Hi Lucky
R ends up as negative for the case of Shure M97xE (Lo=588mH, Ro=1345 Ohm, thus R= -169)!
Is it because of this that you wrote the idea works with some cartridges and it doesn’t work with others?
Any suggested way to go around it?
George
Could you please show us the full schematic of the Grado optimised Aurak please ... as an example to us hu neber en to skul
George:
Thanks, I have one of these cartridges and had the same question!
The math for my AT 440MLa works out a little better:
(Lc=490mH, Rc=820 ohms so R=160 ohms
mlloyd1
Thanks, I have one of these cartridges and had the same question!
The math for my AT 440MLa works out a little better:
(Lc=490mH, Rc=820 ohms so R=160 ohms
mlloyd1
It just means your '75us' is too low which will introduce a HF shelf.
Just use 0R and introduce a corresponding HF lift shelf elsewhere.
This in fact maximises the noise advantage of this technique.
Gotta dig up the really old valve preamps which did this.
I thought R = 2L - Rc law almost covered them all, but evidently not ! For exceptions, rejig to R = 3L - Rc seems the way, which is easy enough. For the single ended Aurak 3 sketch, using standard value parts
R24 = 12k
R25 = 4.7k
R22 = 1k
C10 = 75nF
C13 = 1nF
...... for example.
LD
Thank you, George.
Interesting that some of the claims include armature motion is magnetically damped, with claimed benefit to managing mechanical resonance attributed to tip-mass, as was the common attributation in the day. Discussion also asserts that conventional suspensions implement this damping via suspension elastomer. I think the point is that some damping of this resonance is deemed necessary, and selecting its frequency to be above the audioband is beneficial but not always done.
I would love to see an extended f response plot of a better Grado cart using transimpedance preamp loading. That might tie up a few loose ends for me.
As to overall noise performance of these carts with TI loading, that might be very interesting too with this configuration, I think.
Ultra-low impedance transimpedance loading can be generally good for external noise performance per se in MM/MI carts. Aside from irreducible theoretical preamp/cart noise, there is always general interaction with external environmental sources and for all cart types, certain types of such noise might benefit from low impedance loading in principle.
Even single ended TI loading might provide benefit to certain types of external noise, for MM/MI cart types generally. And immunity from effects of C loading with TI loading makes relatively long cable runs possible, at low noise, even in SE connection.
LD
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Whith poles moving up in frequency, either Lcoil is smaller or Rcoil is larger.
The first one results in less noise, the second one in more noise when Rcoil is the only resistor in series with Lcoil.
When for instance looking at the 580mH and 460 Ohm Cart that I used, I had to insert a 700 Ohm resistor in series to fulfil R= 2*Lcart-Rcart.
This gave a pole at 318 Hz.
Had I left the 700 Ohm, the pole would have been shifted down to 126 Hz and noise generated from the 700 Ohm resistor would have been avoided.
After some recalculation of resistors around the first stage, this would have given a slight improvement in dBA.
However a 700 Ohm resistor generates 3.5 nV/rtHz as against 5.1nV/rtHz for the OPA 1642, so I do not expect anything significant.
R=2*Lcoil - Rcart is good enough and no reason to worry.
Hans
Here it isCould you please show us the full schematic of the Grado optimised Aurak please ... as an example to us hu neber en to skul
Hans
I would love to see any of these. I did an EF184 2 stage version which works well. Single ended, with cathode followers. Hand drawn schematics only though, if I can find them I'll post.
LD
I was more thinking (getting nursery sorted over the weekend so only limited thinking time), that the lower the pole, the more gain you need. Front end gain is a key source of overall noise so there might be a sweet spot. But if the series resistor dominates noise that's not a worry.
Plus I'm overthinking about what can be done downstream in DSP!
Plus I'm overthinking about what can be done downstream in DSP!
Why not give us the exact details of the Carts you have in mind.
With your DSP in mind, proper advices can be given in more detail.
Hans
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Well I have 3 cartridge options to consider
1. AT150Mlx 2k3Ohms, either 350mH or 450mH (need to confirm)*
2. Ortofon Super OM. 1kOhms 580mH
3. mono strapped super OM (as 2. doubled).
*looking around I think I will need to measure it or find someone who has. There are numbers all over the shop.
EDIT working back to DC from the numbers I have (which are not from AT website but vendors) at 1kHz gives 356mH and 530Ohms, which I can believe
1. AT150Mlx 2k3Ohms, either 350mH or 450mH (need to confirm)*
2. Ortofon Super OM. 1kOhms 580mH
3. mono strapped super OM (as 2. doubled).
*looking around I think I will need to measure it or find someone who has. There are numbers all over the shop.
EDIT working back to DC from the numbers I have (which are not from AT website but vendors) at 1kHz gives 356mH and 530Ohms, which I can believe
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Thank you Lucky. (this will serve mlloyd1 too). I will try it.
And if I am not asking too much, I would love to see you show us the methodology of calculating the components around IC1 in relation with the R formula.
Thank you Ricardo. (may I ask you for any reference to the preamplifier you designed which Scott mentions in post #226? )
George
For your AT150MLx a modified Aurak has to be used, because 2*350 - 2300 is a negative figure, but don't worry I can fix that.
With this modified Aurak I can give you the value for the series resistor for both Ortofons.
What I found for the mono Ortofon is 330mH and 700 Ohm, but that may be a different type.
So when having the exact figures for the 3 elements, I will present you a solution for an amplifier that only does the 75usec correction.
Within your DSP you have to do the 318 and 3180usec.
Hans
Yes, soz read the post, did some stuff, then replied. So missed the edit. Seems plausible 530R.
OPA1632 should be great, superb gbw amongst other things, needs good layout of course.
LD
IIRC along the lines of these style arrangements, which have an absolute limit of re/4 for noise floor. So for .28nV (~5 Ohms noise floor) I can't see running it at less than 1mA for no rbb at all. These circuits are clever reuse of current to effect paralleling of devices but in the end device physics rules the day. Every few years we had a customer that wanted an op-amp that had 1nV noise but a total Iq of 100uA.
L C Audio Technology / RIAA/MC Amplifier