Step-Up Transformers v Phono Preamps

[MartinT]

Thanks all for your suggestions.

The GCPH phono amp incorporates two gainclone modules (with passive RIAA in between) and adjustable gain for each stage (rear switches for the first stage, front panel knob for the second stage). As you can imagine, there are enough variables to drive you mad!

I have tried lower gains while turning my main preamp up, but the overall gain keeps noise at listening volumes pretty much the same, as does relative adjustment of each gain stage. I have moved the GCPH around but the hum doesn't change. Removing the TT earth increases hum dramatically. I surmise that the remaining noise & hum are just caused by the high gain being asked of the GCPH. I should stress that this can only just be heard while not playing a disc - it is not audible at all whist listening to music.

I do agree with the suggestion that I should try a SUT and see. I am certainly not going to buy the delicious looking AT-5000 unit, but I am willing to experiment with something highly rated that stands a chance of allowing me to compare sound quality while remaining inexpensive.

Reviews and forum comments have drawn me to the CineMag transformers, with Bob's unit the best looking implementation:

http://www.bobsdevices.com/cinemag.htm

I've discussed the AT33PTG's compatibility with him and he reckons it'll be nigh-on perfect on the 16x gain setting. The price is reasonable and I could always eBay it if it doesn't work out. Should I take the plunge for the sake of experiment? I'm gravitating towards firing an order off.

[Marco]

Why not, Martin - what's the worst that could happen?

Marco.

[Barry]

Hi Guys ,

From what I have read on the web, it would seem that AT33PTG users are loading their cartridges with anything from 20 Ohm (!) to 150 Ohm, with 100 Ohm being preferred. All Audio Technica say is > 20 Ohms

The internal (coil) resistance is 17 Ohm and the cartridge has an output of 0.5mV (at 5cm/sec). So if you are going to use a SUT, then the rule of thumb is to choose a transformer that will present a load impedance 5 -10 x the cartridge's internal resistance. If you choose a transformer with a 20:1 turns ratio, then the 47KOhm impedance of the preamplifier will be transformed to 117.5Ohm or ~7 x the cartidge impedance. A 20:1 transformer will then produce a 10mV signal for the preamp.

Transformers are lower noise devices than pre-preamps, but to work at their best one should choose a design that is designed for a specific source impedance (in this case 17Ohm). Very few transformer manufacturers offer this information. In fact the German Haufe and the British Stevens and Billingham are the only companies I know that provide this information.

So in view of the above I would you suggest you keep to you step-up amp and carefully check, as Chris has suggested, the earthing arrangements.

Trust this is of help
Barry

[The Grand Wazoo]

Martin,
The Cinemags are good transformers - quite sweet sounding.

Bob's version has a couple of switches, & if you buy them, I'd ask for instructions as to how they can be bypassed once you've found the setting that suits best for your gear. It shouldn't be too tricky & you can't beat a straight signal with no switches when you're dealing with such a tiny & delicate signal. These things are usually easy to reverse.

[Barry]

Between reading your initial query and posting my reply, I see that you are thinking of using CineMag transformers with a 16:1 turns ratio. These will transform the 47KOhm preamp impedance to 183.6 Ohm (or 10.8 x the coil resistance), and will boost the output to 8mV.

I can't see any problems here, so if you are willing to give it a go - why not?

Barry

[NRG]

I highly recommend Sowter. Not cheap but then quality never is and you would need to mount them yourself but their performance is outstanding and Sowter can wind them to your exact needs if required...

http://www.sowter.co.uk/phono-cartri...ansformers.php

[Chris]

I agree with Marco - cartridges and Suts should be matched pairs . My DIY haufe based SUT walked all over my Audio Technica AT650 when used with my 103Pro but now I have changed to an Escoed Troika the AT sounds better in spite of the fact that in theory the Haufe should be fine. I feel one thing is the electrical compatibility and another is the sound. If you can audition the alternatives or use other people´s experience with the same combination, great, but if not, as is my case, it´s difficult so I´m going to opt for the Hagerman Piccolo active head amp kit which offers all sorts of flexible settings and seems to be comparable to the Cinemags, sound-wise. In the days of the Troika, SUTs were frowned upon so no-one has much experience with the Troika- hence my decision. Fingers crossed.

[MartinT]

Thanks everyone. I've had further discussions with 'Bob' and his CineMag unit is ideally matched to the AT33PTG with gain and impedance loading all perfect for the PS Audio to receive a healthy 7mV into 47k and the AT loaded to 180 Ohms (thanks Barry for your confirmation). This on the 16x setting.

I have decided to go ahead and order it. I will conduct some comparisons and report back in Strokes of Genius if people are interested. I'm quite intrigued to hear the results myself.

[DSJR]

A very kind gent has loaned me his AT630 SUT for a while. I've yet to listen, but its lower gain should be a great help with the Croft.

I just wanted to express an opinion, not about good ss stages vs good sut's as such, but a comment on the ridiculously priced (to me) AT sut referred to earlier in this thread... I hope the likes of Marco would agree that much "top end" audio is aimed at people to whom a few grand isn't a huge problem. These products, in my opinion, don't offer a huge amount over more basic ones and in the case of solid state products, don't cost much these days to make. What DOES cost is the deluxe casework, the deluxe packaging (in the case of cartridges, the packaging often costs more than the item itself at factory costs [first hand experience here, this bit's not opinion]) and most priceless of all, the status given to the owner - very important I understand in the far east.

having thought about it for a good while, I will happily accept that huge special transformers used in power amps cost a bit, especially when the chrome end caps are fitted () but a little sut for mc cartridges? The fact is I suspect, is that it has to sell for this amount to get a cachet in Hong Kong and other similar places. A phono stage I know of which sells for £2500 here could quite profitably sell for £900 or so in the UK, but is priced where it is because most are sold in the far east. This item is up against units costing £12,500 and out-performs them apparently, but I wouldn't be surprised that the people buying the dearer model wouldn't even look at the cheaper, superior product - a bit like LP12 owners being vitriolic about SL1200's despite never having heard them.

A bit muddled and posts have moved on now, but it's a good idea to try alternatives while keeping the unit you know and love, despite a query regarding one aspect...

P.S. With an output of .5mv @ 5cm/s, wouldn't a gain of 10:1 be better? SS preamps can have funny overload problems and surface ticks and spits can sometimes cause momentary ringing in phono circuits (apparently even in good ones). Just a thought...

[MartinT]

Bob's CineMag SUT arrives tomorrow. I'll report on my findings once I've had a good play with it.