@Tom_S said:
Interesting outcome. I would think any confirmation bias would favor the Mod version, especially since you knew THD was lower and bandwidth was flatter. Could this be an example of how we still can't measure everything our ears prefer?
I did my listening before I did the measurements. I was thinking that maybe I was biased in favor of the stock version because I have read, over the years, several reviews and articles about the negative sonic effects of negative feedback in amplfiers. After I took the measurements, however, I was kind of thinking that I should not have heard any difference at all, since the differences were so small. But I clearly did. So I would think, at this point, that if my mind was making this up, I would conclude that both versions sound the same. But they don't. So, now I have to give Craig's jumper wire idea a test to see where this goes.
I have heard (with my own two ears) negative feedback suck the life out of an amp. I get why you like the original design better. Too bad the gain is so high with the original circuit.
@PWRRYD said:
Forgot to ask... when doing your listening tests what was the input impedance of your power amp (ie how much preamp gain)?
For all listening tests, I was driving into a Lepai 2020 amp, which measures 10k ohms across the inputs. This would have made the preamp gain 14dB for the stock channel and 6.5dB for the mod channel. When I swapped out channels, I adjusted the volume control up and down to compensate and equalize the SPL from the speaker. Interconnect cable capacitance was about 200pF, so probably no HF roll off on either the mod or stock channels.
@PWRRYD said:
Try bypassing the input cap with a jumper.
I attached a short jumper wire and gave it a listen. As you can see in the photos, I soldered the jumper wire to the bottom of the board, but left it unconnected on the top side, roughly 1/16" away from the solder pad connection point. In this way, I was able to use a small wooden stick to quickly connect and disconnect the bypass jumper wire and switch the cap in and out of the circuit almost instantaneously as the music played. Result: No difference at all. The sound was the same with or without the cap.
So, the sound quality difference I am hearing between the mod and stock channels must be due exclusively to the feedback loop. Something about the feedback is sucking the life out of the music, making it sound dead compared to the stock channel.
Well, I think that pretty much wraps up my attempts to modify this version of the 6J1 preamp. I am going to remove the mod perfboard and change it back to the stock circuit. Gain will be about 14dB into a 10k ohm load, which is a little high by some standards, but I am OK with this. I intend to use this preamp with the Pass Labs Pearl I phono stage (59dB gain) in combination with a low output moving coil cartridge. A line amp with about 10-15dB of gain is just about right for that type of setup. If I had built either the Pass Labs Pearl II or III phono stage, then this line amp would probably have a little bit too much gain.
I tend to use low gain amps, like many of the passdiy's are around 15db and a hypex ucd250 with no input buffer that is 13db. Though also use a F4 diy sometimes which is 0db gain but have a FE2022 'preamp' that has some giddy up at 20db gain or so. But with weather getting warmer I use either the hypex or p101 lat mosfet amp (padded that down to 22db). All this preference of mine for lower gain amps likely due to my smallish 2 channel listening room. F5m has been my recent fav, like it much better than original F5, enough so that I sold off my F3. Stock F5m circuit gives nice 2nd harmonic (rather than needing distortion analyzer to set P3).
Have Pearl 3 kit that someday will build out, but my spinning vinyl not that often thus slow to get to it and suffice with mani2 when do, or the (gasp) built in phono stage of marantz receiver (modern) if in fam room.
I should really have a go at building tube power amp, 10w or less fine for me. Though my parts collection not for the high 200-400v used in tube circuits and the lurking subconscious fear of opening new can of worms.
@DrewsBrews said:
Any comparison to the lower voltage one? Maybe try some tube rolling and see if the differences are more or less pronounced?
Thinking about how to do this. I have two versions of the lower voltage preamps. From what I remember, they did not sound the same. One operates off the 12vac 500ma wall wart and uses a 58vdc voltage doubler circuit. The other has a custom "CRCRC" type 58vdc power supply.
We would probably want to start by hand selecting a group of four 6J1(6AK5, etc) tubes that sound the same in just one of the preamps. If we did not do this as a first step, then it would probably be difficult to separate the sound of tubes from the sound of preamp versions. Once the set of four tubes is selected, you would need to quickly switch from one preamp to the other before you forget what the other one sounded like. Would need to either DIY a custom switch box or swap out pairs of RCA interconnects at high speed. Not as easy task. Thoughts?
Comments
Good idea! Will give this a try and report back.
