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Tube line and phono pre amp project >PING Patrick
Patrick,
That is some awesome preamp! In line with Fred's Dogzilla. Without a lot of details, what was your basic philosophy in designing with so much circuitry? What additional benefits did you seek and how does your particular philosophy stand up to the popular "Least circuitry from input to output?" west "Patrick Turner" wrote in message ... A S Goh wrote: Hi Patrick Turner Where is the schematic of the regulated PS I have never posted the schematic of the PS. I did have a solid state regulator to produce +320v at 50mA, and very low ripple, and this was further filtered by the RC infront of each pair of L&R stages. There was also a regulated 12.6 v DC supply for the heaters. Since April 2,000, that amp has been reborn again without regulation. There is really no need for it inh the B+ supply. Now I have 470uF, 20H, 470 uF, then ecah channel has 470 ohms, 470 uF, then another RC before each stage in each channel. The phono stage has an extra RC with 470 uF as the final B+ C. With all that RC, the hum is unmeasurable. This proved to be more reliable and quieter than active regulation. The regulator was series element LM350, but now it's a shunt type, using "amplified" zener diode, ie, a zd to the base of a power transistor, as well as lots of C. The phono input stage has chokes to the heaters, and bypassed to 0V with 0.1 uF caps. I hope to post the whole schematic when I upgrade the website soon. But its not rocket science to make a good line stage. I made mine on a chassis big enough to keep the PS away from the phono input stage. The chassis is 500 mm long. The HT transformer is potted, ex navy, and also the heater trans, Both of these are inside sheet steel case, and the hum from the magnetic fields is just low enough to not affect the fet input stage for MC which I now have instead of the MM 12AX7 stage I have on the 2000 preamp. That phono stage is hard to beat, but with MC at 20 dB lower output, I needed something a little quieter. With a line stage, you don't need to be so darn perfect, because the noise of a PS won't affect the higher signal level so badly as it does with a phono stage. Nevertheless, don't be bashful or shy, build a good PS and you won't regret it. Regulation isn't needed for the B+ for a class A amp. The only supply variations seen are the very tiny slow moving variations of the B+ voltage due to mains levels variations. But with my amp, you can turn it off, then back on after 5 seconds, and hear no sign of the B+ sliding down, then back up. The April 2000 preamp uses shunt FB to control gain of the 12AT7. The tone control uses 12AX7, but its gain is reduced to only 1 with the tone control networks included in what's called a Baxandall network. I don't mind some FB. But for those who don't like shunt FB, or other loop FB, read on, although with a cathode follower, you cannot escape the FB, since all the output voltage is in series with the input; its a case of lotsa FB. But its very local, and somewhat the most blame free form of FB I know. One line stage integrated amp I supplied a customer a couple of years back uses two chassis about 220mm square each, one for PS, and one for the two amp channels. The PS has an EL84 used as a shunt ripple reducer, about as effective as a 30H choke. The line stage uses 1 x 6CG7 per channel. NOS Telefunken give the best sound. The input comes via a 5 way source select twin wafer switch from DACT. I have one triode loaded with 47k, cap coupled to a DACT switched attenuator, then this has a cathode follower output loaded with 47k; 2 uF to the output. Beyshlag resistors and Wima polypropylene caps are used. 6SN7 is the king of the signal triodes, and is electonically identical to the 6CG7. For a slightly warmer sound, use 12AU7. The signal should come straight to the first tube and its level raised from the maximum standard 0.2v line level to 2v. Using 1/2 a 6SN7 with no cathode bypass cap give a gain of 10, (20 dB,). 12AU7 give a gain of around 7, with unbypassed Rk. The 2v is then fed to a pot of attenuator, where the 12 o'clock position reduces the level back down to 0.2v, and that is fed to a cathode follower output stage the same as shown at my site, and you get a nominal 0.18 vrms output. This is plenty for normal listening, and in fact if you measured the input levels used for comfy listening, its usually around 20 mV. If the power amp needs the standard 1v to make full power, then the preamp with a gain of 7 can provide enough gain. Even a gain of 4 is plenty, so balance controls can be also fitted. If I am asked to fit them, I now place them on the input side of the gain tube, with volume after. Its then I'd use 6CG7, gain = 10, or 15 with bypassed Rk to compensate the drop with a balance control inserted. Good systems don't need balance controls. This is a very simple form of SET preamp, and despite trying lots of SRPP and bootsrapped follower ideas, the simple thing sounds very accurate. You have freedom to try ideas and build various things, and listen. thd will be