Assume the typical MM cartridge with 5mV output @5cm/s.
Resonance peak can be as much as 20dB FWIW.
If it happens, it will either clip the following preamp or eventually the power amp, and if you're unlucky you kill a tweeter. IMV it makes no sense to allow a phono stage to emit such high voltages. You just move the problem higher up in the chain.
After reading your post, it's apparent that my description of the preamp's performance isn't nearly sufficient to render a meaningful opinion. I have now made the necessary bench measurements to better define the gain structure and to quantify the maximum signal level at each stage. This data will need to be compiled in order to produce headroom numbers at both ends of the spectrum. I'll come back to this discussion once the work is complete, hopefully in a day or two. Thanks for posting those numbers.
My first iteration of the current project used the first stage in triode mode. Input C must have been truly excessive, because the upper mids were really biting my ears. Reconfiguring in pentode appears to have resolved that issue. Additional work necessary to verify all this (as well as headroom and other characteristics) is upcoming.
With a MM cartridge the resonance peak usually happens in or near the upper audible range. MC resonance peak is much higher of course.
Another - better - solution would be a phono pre with variable gain adjustable to the record you are playing (reduce gain for records cut very loud).
Where did you get that? I never said headroom was too high. I would simply like to compare this to other tube preamps to get a better idea of where it stands. As a matter of fact, I fired up SPICE this evening and reworked the output stage to increase the maximum output swing. I'll make the physical circuit changes in a day or two and take new measurements. I expect this will benefit headroom at lower frequencies. About the resonance issue, the pentode in this front end exhibits a truly tiny amount of input capacitance. In fact, I'll likely need to add 100pF or so to flatten response. If it sounds like I'm not worried, that's why. I certainly do agree regarding the necessity of proper loading, and that's something I'll tackle after the preamp proper is completely vetted.
Yep.
To prevent this just include RIAA equalization into the gain of the input stage (for MM at least). With up to 40 dB less gain at 20 kHz compared to the gain at 20 Hz you reduce the chance to clip the input stage by high frequency signals significantly.
IME the difference in signal level is not that much to warrant this. Variable gain is difficult to implement in a first gain stage with RIAA EQ, and it's not necessary as such a gain stage is not prone to clip in the first place (see my posting above).
Just to add, I'm in the process of re-biasing the output stage. That will increase maximum output capability to something like 25V. The input stage can handle more than 1V from the cartridge. If post #7 and your simplified calcs are accurate, I don't see how this preamp could ever be overloaded.
A lot was discussed earlier in this thread, and I recommend to read the postings from Michael Fidler earlier, e.g.
www.audiosciencereview.com
The problem is that 20dB peak can overload some phono section inputs with certain cartridges.
Please look at the link in this post.
The best way to do it is to load the MM cartridge, thus killing the resonance.
Noise (including Johnson noise) then becomes a problem...
Not necessarily. You could add an emitter follower within the feedback loop to increase output current capability, or something similar.
How would that work with the RIAA characteristic ahead of the input stage?
Nothing - it just makes sure that the headroom is maximised without negative consequences in case of clipping.
Absolutely yes. Still one should apply RIAA EQ in the first gain stage of MM pickups to optimize the headroom. What's wrong with this?
My apologies for dredging up this old thread….
…
it appears the front end can handle a little more than 1V RMS.
Reading through the information here, it seems to be agreed (?) in general terms that a MM phono preamp should be capable of 20dB above a 5mV nominal input (50mV), or better yet, 26dB (100mV). Considering my measurement is an order of magnitude greater than this, I'm wondering if I've misinterpreted the basic foundation of this thread. ...
Please go easy, I'm still new to this.
Jack
