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Old 3rd November 2009, 04:19 PM   #1
syn08 is offline syn08  Canada
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Default HPS 4.0 phono stage

Here's the HPS4.0 phono stage schematic.

For simplicity, I am posting here only the head amp. The rest of gain stages and distributed RIAA correction is identical to HPS3.1, having of course the same performance of +/- 0.1dB.

While the architecture is similar to HPS3.1, there are significant differences:

- Uses only bipolar devices (see the discussion below).
- An emitter follower stage is employed, to provide high input impedance. This stage is coming with a noise penalty, but that's the price that has to be paid.
- Current sources are defining the input stage collector current.
- A new power supply. While the opamps are fed from a standard pair of LM317/LM337 pair of regulators (so no more Jung superregulators), the low noise input stage (while still cascoded) is fed by a pair of onboard parallel regulators. The input stage draws low and almost constant current, so the parallel regulators (on board) can be designed for minimum power dissipation.
- Still zero caps in the signal path.
- Gain

The input followers and the bipolar input gain stage are both running at about 3mA. That's a good trade between low noise and a reasonable input bias current. If the input devices are beta matched, the base currents are cancelling each other and, as a result, the current through the cartridge is minimized. Experimentally, with the Hitachi bipolar pair, I got the current as low as 0.5uA.

Of course, the big question is about the low noise bipolar devices. Which are the best, what can be reasonable used? I have experimented with four complementary pairs:

1) Hitachi 2SC2547/2SA1085

These are the best, but unfortunately, as with other so many great parts, they are now almost extinct. You can still get them, but they are becoming rare and expensive. What makes them unique is not only the low noise, but also the large beta. I was easily able to select a pair with a beta of 400, which is clearly an advantage in terms of input bias current. With these devices, the HPS3.1 head amp noise was 0.28nV/rtHz and the input bias current was 0.5uA.

2) Sanyo 2SC3504/2SA1433

These devices (coming in the extended TO92 case) are part of the Sanyo video devices series. Although not intended for low noise, the FBET polysilicon emitter process is providing extremely good noise performance, at par with the Hitachi pair above. However, beta is not that high; it is difficult to select pairs with a beta over 200. This leads to an input bias current of about 1uA. The noise performance was measured at 0.31nV/rtHz. These devices are in full production at Sanyo and can be purchased through distributors at a very reasonable price.

3) Sanyo 2SC3601/2SA1407

Another FBET pair from Sanyo, in TO126. Consequently, they provide exactly the same noise performance and the same beta, on the low side. I was unable to find any benefit comparing to 2SC3504/2SA1433 above, so this should be an option only if one has these devices at hand. While they are also in full production at Sanyo, this pair is expensive (and 4 pairs are needed).

4) Fairchild KSC3503/KSA1381

A dirt cheap pair from Fairchild, coming in TO126, they cost about $0.6/pair at Mouser. They have very good noise performance (about 0.7nV/rtHz) and decent beta (I was able to easily find pairs with beta 300). With these devices, the noise performance was measured at 0.41nV/rtHz, at par with the best available phono stages available today, while the input bias current was 0.6-0.7uA.

So, pick your poison

The construction is on a single board, the size is about the same as HPS3.1 I'm now looking into building a SMD version. Sanyo is currently manufacturing a FBET device in SMD, but I was so far unable to source it. Otherwise, for top performance, input devices need to be reasonably sorted and matched for beta.

So, which one is better, HPS3.1 or HPS4.0?

From a noise perspective, they are both dead silent, in 97dB efficient horns, ear on grille. From a distortion perspective, HPS4.0 is slightly better, due to a larger loop gain in the head amp. From a RIAA compliance they are identical, provided that 0.5% or better tolerance is provided for the passive devices defining the RIAA response. According to my (sorry, engineer level) ears, they sound identical and wife in the kitchen didn't also comment in any way. More to follow (friends, neighbors, dog, etc...) but at this point I think it's safe to assume they sound pretty much the same. Then, what's left?

Device availability. P channel low noise JFETs from Toshiba are extinct and LSI still has to provide an equivalent (also needs to be validated). Low noise bipolars are readily available, even if they are not intended as such. Of course, there's always the argument of DC through a cartridge as baaaaaaaddddd, but I think that, within reasonable limits, this is just another myth. I can't imagine that each and every supa dupa high end phono stage is nowadays built with low noise JFETs. Either as-is, or by using an input cap, bipolars ought to be used for such low noise applications.

On the flip side, HPS4.0 needs a reasonable level of beta sorting and matching for the input devices. This is for DC purposes only. HPS3.1 doesn't need any kind of sorting, it will work as good with devices out of the tube. However, if you can live with a couple of uA through the MM cartridge, or if you decide for an input cap, then no sorting is required.

