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
oferi OP
12 posts
 
Yes, I am presently using a 10k passive pot for volume control (Incidentally I am using a Sescom mkp-25, which is only 50$). I also have at my disposal an ARC LS2 and the AI module 3a. But the pot arrangement sounds excellent to me. More like the AI than the ARC.

My power amp an Icepower based on the 2 x 125asx2 modules (I also have a CJ MV60SE). The ice amps has a rated 10k input impedance, but based on my experimentation it seem to be higher than that. I plan to try this carefully with the CJ next.

My guess is that the series resistor with the pot is resulting in an overall equalization that is closer to the tube amp setup. But clearly it is not simply because of matching output impedance to that of the tube amp (as noted above regarding the cathode follower arrangement).

Anyway, it is just a fun thing to try to get different (and in my case good) sound that is much cheaper than rolling new equipment.
dtc's avatar
dtc
1,047 posts
 
A 30K output impedance into a 10K input impedance is certainly a little different than normal recommendations. 10K is pretty low. It would be interesting to try an amp with 100K input impedance. It would be interesting to look at the frequency output of the amp.

Interesting experiment.
mechans
3,048 posts
 
So what sounded most like a tube amp and what sounded most like an SS amp??
mapman's avatar
mapman
22,198 posts
 
No doubt impedance alone can account for a lot of what one hears different between SS and tube amplification gear.
oferi OP
12 posts
 
to be clear, the I have a 10K POT for volume control followed by a series resistor (also a pot so I can adjust it). After thinking about it I realized that I made a mistake when saying that the amp is rated for 10K. Since each module operates in fully balanced mode there is a single ended to balanced input buffer. So likely the actual input impedance of the amp I am using is higher than 10K.

The setup with the 10K volume pot and 20K series resistor sounded a lot like the AI preamp. But I wouldn't assume it would be the same with other power amps.
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