Cheap SUT - AT630 upgrade?

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little eddy
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#1 Cheap SUT - AT630 upgrade?

Post by little eddy »

So have been thinking about further vinyl system upgrades, initially by upgrading the SUTs in my current RIAA amp. But then to future-proof myself, (thinking of a D3a phono at some time), change the current phono to have a 47k load, and buy/build a new off-board SUT box.

But what about an AT630 that I was kindly given with the RD40 and Mayware arm, so gripped with enthusiasm decided to take the AT630 apart with a view to some mods:
1. upgrade the output cable
2. upgrade the fitted 'loading' resistors
3. consider the option of load plugs
AT630 small.jpg
It originally might not have been the most expensive purchase, (was sold in a pack with an AT-30E cartridge), but how good/bad might the SUTs be? And is upgrading or reusing a worthwhile considering?

Information I have found includes:
Recommended input impedance 20R
15Hz to 100kHz
Triple permaloy shield (might be when assembled)
I believe the gain to be x10

The next question is where are the loading resistors, (I was expecting to see a couple of 2k loading resistors across the outputs)? Might these be inserted between the two SUTs, currently held together with a clear heatshrink sleeve?

It's going to be very tight to add a loading plug in the existing enclosure. AT630s on slease-bay only make around £50 so the other option is to house the transformers in a new enclosure. Are the transformers good enough to consider this?

Or are there other SUTs I should consider if am going to rebuild? By comparison a pair of Sowter 9570 1:10 Low Z Cartridge Transformer are around £200 new.

I am never going to own a mega expensive turntable nor cartridge therefore unlikely to spend £1k on a couple of high spec SUTs, but on the other hand want to systematically remove the weaker points in my system.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
JohnG
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#2 Re: Cheap SUT - AT630 upgrade?

Post by JohnG »

I am a member of a Group Buy for Cinemag 1245 SUT.
The pair are a very similar cost to your Sowter 9570 Price.
At present there is a delay to the order being produced, which could mean a option might still be available to join, when the production resumes.
Just mentioning this as a further idea on offer for a option.

The Cinemag 1245 should give a SUT with a very similar performance to the Bob's Devices 1131, as the Tranx on the 1131 are the same spec as the 1245.
Indirectly the 1245 would produce a DIY Built SUT, that has a equivalent commercial offering that is a $1000 1131.
Ant
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#3 Re: Cheap SUT - AT630 upgrade?

Post by Ant »

Why not consider building an ss step up, there are plenty of circuits on the internet
Personally, and somewhat controversially I prefer ss step ups having used both types
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little eddy
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#4 Re: Cheap SUT - AT630 upgrade?

Post by little eddy »

JohnG wrote: Thu Apr 30, 2020 8:57 pm I am a member of a Group Buy for Cinemag 1245 SUT. The pair are a very similar cost to your Sowter 9570 Price.
Thanks JohnG. Had a look but all the latest posts look like the GB remains closed. Must admit I would probably go with Sowter if I were to buy, just a British brand I trust.

But I still don't know how the the Audio Technica SUTs might stack up against either the Cinimag or Sowter, and also my current RIAA ones for that matter, (although they are of an unknown manufacturer/standard).

Tending towards building my own initially using the AT SUTs because they don't seem to get a slating anywhere, and a new implementation should hopefully be an improvement on the stock item. Can always upgrade in the future if not a success.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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IslandPink
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#5 Re: Cheap SUT - AT630 upgrade?

Post by IslandPink »

You're not going to go far wrong with Sowter on step-ups.
However for low bass and timing, a SS step-up is going to be better - however it is hard to get that to work for upper mids and treble. I couldn't quite get a jFet front-end to work on my phono, but Nick has done better with his unit. James D had some other ideas which I haven't heard put into practice, but I think there's probably an excellent solution out there with some experimentation.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
little eddy
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#6 Re: Cheap SUT - AT630 upgrade?

Post by little eddy »

So reading here: https://www.vinylengine.com/step-ups-an ... dges.shtml, I take it that there are no resistors in the AT630 and it is wound specifically for a 47k/20R ratio. The gain is therefore 48.5, or 26dB.

Being such high gain, without increasing the 47k load resistor, cannot be adjusted for higher than 20R, not as 'tunable' as say a 1:10 SUT. But perhaps worth trying with my 2 cartridges, MC25FL and a new to me AT30E.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#7 Re: Cheap SUT - AT630 upgrade?

Post by little eddy »

After reading up a bit, I have bought a Pico PC scope and had my first play this afternoon. It comes with a built-in function generator, (think the internal impedance is circa 600R).

Fist up was an open circuit sine wave test of the AT630 and comes in at an amplification factor of circa 15 or 23.5dB. Set a 1V out, the measured output by the scope was 425mVrms.

As I increased the loading, the output voltage of the generator fell, I'm assuming because of the highish internal resistance, and fell to 158mVrms at 47k. At 6k8, fell further to 39mVrms. Amplification factor as expected also fell to 14.4 and 12.3 respectively.

I then turned my intention a square wave test and this didn't go well, I am guessing because of the parameters and robustness of the built-in signal generator. The input square wave did not have good definition. The most 'clear' insight I had was at 10kHz, 6k8 load. The output signal in red looks reasonably critically damped. You can see a little overshoot and oscillation but expect this looks better than actual because the input signal in blue does not rise sharply all the way to the maximum voltage.
20-05-06 AT630 10kHz Square 47k small.jpg
Is this also a function of the generator output resistance, or another shortcoming?

What can/should I do in order to get a better/more real life input signal to the SUT? Can or should I add series resistance to the signal generator output using say a pot, and feed the SUT with a significantly lower signal, perhaps resulting in an input into the SUT of a few mV?

I have read that in order to consider optimising Zobel networks, you need to use a generator with a similar internal impedance. For the 2 cartridges I have, specs say 6 and 10R!
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
little eddy
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#8 Re: Cheap SUT - AT630 upgrade?

Post by little eddy »

Done some playing around with my new toy and seem to be getting somewhere. From this with 47k load:
20-05-10 Image #1 SUT and 47k load small.jpg
To this with added Zobel:
20-05-10 Image #7 SUT, 47k load and 320pF & 5k6 network small.jpg
The frequency of the oscillation/ringing is well above audible frequencies so perhaps I shouldn't do anything?

Resistive loading only to minimise ripple (5k0) reduced the output by 40% so the RC network has significant gain benefits of approaching 3dB.

Adding capacitance (such as trying to mimic the 400pF phono stage input) reduced the frequency of the ripple. Increasing the RC network series resistance generally reduced the magnitude of the ripple. What is evident is that the optimum Zobel network is also dependant upon the phono stage input capacitance - with my parallel ECC83 at circa 400pF, this already dampens the ringing slightly and might minimise the benefit/need of a Zobel network.

If you look carefully, with the Zobel, the rising edge of the square wave is not quite as steep as the undamped waveform so a compromise may be the best solution with a little bit of overshoot, (taking me back to my PID days).

Now on to playing with the installed SUTs, and then trying primary loading to compare.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.
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