Crown FM1 vs Si4735D60
The Crown is one of the few exceptions here. The FM Two is even better. That is one of the better digitals ever made. However, do remember that all that display circuitry and PLL synthesizer wizardry needs extremely high amounts of shielding to not cause RF interference. Crown did shield their tuners well. But this method adds a little more noise floor than the older tuners. But how much depends on the quality of the design and engineering and the components used. People in and near the transmitters they wish to receive hear it less, those like myself in rural areas need every last measure of sensitivity, selectivity, and low noise floor we can muster. Especially when we are trying to fetch a weak signal from that 50-90 miles or more out.
So i picked up the dead FM1, took a good look inside and put it too some tests.
I see what you mean Ken, they could have done a much better job in the shielding/wiring design of the unit.
I broke the PLL loop by pulling the TL071CN in the loop filter board.
I crudely wired up two pots, to the V tune line. Used a course and fine tune pot the later being a heli-trim w/w type. Hooked the frequency counter up to the SP8629 pre-scaler /100 output.
Breaking the control loop removes the PLL generated noise. The 7-seg LCD display multiplexing is also non-functioning.
So now the FM1 is not a digital tuner anymore but a traditional super-het analog tuner using varactor diodes.
Great it works, so I tune to a station that I know is a fringe station, sandwiched between two higher power local transmitters. It pulls it in, some thing that the tuner in a Pioneer SX-950 has a real problem with. Depending exactly where I tune, you can basically remove the adjacent carrier cross talk, however it is not spot on the exact tune frequency. So I can say the FM1 tuner is pretty darn good. When i compare the Si4735 in the exact same setup, it is just as good, if not better than the FM1.
In this test I see where a narrower IF maybe able to help the FM1. Since the ceramic filters are socketed, it is easy to play around. Digikey still has a large selection of Murata CF types available!!
I asked the wife to compare sound quality, she said she could not hear any difference when in mono. When I put the FM1 in stereo, she said it sounded better than in mono. The signal level on the Si4735 was around 20dBf. I have to do some s/w adds to dynamically change the stereo blend settings, as the default thresholds are I believe are to conservative.
I see a few improvements to the FM1, but I really wonder if it is worth it. I think most of them would be margianl improvements however. One day when I am really bored and the snow is 10' high.
Even with the FM1 -65dB noise/hum spec, I can't hear it. I could re-design the controller too, but that is a fair amount of work.
If anyone is an expert on tuner designs, I have a few ? about a few circuit design specifics? like why they have a var voltage going into the AFC summing node? I see some has replaced the CA3189 with a HA11225, are they truly interchangeable?
So there you have it, as far as I can tell, a 10$ chip performs as well if not better than a expensive varactor tuner from the past.
As far as being as good as a T-2 well I can not say unless I do a comparison too. Not that I do not trust NickG, but I would like him to compare using a Si4735 or Si4770 and using an external DAC too. To bad NickG and others live so far away or we can have a tuner shoot out over a few beers.
Cheers
Rick
I think I threw it out the window after a particularly annoying commercial & never looked back....if I had a gun I'd have pulled an Elvis on it :thmbsp:
