V/uS. This is just over half of what we wanted. In our earlier discussions we thought about using 1nF for the dominant capacitor, but then I explained that meant R3 being 1k, and its contribution to noise could be a "bridge too far". So here we're trading noise with bandwidth...

Those of you who have followed the development of Graham Slee Projects Ltd, will have read me going on about trading bandwidth for noise, and the above proves the case! No, it wasn't just some slick advertising glob, I've been talking about reality all the time! While at this juncture, I am sure therefore, questions may be forming about our ultra-fast Era Gold V? How was that so fast? It could only do what it did by choice of op-amp and the 50KHz cutter head knee (using a similar series resistor in the dominant capacitor leg), which also served to improve phase margin. The value of its equivalent R3 was also larger than the low noise Jung example we are working from here... again it traded noise for bandwidth. But the slew rate of the Era Gold V could obviously not be the same as that of the op-amp, loaded by the RIAA filter, as it is - the op-amp's slew rate being 250V/uS! The filter did reduce it down, and the op-amp itself has a slew rate limiting network in its output stage, otherwise it would have been unstable. Also, as I have said above, without output current, slew rate is meaningless - the Era Gold op-amp does 50mA! nearly ten times the LF356. So why aren't we using it here? That's because of the number of "infant mortals", the Era Gold V op-amps failed noise-wise at the rate of 33%. You probably won't have the noise testing facilities we have, and therefore, even with good hearing, would, in all probability, not be able to tell. The LF356 is in a different class regarding noise failures, such that you have a 99.9% chance of having two perfect units in this design.

So, with just over half the slew rate we deduced we needed in a utopian world (even so, it will handle "full-sensitivity" stimuli up to 100kHz well), I think we have here a high performance phono preamp in-waiting. At 5V/uS, it is far faster than the 60's and 70's designs (that still exist today in audiophile disguise) which were lucky to reach 1V/uS.

Next we need to establish the rest of the RIAA filter component values, and then evaluate a few other considerations before arriving at a schematic to try out.


Edited by Graham Slee - 06 Jan 2010 at 7:56am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iamalexis Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jan 2010 at 11:58am
it is very interesting to be given an insight into the design side of things. i didn't realize quite how much is involved in the process. thanks for sharing this information/knowledge with the forum Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RobW Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jan 2010 at 10:36pm
Am trying hard to keep up but at every turn I find myself having to check my compass. Is there a good primer out there for the basics? I'm a mechanical engineer and have this annoying habit of trying to look at this subject through that filter ... electronics seems a bit like alchemy. Advice appreciated - I'm a late blooming enthusiast.
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