Step-ups

Hi,

Running my mc straight into an mc phono requires 600 ohms and 330 pf loading. However it seems from talking to rob very much lower loading is required for a stepup - perhaps 50-100 ohms. Can I do without the capacitance altogether with a stepup or should I put some in? Also which type of cap is best for loading.

I guess a manley steelhead would be handy right about now.

Nick.

 

Hi Nick,

Someone must know the the impedance of the Brinkman cartridge.

Anyone?

This would help hugely.

Sowter have some good info on their site and they'll also custom wind to your spec at little or no extra charge.
You heard Sowters in my phono stage on Saturday.
http://www.sowter.co.uk/2phono.htm

 

Via transformers, impedance is transformed by the square of the turns ratio, so your loading might need to reflect that. So with, for example, a 1:5 step-up loaded with 600ohms on the output will look like (600/25) = 12ohms to the MC driving the input. If this is to drive an MM input, then the stup-up is likely to be rathe rhigher, but then so is the input loading of a typical MM phonostage - 47K. Given a 1:10 step-up that becomes a 470ohms load to the cartridge - probably as close as you need to get. Capacitance will be transformed the same way, but may need a bit of experimentation - the transformer itself (and associated wiring) will provide some stray C.

Best type of caps for signal path stuff like this are polystyrene - or teflon, if you can find them.

 

Internal resistance is 25 ohms - I did post it on the PFM thread.

Step up is 1:10

What brand should I look for?

 

Many hold these in high regard

http://www.stevens-billington.co.uk/page103.htm

 

Thats the step-up I bought

I meant brand of caps!

 

Silver Mica caps are superb but quite large one you go above 100pf. I have them in the RIAA stage and they were very impressive compared to the polystyrenes. Cost is about £10 each and you can find them at RS and farnell.

They are the yellow caps in the pic:

 

Those green caps look big in there!

Hey I have some of that cable.. lol

 

Those green caps look like the russian PIO's. Nice aren't they?

I'm interested to try out the russki teflons I recently bought though.

 

Yes, I couldn't get them on the board! - had to glue them in.

The wire was installed under duress
The Waddies over on the WD forum took one look at the amp with the old thin grey RS cable and demanded that I use something better. Well, the new cable sounds great, just like the old stuff in fact - lol

I made four sets of stereo cables yesterday using that Nato teflon covered silver wire. Jesus is that stuff nasty to work with - I have sore hands today for my efforts but the cable looks beautiful. I reckon I could flog them for £1000 a go

Sorry Nick, hijacked your thread - back to trannies.

 

I'm very impressed with them. There are also two smaller 0.1uf versions in the circuit.

 

Hi,

Just a note, most MM Inputs have 150pF or more capacitance in parallel with 47K. The capacitor is stepped up in value by the square of the stepup ratio, so 150pF on the secondary of a 1:10 stepup capacitor is the same as a 15nF capacitor on the primary.

Hence I normally recommend very short, low capacitance but well shielded cables after a MC Steup, while before the MC Stepup cable capacitance practically does not matter.

So, if a transformer is used into a MM Phonostage usually no extra capacitve loading is neccesary or desirable.

Ciao T

 

Ah ok thats great - solves that issue. I think the tonearm cable and interconnect are about 200 pf or so. Plus whatever the transformer adds - any idea what that would be?

 

Hi,

The transformer itself adds very little, otherwise the bandwidth would stink.

All it does is to transform impedances, thus any inductive or resistive load on the secondary (where there usually is non) is translated downwards in impedance and thus it's aparent value in the primary by the square of the winding ratio.

Equally, for a capacitive load on the secondary the impedance is translated downwards by the square of the stepup/winding ratio, however as a lower impedance in a capacitor implies higher value the aparent value of the capacitance is in effect multiplied by the square of the stepup ratio.

BTW, please make sure to "burn in" the transformers according to the instructions I posted to Arthur Salvatore (www.high-endaudio.com) who reposted it.

Ciao T

 

Nick, following on from what Thorsten has said I'd strongly suggest putting the Borbely circuit into a larger case which can also accommodate the S&Bs. That way you can simply twist the lead-outs from the transformer and solder straight onto the board. The PSU in separately housed so there will be no noise issues.

 

I may put the tx in its own case eventually - as I may upgrade the borbely to the higher spec version if I decide to stick with it.

T - thanks, I will do just that. I presume I can burn in both at once and that each tx is connected to the signal and earth of each cd channel with a resistor accross both primary leads.