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
mapman's avatar
mapman
22,152 posts
 
Oferi,

Many SS amps designed specifically for good performance with all pre-amps, including tube pre-amps, have 60Kohm or higher input impedance to compensate for higher general output impedance of tube pre-amps. That helps keep distortion minimal and retain good dynamics with most all pre-amps out there, SS or tube.
georgehifi's avatar
georgehifi
10,312 posts
 
Oferi your Ice Power amp/s with 10kohm input impedance is not a good match for any passive pre or even some tube preamps.
However your CJ MV60SE is 100kohms input impedance and this is a great match for a 10kohm passive preamp.

Cheers George
mapman's avatar
mapman
22,152 posts
 
This kind of experimenting with various impedance matches is very interesting. Looking forward to whatever general findings or patterns might be detected.
dtc's avatar
dtc
1,047 posts
 
Georgelofi - not a good match by traditional thinking, except it sounds good to him.
georgehifi's avatar
georgehifi
10,312 posts
 
"not a good match by traditional thinking"

It's more than just traditional thinking, it's "Ohms Law", every stage of amplification design is based on on it and Kirchhoff's Law, without them you'd have a mess.
And yes if it sounds good to him he may have stumbled on something that has addressed a problem elsewhere.
Typical example is the Linn Isobarik speaker of yesteryear which was a highly overdamped design (not the ideal .707Q), which worked well with amps that had very mediocre low damping factor such as the Naim 250 at lower than 20 damping factor (highish output impedance). If you drove them with an amp that had a reputation of great controlled powerful bass that had high damping factor (low output impedance) those same Isobarik's had no bass, even though the amp was regarded for it's bass performance.

Cheers George
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