Very simple beginner line preamp
Hello everybody!
I've had some interest in the linestage portion of the preamp I recently built, mainly with some italian friends.
So I decided to make things a little clearer, and to give a response to everyone is seeking for a SIMPLE linestage, with gain and reasonable performance (althought it seems that's more than reasonable...)
here it is:
http://www.giaime.altervista.org/plate_follower.html
I know there are many many people on this forum that eat tubes for breakfast
so comments / suggestions / corrections are welcome!
Maybe following will be a simple regulated power supply and the developement of the Plate Follower Stage II: The Constant Current Loaded One
Hello everybody!
I've had some interest in the linestage portion of the preamp I recently built, mainly with some italian friends.
So I decided to make things a little clearer, and to give a response to everyone is seeking for a SIMPLE linestage, with gain and reasonable performance (althought it seems that's more than reasonable...)
here it is:
http://www.giaime.altervista.org/plate_follower.html
I know there are many many people on this forum that eat tubes for breakfast
so comments / suggestions / corrections are welcome!Maybe following will be a simple regulated power supply and the developement of the Plate Follower Stage II: The Constant Current Loaded One
I'm afraid you need to think again on your Super Plate Follower. What you start with is a mu-follower. That's good. But then you've used the lower output. That's bad. The idea of the cathode follower up top is to provide a high impedance load to the lower valve so as to reduce distortion. Take the output and feedback from the cathode of the upper valve. A well-designed mu-follower with carefully chosen valves gives very low distortion and excellent rejection of power supply noise. In theory, wrapping feedback around it should improve things further. I think you'll have to try it to see whether the feedback is a good idea.
Oh, and input impedance is still tending towards 47k.
Oh, and input impedance is still tending towards 47k.
Hello EC8010, and thanks for your response.
In fact, the super plate follower (as I like to call it) is still an idea, the target was the original simple plate follower. In fact, I'm thinking of developing a solid state CC source, to provide the high impedance load.
Can you please explain me the part
are you suggesting to move both the output of the circuit and the feedback signal from the lower anode to the upper cathode?
And yes, about the input impedance, that was math error
I will correct in the webpage.
In fact, the super plate follower (as I like to call it) is still an idea, the target was the original simple plate follower. In fact, I'm thinking of developing a solid state CC source, to provide the high impedance load.
Can you please explain me the part
are you suggesting to move both the output of the circuit and the feedback signal from the lower anode to the upper cathode?
And yes, about the input impedance, that was math error
I will correct in the webpage.
Hi Giaime,
The plate follower is old hat (sometimes a good thing) and not related to a mu follower at all. (Not quite sure what EC8010 is on about here.)
You don't need R2 in the schematic as shown due the the feedback network and R7. This will net you a bona fide 100K input impedance. (There is still a grid bias path should the pot wiper be intermittent.)
I used a simular output circuit with 12AX7 in some of my early pre-amplifier designs, both with and without a cathode follower.
Incidentally I did not like the sound of the 12AX7 mu follower with feedback wrapped around it, perhaps because of the relatively high level of global feedback extant as I used this as a unity gain output buffer after a LTP configured to allow output polarity reversal.
This circuit is an excellent match with some tweaking to the 5687, with NFB applied for a modest amount of gain you can get source impedances of well under 1Kohm. (Unity gain <150 ohms)
You can also use a T type attenuator network in place of R6 if you need gain and are concerned about the quality of higher value resistors.
The plate follower is old hat (sometimes a good thing) and not related to a mu follower at all. (Not quite sure what EC8010 is on about here.)
You don't need R2 in the schematic as shown due the the feedback network and R7. This will net you a bona fide 100K input impedance. (There is still a grid bias path should the pot wiper be intermittent.)
I used a simular output circuit with 12AX7 in some of my early pre-amplifier designs, both with and without a cathode follower.
Incidentally I did not like the sound of the 12AX7 mu follower with feedback wrapped around it, perhaps because of the relatively high level of global feedback extant as I used this as a unity gain output buffer after a LTP configured to allow output polarity reversal.
This circuit is an excellent match with some tweaking to the 5687, with NFB applied for a modest amount of gain you can get source impedances of well under 1Kohm. (Unity gain <150 ohms)
You can also use a T type attenuator network in place of R6 if you need gain and are concerned about the quality of higher value resistors.
Sorry EC8010, for some reason I did not see the mu-follower on that page the first time I looked at.
Some people do connect mu-followers this way, without feedback and into a following high impedance stage. The argument for doing this is supposedly improved linearity, the follower on top being used solely as a current source.
My comments about 12AX7A based mu-follower still stands, lifeless sound when global feedback is applied. Not sure about other tube types though. I have used this topology in phono stages (no global fdbk) where the gain is useful, but thought the sound a little on the euphonic side of things.
Some people do connect mu-followers this way, without feedback and into a following high impedance stage. The argument for doing this is supposedly improved linearity, the follower on top being used solely as a current source.
My comments about 12AX7A based mu-follower still stands, lifeless sound when global feedback is applied. Not sure about other tube types though. I have used this topology in phono stages (no global fdbk) where the gain is useful, but thought the sound a little on the euphonic side of things.
kevinkr, no apology necessary. I can see the arguments for taking the lower output of a mu-follower into a high impedance stage such as a cathode follower although I've never needed to do it myself.
