Phono Preamp. With transformer or fully active
Ralph, Cartridge loading is another hotly contested subject that is best to save for a new discussion. It seems we both agree that applying a load through the secondary of a SUT has a much greater chance of changing the SUT behavior in an audible way than the cartridge behavior. The inductance question can be answered by looking at SUT behavior when driven from various cartridge impedances. The plot linked below is the 4722 wired 1:18 driven from 2Ω to 100Ω and at 100Ω the -3dB point is ~10Hz which translates to 1.6Hy. http://www.intactaudio.com/images/SUT%20white%20paper/4722%20cart%20z.png I chose the 4722 because it is a known device that represents the typical behavior of a microphone transformer pressed into SUT usage. I agree that the top end behavior of this device leave a lot to be desired. When paired with an EMT or a Denon 103 into a 47K load it is -1dB @ 10Hz and 20kHz and has what many call a "vintage" sound. As an aside, Bell Labs did a lot of work on voice transmission and intelligibility and found that linearity in the phase domain was far more important than linearity in the amplitude domain. They considered the range from 600hz to 4Khz the critical range and found that phase shifts creeping down below 4kHz tended to muddle the voice. I agree that the audio bandwidth is much larger than that required for voice but it is interesting to note that the phase shift of the 4722 does creep down into that "sacred area" that Bell Labs found had an impact on intelligibility. I just measured some 900:900 Western Electric repeat transformer that had a specified frequency range of "Voice" and 425Hz-1615Hz was mentioned. When fed from a 50Ω source and left unloaded the -1dB points were 18Hz and 2.4Mhz. I don't think the crazy high top end bandwidth was an accident and the phase stays flat to 100Khz. http://www.intactaudio.com/forum/files/phase_385.png |
I chose the 4722 because it is a known device that represents the typical behavior of a microphone transformer pressed into SUT usage.18:1 seems like a lot of stepup to me, but at the same time I suspect that 4722 is intended for an input inductance that is a bit different, being that its a mic transformer. |
18:1 is a bit on the high side for a 103 but not terrible. Problem with the higher step up and the 40Ω cart is any capacitance on the secondary claims the 20kHz+ info quickly. The 103 is a 0.3mV cart and into a 1:18 loaded with 47K you get really close to the 5mV "Norm" for most mm inputs. Now 1:70.... that is high... but also a perfect fit for a 0.05mv cart into the right cable / pre combo. I don't see where the inductance comes into play here. as long as there is sufficient inductance to cover the low end without going too low it falls out of the picture. An interesting anecdote about the 4722 and its ilk which can be wired 1:18 / 1:36 is you get just about the same output due to the 1:36 loading the cartridge below its internal impedance and the frequency / phase plots are quite similar with the 1:36 losing a little more than 1/2 a dB @ 20Hz & 20kHz. I know of a few people who simply prefer the 1:36 hookup and given the similarity of the measured response of each it makes me wonder if any of this preference is due to the drastic load improving traceability ala Moncrief. I know this seems in contradiction to what I said above but in this case the loading in both cases is right where the transformer was specified (ie 50K load) and the measured responses are similar so I see a window of possibility. An unfair test would be to load the 1:18 setting with an additional 16K on the secondary and compare it to the 1:36 with a typical 47K. The proper way to test it would be to simply parallel 55Ω with the primary of the 1:18. The gain would be down around 4-5dB but the frequency response would minimally change. http://www.intactaudio.com/images/SUT%20white%20paper/4722.png' dave |
@intactaudio When I look at the specs of the 4722, it seems its bandwidth might be limited by the simple fact that it is meant to deal with a signal much stronger than that of a LOMC cartridge. Or it might simply be that Altec didn't intend the application for much more than public address. But whatever the reason, by comparison on paper, any Jensen SUT transformer outperforms it in every way- take a look: https://www.jensen-transformers.com/transformers/moving-coil/ There's a bit of cult around the 4722, isn't there? One not from actual performance, right? |