below 0.1%. I have used CCS instead of the 47 k loads for the triodes. I liked the sound of that. I think its tube choice which governs the sound heard the most. So if you make a circuit with ninepin sockets, different tubes can be tried, providing the heater voltage can be altered for either 6CG7 and 12AU7. The use of good quality R&C also minimise likelyhood of any cloudiness otherwise heard. Patrick Turner. Thanks "Patrick Turner" wrote in message ... Spade wrote: Hello, I planning to build tube line and phono pre amplifiers for my hi-fi systems. Can you recommend me some good and simple schematics that are proven to work fine. I like to use tubes that can take 12,6 on heaters, like ECC81-83, since I have couple of transformers that give 230VAC + 12VAC, but sure I can regulate heater supply to 6,3VDC... Any ideas? Than you! Best regards, Spade You could look at the complete preamp at http://www.turneraudio.com.au/htmlwe...tubepreamp.htm Then see the 12AT7 line stage which has a gain of 8, toprhs of the schematic. There is a volume control pot, log, with added balance pot, series, after a tone control stage which can be deleted altogether. My pre has two pair of LR outputs to suit trialing different amps, but you could have just one cathode follower output stage per channel. Patrick Turner. |
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west wrote: Patrick, That is some awesome preamp! In line with Fred's Dogzilla. Without a lot of details, what was your basic philosophy in designing with so much circuitry? I needed something with all the features to test speakers or amps in AB comparisons, hence the two pairs of LR outs. To adjust gain, I have a deletable gain stage. The tone control stage is also deletable. But to achieve that all stages are high R in, low R out, and I like the SRPP, or bootstrapped follower topology, so there are twice the number of triodes. Its really hard to find better sound IMHO. What additional benefits did you seek and how does your particular philosophy stand up to the popular "Least circuitry from input to output?" west Well, that pre of mine withen switched to its minimum bells and whisle mode, ( MB&WM), when used with a line level source has only a volume control, cathode follower in front of the power amp. The latest version of the phono stage has a 2SK369 fet for the input, because its 15 dB quieter than any normal tube with only 5 mA of drain current. Then I have a 6EJ7 in triode in cascode with the fet, then a 12AT7 second stage in bootsrtapped follower. The fet's Gm is 40 mA/V, and way more than most tubes! That sounds excellent, IMHO, even with low signal MC carts. thd is less than 0.1% even at 10vrms output, mostly 2H. And not a drop of loop FB, since RIAA is passive off the 6EJ7, and in that mode, its a current source. So there is as much circuitry in the phono amp as the line and power amp. So what, it works. The minimum phono amp needs an MM cart, or MC with step up tranny. Its got to be at least an EF86, then that's highish Ro, and so you have to be very careful to have the pre feeding high Rin of the volume control before the power amp. The power amp can be an SET with 12AX7 drive, and so its vinyl to speaker in 3 tubes. Be my guest to try all that. I use NFB around my power amps, so I have at least 3 tubes in the power amps, and I don't much like EF86. I'd prefer two halves of 12AX7 at least for phono, but you then have high impedance circuits everywhere, which I don't like too much. Compared to a McIntosh or EAR amp, my power amps are simpler. And many preamps are far more complex. Try an Electroaccompaniet SS preamp. I counted 96 SS devices, and some were opamps. I'd rather have two low U triodes in series than have a high U triode or a pentode for gain. When listening to CDs, its all done with just a volume pot and CF in series with the power amp. The cables don't affect the sound. The power amps are FB types, and so have an extra tube compared to a mimimalist's amp. But to me, the extra triode and FB makes the sound better than without them. Not going to lynch me from an oak tree just because I am using one lousy extra tube are yer? PITA Turner. "Patrick Turner" wrote in message ... A S Goh wrote: Hi Patrick Turner Where is the schematic of the regulated PS I have never posted the schematic of the PS. I did have a solid state regulator to produce +320v at 50mA, and very low ripple, and this was further filtered by the RC infront of each pair of L&R stages. There was also a regulated 12.6 v DC supply for the heaters. Since April 2,000, that amp has been reborn again without regulation. There is really no need for it inh the B+ supply. Now I have 470uF, 20H, 470 uF, then ecah channel has 470 ohms, 470 uF, then another RC before each stage in each channel. The phono stage has an extra RC with 470 uF as the final B+ C. With all that RC, the hum is unmeasurable. This proved to be more reliable and quieter than active regulation. The regulator was series element LM350, but now it's a shunt type, using "amplified" zener diode, ie, a zd to the base of a power transistor, as well as lots of