Bearing in mind that a mu-follower is a series amplifier, the HT needs to account for the Va of upper valve plus lower valve plus the drop across the load resistor, so choosing a high-mu valve (which invariably requires more Va than a low-mu valve) seems doomed to failure unless you have an extraordinarily high HT. Perhaps being forced to operate with a lower-than-ideal Va is why you didn't like the results from your experiment?
By the way, I edited the title of the thread as I'm sure the threadstarter wanted to infer that the line pre-amp was very simple, rather than the beginner building it...
Bearing in mind that a mu-follower is a series amplifier, the HT needs to account for the Va of upper valve plus lower valve plus the drop across the load resistor, so choosing a high-mu valve (which invariably requires more Va than a low-mu valve) seems doomed to failure unless you have an extraordinarily high HT. Perhaps being forced to operate with a lower-than-ideal Va is why you didn't like the results from your experiment?
By the way, I edited the title of the thread as I'm sure the threadstarter wanted to infer that the line pre-amp was very simple, rather than the beginner building it...
Hi Giaime !
As a good Italian, can you give me some help?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=80085
I'm Italian descendent, but I don't know anything in italian ! But if you are like my family, everybody is very gentile in help one each other!
Nice site, very nice point of view about the tubes, I agree, they are much more human than transistors !
Grazie il mio amico! (by google heheh!)
As a good Italian, can you give me some help?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=80085
I'm Italian descendent, but I don't know anything in italian ! But if you are like my family, everybody is very gentile in help one each other!
Nice site, very nice point of view about the tubes, I agree, they are much more human than transistors !
Grazie il mio amico! (by google heheh!)
Ciao Giaime/all
I made somehing like that a while ago (signal from the lower plate). It was not a mu-follower but a SRPP (6H30). Later on I tried a mixed SRPP (6H30+EF184) and now a SRPP loaded with a choke (my current choice)... you get rid of a cap with the SRPP. err ... no feedback.
SRPP 6H30 ... I can't remember that much.
SRPP 6H30+EF184 ... I liked more the sound taking the signal out of the lower plate but on the o/scope the performace was better when taken from the upper cathode: 10kHz square wave was clearly steeper with only small rings (NB: that SRPP was feeding an high-impedance interstage tranny). That was just an idea/experiment for an active load on the 6H30.
SRPP 801+807+choke ... same results as above. BTW I measured the "plate follower SRPP" but I actually listened only to the "traditional SRPP" as I went straight that way. And I am very happy with it. I am probably wasting lots of "resources" with this set-up ... but ... do I care? No I dont.
Hope it can help.
Ciao
Gianluca
I made somehing like that a while ago (signal from the lower plate). It was not a mu-follower but a SRPP (6H30). Later on I tried a mixed SRPP (6H30+EF184) and now a SRPP loaded with a choke (my current choice)... you get rid of a cap with the SRPP. err ... no feedback.
SRPP 6H30 ... I can't remember that much.
SRPP 6H30+EF184 ... I liked more the sound taking the signal out of the lower plate but on the o/scope the performace was better when taken from the upper cathode: 10kHz square wave was clearly steeper with only small rings (NB: that SRPP was feeding an high-impedance interstage tranny). That was just an idea/experiment for an active load on the 6H30.
SRPP 801+807+choke ... same results as above. BTW I measured the "plate follower SRPP" but I actually listened only to the "traditional SRPP" as I went straight that way. And I am very happy with it. I am probably wasting lots of "resources" with this set-up ... but ... do I care? No I dont.
Hope it can help.
Ciao
Gianluca
76 or 6P5GT
Simple three tube 6P5GT or 76 with Mullard EZ81. Use a PS with a LC to two seperate LC sections (one for each channel).
Small chokes are inexpensive- use a 40 ma 8HY or higher value for the first choke & 20 ma & 15HY choke for each channel.
Apply about 200 vdc to the 6P5GT 60K plate resistor (one per channel) with 2.8K cathode resisitor (one per channel). Do not use any cathode caps.
Use a polyproplyene or motor run oil cap on the last capacitor (closest to 6P5). 20uF is good. A 20uF motor run would be great for the first cap, but electrolytic is ok. A pair of 1uF coupling caps of high quality will make this simple amp outperform Bruces complicated but good sounding GG. Been there, done that.
Simple three tube 6P5GT or 76 with Mullard EZ81. Use a PS with a LC to two seperate LC sections (one for each channel).
Small chokes are inexpensive- use a 40 ma 8HY or higher value for the first choke & 20 ma & 15HY choke for each channel.
Apply about 200 vdc to the 6P5GT 60K plate resistor (one per channel) with 2.8K cathode resisitor (one per channel). Do not use any cathode caps.
Use a polyproplyene or motor run oil cap on the last capacitor (closest to 6P5). 20uF is good. A 20uF motor run would be great for the first cap, but electrolytic is ok. A pair of 1uF coupling caps of high quality will make this simple amp outperform Bruces complicated but good sounding GG. Been there, done that.
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