Amplifier input tech question
Ska_man - not only, as Al said, you shouldn't lower your input impedance but it's even impossible because in order to get any meaningful attenuation like 10dB input impedance, in your case, would have to be about 30 ohms and preamp would distort a lot driving 90 ohm load. Buy attenuators that Dave mentioned. |
As Bob very correctly indicated, the 8 volt maximum output of the preamp is irrelevant. A preamp with a significantly smaller maximum output specification would not help the problem you are trying to address (everything else being equal), and if that number were much lower than 8 volts a new problem might even result, clipping of its output stage on high volume peaks. What is relevant is the preamp's gain, which as Dave indicated is a factor of 5 (200mv in producing 1v out), which is a gain of 14db. That is certainly NOT particularly high. Also as Bob correctly indicated, the power amp input sensitivity of 1.68 volts is a fairly typical number, and most power amps with similar output power ratings (200W in this case) will therefore have similar voltage gains. Your speakers appear to have a sensitivity rating of 91db, which is higher than average but just slightly. Therefore to the extent that it is meaningful to identify a "culprit," Bob's suspicion is probably correct. The DAC is probably putting out an excessively high voltage. But whether it is or not, as I indicated in my previous post a simple solution is to insert in-line attenuators. Regards, -- Al |
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Dave makes really good points, and 8v seems like an awful lot of output for a preamp. The parameter you want to compare, when considering amps, is the input sensitivity. Start looking at different models and see how they vary from the Aragon. If they don't vary much, the issue may be with the output of the JC-2. In any case, I would call Parasound to discuss this. I hear the JC-2 is a very good pre. Sometimes there is an easy adjustment to reduce the output of a pre, but hopefully you can find an amplifier with a higher input sensitivity. Perhaps Parasound can recommend the spec you should be seeking. |
Ska_man, your problem is not uncommon, and is easily fixed without changing any of your major components (although I recognize from your posts that you appear to want to do that anyway). Just get a pair of Rothwell attenuators (made by the company whose website Dave linked to). An Audiogon seller (with whom I have no connection) is offering them here: http://www.audiogon.com/cgi-bin/cls.pl?accstwek&1260621019&/Rothwell-10-dB-attenuators--rc. Contrary to what has been said, DO NOT address the problem by selecting a component with a substantially lower input impedance. That will most likely introduce frequency response irregularities into the system, because the output impedance of the component driving that input will most likely not be constant with frequency. Given that the problem is simply that your volume control needs to be set at a non-optimal point, and that there is no indication of clipping or other severe distortion, I would add that the root cause of the problem is that the overall combination of preamp voltage gain, power amp voltage gain, output voltage from the source component, AND speaker efficiency/sensitivity, is too high. In other words, it is not generally possible to assure that this kind of problem will be avoided simply by constraining the relationship of preamp output voltage to power amp input voltage, or by focusing just on any other specific interface between components. It is a system-level issue. Good luck! -- Al |
>>"My guess is that there is also a relationship between the preamp output voltage"<< That's the relationship you want to look at not impedance The Aragon 8008 amp has an input sensitivity of 1.68 volts according to the factory spec That means a signal of 1.68 volts will drive the amp to it's full rated output of 200watts Your Parasound JC2 pre can put out as much as 8 volts which is 4 times more than you need to drive the Aragon It seems that the Parasound's output voltage is directly related to it's input sensitivity From the Parasound site it states that a 200MV input signal will output 1 volt at the JC2's output ... so if your source (Dac) is putting out 400MV it would probably be just about right to drive the pre so the pre would output about 2 volts and not drive your amp into overload But your PS Audio dac probably puts out 2 volts to the JC2 pre and causes the JC2 to output the full 8 volts This 8 volt output from the JC2 is way to much for the 1.68 volt input sensitivity of your Aragon ... so you have to turn the volume control on the Pre down, so as not to dive the amp into overload This is probably why you can't turn the volume control on the Pre up pass 9 oclock Simply put the 2 volt output of your dac is driving the JC2 pre amp to full gain and outputs 8 volts to your amp which is way to much ... so you turn the volume control down to 9 oclock so as not to over drive the amp Volume controls are attenuators and can only turn volume down by adding resistance ... the more you turn the volume down the more resistance you add to the circuit The more resistance added the less transparant the signal will be So ideally you would like to have your volume control turned all the way to 1 oclock where there is little to no attenuation or resistors in the circuit to reduce transparency Here are two links which may explain it better than I did http://www.rothwellaudioproducts.co.uk/html/attenuators.html http://www.rothwellaudioproducts.co.uk/html/how_attenuators_work.html HTH Dave |
Tpreaves, The JC 2 manual suggests setting the amplifier gain controls (if any) so that the volume control on the JC 2 is between 11 o'clock and 1 o'clock during normal listening. That is (according to the manual) the "sweet spot", where the channel tracking is closest & the distortion is lowest. As stated above, I intend to purchase a new stereo amp or two mono block amps. The intent of the question, is to learn about the impedance and voltage measurements and what they mean, as far as finding a good match. I will ultimately choose the amp or amps by ear, but I do want to consider getting the JC 2 into its "sweet spot". |
If I were you, check the type of volume pot you have on the preamp. If it is 250K, make it 100K. If it is 100K, make it 50K etc. For sure, you will increase the position of the volume knob without changing the output level. You have to look at the pot and determine how the voltage divider is implemented in relationship to the pot wiper. It is cheaper and easier to do this than buying a preamp no? |