I did my listening before I did the measurements. I was thinking that maybe I was biased in favor of the stock version because I have read, over the years, several reviews and articles about the negative sonic effects of negative feedback in amplfiers. After I took the measurements, however, I was kind of thinking that I should not have heard any difference at all, since the differences were so small. But I clearly did. So I would think, at this point, that if my mind was making this up, I would conclude that both versions sound the same. But they don't. So, now I have to give Craig's jumper wire idea a test to see where this goes.
Ha Ha, waiting for yo to catch the rabbit down the hole...
this is all very interesting, though i am absorbing just maybe 10%....
I have heard (with my own two ears) negative feedback suck the life out of an amp. I get why you like the original design better. Too bad the gain is so high with the original circuit.
Forgot to ask... when doing your listening tests what was the input impedance of your power amp (ie how much preamp gain)?
For all listening tests, I was driving into a Lepai 2020 amp, which measures 10k ohms across the inputs. This would have made the preamp gain 14dB for the stock channel and 6.5dB for the mod channel. When I swapped out channels, I adjusted the volume control up and down to compensate and equalize the SPL from the speaker. Interconnect cable capacitance was about 200pF, so probably no HF roll off on either the mod or stock channels.
I attached a short jumper wire and gave it a listen. As you can see in the photos, I soldered the jumper wire to the bottom of the board, but left it unconnected on the top side, roughly 1/16" away from the solder pad connection point. In this way, I was able to use a small wooden stick to quickly connect and disconnect the bypass jumper wire and switch the cap in and out of the circuit almost instantaneously as the music played. Result: No difference at all. The sound was the same with or without the cap.
So, the sound quality difference I am hearing between the mod and stock channels must be due exclusively to the feedback loop. Something about the feedback is sucking the life out of the music, making it sound dead compared to the stock channel.
Well, I think that pretty much wraps up my attempts to modify this version of the 6J1 preamp. I am going to remove the mod perfboard and change it back to the stock circuit. Gain will be about 14dB into a 10k ohm load, which is a little high by some standards, but I am OK with this. I intend to use this preamp with the Pass Labs Pearl I phono stage (59dB gain) in combination with a low output moving coil cartridge. A line amp with about 10-15dB of gain is just about right for that type of setup. If I had built either the Pass Labs Pearl II or III phono stage, then this line amp would probably have a little bit too much gain.
I tend to use low gain amps, like many of the passdiy's are around 15db and a hypex ucd250 with no input buffer that is 13db. Though also use a F4 diy sometimes which is 0db gain but have a FE2022 'preamp' that has some giddy up at 20db gain or so. But with weather getting warmer I use either the hypex or p101 lat mosfet amp (padded that down to 22db). All this preference of mine for lower gain amps likely due to my smallish 2 channel listening room. F5m has been my recent fav, like it much better than original F5, enough so that I sold off my F3. Stock F5m circuit gives nice 2nd harmonic (rather than needing distortion analyzer to set P3).
Have Pearl 3 kit that someday will build out, but my spinning vinyl not that often thus slow to get to it and suffice with mani2 when do, or the (gasp) built in phono stage of marantz receiver (modern) if in fam room.
I should really have a go at building tube power amp, 10w or less fine for me. Though my parts collection not for the high 200-400v used in tube circuits and the lurking subconscious fear of opening new can of worms.
Any comparison to the lower voltage one? Maybe try some tube rolling and see if the differences are more or less pronounced?
Thinking about how to do this. I have two versions of the lower voltage preamps. From what I remember, they did not sound the same. One operates off the 12vac 500ma wall wart and uses a 58vdc voltage doubler circuit. The other has a custom "CRCRC" type 58vdc power supply.
We would probably want to start by hand selecting a group of four 6J1(6AK5, etc) tubes that sound the same in just one of the preamps. If we did not do this as a first step, then it would probably be difficult to separate the sound of tubes from the sound of preamp versions. Once the set of four tubes is selected, you would need to quickly switch from one preamp to the other before you forget what the other one sounded like. Would need to either DIY a custom switch box or swap out pairs of RCA interconnects at high speed. Not as easy task. Thoughts?
did someone have a link to the case that works for the 6J1 preamp board?
Do you mean the polycarbonate ones?
Yes, I see a few on Amazon, do they work OK?
Wish I could tell you. I put mine in an old aluminum enclosure.
For the low voltage kit? The suggested replacement caps are taller and the tops contact the case top, but otherwise seems to fit fine.