My 12ax7 zero negative feedback preamp, with measurement
All of my previous projects are Opamp based pre and power amps. Now I have tried tubes and I really love it! I am a tube newbie so I hope I can learn more.....
My 12AX7 zero negative feedback pre-amp : http://www.shine7.com/audio/12ax7_pre.htm
Here are some photos and measurements from the above page :
All of my previous projects are Opamp based pre and power amps. Now I have tried tubes and I really love it! I am a tube newbie so I hope I can learn more.....
My 12AX7 zero negative feedback pre-amp : http://www.shine7.com/audio/12ax7_pre.htm
Here are some photos and measurements from the above page :
SY,
if you ever make it to HongKong you have to visit the store where I saw those solder stands.
Bush Electronic Supply, in an alley behind a pretty big street in Hong Kong central or the Centre.
You enter the alley, then walk up some steps and you enter the shop. Amazing, the contrast.
http://hktubeaudio.homestead.com/files/Central_tube.htm
Daan
if you ever make it to HongKong you have to visit the store where I saw those solder stands.
Bush Electronic Supply, in an alley behind a pretty big street in Hong Kong central or the Centre.
You enter the alley, then walk up some steps and you enter the shop. Amazing, the contrast.
http://hktubeaudio.homestead.com/files/Central_tube.htm
Daan
similar PCB is available on E-Scam
http://cgi.ebay.com/MULTI-PURPOSE-T...eZWD1VQQtrksidZp1638.m118.l1247QQcmdZViewItem
not associated with the seller.
http://cgi.ebay.com/MULTI-PURPOSE-T...eZWD1VQQtrksidZp1638.m118.l1247QQcmdZViewItem
not associated with the seller.
Gorgeous work alexw88, it suggests a professional's hand. That harmonic graph is a little strange though, specifically the 20 dB drop in noise floor above the fundamental. Any idea what that's about? RMAA can be a little flakey at times.
Incidentally, if you're not afraid of solid state, replacing the front end plate load and cathode follower cathode load with something like an IXYS CCS will probably drop those spikes much lower. And thanks for the board lead!
Incidentally, if you're not afraid of solid state, replacing the front end plate load and cathode follower cathode load with something like an IXYS CCS will probably drop those spikes much lower. And thanks for the board lead!
daanve said:Bush Electronic Supply, in an alley behind a pretty big street in Hong Kong central or the Centre.
An excellent shop! Sadly, Mr Chan (who previously ran it) died three years ago. Further details:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=21588&highlight=
Nice construction, by the way.
Thanks everyone for the comments
I *guess* the 20dB noise floor at lower frequency is caused by the regulator capacitors charging and discharging pattern. I tried removing one tube (to lower current draw) and do the measurement again, that 20dB noise started to drop at 1kHz instead of the current 1.5 kHz. I think if I increase the supply capacitance, I can make the noise lower at lower frequency.
What is IXYS CCS? I am new to tube design.....
Oh, so I can't claim it is a zero negative feedback amp...
The Jensen electrolytics are for the B+/12.6V supply filtering. The Blackgate NX is to bypass the cathode resistor. And this one need to be large, so I have no other choice.
I think the gain is more than enough, however it is ok in my system as my speaker is 86dB and I have just changed the gain of my power amp by re-wiring its input transformer (LL1545A) to a 2:1 ratio.
rdf said:That harmonic graph is a little strange though, specifically the 20 dB drop in noise floor above the fundamental. Any idea what that's about?
Incidentally, if you're not afraid of solid state, replacing the front end plate load and cathode follower cathode load with something like an IXYS CCS will probably drop those spikes much lower.
I *guess* the 20dB noise floor at lower frequency is caused by the regulator capacitors charging and discharging pattern. I tried removing one tube (to lower current draw) and do the measurement again, that 20dB noise started to drop at 1kHz instead of the current 1.5 kHz. I think if I increase the supply capacitance, I can make the noise lower at lower frequency.
What is IXYS CCS? I am new to tube design.....
abzug said:There's NFB inside the 12AX7
Also, I see electrolytics in the signal path
Oh, so I can't claim it is a zero negative feedback amp...
The Jensen electrolytics are for the B+/12.6V supply filtering. The Blackgate NX is to bypass the cathode resistor. And this one need to be large, so I have no other choice.
salas said:Is the gain ok? Or is it excessive? What's the gain of your main amp and the SPL/2.83V sesitivity of your speakers?
I think the gain is more than enough, however it is ok in my system as my speaker is 86dB and I have just changed the gain of my power amp by re-wiring its input transformer (LL1545A) to a 2:1 ratio.
Hello.
First I would like to echo the others in saying beutiful job on that preamp.
I think you can justify saying no negitive feedback. While triodes
by their natture do have some built in negative feedback, you haven't added any extra feedback, and you have fully bypassed the cathode resistor to minimise the feedback as much as you could. As a bonus, bypassing also lowers the output mpeadeance
of the stage.
Have you considered battery or LED bias? This way no bypassing is needed and, (if I am wrong someone please correct me) there might be even less feedback. This circuit is definatly going to give you the classic tube sound.
Cheers!
Rolf
First I would like to echo the others in saying beutiful job on that preamp.
I think you can justify saying no negitive feedback. While triodes
by their natture do have some built in negative feedback, you haven't added any extra feedback, and you have fully bypassed the cathode resistor to minimise the feedback as much as you could. As a bonus, bypassing also lowers the output mpeadeance
of the stage.
Have you considered battery or LED bias? This way no bypassing is needed and, (if I am wrong someone please correct me) there might be even less feedback. This circuit is definatly going to give you the classic tube sound.
Cheers!
Rolf
It ain't pretty -- but I breadboarded AlexW's line stage to check it out -- for one thing using 680R + 470uF on the cathode of the input results in a bit lower THD% than an LED -- THD measures about 0.14% using 80kHz bandwidth -- and I think that this is high because the device isn't in a more usual aluminum habitat. The third harmonic is down about 44dB from the second (better than Alex shows -- measured in linear mode on my HP3577). I used WIMA polypropylene's and carbon resistors and my regulated Heath Supply as that is what I had on hand, and I plugged in a rather ancient Dynaco 12AX7 since that was the first one I reached for:
An externally hosted image should be here but it was not working when we last tested it.
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