need some help with some calculations...

[matthewmckay]

Hello Fellow Bottleheads,

I have an issue and I need some help.  I have a turntable/seduction, hooked up to a Quickie(with PJCCS), which is hooked up to a yamaha amp.  There is a serious gain issue because the amplifier has a very high sensitivity (127db).  I want to replace the resistors that are at the inputs on my amplifier(580 ohm) with a value that will give me a more appropriate mating to the quickie.  I read the PJ article "signals, noise, and signal to noise ratio", which just made my head spin... (i am a chemist who barely scraped by in mathematics).

I do not know what information is required or important here, but this is what I know.

SPEAKERS
Yamaha soavo-1

impedance ..........................6ohms
nominal input power...............50W
Maximum input power.............200W
Sensitivity............................89 dB/2.83 V/m

AMPLIFIER
Yamaha M-65

Minimum RMS Output Power Per Channel
  20~20,000 Hz      0.007%THD, 6 ohms ............195W
Dynamic Headroom (IHF, 6 ohms)................... 2.08 dB
Dynamic Power per Channel .............................. 315
Input Sensitivity/Impedance...............1.28V/20 K-ohms
Signal-to-noise Ratio......................................127 dB

My listening room is fairly small, and I sit about 9 feet away from the speakers.  I would say that the room is slightly "live". 
If anyone can help me figure this out, I would be very much obliged!

 

[Paul Joppa]

You want a nominal 82dB, which is 7dB less than the speaker sensitivity, so a nominal 0.2 watts will be about right. (Instantaneous peaks will be much greater than that, maybe 20 watts with an audiophile CD, which is why you have a powerful amp). The amp puts out 195 watts with 1.28v input, which is about 10oo times the nominal needed, so the voltage needed is the square root of 1000 times 1.28, or 0.040 volts.

Seduction has a gain of 100 which is 40dB, if you have the C4S boards. That means a nominal output of 0.40 volts if your cartridge is 4mV nominal. The instantaneous peak is about 2v for well-recorded vinyl (14dB peak to nominal).

Let's assume that in normal operation you are using the Quickie turned down 12dB from the maximum, so there is some room for quiet recordings, or parties, or digging into a quiet passage. That gives a net gain of unity, so the Quickie output is also 0.4v nominal. Do, you need a 1:10 voltage attenuator, or 20dB, at the power amp.

The first choice is to put the attenuator inside the power amp. It has a 20K input impedance, so a 180K resistor  to replace the wire that attaches to the RCA jack will do the job. Very simple, unless the RCA jack is soldered directly to a PC board - then it might get a bit tricky.

If you want to leave the Yamaha unmodified, a reasonable second choice is to put the attenuator in the Quickie. This requires that you keep the resistances lower, but a voltage divider of 18K + 2K will do the job and have an output impedance of less than 2K, which is half the impedance of a quickie with PJCCS. You could easily go twice that impedance.

What I would do, myself, would be to get a pair of potentiometers, 20K to 50K, and install them inside the Quickie. You can probably glue them to the chassis next to the output jacks. That way you can adjust the gain in each channel, allowing for tube gain differences, and experiment to find the attenuation that works best in your system. The PEC pots are relatively inexpensive and quite nice sounding. You can use a smaller value, say 5K, with a series resistor so it adds up to at least 20K - that way the adjustment is less tricky and you can easily use linear pots.

 

[matthewmckay]

Hey Paul,

Thankyou so much for the help.. there is no way I would have figured that out.  I had to recently replace the RCA jacks on the yamaha because they broke off their flimsy PCB.  I think I will try the 180K ohm resistors first, at the new input jacks, since I already had mod the case by drilling new holes.  I will try the pot suggestion next if I am not satisfied with what I hear.

Again..  Many Many Thanks!

 

[intojazz]

Damn Paul,
That's just flat impressive, and probably off the top.
John