Hi,
Has anyone tried the 12ax7/12au7 tube combo for a preamp?
The akido pcb is on it's way. I would appreciate your help to get
get me started.
thanks!
Has anyone tried the 12ax7/12au7 tube combo for a preamp?
The akido pcb is on it's way. I would appreciate your help to get
get me started.
thanks!
People around here are always willing to help, based on basic knowledge until expert knowledge. Thing is, Aikido has been discussed a lot already and you will probably learn lots from reading old threads, and off course tubecad.com
IN the case of a 12AX7 as the first tube... you will get a gain of about 50 from this preamp, this is way to much for preamp duties. Normal CD players have an output of 2VRMS. The aikido using 12AX7 could theoretically amplify this to 100VRMs, while the power amp may need maybe 1VRMs. Do you understand this?
Erik
IN the case of a 12AX7 as the first tube... you will get a gain of about 50 from this preamp, this is way to much for preamp duties. Normal CD players have an output of 2VRMS. The aikido using 12AX7 could theoretically amplify this to 100VRMs, while the power amp may need maybe 1VRMs. Do you understand this?
Erik
Hi,
yes, thanks for your reply. I did search and have been reading up on it.
I understand the 12ax7 has a high gain. I have these tubes. My Jolida 502B uses the 12ax7 and 12at7 for the preamp stage.
The prima luna preamp also uses the 12ax7. Are they using a lower B voltage to decrease the gain?
yes, thanks for your reply. I did search and have been reading up on it.
I understand the 12ax7 has a high gain. I have these tubes. My Jolida 502B uses the 12ax7 and 12at7 for the preamp stage.
The prima luna preamp also uses the 12ax7. Are they using a lower B voltage to decrease the gain?
A 12AX7 is indeed a high gain tube, and is used as such to drive power tubes, as in the amps you mention. Example: an EL34 requires something like 30VRMS, so the driver tube must amplify the 2VRMs coming from the CD player to 30VRMs needed through the EL34. This is about 15x gain, less than the 12AX7 develops. No problem actually, because other factors, as Negative feedback which will reduce the gain of the 12AX7 tube.
What I say is not completely right, other factors also influence it and so on, but it is just to give you an idea. If you are really interested in learning 'why', and not just 'how' I recommend you to buy Morgan Jones 3rd edition of Valve amplifiers, available at Amazon.
The use of 12AX7's in line preamps is common practice in comercial amps, but you will not see this much among DIY'rs. For line level duties there are much better tubes around, most having lower gain and other characteristics that make them suitable to drive long cables (the only use of a preamp, IMHO).
Erik
What I say is not completely right, other factors also influence it and so on, but it is just to give you an idea. If you are really interested in learning 'why', and not just 'how' I recommend you to buy Morgan Jones 3rd edition of Valve amplifiers, available at Amazon.
The use of 12AX7's in line preamps is common practice in comercial amps, but you will not see this much among DIY'rs. For line level duties there are much better tubes around, most having lower gain and other characteristics that make them suitable to drive long cables (the only use of a preamp, IMHO).
Erik
Definitely buy the Morgan Jones book. Then buy his other book, Building Valve Amplifiers, as it focuses on the actual build process once you have a circuit.
I would have been completely lost without them.
I would have been completely lost without them.
Agreed the 12AX7 probably has more gain than you need. 12AU7-12AU7 might work well. You mentioned 12AT7, it might work as a second tube but I have not looked into it…John
I just ordered both books. I will stick with both tubes as 12au7's to start.
I'm looking at the chart for the 12au7. The output gain is about 1db difference between 100V and 300V for the B voltage.
What's the advantage of using a higher B voltage?
is anyone using a soft start circuit? I'd like to have no thumps on the speaker when powering on/off.
also, the cap between R15/R16 can be from 1-10uF. How does changing this value affect the sound?
there's no place for input coupling caps, are they left out for a reason?
thanks.
I'm looking at the chart for the 12au7. The output gain is about 1db difference between 100V and 300V for the B voltage.
What's the advantage of using a higher B voltage?
is anyone using a soft start circuit? I'd like to have no thumps on the speaker when powering on/off.
also, the cap between R15/R16 can be from 1-10uF. How does changing this value affect the sound?
there's no place for input coupling caps, are they left out for a reason?
thanks.
Hi Bengali
Do you already have the 12AU7 in hand? Otherwise I would go for the 12BH7A or the 6CG7, or still other noval with similar gain...all this because the 12AU7 has higher distortion figures... the more experienced people around here do not like them
But if you have them, go for it. You can always change for another tube later. Higher B+, within limits, means lower distortion, and as you have seen the gain is not influenced much. I am sure John Broskie got values for B+ and resistors to use together with the 12AU7!