Bottom line, HPS4.0 has better noise specs compared to HPS3.1. It should though be noted that this level of performance applies strictly for low impedance MC cartridges. For MM cartridges, the input current noise may contribute significantly, degrading the overall noise performance. But then certainly a MM does not need 0.28nV/rtHz, anything under 1-2nV/rtHz will do just fine, so the complexity of this head amp can barely be justified anyway.

I'll be back ASAP with pictures, measurements, etc... As usual, schematics, Gerbers, measurements will be available on my web site. Meantime, comments are welcomed BTW, for those scared of 0.5uA through the MC cartridge, there's a placeholder for an input cap of your choice

Click the image to open in full size.

Click the image to open in full size.

P.S. Voltage supplies are +/-22V, while the parallel regulators are providing +/-15V for the input stage. These voltages are providing the same headroom/dynamic range of 32dB as HPS3.1

Last edited by syn08; 3rd November 2009 at 04:21 PM.
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Old 3rd November 2009, 04:30 PM   #2
h_a is offline h_a  Europe
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Congratulations to yet another great iteration of your design! Leaves me still ashamed that my power amp is still unfinished while you iterate happily.

Only thing that seems missing for world wide cloning is the option to hook up either MM or MC (switchable gain?).

Have fun, Hannes
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Old 3rd November 2009, 04:35 PM   #3
syn08 is offline syn08  Canada
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Quote:
Originally Posted by h_a View Post
Congratulations to yet another great iteration of your design! Leaves me still ashamed that my power amp is still unfinished while you iterate happily.

Only thing that seems missing for world wide cloning is the option to hook up either MM or MC (switchable gain?).

Have fun, Hannes
This is not for MM at all... The input current noise will significantly degrade the noise performance. OTOH, you don't need 0.28nV/rtHz for MM so there's really no incentive to build this phono stage.
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Old 3rd November 2009, 04:51 PM   #4
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As regards the Hitachi 2SC2547/2SA1085, Radionics sell a 'Magnatec' version. Are these the same or just a bad copy? I have never heard of Magnatec myself and Radionics have no spec sheets either?

Magnatec | Semiconductors | Discretes | Bipolar Transistors | NPN Small Signal |2SC2547E
and
Magnatec | Semiconductors | Discretes | Bipolar Transistors | PNP Small Signal |2SA1085E
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Old 3rd November 2009, 04:58 PM   #5
syn08 is offline syn08  Canada
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Originally Posted by DubbLinn View Post
As regards the Hitachi 2SC2547/2SA1085, Radionics sell a 'Magnatec' version. Are these the same or just a bad copy? I have never heard of Magnatec myself and Radionics have no spec sheets either?

Magnatec | Semiconductors | Discretes | Bipolar Transistors | NPN Small Signal |2SC2547E
and
Magnatec | Semiconductors | Discretes | Bipolar Transistors | PNP Small Signal |2SA1085E
Don't know what to say, but if they are not genuine Hitachi I wouldn't touch them with a pole.

I was offered these devices from Dalbani in the UK, but then I decided I have already enough for my life expectancy . They still have them in stock. But then these parts are sooner or later going to dissapear anyway, so if I would have to choose between these and the Toshiba JFETs, I'll go for Toshiba.
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Old 3rd November 2009, 07:04 PM   #6
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Just curious. Why did you rule out the Fairchild KSA992/KSC1845 pair. I know you are aware of these devices and that are cheap and available and have fairly high beta. I'm sure you had a reason. What are the shortcomings of this pair?
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Old 3rd November 2009, 07:21 PM   #7
syn08 is offline syn08  Canada
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Originally Posted by Steve Dunlap View Post
Just curious. Why did you rule out the Fairchild KSA992/KSC1845 pair. I know you are aware of these devices and that are cheap and available and have fairly high beta. I'm sure you had a reason. What are the shortcomings of this pair?
Like many other so-called "low noise" devices (including 2SC2240/2SA970) they have Rbb~40ohm. That's an order of magnitude worse than what is required here.
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Old 3rd November 2009, 09:38 PM   #8
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Thanks. That, like much other information, isn't given on many spec sheets.
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Old 3rd November 2009, 10:14 PM   #9
syn08 is offline syn08  Canada
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Originally Posted by Steve Dunlap View Post
Thanks. That, like much other information, isn't given on many spec sheets.
You can always do some (more or less) precise calclations based on the iso-NF curves in the datasheet. However, you can tell if the device is low noise at a glance. If the iso-NF curves are not specified towards low source impedances (10-100 ohm) then that device has high Rbb and consequently high noise (for a MC phono stage).
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Old 3rd November 2009, 11:31 PM   #10
Bonsai is offline Bonsai  Taiwan
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Syn08, any thoughts on the THAT matched pairs, or the MAT series devices in place of the types you used in your head amp?
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