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12ax7 12au7 preamp

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#4
Really strange design! It starts with a high gain high impedance valve (12AX7) but then throws away the gain by using strong degeneration and a relatively low anode resistor. Having done this, the sensible thing to do would be to follow with a CF but this uses another similar stage. The resultant high gain and high output impedance then has to be tamed by using feedback. Not enough feedback to really suppress higher-order products, though.

It looks like a valve circuit designed by someone brought up on BJTs.
 
#6
The PCB and basic topology is usable though. There's nothing to stop the OP using an ECC82 or ECC81 as an alternative, and either eliminating or changing the cathode degeneration and feedback loop. You could even ignore the second stage and arrange the first stage as a cathode follower for a unity gain buffer. Would make an neat and simple platform for a beginner to play with.
 
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#7
spend some time at TUBECAD and study the different aikido, cathode follower, and other topologies discussed there.

I learned a heck of a lot by doing so and ended up ordering a simple 5687 aikido preamp kit. I am also a repeat customer because his products are proven, very good quality, and support is wide in this forum.

Not saying the preamp link you posted is bad - BUT if you are just starting out I would hate to see frustration come out of a build if you are.

The manuals have all kinds of good information too since his circuits allow for tube rolling with minor resistor value tweaks.

People have very good luck with using tubes like the 6dj8, 6h30, 6cg7, 6n1p to name a few. A gain that seems to generally work well for hi-fi ranges between 5 - 15 at most.

For a typical preamp a pot value of 50K or 100K should work just fine. I typically use Alps 100K stereo pots available at mouser. A stepped attenuator in that value range would be nice as well.
 
#8
cjkpkg said:
spend some time at TUBECAD and study the different aikido, cathode follower, and other topologies discussed there.

I learned a heck of a lot by doing so and ended up ordering a simple 5687 aikido preamp kit. I am also a repeat customer because his products are proven, very good quality, and support is wide in this forum.
Click to expand...

I am looking for a preamp for my MyRef Gainclone and see-sawing between the Grounded Grid and Aikido.

I heard a borrowed GG in my setup and funnily (and wonderfully) it made everything in between the CD and speakers disappear. Then I've had little luck in finding good 12AU7s amongst many NOS I tried so want to move away from 12AU7s. Also the Aikido promises to be cheaper.

How would you rate the Aikido on delivering excitement and ability to lift veils from the music?

---G0bble
 
#9
yeah, I confess, I bought it.

I don't have intimate knowledge of all the topologies,yet.
I've built several aikido's though.

I figure some one here would have a great circuit to replace
the original one if it's really that bad. The board is nice enough to
make mods.

Thanks
 
#10
It is the only preamp I have built where I do not feel the need for any kind of equalization. I have built some op-amp pre's and a Pass JfetBOZ. In my opinion what is recorded at the source is what comes over the speakers.

The aikido sounds just as good with a 6W SE amp as well as a MOSFET amp like a Pass F5. The beauty is being able to swap input tubes to fine tune the gain. I typically roll between 6CG7's and 6DJ8's depending on the power amp.
 
#12
Why not use the board for a building block? Sub the 12AX7 and 12AU7 for 6Fq7's and build Franks line stage. Franks line stage used 6SN7's I believe the 6FQ7 is the small 9 pin version of the 6SN7. Then you would have something worth listening to.
 

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#13
Well, for building an actual amp/preamp, there are universal driver boards available from a couple of members here. You would get a proven quality product from someone able and willing to provide support. I would much rather buy from a member than some unknown Chinese source.
 
#15
Well, not just to support our fellow DIYers. When you buy a PCB from Millett, Tubelab and all the others here, you have the confidence that the board actually works and is of reasonable quality. Not so much with an anonymous Chinese board.

You also have someone to ask questions and have a reasonable assurance that you WILL get an answer and a competent one at that 😉
 
#16
The design would be suitable for low level signals with high gain. It can be made into a very fine amp. for a current output DAC with a gain of around 100 allowing a low value resistor for I/V conversion (around 20R for an AD1865). R3/R7 is probably a typo. and should read 3K. In the first instance, I would change R3/R7 and R2/R6 to 1K, R10/R11 to 150K, and R4/R5 to 220K. Output impedance is a little high, maybe 3 or 4K. To greatly improve the situation, you could use ECC88 in the output stage using 10K (2W or higher) for R14/R15 together with an Avago HLMP6000 LED, bypassed with 10nF, instead of R13/R16. You'd also need a 100R grid stopper and 22R anode stopper soldered right at the ECC88 pin connections (this is a lot cheaper and more effective than tube rolling). Most of the sonic colourations from valves and C3/C4 will be eliminated by the feedback. You'd still need a good quality capacitor for C5/C6 though. Oh, and be careful with the different heater connections/voltages with ECC88.
I haven't looked too much into the time constants but it should have a stability margin above 90 degrees with the tweaks and shouldn't be too fussy about reasonably long interconnects. It would work OK with a 50K pot. in the power amp.
I ordered two of these PCBs today to alter for my own design which includes all capacitors in the feedback loop and is critically damped with the interconnect capacitance, so I won't be testing this circuit with my proposed tweaks. Maybe fun for someone else. I don't like the DC feedback from V2 anode - some might. In this design, C5/C6 can be safely increased to give better LF response but it's perfectly OK as is.
Regards,
Bob
 
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