C. The phono input stage has chokes to the heaters, and bypassed to 0V with 0.1 uF caps. I hope to post the whole schematic when I upgrade the website soon. But its not rocket science to make a good line stage. I made mine on a chassis big enough to keep the PS away from the phono input stage. The chassis is 500 mm long. The HT transformer is potted, ex navy, and also the heater trans, Both of these are inside sheet steel case, and the hum from the magnetic fields is just low enough to not affect the fet input stage for MC which I now have instead of the MM 12AX7 stage I have on the 2000 preamp. That phono stage is hard to beat, but with MC at 20 dB lower output, I needed something a little quieter. With a line stage, you don't need to be so darn perfect, because the noise of a PS won't affect the higher signal level so badly as it does with a phono stage. Nevertheless, don't be bashful or shy, build a good PS and you won't regret it. Regulation isn't needed for the B+ for a class A amp. The only supply variations seen are the very tiny slow moving variations of the B+ voltage due to mains levels variations. But with my amp, you can turn it off, then back on after 5 seconds, and hear no sign of the B+ sliding down, then back up. The April 2000 preamp uses shunt FB to control gain of the 12AT7. The tone control uses 12AX7, but its gain is reduced to only 1 with the tone control networks included in what's called a Baxandall network. I don't mind some FB. But for those who don't like shunt FB, or other loop FB, read on, although with a cathode follower, you cannot escape the FB, since all the output voltage is in series with the input; its a case of lotsa FB. But its very local, and somewhat the most blame free form of FB I know. One line stage integrated amp I supplied a customer a couple of years back uses two chassis about 220mm square each, one for PS, and one for the two amp channels. The PS has an EL84 used as a shunt ripple reducer, about as effective as a 30H choke. The line stage uses 1 x 6CG7 per channel. NOS Telefunken give the best sound. The input comes via a 5 way source select twin wafer switch from DACT. I have one triode loaded with 47k, cap coupled to a DACT switched attenuator, then this has a cathode follower output loaded with 47k; 2 uF to the output. Beyshlag resistors and Wima polypropylene caps are used. 6SN7 is the king of the signal triodes, and is electonically identical to the 6CG7. For a slightly warmer sound, use 12AU7. The signal should come straight to the first tube and its level raised from the maximum standard 0.2v line level to 2v. Using 1/2 a 6SN7 with no cathode bypass cap give a gain of 10, (20 dB,). 12AU7 give a gain of around 7, with unbypassed Rk. The 2v is then fed to a pot of attenuator, where the 12 o'clock position reduces the level back down to 0.2v, and that is fed to a cathode follower output stage the same as shown at my site, and you get a nominal 0.18 vrms output. This is plenty for normal listening, and in fact if you measured the input levels used for comfy listening, its usually around 20 mV. If the power amp needs the standard 1v to make full power, then the preamp with a gain of 7 can provide enough gain. Even a gain of 4 is plenty, so balance controls can be also fitted. If I am asked to fit them, I now place them on the input side of the gain tube, with volume after. Its then I'd use 6CG7, gain = 10, or 15 with bypassed Rk to compensate the drop with a balance control inserted. Good systems don't need balance controls. This is a very simple form of SET preamp, and despite trying lots of SRPP and bootsrapped follower ideas, the simple thing sounds very accurate. You have freedom to try ideas and build various things, and listen. thd will be below 0.1%. I have used CCS instead of the 47 k loads for the triodes. I liked the sound of that. I think its tube choice which governs the sound heard the most. So if you make a circuit with ninepin sockets, different tubes can be tried, providing the heater voltage can be altered for either 6CG7 and 12AU7. The use of good quality R&C also minimise likelyhood of any cloudiness otherwise heard. Patrick Turner. Thanks "Patrick Turner" wrote in message ... Spade wrote: Hello, I planning to build tube line and phono pre amplifiers for my hi-fi systems. Can you recommend me some good and simple schematics that are proven to work fine. I like to use tubes that can take 12,6 on heaters, like ECC81-83, since I have couple of transformers that give 230VAC + 12VAC, but sure I can regulate heater supply to 6,3VDC... Any ideas? Than you! Best regards, Spade You could look at the complete preamp at http://www.turneraudio.com.au/htmlwe...tubepreamp.htm Then see the 12AT7 line stage which has a gain of 8, toprhs of the schematic. There is a volume control pot, log, with added balance pot, series, after a tone control stage which can be deleted altogether. My pre has two pair of LR outputs to suit trialing different amps, but you could have just one cathode follower output stage per channel. Patrick Turner. |
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