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Old 21st September 2005, 07:36 PM   #1
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Default Perceived sound vs. output impedance?

Is there a link between perceived sound and output impedance of an amp?

As some may know I listen to a modified version of the great Leach amp.

I messed around with the circuit and found that I liked the sound best when the open loop gain was lowered a bit compared to the original. Iím unsure if this is linked to the fact that the output impedance of the amp rises as the open loop gain is lowered.

Could it be that peopleís obsession with the sound of no feedback designs is in fact linked to a higher output impedance of these amps. Iím wondering if it is possible to make a non feedback amp with as low output impedance as one with tons of global feedback.

Your thoughts

\Jens
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Old 21st September 2005, 08:23 PM   #2
hughmon is offline hughmon  United States
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Seems like you could simulate the amp having a higher ouput impedance by just placing a resistor of desired test value in series with the speaker, then bump up the volume control to make up for the drop in level.
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Old 22nd September 2005, 02:51 AM   #3
jcx is offline jcx  United States
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Old 22nd September 2005, 07:22 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
small values of resistor in series with the speaker (0r05 to 1r0) will increase the Q of the bass driver. This is very noticeable in the sound balance coming from a full range speaker.
It is an acceptable method for correcting slight Q mismatch between stereo pairs.
Resistors towards the higher end of the range will give the bass much more low end extension and can change a bass light speaker to a bass strong version.

These same effects could be mimiced ( that does not look right - mimic with an ed) by designing the amp output impedance to suit the speaker set-up.
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Old 22nd September 2005, 07:33 AM   #5
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Default Re: Perceived sound vs. output impedance?

Quote:
Originally posted by JensRasmussen
Is there a link between perceived sound and output impedance of an amp?

As some may know I listen to a modified version of the great Leach amp.

I messed around with the circuit and found that I liked the sound best when the open loop gain was lowered a bit compared to the original. Iím unsure if this is linked to the fact that the output impedance of the amp rises as the open loop gain is lowered.

Could it be that peopleís obsession with the sound of no feedback designs is in fact linked to a higher output impedance of these amps. Iím wondering if it is possible to make a non feedback amp with as low output impedance as one with tons of global feedback.

Your thoughts

\Jens
Good thoughts but compare this with First Watt and 60 ohms out accordning to rumours.
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Old 22nd September 2005, 11:00 AM   #6
inguz is offline inguz  United States
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Default Not |Zout| but...

My theory-du-jour is that the size of the imaginary component of ZOut is important (perhaps more than the magnitude). Not supported by experience yet. :-)

Blumlein, 1935, says match impedance:
http://www.doramusic.com/patents/425553.htm
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Old 22nd September 2005, 11:41 AM   #7
tlf9999 is offline tlf9999  United States
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What if you increase Zout to infinity? as in a current amp. Will that sound better?

I personally haven't found any difference there.
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Old 22nd September 2005, 12:14 PM   #8
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Dear,

If Zout increase, the dampingsfactor become way lower. So basscontrol is less. It can look like there is more bass, but it is just the out of controle resonating woofer.

Try it yourself. Take a woofer (without filter etc.) and short the two terminals. Give a solid pop with you're finger on the woofer membrame. You will hear a tight and short pop. Now short the woofer with a 1 Ohm or higher resistor and pop again. Hear the difference...

A woofer has to be shorted if the sinwave is passing the zero point. The higher the output impedance, the more the woofer is out of control on the zero point.

With kind regards,
Bas
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Old 22nd September 2005, 01:01 PM   #9
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The damping factor seems to be related with the speaker impedance curve. As far as understand, speaker makers tune their sound, considering the drivers impedance peaks which are common in low frequency zone, and presuming the low damping factor of the power amp (voltage source amp).

For this reason, if the amp output impedance is increasing (to a current source amp with low damping factor), more current would occur especially in the low frequency area so that the speaker might sound like the uncontrolled bass.

Recently I contacted the speaker designer of German Audio Physic, and they confirmed this. In addition, I got the same confirmation myself after having built Pass F1 Amp (of low damping factor).

Anyhow, I solved the excessive bass by attaching and adjusting a low-cut filter at the input of my pre amp. Wonderful bass!

DIY is full of fun!

Regards
jH
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Old 22nd September 2005, 02:59 PM   #10
Eva is offline Eva  Spain
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Increasing the output impedance of the amplifier is always going to change the frequency response of the loudspeaker. When a single driver is employed, both the LF roll-off due to resonance and the HF roll-off due to voice coil inductance will vary.

When a multi-way system with passive crossover is employed, crossover points will be shifted upwards or downwards and the Q of the filters will also change just by increasing the output impedance of the amplifier.

In other words: Jens, you have been probably just fiddling with your speaker system :P
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