Hi Marko, et.al.,
Some issues to consider:
1. Please review carefully what a phono circuit does. Here's a great article I found that discusses the issue in understandable terms.
http://www.graniteaudio.com/phono/page5.html
Note the RIAA curves specified at the bottom of the page - they only go out from 35-15khz because that's what RIAA specified in 1955 and the RIAA test record only checks that.
A phono circuit equalizes the output of a phono cartridge. It has to have a specified curve and great manufacturers provide a frequency range and tolerance for their product. The 16.5 lists a 30-15khz range at +/- 0.20 db!, by comparison, the Parasound unit you referenced was 20-20kz at +/- 0.25db. Now please understand that the circuits are not a brick wall, that they don't stop allowing signal to go through below 30 hz and above 15 khz that's specified. The specification is primarily a function of what was common and normal in the late 70's when this unit was designed and sold and it was common to only specify out from 30-15khz for many pieces. The phono circuit will allow signals beyond that to pass, it's just that it wasn't part of the published specification. Likely, if you could check it, you would see that the signals are passed within some +/- x.xx db specification, but it would be there. My guess is the phono preamp can pass from about 15-25kz from just comparing the circuit in general to typical RIAA feedback circuits.
2. It is much more important to check the accuracy of the RIAA phono system because midband deviations are much, much more audible than the frequency extremes. You could use the MM or MM/MC RIAA test cd here and a good volt meter to check yours easily. see:
http://www.graniteaudio.com/phono/page2.html
This is an interesting test cd - but note: the test tones go from 100 to 15 khz, because that is about the limits of resolution of the RIAA curve. There are other test systems or RIAA reverse circuits that you could feed a constant test level into to check your circuits. However, I don't personally feel it would be worth it - the deviations in the phono preamp circuitry, assuming all is working well, are much smaller than the rest of the reproducing chain.
3. If you are really concerned, have your tech run tests on your phono preamp section to determine if it is in specification from 30-15khz, and then if he has the right test circuit, he can check from 20 to 30 hz, and from 15khz to 20khz or beyond. You might want to be there to see if his speakers or your ears can hear those frequencies.
4. Please read this next section carefully because it is objective and not meant as any criticism at all. No offense is meant or intended.
Please look at the other parts of our/your system and understand the limitations in all the pieces. We all have parts that are similar with restriction.
Grado Gold (Nice cartridge!) - Grado specifies it at 10-60kz but no +/- tolerance and no curve at all. May be somewhere but not on their website. No major problem, I trust their product. However, all phono cartridges are, to some degree, sensitive to their loading, ie the phono capacitance and resistance they see when plugged into a circuit. The phono capacitance is from the combined wire/cable from cartridge pins to the input jack of the preamp and the loading cap inside, if there is one. There can be subtle changes in overall response. Grado specs a 47k input impedence, the 16.5 has a 50k input impedence, well within a 5% spec. The input capacitance is not specified by either Grado or Concept.
Speakers - Yours happen to be the Triangle Celius 202. Look and likely sound to be great speakers. Take a look at the published specs on your speakers. They are Frequency range 45-20 (+3dB Hz-kHz) from here:
http://www.audioconnection.com.au/product.asp?id=327
Does this mean that your speakers can't pass anything below 45hz or above 20khz? No, but it does mean that it likely is outside of the +/- specification. As the system drops lower the output also drops. Likely by 30 hz, your specific system is at least 6-10db down from 45 hz. Also, at 20 hz, its likely to be at least 12 db down if it follows the typical 12 db/octave rolloff most speakers exhibit at their lower frequencies. Thats a subjective reduction of almost 20 times lower.
So even if the phono system were able to pass down to 20 hz, your speakers may not be able to output that signal in any meaningful or audible way. Not a failing at all, it takes some big or huge subwoofers to really get down below 35 hz or so for real.
Also, for phono systems, most LP's, even direct to disk, cannot record much real music below 30 hz because of the interaction of the cutter head, disc material, recording electronics and then the record (warp, surface noise), turntable rumble (noise inherent in the motor, drive system - idler, belt, direct, bearings, tonearm/cartridge reasonance interactions) and tracking forces. All of the above contribute a lot of low frequency junk that most systems would waste a lot of amplifier power trying to reproduce. If you have ever seen how woofers "breathe" while trying to track a record surface warp or arm/cartridge resonance (in the range of 5-15 hz) you can see that it is useless trying to get that passed - it wastes energy for no useful audio output. Thats why many early phono systems had "rumble filters' to filter out that stuff if needed
Cassette: I checked my Audio magazine 1978 directory - the Akai GXC-730D deck was a nice unit that specs at 30-17khz +/- 3db. You can be pretty sure that the +/* 3db points are at 30 and 17kz, a wide variation on a flat curve and way more coarse than the phono circuits of the 16.5. Even the best Nak and Tandberg decks of the time had wider but similar specs.
Last, the ears. All of us have different hearing. Many of us can sense but not exactly hear the frequency extremes. You may have to turn up the system really loudly at low bass and very high frequency to really hear any output there, assuming your speakers (or any speakers) play there.
Therefore, we may be chasing specifications rather than really concentrating on music reproduction which is really a function of the 40-15kz passband for the most part. Although it may not be much comfort, the best sounding systems play that passband extremely well and tend to sound more alive and muscial than those that can reach the extremes but can't do the middle well. I would suggest not worrying so much about the phono system and enjoy what you have without mods. If you must have a phono system separate, I endorse the idea of an external switch to switch out the cassette, just be sure that its a very high quality switch..
My 0.04 cents for today. Again, a general commentary, not attacking you, my system has exactly the same limitations and then some! My 51 year old ears are also not well calibrated anymore either!
Regards,
Bart
