Those of us who own preamps with phono stages are familiar with the phrase, “will handle MM cartridges and most MC cartridges”. The word “most” becomes critical when considering many of the low-output pickups available. My Kora Eclipse, for example, has separate MM and MC inputs and the MC stage is rated at .4mV/100ohm with a gain of 55dB. At what point is that 55dB gain insufficient to handle EFFICIENTLY a very low-output pickup? .5mV, .6mV? I know that it does depend upon actual output of the pickup and there are no hard and fast rules. I was given the following general guidelines some time ago which I post below. Does this seem like a good “rule of thumb” or is someone prepared to give it the finger, instead? Thanks.
0.2-0.5 mV needs 63dB gain 0.6-1.9 mV needs 50dB gain 2.0-5.0 mV needs 37dB gain
Maxgain, I have heard the stories about this being the case, but I have not found it to be true in my situation. A good step-up transformer should not get in the way of the sound at all. In fact, in some ways, it makes a better coupling than going straight into the phono section.
I think that sometimes people don't know how to use them, and then they sound bad. For instance, the loading is different when you load at the transformer, than when you load at the phono stage. Generally the loading at the transformer is much lower than what would be recommended going straight into the preamp. Alot of folks don't know this. In other cases, like cheaper transformers, they don't have loading capability at all, so they won't even have hardly a chance of sounding right. Or sometimes people don't know that there might be a pre-set loading on the transformer, and then they load the phono section too, and screw everything up that way. I think that user error is more likely the reason that some people don't think transformers work well for cartridge step-up.
It's a lot different than an amplifier output transformer. The cartridge signals are so low, that there is no way you're going to saturate that transformer, or roll anything off, as long as it's a decent transformer. And transformers in the right application are a really good thing. Look at the new Audio Note $15k preamps. They have something like 8 transformers in them. Input trannies, output trannies, interstage trannies. Good transformers in the right application can be very good performers.
And if you want to compare with direct input to the phono section, let's face it, you have to get the gain somewhere. If you don't have a transformer, you have to have alot more gain stages in the phono section. That can also lead to signal losses or more noise. So even if the transformer is not perfect, neither are the extra gain stages in the phono section. Also, by having the transformer located close to the pickup, it boosts those low level signals earlier, so their less likely to have loss. Take your pick. I like the transformer for my money.
I'll guarantee that anybody listening to my system is not going to be saying anything about signal losses in my step-up transformer clouding up the signal. This system is crystal clear.
Twl, don't most transformers negate the performance advantages of low output MC's. I know that their are some out their that must be good. I was told that the Jensen transformers made in england are very good but where would you get one?. I have experience with mostly old stuff like the Varion(I think that's how it is spelled) and some old ortofon transformers.I never liked what they did. Bring me up into this century please!
One might also consider the system as a whole when trying to determine "sufficient gain." Obviously, the cartridge output, phono gain and pre-amp gain are important. Remember, your amplifier input sensitivity & gain, and speaker sensitivity will also play a role in what is perceived as sufficient source gain in terms of spl. Also, keep in mind that most volume controls (trimpots) will sound better when used in their upper regions as you are removing more of their conductive material from the direct signal path. Enjoy.
4yanx, I think the table you were given is an acceptable minimum for those who strive for lowest noise in the phono stage. I myself would want even more gain if the noise floor of the phono gain circuit was up to it. Matching gain, impedance and noise is very important early in the signal path. Small imperfections in the source end are amplified and add up to big imperfections in sound. Please correct me if I am wrong but I reckon it make better engineering sense to use a low noise high gain phono stage designed to add little noise of its own and that of the MC cartridge (thermal noise) than relying on a high gain line stage which would amplify the noise of a low gain phono stage. Thus I prefer less gain (<20db) in the line stage of a preamp. With (SA)CD players and tuners easily putting out 2+ volts, more than 8 db in the line stage just isn't necessary to drive most amps. Perhaps an adjustable gain setting in the line stage is a sensible option.
Yes, Marty, I had had been in e-mail discussions with Tom with respect to the Shelter pickups, among others. I posted here because I thought others may have the same general issues.
I had come to some conclusions, based on faulty info I had received stating that the gain for the MC phono section in my Eclipse was 70dB. I have since found out from Kora that the actual rating is 55dB. As such, the suitability of the Shelter came into question. I had not figured on the "additive" effect of the line stage gain and have a message in to Kora to determine this specification (the manual only provides line input S/N of 112 dB).
If the gain of the line stage is in the 20dB range, added to the 55dB of the MC phono stage, I am assuming that things should be hunky-dory, given the .4mv/100ohm specs of the MC phonostage in conjunction with the attributes of the Shelter.
Does ANYONE have any thoughts as to whether the general guidelines for gain posted above have any relevance?
Marty, I tried using the Shelter at 100 ohms, which is about the best load I found for it. It just didn't have enough "oomph" with my MFA preamp(66db total gain). The Shelter doesn't do as well when it is unloaded, in my opinion anyway.
I am hoping that David can get enough gain to get the right result with the 100 ohm loading that seems to work best with the Shelter. He is primarily talking about the Shelter cartridges in this question. I know, because we have talked off the forum about this.
I am not sure what I am geting from the line stage, Tom, but I will check. But, if I am getting the same 20dB as you are, then I would have a total of 75dB and be in the same "league", no?
David, as Marty pointed out to me in another thread, it will be the phono stage gain combined with the line stage gain that will be the key to this.
My MFA Magus preamp has 46db gain in the phono, and 20 in the line stage, making a total of 66db gain for the entire preamp system. This was not sufficient for the Shelter 501, and I had to resort to using my Cotter step up transformer. The Cotter gave a 20db boost, to bring the total gain to 86db, which is now more than sufficient, and I can only open up my volume knob to about 9 o'clock before I am at full volume. I think with the Shelter cartridges, you will need at least a good 75db total gain in the preamp system to handle this properly.
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