Are you using SS amps or valve amps? If SS you are right to worry about thumps, but just when you turn both amps on together. You are safe when you turn the SS amp on after half a minute after turning on the aikido. You can also include a relay connected between the output signal and the ground of the Aikido...this relay should be NC and powered by a delay circuit, with a 555 timer, for example. You can use the filament voltage for this circuit, given you are using DC on filaments - or rectify the AC and smooth with a big cap, I would not worry much about regulation. Or the soft start circuit, but that makes the PS more complexe.
I thins the cap between R15 and R16 must be the output cap. It works as a high pass filter together with the input impedance of the power amplifier. I am sure Broskie explains this...but for SS power amp I would stick with at least 4,7uF - but you are free to experiment.
An input coupling cap is not needed, as the gird of the first tube is kept at gorund level through the gridleak resistor.
Good luck with it
Erik
Do you already have the 12AU7 in hand? Otherwise I would go for the 12BH7A or the 6CG7, or still other noval with similar gain...all this because the 12AU7 has higher distortion figures... the more experienced people around here do not like them
But if you have them, go for it. You can always change for another tube later. Higher B+, within limits, means lower distortion, and as you have seen the gain is not influenced much. I am sure John Broskie got values for B+ and resistors to use together with the 12AU7!
Are you using SS amps or valve amps? If SS you are right to worry about thumps, but just when you turn both amps on together. You are safe when you turn the SS amp on after half a minute after turning on the aikido. You can also include a relay connected between the output signal and the ground of the Aikido...this relay should be NC and powered by a delay circuit, with a 555 timer, for example. You can use the filament voltage for this circuit, given you are using DC on filaments - or rectify the AC and smooth with a big cap, I would not worry much about regulation. Or the soft start circuit, but that makes the PS more complexe.
I thins the cap between R15 and R16 must be the output cap. It works as a high pass filter together with the input impedance of the power amplifier. I am sure Broskie explains this...but for SS power amp I would stick with at least 4,7uF - but you are free to experiment.
An input coupling cap is not needed, as the gird of the first tube is kept at gorund level through the gridleak resistor.
Good luck with it
Erik
thanks Eric for the good input. I really value that you mentioned the 12au7 have higher distortion. I'll stick with the 6CG7 then.
Yes, I plan to use with a SS amp. thx.
Yes, I plan to use with a SS amp. thx.
12AX7s in an Aikido
12AX7s are quite useful in an Aikido circuit if you are building a phono preamp (e.g., two Aikidos with a RIAA eq circuit between the two). There, you will certainly need the gain. If you need to support a MC cartridge, you will probably need a MC step-up transformer as well.
12AX7s are quite useful in an Aikido circuit if you are building a phono preamp (e.g., two Aikidos with a RIAA eq circuit between the two). There, you will certainly need the gain. If you need to support a MC cartridge, you will probably need a MC step-up transformer as well.
Hi Erik,
Is an expensive Hammond transformer necessary for the best audio? I read on this forum some one used a low cost isolation transformer and reversed it for 220V out.
I know the Hammond transformers are especially designed for tubes so do they produce superior sound vs. trying to use a cheap alternative?
Do you know if having seperate transformers(small one for the heater and large one for power) vs. one transformer for all voltages reduces the ac noise level?
thx.
Is an expensive Hammond transformer necessary for the best audio? I read on this forum some one used a low cost isolation transformer and reversed it for 220V out.
I know the Hammond transformers are especially designed for tubes so do they produce superior sound vs. trying to use a cheap alternative?
Do you know if having seperate transformers(small one for the heater and large one for power) vs. one transformer for all voltages reduces the ac noise level?
thx.
Hi Bengali.
I ended up using a dedicated transformer for B+ (300vdc) as well as a seperate transformer for the heater (12.6vdc). The reason why I chose a seperate filament transformer is that it allows me to use one of those octal delay tubes - the filaments are on for 60 seconds prior to the B+ transformer kicking on. You could use two switches and a single transformer for soft starts.
As for Hammond, they are reasonably good and relatively cheap (e.g., $35-40) compared to some of the other available mains transformers. Given that I needed 300vdc, the isolation transformer route doesn't work for me (though I suppose the voltage doubler route could work).
I ended up using a dedicated transformer for B+ (300vdc) as well as a seperate transformer for the heater (12.6vdc). The reason why I chose a seperate filament transformer is that it allows me to use one of those octal delay tubes - the filaments are on for 60 seconds prior to the B+ transformer kicking on. You could use two switches and a single transformer for soft starts.
