There are numerous RIAA configurations one amp with feedback is only one of them. I would think that one not working would be obvious.
Sorry when Richard squared the turns ratio to make the equivalent resistances it worked out to the same answer. SPICE comes up with the same 1.8dB SNR. I tried the Lundahl LL2912 ribbon mic transformer with a 3 Ohm source unloaded and got 0.2dB (1:37 and 0.05:59 Ohms resistance). I found several folks rebuilding classic ribbon mics with these as an upgrade. With an 0.7 Ohm ribbon the SNR is still only 0.5dB unless there are other issues I am missing.
These transformers are absurdly small BTW.
https://www.lundahltransformers.com/wp-content/uploads/datasheets/2912.pdf
Remarkable result for such a small device.
You can use two different ways to get the required dBV result.
Noise after 1:37 with ideal trafo: 8.251nV/rthz for a 3R Cart unloaded.
Noise after 1:37 with 0.05/59 Ohm: 8.378 nV/rtHz
Degradation: 20*log(8.251/8.378)=-0.13dBV S/N
Or use Richards method:
10*log[3*37^2/(3*37^2+0.05*37^2+59)]=-0,13dBV S/N
Richards 10*log seems a bit confusing at first sight, but the result is of course also in the same dBV and not dBW.
Attaching this Cart plus Lundahl to a AD743 with 2.9nV/rtHz brings the noise to 8.866nV/rtHz or to -0.6dBV S/N
This is -0.6dBV quite a lot better as -1.7dBV example with the 150R /150K loaded microfone still without amp attached.
Hans
well if you get the mumetal screened version its €90 each and you do have to mess around with HF loading a little bit in some cases to prevent ringing.
30dB gain is a little higher than most would use for an MC, but if it doesn't have overload issues I suspect that would open up some interesting possibilities for the next gain stage if one ignores the usual use of a headamp/transformer to get things to MM levels. Quick envelope suggests the following stage would only need around 15dB of gain for my personal nefarious purposes.
Intriguingly the lundahl transformers for MC cartridges are about 25% more than the ribbon transformers. At least if you don't go for the silver wound version.
30dB gain is a little higher than most would use for an MC, but if it doesn't have overload issues I suspect that would open up some interesting possibilities for the next gain stage if one ignores the usual use of a headamp/transformer to get things to MM levels. Quick envelope suggests the following stage would only need around 15dB of gain for my personal nefarious purposes.
Intriguingly the lundahl transformers for MC cartridges are about 25% more than the ribbon transformers. At least if you don't go for the silver wound version.
Thank you HansHans Polak said:
You've saved me a beating by the fierce pedant, Guru Wurcer ______________________________
The missing issue is LF response.
It's missing on their later ribbon datasheets but is on the older LL1927A. The reason why good transformer makers don't like specifiying LF response is cos it (the Inductance) changes A LOT (and non-intuitively) with level. But you still need some idea to start design.
LL1927A looks like a slightly bigger LL2912 so they are probably designed to match the same 0R3 ribbons and have the same (I'll assume) -1dB @ 10Hz LF response.
If you use a 3R source on a transformer designed for 0R3, the LF cutoff will be 10x greater .. ie -1dB @ 100Hz (and -3dB @ 50Hz assuming 1st order from just the source DC and the transformer inductance).
To get the same LF response as before, you need 10x the inductance .. ie rt(10) = 3.14x more turns (cos Inductance proportional to turns^2) .. so 3.14x longer wire in the primary winding.
Also as these extra turns / longer wire must fit in the same bobbin space, the wire x-section must be 3.14x less. So thinner wire with 3.14x more resistance/m. The primary DC resistance becomes 3.14 x 3.14 = 10x greater and the primary NF is as it was before.
It turns out that for a given core size & material, the LF vs noise compromise is always the same ... regardless of design impedance .. provided good design rules are followed.
It breaks down at very low R cos the difficulty of filling the bobbin with thick wire. Lundhal use loadsa sections paralled .. but this may be cos they want to offer a choice of ratios. Today, I suspect a SMPS transformer maker that actually designs them (rather than just follow some black magic book) would use Cu tape/ribbon .. but it all hinges on the maker's experience & plant .. and if they know what they're doing.
