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#1
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Stages of phono preamp
Hi all!
I'm playng around with the project of my phono preamp. After many schematics, now i'm thinking about a EC8010 -- RIAA network -- RCA 76, both tubes CCS loaded. Maybe this isn't a good design, but i'm a rookie in this field, so be patient... At the operating point the plate current for the EC8010 is 25 mA and 4mA for the 76, both are about at 120V on plate. Therefore the EC8010 drives more power than the 76. I wonder if this makes sense. Maybe i'm wrong, but it seems to me that usually every stage drives a following more powerful stage. It has to be so, or there is no relation between the power dissipated in a stage vs. the power dissipated in the following stage? I' didn't found much information about this. |
#2
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V-Twin wrote: Hi all! I'm playng around with the project of my phono preamp. After many schematics, now i'm thinking about a EC8010 -- RIAA network -- RCA 76, both tubes CCS loaded. Maybe this isn't a good design, but i'm a rookie in this field, so be patient... At the operating point the plate current for the EC8010 is 25 mA and 4mA for the 76, both are about at 120V on plate. Therefore the EC8010 drives more power than the 76. I wonder if this makes sense. Maybe i'm wrong, but it seems to me that usually every stage drives a following more powerful stage. It has to be so, or there is no relation between the power dissipated in a stage vs. the power dissipated in the following stage? I' didn't found much information about this. Some folks use a lot of current in the input stage of their phono amp to allow the chosen triode to work where its Gm is the highest, and its noise the lowest. Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. I used to use the circuit shown at http://www.turneraudio.com.au/htmlwe...tubepreamp.htm but now I have a 2SK369 in cascode with a frame grid pentode, the 6EJ7, running in triode, with 6 mA of Ia and this gives me 20 dB less noise when MC is used, and 10 dB less noise with MM. Once you have got past the first stage, there is no need for high current following stages, since the SNR isn't a problem, and a gain stage with 3 mA is ample. CCS loadings can be achieved with transistors, j-fets, or pentodes. The CCS load is a vet high impedance load, and thus the gain of the triode becomes cloas to its Mu, and the thd falls to the lowest it can be without using a balanced circuit, which reduces the thd even further. IN phono input stages, thd isn't a main concern, and typical thd figures at 100 mV of of output is maybe 0.01% of 2H, even when the load is a resistor of say 5 times the RA of the tube. The noise of the stage is determined by the grid input noise, the shot noise, and the flicker noise of the tube, which is worst at LF, and spoils many tube phono amps, so tube choice and careful selection is very important. The CCS needs to be low noise, as would the R used instead, but the Ra is in shunt with the anode R load, so R has little effect if Ra is a small fraction of it. Same goes for the following grid bias R, which is effectively in parallel with the load R supplying DC to triode. Patrick Turner. |
#3
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Hi RATs!
