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Designing a universal diff-in/diff-out Head Amp
Designing a universal diff-in/diff-out Head Amp
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Old 9th May 2018, 05:30 PM   #1
Hans Polak is offline Hans Polak  Netherlands
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Default Designing a universal diff-in/diff-out Head Amp

There are quite a number of circuit diagrams for (differential) Head Amps available;
point is that most of them are either very complex, consuming large amounts of energy, or to be assembled from a large number of selected Fets as input devices.
To make a difference, I used the following points:
  • Diff in and diff out
    No caps in the signal path
    To be either supplied by batteries or mains supply.
    As simple, yet as powerful as possible.
    Configurable as a voltage or as a current input circuit.
  • High grade differential MM preamps can be as good as having only ca 4nV/rtHz A-weighted equivalent input noise (566nV from 20 to 20Khz) with the Cart connected, equalling a S/N of 20Log(5mv/566nV)= 78.9 dBA.
  • When connecting a low output impedance Head Amp instead of a MM Cart, this noise from the MM preamp will drop some 4 dB, going to around 350nV or 2.5nV/rtHz.
    Output noise from the Head Amp will have to be a bit higher, not to drown in this MM Preamp noise.
  • There are two main groups of low output MC carts, one producing around 0.5mV and the other group only 0.25mV, both at 5cm/sec@1Khz.
    Since one can regard a S/N of 75 dBA as sufficient, this calculates into 89nVnoise for the 0.5mV version and 44nV for the 0.25mV one, both from 20Hz to 20Khz, or the equivalent of resp. 0,63nV/rtHz and 0.32nV/rtHz for the two versions.
  • ]For the 0.5mV Cart, a gain of 10x or 20dB will be needed for the pre-pre. The 0.25mV Cart should be amplified 20x or 26dB.
Now the big thinking mistake that I made was to calculate down into a flat linear fashion and looking for 0.63nV/rtHz and 0.32nVrt/Hzsolutions. Dead wrong !! The point is that the noise from the Head Amp will also have to be measured Riaa corrected and A-Weighted, effectively leading to at least halving the equivalent input noise figure!
  • So for 0.32nV/rtHz equivalent input noise, you only need a Head Amp with a flat equivalent input noise of 0.64nV/rtHz. For a 0.5mV Cart a Head Amp with 1.26nV/rtHz will be adequate.
    Even a dual AD797 in differential setting will be adequate to meet the goal for a 0.5mV Cart, either configured as a current input amp, but also as a the more usual voltage amp with a Cart’s termination resistor.
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Old 9th May 2018, 05:36 PM   #2
Hans Polak is offline Hans Polak  Netherlands
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The difference between a current input and a voltage input diff amp, is rather small, as can be seen below.

Edit: Image is posted on #6

With a handful of components, both can be realised in one go. There are two inputs visible. When connecting the floating Cart to input 1 and a short circuit to input 2, you will have a current input amp. R1 and R2, in combination with the Cart’s internal resistance will determine the gain. Connecting resistor R6 to input 1 and the Cart to input 2, will result in a voltage input amp, where the ratio 1+(R1+R2)/R6 will set the Gain and R3 + R4 serving as the Cart termination.

As mentioned in point 6), for a 0.5mV Cart, U1 and U2 can be the 0.9nV/rtHz AD797 and you are ready to go with a minimum amount of components, yet hitting the target of 75dBA S/N with a 20dB Gain setting with a THD below -90dB up to 20 Khz.

For a 0.25mV Cart, you will need amp’s with halve the noise as produced by the AD797. For this I used John Curl’s patent, dating back more than 40 years ago, but now modernised with components that weren’t available at that time. Basically the circuit looks like:

Last edited by Hans Polak; 9th May 2018 at 05:58 PM.
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Old 9th May 2018, 05:41 PM   #3
Hans Polak is offline Hans Polak  Netherlands
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The circuit is fed from a double 1.5 Volt supply. John Curl used 8 paralleled transistors in his design, but with modern ZTX851/ZTX951, two transistors are adequate to meet the noise target of 0.64nV/rtHz after Riaa and A-weighting. In said Patent, the circuit was used in a current input setting, with a resistor directly between in and output. But the circuit can just as well be used in a voltage input mode.

There are a few flaws to be mastered however, being uncontrolled current to flow into the Cart and the far from zero output impedance. When powered by batteries, but also when powered from a mains supply, the slightest difference in V+ and V- or a difference in Hfe or Vbe between both transistors will cause a current to flow into the Cart. It might be harmless as some say, but it is better to stay on the safe side. This can be solved with an LT1884, having only 50uV input offset, regulating the positive supply with a Fet. This is a very simple and effective mechanism, keeping the Cart current below 1uA at all time.

Second point is the high output impedanceof the circuit that heavily affects gain of the circuit when loaded by some feedback resistor. To overcome this, I have added a LT6203 op-amp as buffer, a 1.5 nV/rtHz videoamp that can operate from the same supply, thereby giving a low output impedance.

In a differential setting, meaning doubling the amount of all components, total current is ca. 17mA. When using 1.5 Volt Alkaline penlight’s, you could power this circuit for almost 90 hours. With 1.5 Volt Alkaline D cell’s even 900 hours !
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Old 9th May 2018, 05:49 PM   #4
Hans Polak is offline Hans Polak  Netherlands
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The complete sub-circuit in its most complex form with these added features is shown below. One could however leave the LT6203, only making it a bit more complex to find the right value for a feedback resistor but with hardly any effect on THD. So in its most basic form, only U1 is stronglyadvised !

