2. amps-preamps
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An interesting experiment

I have recently been experimenting with different preamps, as I found a while ago that this seems to be the component that influences the sound of my system the most. Being an EE (and a physicist by training)I was trying to figure out the source of the different sounds that tube and SS give me. I am a firm believer that good designs with modern electronics all sound the same (at least to my ears). As tubes are new to me I studied their specs a little, and recently realized that one big difference vs. SS is their output impedance. To be clear, I am talking about minimalist tube design, where the output stage is driven directly by the tube. As it turns out, tube designs have much higher output impedance. For example a 6SN7/6922 based preamp probably has an output impedance on the order of 8K/3K ohms respectively. While a typical SS would be less than 500 ohm. So as an experiment I put together a passive preamp with a series potentiometer that allows me to change the output impedance of the source. I connected my CD output through this "preamp" directly to the power amp. At this stage after listening for awhile I believe that my new setup sounds a lot like my tube setup (which has an audible illusions M3A pre, and magnepan 20.1s). The curious thing is that I had to dial the output impedance to around 30K ohm, which I believe is significantly higher than the tube pre. I know this setup by all accounts should be bad....

Regardless, to my ears this strange setup sounds excellent. I dare even say, better than with the preamp in the loop.
oferi
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almarg
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Cymbop, thank you most kindly! However, I respectfully decline the nomination, as I'd much rather spend time listening to my stereo than dealing with being President :-)

George, thanks. I haven't checked your calculations, but I know from past threads that you are very accurate in these matters. I would just add two points that others should keep in mind.

First, the obvious one that many people will require significantly longer lengths than 1 meter. And cable capacitance will increase in direct proportion with length, which will decrease bandwidth correspondingly.

Second, the bandwidth of the low pass filter that is formed by the interaction of output impedance and cable capacitance arguably needs to be at least several times greater than the 20 kHz nominal bandwidth of our hearing, to eliminate the possibility that phase shifts caused by that filter may have audible consequences.

Oferi, thanks for an interesting thread. Enjoy your experiments!

Regards,
-- Al
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almarg
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George, after submitting my post just above I did notice something in your calculations that prompted me to look at them further. A minor correction: The attenuation at half the 3db bandwidth frequency won't be 1.5 db.

"-1.5 db" (in three places) should actually be about "-0.97 db," corresponding to:

20log(1/(Square root(1 + (f/f3db)^2)))

f/f3db, the ratio of frequency to 3db bandwidth, being 0.5 in your three corresponding examples.

Regards,
-- Al
bifwynne
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Cymbop +5 re Al for President. My next comment is not intended to be technical response because I am not a EE.

To the extent Al and Oferi are speaking about input/output impedance matching, I kinda tripped into that issue a while back. Most ARC linestages should not be presented with a combined output load of less than 20K ohms. As I learned by accident, I inadvertently overloaded my linestage.

My amp's input impedance is rated at 300K ohms (XLR input). However, I also loaded a subwoofer off one of the SE Main outputs of my linestage. The sub's input impedance was 20K ohms. OOppss. If you do the math, I think the combined impedance was less than 20K ohms. Plus, as ARC explained, loading the amp off the linestage's XLR output (Main #1) and the sub off the other SE output (Main #2) "asymmetrically" loaded the linestage. ARC said that was a big no-no.

The solution was Tom Tutay who custom designed and built an impedance buffer for me that summed the linestage's L/R channels off the Main #2 XLR output without shorting the Mains AND raised the impedance level of the buffer's input to 330K Ohms. If you do the math, I think the combined input impedance presented to the linestage is 157K ohms.

My linestage is happy and I'm very happy. And Kal at ARC is extremely happy too. We're all happy. ;>')
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almarg
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Thanks Bruce (Bifwynne)!

Everything in your post is correct. For two impedances in parallel, assuming they are purely resistive (which is usually a good approximation when it comes to input impedances) their combined impedance is the product (multiplication) of the two numbers divided by their sum.

The answer will always be smaller than each of the two numbers.

Best,
-- Al
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