I used a SV572-30 to drive a 2A3. Nothing to do with phono amps, but, I just like telling that one Happy Ears! Al Alan J. Marcy Phoenix, AZ PWC/mystic/Earhead |
#4
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Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc,
and this supposedly allows the use of moving coil carts, without a step up tranny. But that won't work with a total max. gain of 600 x ..... Ec8010 (on ccs) x # 76 (on ccs) = 60 x 10 = 600 ..... Way to low ! Even for MM ...... Ronald . |
#5
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OK, what am I missing? My Sumiko, which is a high output MC, has an output
of 3mV times a gain of 600 is 1.8V. Add a line stage and you have more than enough gain for any amp. My amp drives to full output with a gain of .8V so no line stage would be needed with this phono stage. Sandy Brown Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. But that won't work with a total max. gain of 600 x ..... Ec8010 (on ccs) x # 76 (on ccs) = 60 x 10 = 600 ..... Way to (sic) low ! Even for MM ...... Ronald . |
#6
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"Centifolia" OK, what am I missing? ** Wait for it. My Sumiko, which is a high output MC, has an output of 3mV times a gain of 600 is 1.8V. ** The output may be 3mV - but at 1 kHz !!! At 25 Hz it will be only 0.3 mV - due to the RIAA recording curve. A phono preamp needs at least 1000 times gain at low frequencies - even when no NFB is used - much more if it is. .......... Phil |
#7
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"Phil Allison" ha scritto nel messaggio u... At 25 Hz it will be only 0.3 mV - due to the RIAA recording curve. A phono preamp needs at least 1000 times gain at low frequencies - even when no NFB is used - much more if it is. I said i'm a rookie...i forgot to consider the 20dB loss of the RIAA network... ) |
#8
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And it'll sound like sh*t since you don't use a Riaa correction .... ;-))
A total (unfiltered) gain of about 3600 for MM is quite normal .... "Centifolia" schreef in bericht news:Bchsb.128916$ao4.403701@attbi_s51... OK, what am I missing? My Sumiko, which is a high output MC, has an output of 3mV times a gain of 600 is 1.8V. Add a line stage and you have more than enough gain for any amp. My amp drives to full output with a gain of .8V so no line stage would be needed with this phono stage. Sandy Brown Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. But that won't work with a total max. gain of 600 x ..... Ec8010 (on ccs) x # 76 (on ccs) = 60 x 10 = 600 ..... Way to (sic) low ! Even for MM ...... Ronald . |
#9
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Ronald wrote: And it'll sound like sh*t since you don't use a Riaa correction .... ;-)) A total (unfiltered) gain of about 3600 for MM is quite normal .... "Centifolia" schreef in bericht news:Bchsb.128916$ao4.403701@attbi_s51... OK, what am I missing? My Sumiko, which is a high output MC, has an output of 3mV times a gain of 600 is 1.8V. Add a line stage and you have more than enough gain for any amp. My amp drives to full output with a gain of .8V so no line stage would be needed with this phono stage. Sandy Brown Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. But that won't work with a total max. gain of 600 x ..... Ec8010 (on ccs) x # 76 (on ccs) = 60 x 10 = 600 ..... Way to (sic) low ! Even for MM ...... Ronald . I have used a 6DJ8 with a passive RIAA, followed by a 12AX7, so total gain at 1 kHz is 30 x 70 /10 = 210, and the divide by 10 is the loss at 1 kHz of the RIAA filter. So if you have a 5 mV @1kHz cart at 1 kHz, you just over 1,000 mV at the output. But a cart of 2,5 mV, and a second phono stage gain of 35 would still make 250 mV. But not so good with a Denon 103 MC, whose output is only 0.3mV. Then you need more gain, or a step up transformer. FB type EQ might have even greater demand for gain since even at 20 Hz there is maybe 12 dB of FB operative, and 32 dB at 1 kHz, so an open loop gain of 4,000 would be appropriate. Many old phono stages just had two halves of 12AX7 cascaded for an open loop gain of about 4,000 at LF. Some of the better ones had a CF stage added to the output of the second gain 1/2 of the 12AX7, thus making the stage very low Ro, even at LF. I prefer the passive RIAA, after a cascode stage, with mu follower topology following . Patrick Turner. |
#10
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"Patrick Turner" ha scritto nel messaggio ... Some folks use a lot of current in the input stage of their phono amp to allow the chosen triode to work where its Gm is the highest, and its noise the lowest. Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. Therefore is a good idea, or? Once you have got past the first stage, there is no need for high current following stages, since the SNR isn't a problem, and a gain stage with 3 mA is ample. Ok. The noise of the stage is determined by the grid input noise, the shot noise, and the flicker noise of the tube, which is worst at LF, and spoils many tube phono amps, so tube choice and careful selection is very important. I only have very basic knowledge about this, i need to learn more. Any suggestion (books, web sites...)? |
