Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Advice for grounded grid input stage
Hi RATs,
I'm considering a grounded grid stage as the first input stage in my continuating first power amp project. Considering that my soundcards (Lynx 2 and Terratec EWX-24/96) both have low output impedances (100 and 200 ohm resp), it seems that this would be possible from a load perspective. I am considering a rather high gain config with a 12AU7 that according to my calculations should yield an input load of about 1.4K. So a few questions: 1)Seems like a reasonable impedance matching for soundcards but will this also be compatible for other pre-amps ect? 2)I believe that the input would see the low DC bias voltage of a few volts. Can I direct couple this without the soundcard getting unhappy, or would a soundcard need to be shielded from this small voltage? 3)I want to run feedback to this stage. It seems to me that you could maybe put a small resistance between the grid and ground and put the feedback between them. Is this possible? Thanks. I'm coming along in this tube stuff. Lots of fun. Wessel |
#2
|
|||
|
|||
Wessel Dirksen wrote: Hi RATs, I'm considering a grounded grid stage as the first input stage in my continuating first power amp project. Considering that my soundcards (Lynx 2 and Terratec EWX-24/96) both have low output impedances (100 and 200 ohm resp), it seems that this would be possible from a load perspective. I am considering a rather high gain config with a 12AU7 that according to my calculations should yield an input load of about 1.4K. So a few questions: How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k? 1)Seems like a reasonable impedance matching for soundcards but will this also be compatible for other pre-amps ect? No. 2)I believe that the input would see the low DC bias voltage of a few volts. Can I direct couple this without the soundcard getting unhappy, or would a soundcard need to be shielded from this small voltage? Use a coupling cap to isolate the tube DC from the sound card output. What value C will you need to get a LF pole at 7 Hz? 3)I want to run feedback to this stage. It seems to me that you could maybe put a small resistance between the grid and ground and put the feedback between them. Is this possible? Yes, this is possible, and phase of the FB has to be correct. The Rin to the cathode will rise, and so will the required input voltage. Thanks. I'm coming along in this tube stuff. Lots of fun. Before you award yourself a medal for your achievements, be sure you know what you are doing :-). Patrick Turner. Wessel |
#3
|
|||
|
|||
Patrick Turner schreef: Wessel Dirksen wrote: Hi RATs, I'm considering a grounded grid stage as the first input stage in my continuating first power amp project. Considering that my soundcards (Lynx 2 and Terratec EWX-24/96) both have low output impedances (100 and 200 ohm resp), it seems that this would be possible from a load perspective. I am considering a rather high gain config with a 12AU7 that according to my calculations should yield an input load of about 1.4K. So a few questions: How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k? As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the input impedance formula and I'm only getting about 300 ohms or so now. Disappointed though. I was hoping I was on to something. 1)Seems like a reasonable impedance matching for soundcards but will this also be compatible for other pre-amps ect? No. For future reference, what is a typical output impedance for a preamp or CD player? 2)I believe that the input would see the low DC bias voltage of a few volts. Can I direct couple this without the soundcard getting unhappy, or would a soundcard need to be shielded from this small voltage? Use a coupling cap to isolate the tube DC from the sound card output. What value C will you need to get a LF pole at 7 Hz? Ok, also for future reference then. I guess you are suggesting that it is a good idea to cap couple the input. Primary 1st order pole at 7 hz would seem like a good filter point. Good LF bandwidth but not too rediculous high value of C. 3)I want to run feedback to this stage. It seems to me that you could maybe put a small resistance between the grid and ground and put the feedback between them. Is this possible? Yes, this is possible, and phase of the FB has to be correct. The Rin to the cathode will rise, and so will the required input voltage. Humm, this would raise Zin then as well but not enough to help Zin without killing all the gain. Oh well . . . Thanks. I'm coming along in this tube stuff. Lots of fun. Before you award yourself a medal for your achievements, be sure you know what you are doing :-). Considering my nonsense calculation of 1.4k for Zin, I can understand this caveat. Yet the addage " . . . know just enough to get yourself into trouble" is something I thrive on during the learning process. Thanks for your input Patrick. Wessel Patrick Turner. Wessel |
#4
|
|||
|
|||
