|   pre-amp output | guy
Jul 16, 2001 11:38 AM | | hi,
i need to know what is the difference between pre-amp with output level of 1v/1.2 kohm and pre-amp with output level of 1v/100 ohm?
BTW my power amp input sensitivity is 1v/100 ohm.
which output level is best for my amp and why?
thanx.
guy |
| re: pre-amp output | Trevor Wilson
Jul 16, 2001 11:54 AM | | The lower the output impedance, the better (all things being equal). The one with 1 Volt @ 100 Ohms is the best choice.
Low output impedance figures are better, for several reasons:
1) They allow long interconnect runs.
2) Distortion, due to power amp loading doesn't occur.
3) It is good engineering practice to ensure that the output impedance of the source, is at least 10 times the load impedance.
BTW: It is extremely unlikely that your power amp has an input impedance of 100 Ohms. |
| re: pre-amp output | guy
Jul 16, 2001 12:02 PM | | hi,
thanx for your reply.
you are right,my mistake,my power amp input sensitivity is 1v/33 kohm.
would i be able to hear any difference between the 1v/100 ohm and the 1v/1.2 kohm?
guy. |
| re: pre-amp output | Trevor Wilson
Jul 17, 2001 2:07 PM | | IF ALL THINGS ARE EQUAL, then, probably. The low impedance source, will, inevitably sound better, IME. Of course, things are not always equal.
Trevor |
| re: pre-amp output | robertony
Jul 17, 2001 8:06 AM | | Why must the Zout be so large? Do we need to consider the "maximum power theorem" in this case?
What is the best combination of Zout, Zinterconnect and Zin in general ratio/values?
Thanks |
| re: pre-amp output | Trevor Wilson
Jul 17, 2001 2:06 PM | | robertony wrote:
"Why must the Zout be so large?"
No, Zout must be much LOWER than Zin.
"Do we need to consider the "maximum power theorem" in this case?"
No, because we are no interested in power. We are only interested in Voltage. In any case, with audio equipment, maximum power transfer is not applicable. Remember damping factor? The ouput Z of a power amplifier is always much lower than the impedance of the loudspeaker. Same deal with line level stages. Loading on the source, should always be as low as practicable.
"What is the best combination of Zout, Zinterconnect and Zin in general ratio/values?"
Personally, I prefer to keep a minimum of 100:1 ratio of Zin:Zout. In this instance, more is always better. 10:1 would be an absolute minimum figure. A low impedance system (all things being equal) will always be better than a high impedance system. A low Z system will ensure the best S/N ratio and highly capacitive interconnects will not cause problems.
So, a (say) 1 Ohm source impedance (open loop), driving a (say) 600 Ohm load impedance, would provide excellent results.
Another factor, which many forget, is the fact that most SS amps, use possess an impedance of around 1,000 or 2,000 Ohms, on the negative feedback leg of the front end differential pair. Thus, for the most symetrical operation of that part of the circuit, the input impedance of the amplifier SHOULD be between 1,000 and 2,000 Ohms. Usually, it is much higher. It is this imbalance (common to 99% of SS amps) which leads to poor performance.
Trevor |
| re: pre-amp output | guy
Jul 17, 2001 4:01 PM | | i'm still confused,would i be able to hear any difference between the
two,better sound quality,higher volume? |
| re: pre-amp output | Trevor Wilson
Jul 18, 2001 10:21 AM | | If all other factors are equal and you are using a low(ish) capacitance interconnect, then possibly not.
However, as I stated before, IF all other factors are equal, then a low impedance output, is preferable to a high impedance output.
Trevor |
| re: pre-amp output | paule
Jul 22, 2001 11:19 AM | | Ive been following this thread with interest..I just bought an audiosource amp3, which is driven by my STRV-333ES ht receiver for better front L&R music output via its front preouts. The preouts are listed at 2v and 1kohm. The inputs to the amp are listed at .8v and 30kohms. Can you make any determinations from these numbers? |
| re: pre-amp output | Trevor Wilson
Jul 22, 2001 1:42 PM | | Not really. They should work together quite satisfactorily, but I suggest you keep away from long interconnects.
Trevor |
| re: pre-amp output | robertony
Jul 17, 2001 4:14 PM | | "Zout must be much LOWER than Zin."
Please explain more about Zout(pre-amp) < Zin(amp),
Zout(amp) < Zin(load:speaker)
What happens to the case Zout > Zin?
Are they true in any case of SS & tube equipments?
Thanks |
| re: pre-amp output | Trevor Wilson
Jul 18, 2001 11:42 AM | | robertony wrote:
"Please explain more about Zout(pre-amp) < Zin(amp),
Zout(amp) < Zin(load:speaker)
What happens to the case Zout > Zin?"
In the case of a power amp - speaker system, the result, with most loudspeakers, will be a severely compromised frequency response.
Consider the following, extreme example:
Amp O/P Z = 2 Ohms (possible, with some SET amps).
Speaker Z = 8 Ohms, with dips to 2 Ohms (easily possible).
When the speaker impedance dips to 2 Ohms, 50% of the amplifier power will be dissipated within the amplifier output impedance and 50% within the speaker (IOW: A -6dB variation will occur). When the impedance rises to 8 Ohms, the variation will be -1.9dB. Of course, with a 0 Ohms output Z, the variation will be 0dB.
It is important to note that speaker which exhibit a purely resistive load (quite rare) will not suffer the same frequency response problems.
Given that purely resistive loads are not much of a problem, you can see that freequency repsonse issues are not a problem, with line level situation.
"Are they true in any case of SS & tube equipments?"
Of course. The problem is more an issue with certain tube amplifiers (SETs) than with SS amps. However, SS amps are not blameless, since most enjoy a rising output impedance, WRT frequency and will thus suffer a poor frequency response, with some, real world loudspeakers.
See attached speaker impedance curve. Note impedance dips at high and low frequencies. At low frequencies, tube amps will suffer greatly and at high frequencies, SS amps will suffer greatly.
Trevor |
| What a resource! | dmb_fan
Jul 19, 2001 9:06 AM | | Trevor, wow. Thanks for explaining that so well. But now my question is, what is the solution?
I mean, if impedence variations in speakers cause power from the amplifier to dissipate before getting to the speakers, is the solution to use higher-powered amplifiers that will be less suceptable to this dissipation? |
| Blush. | Trevor Wilson
Jul 20, 2001 12:43 AM | | No, the solution is to demand a graph of output impedance from the manufacturer. Look for an amp, which has either a flat output impedance curve, over the full audio band, or one which has a quoted Damping Factor, which is satisfactorily high, from 20Hz to 20kHz.
Trevor |
| Trevor, how do the DIN connectors work? | Pat D
Jul 19, 2001 8:51 AM | | I notice that my preamp has an option on the Tape Outs for a 30K output impedance for use with the DIN connections, which I don't use. How does this work? |
| Trevor, how do the DIN connectors work? | Trevor Wilson
Jul 20, 2001 12:45 AM | | I doubt that the output impedance of the DIN connectors, is that high. It could easily be in excess of 5k Ohm, though. I'll have a look throungh some technical stuff, tomorrow and report. A 30k Ohm output impedance, would certainly lead to serious HF loss, with even modest length cables.
Trevor |
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