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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georgehifi
10,312 posts
 
If you use a passive volume control that is 30kohm it's approx highest (worst) output impedance is about 20kohm.
This 20kohm combind with high (poor quality) cable capacitance of 100pF per foot, 300pf total for 1mt will give you a HF roll off of -3db at 27khz (-1.5db at 13.5khz).
As you can see this is cutting well into the audio band. And if the interconnect's capacitance is even higher than 100pf per foot, thing are even worse.

However if you used a 10kohm passive volume control things change for the better, the -3db HF roll off is now at 54khz (-1.5db at 27khz).

And if you use better quality interconnects like 50pf per foot capacitance the figures for a 10kohm passive are -3db at 108khz (-1.5db at 54khz)

Cheers George
oferi OP
12 posts
 
Almarg,
I completely agree with you. In fact when I thought of trying this, the effect you mentioned is exactly what I was looking for. clearly if one is looking of a high fidelity setup this is not the way to go. However, sometimes playing with the equalization makes things sound better, and at the end of the day this is what counts. As I mentioned earlier, the ICE amps input is probably not 10K due to the added input buffer (single ended to differential (balanced)).
For fun I also tried a 100K series pot, with the idea of exposing the nature of the amp load and IC cable. With such a high impedance I expect the cable to have significant contribution. With such a high resistance I was basically controlling the volume with the series resistor. The effect was interesting, with rolled off highs.

Anyway, I am not going to make this my permanent setup, but it is a nice arrangement to have in the toolbox.
almarg's avatar
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almarg
9,670 posts
 
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
9,670 posts
 
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
2,206 posts
 
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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