Wessel Dirksen wrote: Patrick Turner schreef: Wessel Dirksen wrote: Hi RATs, I'm considering a grounded grid stage as the first input stage in my continuating first power amp project. Considering that my soundcards (Lynx 2 and Terratec EWX-24/96) both have low output impedances (100 and 200 ohm resp), it seems that this would be possible from a load perspective. I am considering a rather high gain config with a 12AU7 that according to my calculations should yield an input load of about 1.4K. So a few questions: How did you calculate the Rin to a 12AU7 with grounded grid was 1.4k? As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the input impedance formula and I'm only getting about 300 ohms or so now. Disappointed though. I was hoping I was on to something. If you can't remember all formulas, think in basic terms. Say your grounded grid input stage has 1/2 a 12AU7 with 47k anode load and 2.7k Rk, and you wish to connect to the cathode. The tube open loop gain is µ x RL / ( RL +Ra ) For 12 AU7, A = 14 approx. So for 47 volts of change at the anode, you'd get 1 mA of of I change. Since A = 14, you'd need ( 47 / 14 ) v at the cathode, = 3.35, and since there *must* be 1 mA change there too the Rin = ( 3,35 / 1.0 ) k ohms = 3.35k In other words, Ri = LL at a / A. BUT, we also have 2,7 k in parallel with Rkin, so Ri = 3.35k parallel to 2.7k = 1.498k, or say 1.5k. If you were to use a CCS from oV to the cathode, then Ri at k is simply 3.35k. 1)Seems like a reasonable impedance matching for soundcards but will this also be compatible for other pre-amps ect? No. For future reference, what is a typical output impedance for a preamp or CD player? Ro = low = 600 ohms typically, Ri = high = over 10 kohms typically. Output Z should be low so the voltage output is unaffected by the load of something connected to it. All devices should follow the rule of low Ro and high Rin. 2)I believe that the input would see the low DC bias voltage of a few volts. Can I direct couple this without the soundcard getting unhappy, or would a soundcard need to be shielded from this small voltage? Use a coupling cap to isolate the tube DC from the sound card output. What value C will you need to get a LF pole at 7 Hz? Ok, also for future reference then. I guess you are suggesting that it is a good idea to cap couple the input. Primary 1st order pole at 7 hz would seem like a good filter point. Good LF bandwidth but not too rediculous high value of C. -3 dB point at LF is where the recatance of the cap in ohms = Rout + Rin. If you have 600 ohms feeding 1.5k, you have a total R = 2.1k. To get a reduction of signal current by 3 dB you need C = 1,000,000 / ( 2,100 x 6.28 x 7 ) uF = 10.8 uF. So you'd ask for a 100 volt rated 12 uF cap at the counter of the parts store. 3)I want to run feedback to this stage. It seems to me that you could maybe put a small resistance between the grid and ground and put the feedback between them. Is this possible? Yes, this is possible, and phase of the FB has to be correct. The Rin to the cathode will rise, and so will the required input voltage. Humm, this would raise Zin then as well but not enough to help Zin without killing all the gain. Oh well . . . Your power amp should be configured to make full power at a volt input including NFB. The soundcard can make up to a volt output, no? Thanks. I'm coming along in this tube stuff. Lots of fun. Before you award yourself a medal for your achievements, be sure you know what you are doing :-). Considering my nonsense calculation of 1.4k for Zin, I can understand this caveat. Yet the addage " . . . know just enough to get yourself into trouble" is something I thrive on during the learning process. Thanks for your input Patrick. Wessel In time, you will become use to sorting out all the factors affecting a simple circuit. Learning the basics about triodes and some RLC network theory are the most important things when building analog amp stages. I tell Mr Yeager that all the time, and finally I hope to not have to tell him anything; he'll always get it right. Patrick Turner. Patrick Turner. Wessel |
#5
|
|||
|
|||
Patrick Turner schreef: Wessel Dirksen wrote: Patrick Turner schreef: Wessel Dirksen wrote: Hi RATs, I'm considering a grounded grid stage as the first input stage in my continuating first power amp project. Considering that my soundcards (Lynx 2 and Terratec EWX-24/96) both have low output impedances (100 and 200 ohm resp), it seems that this would be possible from a load perspective. I am considering a rather high gain config with a 12AU7 that according to my calculations should yield an input load of about 1.4K. So a few questions: How did you calculate the Rin to a 12AU7 with grounded grid was 1=2E4k? As it turns out calculated incorrectly. I have rp+Ra/mu+1 as the input impedance formula and I'm only getting about 300 ohms or so now. Disappointed though. I was hoping I was on to something. If you can't remember all formulas, think in basic terms. Say your grounded grid input stage has 1/2 a 12AU7 with 47k anode load and 2.7k Rk, and you wish to connect to the cathode. The tube open loop gain is =B5 x RL / ( RL +Ra ) For 12 AU7, A =3D 14 approx. So for 47 volts of change at the anode, you'd get 1 mA of of I change. Since A =3D 14, you'd need ( 47 / 14 ) v at the cathode, =3D 3.35, and since there *must* be 1 mA change there too the Rin =3D ( 3,35 / 1.0 ) k ohms =3D 3.35k In other words, Ri =3D LL at a / A. BUT, we also have 2,7 k in parallel with Rkin, so Ri =3D 3.35k parallel to 2.7k =3D 1.498k, or say 1.5k. If you were to use a CCS from oV to the cathode, then Ri at k is simply 3.35k. This is the stuff I like. Appreciated. 