As for Hammond, they are reasonably good and relatively cheap (e.g., $35-40) compared to some of the other available mains transformers. Given that I needed 300vdc, the isolation transformer route doesn't work for me (though I suppose the voltage doubler route could work).
if you don't mind me asking, currently what tubes are you using for the 300V? I'm going to use the 6GC7/6Dj8 tubes and per the aikido table requires 200V. I guess I could get away with using the isolation transformer. I just don't know if there is any audible difference with one transformer vs. another(noise issues).
Hopefully that valve amp book you suggested has some tube rectifier voltage regulator circuits. I'm sure I will get my fill of answers from those books.
I never heard the 6GC7/6DJ8 tubes before, they sound pretty good? I'm only familiar with the sound of 12at7/12ax7's.
thx.
Hopefully that valve amp book you suggested has some tube rectifier voltage regulator circuits. I'm sure I will get my fill of answers from those books.
I never heard the 6GC7/6DJ8 tubes before, they sound pretty good? I'm only familiar with the sound of 12at7/12ax7's.
thx.
I am building an Aikido-based phono preamp for a very low output MC cartridge (Linn Troika - 0.1mV). I am using the following configuration: Peerless 4722 MC step-up/12AX7/12AU7/RIAA/12AX7/12AU7. B+ will be 300vdc.
I am not sure that you need to go the voltage regulator path - it should be pretty easy to build a PS that gives you 200vdc +- a volt or two.
6DJ8/6922s are commonly used in phono preamps. They should sound fine.
BTW - I am using a 115N060 delay relay with the two PS transformers.
I am not sure that you need to go the voltage regulator path - it should be pretty easy to build a PS that gives you 200vdc +- a volt or two.
6DJ8/6922s are commonly used in phono preamps. They should sound fine.
BTW - I am using a 115N060 delay relay with the two PS transformers.
Relabelled 6ES8 is what gave ECC88/6DJ8 a bad rep in audio for a long time. It's a variable mu tube, designed to be nonlinear.
I guess I will throw them against the wall and listen to them pop as they hit. Add to list tube is junk.
For the PSU, I too have a very simple design, CLCLC:
250V mains transformer
5ar4 rectifier
3.3uF cap (shunt)
15H choke
120uF cap (shunt)
15H choke x2(one for each channel)
20uF cap (shunt) x2(one for each channel)
The chokes can be replaced by resistors, altho the regulation is better with the chokes. I used PSUD2 to model the whole supply. That first choke is the most important, tho, and I really wouldn't skip it. And, PSUD2 is awesome for determining what values you need to hit a specific B+. I get 315V out of the above configuration, but if you add some power resistors or a smaller first cap you can lower the effective B+.
And, I had a big thump on turn-on/turn-off when I had too little capacitance in the PSU...bumping up that middle cap to 120uF from 20uF eliminated the thumps completely.
Morgan Jones has some darn good stuff in his book for PSU designs, but they are more for power amps that having varying current draws. The Aikido design has a stable current draw, so you don't need regulation or anything special in terms of hitting a specific voltage.
Heck, the best part of the design I have above is that I can sub in various rectifiers and operate the preamp at a different B+ (if I use a 5u3, I get about 280V) and a slightly different sound (tho the Aikido design is relatively immune to different rectifiers due to the cancellation properties inherent in the design...I think the small differences I hear are more due to the change in B+ than the actual rectifier type).
250V mains transformer
5ar4 rectifier
3.3uF cap (shunt)
15H choke
120uF cap (shunt)
15H choke x2(one for each channel)
20uF cap (shunt) x2(one for each channel)
The chokes can be replaced by resistors, altho the regulation is better with the chokes. I used PSUD2 to model the whole supply. That first choke is the most important, tho, and I really wouldn't skip it. And, PSUD2 is awesome for determining what values you need to hit a specific B+. I get 315V out of the above configuration, but if you add some power resistors or a smaller first cap you can lower the effective B+.
And, I had a big thump on turn-on/turn-off when I had too little capacitance in the PSU...bumping up that middle cap to 120uF from 20uF eliminated the thumps completely.
Morgan Jones has some darn good stuff in his book for PSU designs, but they are more for power amps that having varying current draws. The Aikido design has a stable current draw, so you don't need regulation or anything special in terms of hitting a specific voltage.
Heck, the best part of the design I have above is that I can sub in various rectifiers and operate the preamp at a different B+ (if I use a 5u3, I get about 280V) and a slightly different sound (tho the Aikido design is relatively immune to different rectifiers due to the cancellation properties inherent in the design...I think the small differences I hear are more due to the change in B+ than the actual rectifier type).
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