[chadbang]

The rear jacks on my old Harman Kardon Citation 21 preamp all are broken. I really like the sound of the preamp in my system. It's kind of dark, which works well with my Spica TC-50s. So I decided to find another one to replace it. What I stumbled upon and bought instead was a HK Citation 25 preamp. Same vintage and design unit, but with a few more bells and whistles. Most of the specs are the same except for one - which actually looks kind of important. My old preamp was rated at am Output impedance of 330 ohms. My new one is rated at an Output impedance of 600 ohms. What does will that mean in real world performances differences? Sound? Any knowledge you can share will be appreciated.

 

[JaZZ]

Quote:

Originally Posted by chadbang My old preamp was rated at an Output impedance of 330 ohms. My new one is rated at an Output impedance of 600 ohms. What does will that mean in real world performances differences? Sound?

It doesn't have any adverse effect on the sound. Also 330 and 600 ohm are still roughly in the same category. Nothing to worry about as long as the load impedance (the power amp's input impedance) is at least 6 kOhm. You'd have a hard time finding any power amps with input impedances below 10 kOhm.

 

[chadbang]

Thanks, Jazz.

 

[Tyll Hertsens]

I think this is a subject worth a little dialog.

Back in the day, it was a big deal just to get a signal from one place to another. Could be audio, but could be FM or radar, too. If you've got to pump a signal from one place to another over a long cable or waveguide, you've got to impedance match it.

In audio, that meant the everything had 600 ohm transformers on either end. In FM, radar, or cable TV, it means everything was properly terminated so that there was no VSWR losses. VSWR means "voltage stanging wave ratio." In the RF world, when signal transmittion distances are longer, or a significant fraction of, a wavelength of the signal involved, you have to properly move energy from one place to another efficiently because if you are not efficiently coupling signals you get reflections of energy. This reflected energy "beats" against the incoming signal and creates standing waves on the transmittion line. This "voltage standing wave" can mess with detection of the signal of interest (amplitude, frequency, or phase modulated signals). So it is very often that designers couple by "impedance match" from place to place when efficient transmittion is important.

However, if you're talking about impedance matching an audio signal, the VSWR is of no interest because 20kHz at the speed of light is a very long wavelength, and no "standing wave" type interference would come into play. In audio you are really talking about using a 600ohm transformer balanced method to ship around michophone signals with a lot of common mode rejections from intereference. But, these transformer coupled techniques have some resonant behaviours that effect the sound.

To get rid of that you use a "bridging" method of impedance matching. "Bridging" is generally considered to be the point where the output impedance is at least ten times smaller than the input impeance of the driven device. Generally speaking input impedances of audio equipment (Power amps, pre-amps, headphone amps) is at least 10kOhms---usually more like 50kOhm.

So, you're 300 or 600 ohm ouput impedance pre-amp is driving a 500-1000 times bigger 30kOhm input impedance amp. Which means it's bridging. In this case, the pre-amp has no problem delivering the voltage required to drive the load of the amp input. The low output impedance of the amp acts to short out any reactive difficulties driving the load. This is called being well damped.

This is why headphone amps tend to be good pre-amps: they are authoritative in thier ability to drive the input of another device like a power amp.

 

[Shakey]

Hi Tyll,

Thanks for the information on impedance matching. I'm not a technical person at all so I was hoping that you could help me through this again. Basically, as per Ohm's law (V = RI), are you saying that a headphone amp makes a good preamp because it has a relatively high current output?

Also, are you saying that it's always/usually beneficial to have a low impedance output going into a high impedance input?

Thanks again and sorry if I sound confused here!

 

[JaZZ]

Quote:

Originally Posted by Shakey ...are you saying that a headphone amp makes a good preamp because it has a relatively high current output?

(Although it's not me who's addressed...) Not directly so, rather because it has low output impedance making for a high damping factor, = unsensitivity to uneven load impedance response, which would cause frequency-response distortions.

Quote:

Also, are you saying that it's always/usually beneficial to have a low impedance output going into a high impedance input?

Yes. The higher the ratio, the better. But a ratio of 1:10 is usually enough, any further increase will be academic -- at least as long as we're talking of source devices and amps. With amps and headphones on the other hand a ratio of 1:10 may not be enough. There's possibly an audible difference between 5 ohm and 30 ohm output impedance of an amp driving a 300-ohm headphone such as an HD 650. The reason is that headphones can easily show double the nominal impedance at their resonance frequency. So a damping factor of higher than 20 can be beneficial. The same applies to speakers with their even more extreme impedance curves.
.

 

[machead]

Quote:

Originally Posted by Shakey Basically, as per Ohm's law (V = RI), are you saying that a headphone amp makes a good preamp because it has a relatively high current output?

The important factor here is not so much the ability to deliver high current per se, but the ability of the amplifier to deliver the desired output voltage even if the current drawn by the load (e.g. headphones) changes significantly.

All headphones have a load impedance that varies with frequency. If the amplifier has a low output impedance, its output voltage will be less affected by the variable headphone load than an amplifier with a high output impedance. Put another way, there will be less interaction between the amplifier and the load. This is the environment assumed by most headphone designers when publishing their response curves.

The easiest way to reduce the output impedance of an amplifier is to employ negative feedback. Many amplifier designers, however, believe that feedback degrades the sound quality and they blithely place the burden of dealing with the resulting higher output impedance on the shoulders of their customers. The black art of synergy between amplifiers and headphones involves finding combinations of equipment where the interactions between source and load are deemed to have a euphonious effect.

And to complicate matters still further, the foregoing comments about variable headphone loads also apply, albeit to a lesser extent, to interconnects.

 

[Dominode]

Oringinally posted by Tyll Hertsens Quote:

Back in the day, it was a big deal just to get a signal from one place to another. Could be audio, but could be FM or radar, too.

It's still a big deal,and for some members here a minor obsession -or perhaps yet another outlet for their passion...Witness (as you know) the IC/cable upgrade paths or options available for carrying that precious signal,regardless of the source. You could treat your floor system speakers (and your wallet) to cables that retail for about a set of UE10's per foot...
A modest marvel of of the signal chain can be found in that frequently obnoxius (excluding mine,of course) contemporary totem known as the cellphone.
The ubiquitous cellphone,some of which can now play music -sort of - is a trickle-down product of originally solving a satellite-to-earth challenge of sending a data stream (pick a large number) millions of miles and having the signal/data stream intact and decipherable (error creep) at the receiving end. The practical development of convolutional and Turbo coding (Dr.Peter Elias of MIT was the world's best in this arcane and esoteric field; he also chaired both the EE and Computer Science Depts.) made that possible.
So the next time you're hooked up to your system and in the midst of some impedance-matched sonic reverie,you might give a quick silent "thank you" (between tracks) for good signal mangement. And make sure your damn cell phone is turned off

.

 

[Shakey]

Wow, thanks for the clarification, guys. Muchly appreciated!