Both the LT6203 and the LT1884 are duals,so only two SOIC packages have to be used, adding almost nothing to the neededPCB space.

There might be some concern that at powerup some uncontrolled current could flow into the Cart. However, because of thedifferential set up with no connection to Gnd, this is avoided and needs nofurther attention since being far under 100uA Imagine having a 10 Ohm Cart, that is used in current mode. 0 dB@10Khz, means ca. 5mV rms. In that case+/- 700uA peak flows through the Cart. So one can safely assume that start-upcurrents below 100uA are harmless.
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Old 9th May 2018, 05:55 PM   #5
Hans Polak is offline Hans Polak  Netherlands
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Now the proof of the pudding. Four configs for resp. current / voltage / 0.25 mVolt and 0.5 mVolt all put in a Simulation.

To start with, a 4nv/rtHz differential input MM was configured with a 580mH/460 Ohm Cart. This was done by means of two input buffers with parametrised op-amps set to the exact noise power.

The 0.25 mVolt current input version had lower noise as its voltage brother, just as expected. But for a reason that is unclear to me, theAD797 produced more noise in the current mode. This current mode version however performed just 0.25dBA below the 75dBA target, an insignificant and still acceptable difference.

All versions produced a very low THD, below120dB from 20Hz to 20Khz at 0 dB. CMRR was way below 180dB up to 20Khz, because of the perfect match of allresistors. In practice one can easily keep this under 80dB with 0.01% resistors at vital positions. To put this in perspective, one can touch the Cart’s wires with bare fingers, without hearing anything at all coming from your speakers.

For the simulation results, see images below.
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File Type: jpg Curl7.jpg (617.7 KB, 373 views)
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Old 9th May 2018, 05:57 PM   #6
Hans Polak is offline Hans Polak  Netherlands
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Since the image in posting #2 didn't seem to come through,
I have posted it here again.
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File Type: jpg Universal.jpg (177.5 KB, 368 views)

Last edited by Hans Polak; 9th May 2018 at 05:59 PM.
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Old 9th May 2018, 09:56 PM   #7
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Hi Hans,

Couple of questions. Have you considered the case of digitising case where you are going flat into the ADC and only need around 20dB for MM and 40dB for 0.5mV MC?

Also regarding CMRR. You don't gain anything until you actually do a conversion. One idea I have seen is this http://www.thatcorp.com/datashts/dn140.pdf#page=14 . Contentious outside of pro circles but interesting to consider.
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Old 10th May 2018, 08:23 AM   #8
Hans Polak is offline Hans Polak  Netherlands
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Hi Bill,

I concentrated mainly on the function of a Head Amp, being to bring its input voltage to the level of an MM Cart. However, there is absolutely no problem to give the 0.5mV version, using the AD797's, a gain of 40dB. Just a matter of changing 2 resistors. But it would also be easy to make a second diff-in/SE-out 20dB gain flat amp suited for MM Cart's, for instance with an OPA1642 a 1652 and even a 1632.

Maybe you can go in a bit more detail regarding CMRR. If I understand you correctly, your point is that CMRR is dependant on the whole chain involved, including the diff to SE conversion, a point that I fully support. But I don't get the message so much how this relates to page 14 in your link in this very case. What ADC do you have mind, does it expect a diff input signal and does it support CMRR, or do you have to remove the CM signal before offering it to the ADC ? It's all possible, op amps to perform the needed functions are all on the shelves.

Hans
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Old 10th May 2018, 09:49 AM   #9
billshurv is offline billshurv  United Kingdom
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Designing a universal diff-in/diff-out Head Amp
Hi Hans,

No problem, as it's something I still worry about and may have missed the point. If you are straight balanced all the way through, then overall CMRR can be destroyed by one poorly selected resistor*. The Cohen cross coupled circuit, whilst originally designed for receiver purposes can be used as the output drive stage, removing CM signal so you effectively get a clean balanced and differential output.

If this matters other than paranoia is of course a different issue

*There is the Whitlock INgenius designs for that of course.
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Old 10th May 2018, 07:11 PM   #10
Hans Polak is offline Hans Polak  Netherlands
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Bill,

I have the feeling that you are overly worried for CMRR problems. Just to respond to your question, I have modified the MM preamp into a real world flat 20dB MM preamp, with the OPA1652. This circuit still has the 4nV/rtHz equivalent input noise after Riaa and A-weighting with a 580mH + 460 Ohm Cart.

The input circuit amplifies 40dB, and has an excellent CMRR of 40dB insensitive to resistor values deviating a few percent from nominal. See R3 vs R22, one has the correct 4k95 value needed for a gain of 40dB and the other is 4k5, 10% off.

This input circuit is followed by a diff to SE convertor, attenuating -20 dB. This circuit is very sensitive to a correct resistor value. But as you can see I made R17 1K1 instead of 1K, again 10% off the ideal value. Therefore this convertor only has 20dB CMRR. However, in combination with the input circuit, CMRR becomes 60 dB, see image below. When using a 1% instead of a 10% resistor, you already get 80dB CMRR for this MM circuit alone. Measuring CMRR from the Cart upwards, adds another 20dB because of the Gain of the Head Amp.

So with only 1% resistors a 100dB CMRR, how hard can it be ?

Hans
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