A similar (but more precise since it doesn't 'change' with level) relation holds for LF THD & overload but in these small signal ribbon & MC transformers, if the response is OK, so will the LF THD
___________________
Can we use NF rather than S/N please. The NF is the degradation in S/N.
In these examples, we don't know what the signal is .. only the DC of the source .. so we can't know S/N.
But we can work out how much the S/N degrades. And because it's a relative measurement, its simple dB rather than dBV or dBW.
It appears some of the lesser semiconductor makers use NF too
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The step up transformers I could find in Stereophile were not exactly cheap.
Bob’s Sky40, $1250,- / Enia, $2400,- / Auditorium T1&T2, $4995,-.
So either these guys make a huge profit, or it isn’t that easy to make a good MC step up transformer ?
Hans
P.S. @Richard, which Lundahl or Sewter step up transformer do you regard as suitable for MC carts.
Bob’s Sky40, $1250,- / Enia, $2400,- / Auditorium T1&T2, $4995,-.
So either these guys make a huge profit, or it isn’t that easy to make a good MC step up transformer ?
Hans
P.S. @Richard, which Lundahl or Sewter step up transformer do you regard as suitable for MC carts.
looking here MC PHONO CARTRIDGE SUT TRANSFORMERS if it were me I would get the 1480 148O PHONO CARTRIDGE SUT as it has a centre tapped output and I have an illogical like of balanced inputs. What I have not yet worked out is why its predecessor the 8055 needed a Zobel on the output and this one doesn't, unless they built it into the case. £111 each. So I think someone makes a big markup.
Hmm they do an MM stepup as well now. How curious. I might start a thread on that.
Edit: I did find SYs his master's noise preamp article an useful one for how to optimise your transformer to your preamp, albeit tube.
Hmm they do an MM stepup as well now. How curious. I might start a thread on that.
Edit: I did find SYs his master's noise preamp article an useful one for how to optimise your transformer to your preamp, albeit tube.
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Thank you Bill,
This is great news, I'll give it some thoughts.
Bob's Sky40 is probably exactly this step up transformer
Bob's Devices Inc. Step Up Transformers
Hans
The folks I saw were using super thin foil at more like .7-.8 Ohms per ribbon and were reporting great bass performance.
The level dependent response of transformers or cored inductors for that matter is a problem. The Pass Labs folks are using line transformers as gain elements and this is where I ran across it. This is not new, I noticed some comments on the Sowter site about doing this.
At very low levels I have not seen a noticeable issue. I have used (abused) some miniature microphone matching transformers in bench experiments like making differential signals to test pre-amps and calibrating the signal path things always looked OK. I don't go to absurdly low THD levels but at the -80 to -90dB level never had a problem.
Hi,
Lundahl still does mc trafos too.
Moving coil step-up audio transformers • Lundahl Transformers
/örjan
Lundahl still does mc trafos too.
Moving coil step-up audio transformers • Lundahl Transformers
/örjan
Boy have we drifted off course this week. Can anyone still see land
Very true, but wasn’t it a very instructive journey ?
I wonder why SUT’s are so underrated.
Hans
What a huge assortment of almost similar SUT’s.
I can’t see the wood through the trees.
How can a mortal make his best choice ?
It just frightens me.
Hans
Generally you either buy a cheap one (like I did 30 years ago) find you don't like it and give up OR you spend all your money collecting every type out there based on random internet reviews.
If I ever complete all the SS amplifiers on the backlog and decide to play with transformers It would be Sowter for the history.
If I ever complete all the SS amplifiers on the backlog and decide to play with transformers It would be Sowter for the history.
S&B seem to have got out the MC stepup market, but Jensen are still making them. Boy have we drifted off course this week. Can anyone still see land
Ok, back to the topic
. Did I mention I hate transformers?P.S. The 15V regulator in the middle is to allow a garden variety of wall warts to feed this baby, without overheating the on board current sources. Currently running on a 24V model.
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