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In Reply to: Re: Input Loading for MA1 mk. III & MP3 posted by Lew on March 22, 2006 at 08:13:09:
Thanks for the reply Lew. I did do a little experimenting and got side tracked by something else and I am just now getting back to it. I had read about the 600 ohm load making ic cables have less impact on the sonics, etc.I do have a single 6SN7 per channel in the Circlotron circuit. I first tried loading at 600 Ohm and noticed compression and a lack of dynamics and probably a little roll-off as well. I increased to 1200 Ohms for the reasons you stated above and noticed an improvement vs. 600 ohm. Everything seemed to fairly dynamic and tight.
As usual in search of more perfection, I realized I had put the 1200R in and had forgotten about them. Last night I decided to pull them out and use the factory loading. I thought this was 100K which is why I mentioned it in my post.
What I found sonically with the 1200R in place was the image seemed to be better defined, much sharper with pinpoint precision. The music seemed to be more accurate, but less musical if that makes sense. Almost sterile and squeaky clean.
After removing the 1200s from the system, the music lost a little detail and precision, but sounded a little more romantic and engaging. Not rolled off, just not as tight in focus with a little more bloom.
I was going to experiment with different values, but thought I would ask here in case someone has been through it before.
Follow Ups:
I was just re-reading your posted description of the sonic differences between the 600R and 1200R load and then between the 1200R load and the 100K load. Based on your choice of adjectives, it seems as though there was at least some similarity in character between 600R and 100K, as compared to 1200R, albeit you did make it clear that the 600R load was inferior to the other two alternatives. But is my conclusion accurate?
When you removed the 1200R resistors, presumably the standard 100K ohm grid resistors were in place and you were driving that essentially 100K ohm load. (Under AC conditions, there are other factors that contribute to the load impedance, so Z is not precisely 100K ohms and varies slightly with frequency.) Your observations are valuable to all of us who have not "tried it both ways". Most importantly (to me), your MP3 DC servo circuit did not become unstable into the 100K load, apparently. If you are up to further experimentation, you might want to try increasing R from 1200 ohms in two-fold increments, as I suggested above, until you have the best of both worlds. An easy way to experiment would be to use 10K-ohm potentiometers, then start at 10K and go downwards. Whatever setting sounds best could then be duplicated in the form of discrete resistors.
I plan to do some listening this weekend and try some different resistor values to hear the effect.I can already say, several distinct differences after removing them completely. As I mentioned in my other post, the sound was a little warmer and more musical. However, this was at the expense imaging, detail and most noticably the noise floor was much higher. The background has so much going on that I think it it makes you think the sound is warmer, more euphonic. After listening for a little while I realized this won't work for me.
I started with 300 Ohms per phase (600 across 2 & 3). That was to low for the MP3. I switched to 600 per phase (1200 total) and that worked much better. That is what I have been using until I removed them completely to hear the difference.
Hi guys--Really dumb question here. If I want to terminate my MP3 into 600 ohms (balanced), then what values of resistance do I use per phase to ground?
Hi,I searched the ASOG archives and came up with a thread suggesting that the MP3 would do better with 1200 ohms (600 ohms per phase) than 600 ohms total (300 ohms per phase). I installed 300 ohms per phase on the balanced inputs of my M60s years ago, and now wonder if I should give 600 ohms per phase a try. Guess I answered my own question about the termination resistance calculation.
Well it took some time to test several different resistor values. I finally ended up at 2K. Below 1200 the dynamics became compressed. Above 4K , I noticed the noice floor started to increase ever so slightly. I ended up using Caddock 2K's because the next value the Caddock's were available in was 5K.Overall, I would say somewhere between 2K - 3K is about right for a MP3. This is a significant improvement over not running any resistance at all.
Interesting. With my MP-3/M-60 combination, I settled on 1.5K after a good deal of experimenting. I would say we were pretty close! I imagine 2K would be just fine in my setup as well. Thanks for letting us know your findings--
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