1)Seems like a reasonable impedance matching for soundcards but will this also be compatible for other pre-amps ect? No. For future reference, what is a typical output impedance for a preamp or CD player? Ro =3D low =3D 600 ohms typically, Ri =3D high =3D over 10 kohms typically. Output Z should be low so the voltage output is unaffected by the load of something connected to it. All devices should follow the rule of low Ro and high Rin. 10 to 1 good enough then? Or is some overkill still a good idea? 2)I believe that the input would see the low DC bias voltage of a few volts. Can I direct couple this without the soundcard getting unhappy, or would a soundcard need to be shielded from this small voltage? Use a coupling cap to isolate the tube DC from the sound card output. What value C will you need to get a LF pole at 7 Hz? Ok, also for future reference then. I guess you are suggesting that it is a good idea to cap couple the input. Primary 1st order pole at 7 hz would seem like a good filter point. Good LF bandwidth but not too rediculous high value of C. -3 dB point at LF is where the recatance of the cap in ohms =3D Rout + Rin. If you have 600 ohms feeding 1.5k, you have a total R =3D 2.1k. To get a reduction of signal current by 3 dB you need C =3D 1,000,000 / ( 2,100 x 6.28 x 7 ) uF =3D 10.8 uF. So you'd ask for a 100 volt rated 12 uF cap at the counter of the parts store. 3)I want to run feedback to this stage. It seems to me that you could maybe put a small resistance between the grid and ground and put the feedback between them. Is this possible? Yes, this is possible, and phase of the FB has to be correct. The Rin to the cathode will rise, and so will the required input voltage. Humm, this would raise Zin then as well but not enough to help Zin without killing all the gain. Oh well . . . Your power amp should be configured to make full power at a volt input including NFB. The soundcard can make up to a volt output, no? Yes that is what I strive for. I have gone to direct connection from quality soundcard because a preamp is simply not necessary. The Lynx is amazing. +/-100 ohm Zout. Yet apparently designed to function perfectly driving 1V into a 600 ohm load or higher. Thanks. I'm coming along in this tube stuff. Lots of fun. Before you award yourself a medal for your achievements, be sure you know what you are doing :-). Considering my nonsense calculation of 1.4k for Zin, I can understand this caveat. Yet the addage " . . . know just enough to get yourself into trouble" is something I thrive on during the learning process. Thanks for your input Patrick. Wessel In time, you will become use to sorting out all the factors affecting a simple circuit. Learning the basics about triodes and some RLC network theory are the most important things when building analog amp stages. I tell Mr Yeager that all the time, and finally I hope to not have to tell him anything; he'll always get it right. Patrick Turner. Patrick Turner. Wessel |
#6
|
|||
|
|||
All devices should follow the rule of low Ro and high Rin. 10 to 1 good enough then? Or is some overkill still a good idea? 10:1 is only 600 to 6k. Overkill is better. Tube amps have typically 100k input Z and CF output from preamps = 600 ohms. SS pres have opamps, maybe Ro = 20 ohms, but usually they place a 470 ohm R in series to the output terminal which stops the opamp fusing itself dead if a shorted output occurs. Your power amp should be configured to make full power at a volt input including NFB. The soundcard can make up to a volt output, no? Yes that is what I strive for. I have gone to direct connection from quality soundcard because a preamp is simply not necessary. The Lynx is amazing. +/-100 ohm Zout. Yet apparently designed to function perfectly driving 1V into a 600 ohm load or higher. If that is possible, then its ok to drive low impedances of 600 ohms. It is very easy to make a robust output stage after an opamp which will have low thd and able to put maybe 5 vrms into 100 ohms. Its only 50 mA rms of current, and easy from a pair of class AB bjt transistors after an opamp and included in the loop of NFB. Patrick Turner. |
#7
|
|||
|
|||
In article , Patrick Turner
wrote: Overkill is better. Tube amps have typically 100k input Z and CF output from preamps = 600 ohms. SS pres have opamps, maybe Ro = 20 ohms, but usually they place a 470 ohm R in series to the output terminal which stops the opamp fusing itself dead if a shorted output occurs. What is the typical input Z of a modern SS amp. both pres and integrated? Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#8
|
|||
|
|||
John Byrns wrote: In article , Patrick Turner wrote: Overkill is better. Tube amps have typically 100k input Z and CF output from preamps = 600 ohms. SS pres have opamps, maybe Ro = 20 ohms, but usually they place a 470 ohm R in series to the output terminal which stops the opamp fusing itself dead if a shorted output occurs. What is the typical input Z of a modern SS amp. both pres and integrated? Quad 405 Rin = about 20k; many SS amps tend to be around this figure, including preamps, and this keeps them a little quieter, since noise is worse with high Rin with nothing connected. Rout is typically less than 0.1 ohms from power amps because often there is 110 dB of local and global NFB applied, and what actually makes up the 0.1 ohms may be the LR zobel network and the fuse soldered into the output circuit if there is one. ( it does need to be soldered in, not clipped in, since fuse clips and fuse holders are notorious for increasing thd from 0.001% to to 0.01%. Rout from preamps is often between 220 and 600 ohms, and is made up with the opamp with Ro less than 100 ohms, and a series R added to protect the opamp from shorts. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Which 6550 for SVT reissue? | Vacuum Tubes | |||
Simple AM tuner? | Vacuum Tubes | |||
FS: SOUNDSTREAM CLOSEOUTS AND MORE!! | Car Audio | |||
How Important Is the Sound Of the Phase Inverter? | Vacuum Tubes | |||
What is a Distressor ? | Pro Audio |