#11
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V-Twin wrote: "Patrick Turner" ha scritto nel messaggio ... Some folks use a lot of current in the input stage of their phono amp to allow the chosen triode to work where its Gm is the highest, and its noise the lowest. Hence the use of 6C45pi, 417A, and paralleled 6DJ8 tubes, etc, and this supposedly allows the use of moving coil carts, without a step up tranny. Therefore is a good idea, or? Not necessarily. Some folks like this way of doing an amp. Some folks don't, so I suggest you find by using a soldering iron and your ears, and a CRO. Once you have got past the first stage, there is no need for high current following stages, since the SNR isn't a problem, and a gain stage with 3 mA is ample. Ok. The noise of the stage is determined by the grid input noise, the shot noise, and the flicker noise of the tube, which is worst at LF, and spoils many tube phono amps, so tube choice and careful selection is very important. I only have very basic knowledge about this, i need to learn more. Any suggestion (books, web sites...)? There is a fair bit around the web on phono tube preamps. The old book like RDH4 don't have 6DJ8 circuits within it, but it does have lots of applicable theory, which you should know about, if you want to fiddle around with tubes. Patrick Turner. |
#12
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I use a 76 directcoupled 6be8 CF. The 76 sounds marvelous when
feeding another driving tube. It sounded even better without the CF by a tad, but lower freq's really rolled off when driving cables to the amp (100k input resistor). Maybe the CCS loading will help in this regard. Let me know if it does, as the 76 is a really nice-sounding tube. Bob Hedberg "V-Twin" wrote: Hi all! I'm playng around with the project of my phono preamp. After many schematics, now i'm thinking about a EC8010 -- RIAA network -- RCA 76, both tubes CCS loaded. Maybe this isn't a good design, but i'm a rookie in this field, so be patient... At the operating point the plate current for the EC8010 is 25 mA and 4mA for the 76, both are about at 120V on plate. Therefore the EC8010 drives more power than the 76. I wonder if this makes sense. Maybe i'm wrong, but it seems to me that usually every stage drives a following more powerful stage. It has to be so, or there is no relation between the power dissipated in a stage vs. the power dissipated in the following stage? I' didn't found much information about this. Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#13
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"Bob Hedberg" wrote in message ... : I use a 76 directcoupled 6be8 CF. The 76 sounds marvelous when : feeding another driving tube. It sounded even better without the CF : by a tad, but lower freq's really rolled off when driving cables to : the amp (100k input resistor). Maybe the CCS loading will help in : this regard. Let me know if it does, as the 76 is a really : nice-sounding tube. : : Bob Hedberg Your price, Sir |
#14
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: : Bob Hedberg : : Your price, Sir Err, never mind, see you've made yours up Rudy |
#15
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Pardon?
"Ruud Broens" wrote: : : Bob Hedberg : : Your price, Sir Err, never mind, see you've made yours up Rudy Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#16
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(Think he meant "prize", referring to the contest)
Bob Hedberg wrote: Pardon? "Ruud Broens" wrote: : : Bob Hedberg : : Your price, Sir Err, never mind, see you've made yours up Rudy Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects, Vacuum Tubes & other stuff: | | http://www.dogstar.dantimax.dk | +--------------------------------------------+ |
#17
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it's hard to beat EC8010 with 20 to 25mA in any topology with any tube.
it's ****in' beast ,and I have just a pair-in my WOT ;((( who want to donate at least 20 pieces to poor ZM? -- -- .................................................. ........................ Choky Prodanovic Aleksandar YU "don't use force, "don't use force, use a larger hammer" use a larger tube - Choky and IST" - ZM .................................................. ........................... "V-Twin" wrote in message ... Hi all! I'm playng around with the project of my phono preamp. After many schematics, now i'm thinking about a EC8010 -- RIAA network -- RCA 76, both tubes CCS loaded. Maybe this isn't a good design, but i'm a rookie in this field, so be patient... At the operating point the plate current for the EC8010 is 25 mA and 4mA for the 76, both are about at 120V on plate. Therefore the EC8010 drives more power than the 76. I wonder if this makes sense. Maybe i'm wrong, but it seems to me that usually every stage drives a following more powerful stage. It has to be so, or there is no relation between the power dissipated in a stage vs. the power dissipated in the following stage? I' didn't found much information about this. |
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