what do you think of this preamp?

[Danko]

Hi all!

I have a solid state power amp, and I would like to build a simlpe tube preamplifier for this.
I found this schematic on the net: http://sziget.mine.nu/~danko/aramkor/tube-amp/ecc83_preamp.jpg

What is your opinion about this? This will be my first "tube-experience", but I have worked with such high voltages.
The powersupply: unfortunatelly, I don't have such transformer, that can produce me high voltage, so I have to use a small DC/DC converter, wich makes from about 15VDC -> 300VDC. The controller IC is MAX1771. It can be ordered from maxim's site, for free, as a free sample. After this step-up converter there come's a simple emitter-follower stabilizer, wich stabilizes the output voltage to 260V (or whatever needs). I hope, this emitterfollower will suppress the noise, which could come from the DC/DC converter.

I also made a PCB-drawing for the schematic -> http://sziget.mine.nu/~danko/aramkor/tube-amp/Screenshot-120.jpg

In the center, there's a huge capacitor: 220uF/400V, this is a common capacitor for both the left and the right side. On the top side, the big plane is not connected to anywhere, this plane will be connected to the central star-ground, with a wire.

Bye and thanks: Danko

 

[SY]

What's the input impedance of the power amp and how long is the cable between pre and power amps? I ask this because the 12AX7 is a rather weedy tube to use driving significant cable capacitance and/or a low input impedance amp.

 

[Danko]

I measured the resistance with a multimeter, it's 22kOhm. (the impedance is lower, I think)
When the preamp is finished, I will put it on my power-amp, so the cable can very short.
Is the input inpedance too low for the tube? What about putting an OPA after the tube? For example one of TI's OPAxxxx, in voltage follower mode?

btw, what's the difference between ECC83, and 12AX7?


--
Danko

 

[Zen Mod]

I measured the resistance with a multimeter, it's 22kOhm. (the impedance is lower, I think)
When the preamp is finished, I will put it on my power-amp, so the cable can very short.
Is the input inpedance too low for the tube? What about putting an OPA after the tube? For example one of TI's OPAxxxx, in voltage follower mode?

btw, what's the difference between ECC83, and 12AX7?
--
Danko

22K is way too low for wimpy 12AX7 (european (or Tungsram ) version is ECC83) .
using any SS buffer after line stage is redundant in your case,'couse you have way too much gain "to waste" in feedback.good if you are lover of Audio Research gadgets-which I'm not
it's better to look for some ECC88 (or PCC if you wish) based preamps-you need (generally) lover voltages but what you get is lover gain (mu is around 33 with some 90V A-K and 10-15mA per system). for SS amp is posible to make preamp with one tube per channel-one anode and one cathode folower DC coupled.
you can even make nice one with 12AU7 or 6SN7.........
ask more when you do little more search.......

 

[SY]

A 22K load is marginal for that design. Yes, a buffer will help, but why use an opamp if you're taking the trouble to build a tube preamp? If it were me, I'd look at a different design, something with better load driving capability. If you can detail what you're looking for (i.e., how much gain, budget), I'll bet people here will have some suggestions.

ECC83 is the European number; 12AX7 is the American number. Same tube.

edit: I see Choky had the same thoughts while I was typing...

 

[Danko]

Thanks for your answers.

So, i will need a buffer, becouse the ECC83 tubes don't have enough current. What about putting one ECL86, as a buffer? I have four ECL86, from an amplifier. Can one ECL86 per channel good for my porpuses?

The details, about what i would like:
- A tube preamp for my SS poweramp
- low distirtion
- I don't really need "amplification", becouse my computer (Okay, it's not hi-end ) can drive my SS poweramp properly, I just would like a simple valve-preamp, which will (or won't?) put some "tube-sound" into the sound. I hope, you understand this
- no output transformer!! (becouse of low budged, and simplifying the project)
- low budget (i will buy, what I really have to, but I won't buy 99.9999% pure silver cables, and such things)

I got 4 ECL86 from an amplifier, and also got 2 ECC82 from this amp. But I heard from one of my friend, that ECC83 is better than ECC82.

Thanks!

 

[SY]

I think just a simple volume control and cathode follower will do fine for you. Look for a design using ECC88 or 6SN7.

But I heard from one of my friend, that ECC83 is better than ECC82.

You need to get smarter friends. These are completely different tubes with different characteristics made for different functions. Saying that one is "better" than the other is like saying that a 42 tooth 50mm brass gear is "better" than a 21 tooth 25mm steel gear.

 

[Zen Mod]

ok-jst google it for tube preamp and read;
I'm not lazy -but it's better that you find something and post your questions and thoughts here ,than I (or any other member ) give you finished design;
your goals are :
gain in range 3 to 15
Rout in range of 300E to few K
simple but not simplest than necessary
not to much feedbackmjust because in tube world that lead to typical SS (in pejorative meaning) sound;

in that circumstances-your choice of tubes is pretty miniscule-
ECC82
ECC88
EC900
ECC85
6SN7 (6H8 russian ,if I remember corectly)

you have two pieces of 82;
in that case-you can build anode folover DC coupled with cathode folower per channel , or
you can use two triodes from one tube in one channel as anode folower and in that case you have doubled transcoductance ,lower noise ,halved Rout comparing to just one triode per channel

just few thoughts for start
write again

 

[Sch3mat1c]

Buffer ain't necessary, look at the NFB loop already - it has around 10k impedance, loading it internally. As lightweight a tube as 12AX7 is, a mere couple of volts output is nothing to it.

Tim

 

[Bare]

Reading your posts with some interest.... Have you considered looking up the Bottlehead Foreplay Preamp??. (bottlehead.com) as a starting pont.. these are V simple 12au7 direct coupled/Triode/cathode follower designs.. Very good (astonishingly good actually) sound, uses an absolute minimum of parts. NO impedance matching issues Maker ( it was a George Wright design of a few years ago) has recently 'upgraded it to Tube rectification and a tripling in price to $300..
But the original design, in no need of Tube rectification has a Total parts cost of approx $50, geography dependant.
Currently available Kits from the above site are significantly more , but still in the Bargain area of Audio.. at least for 'proven' genuinely Good sonics.
As always there are LOTSA solutions.

 

[Steve Eddy]

A 22K load is marginal for that design. Yes, a buffer will help, but why use an opamp if you're taking the trouble to build a tube preamp? If it were me, I'd look at a different design, something with better load driving capability.

What's up with that?

I mean, 600 ohm loads were commonplace in the telecommunication, broadcast and recording industries long before transistors came on the scene. Yet today, if you mention even a 10k ohm load, let alone 600 ohms, bottleheads tend to shriek in horror.

Why is that? Was there some "ancient wisdom" that was somehow lost along the way?

se

 

[Danko]

Thank you, again, your answers!

I found this schematic: http://headwize.com/images5/ahammer4.gif
The article is here: http://headwize.com/projects/showfile.php?file=ahammer1_prj.htm

What about this?
I don'h have exactly EL84, I hace it's russian type: 6PI14PI
Here they are: http://sziget.mine.nu/~danko/aramkor/tube-amp/parts-1/dscn3815-small.jpg

I think, this headphone-amplifier can drive my SS amplifier properly, becouse the article says, this amp can drive even 30ohm load.

What about this? Is it sufficient? How much power will the EL84 dissipate? A few watts for the heating (6.3*760mA), and how much the rest? I see, 210Volt wil ldrop across the tube, and 35.5mA is flowing through the tube, so, if I'm right, then the tube will dissipate about 210*0.0355+6.3*0.76 = ~13Watts. Doesn't this much dissipation shorten the tube's life much?

What about decreasing a little bit the quiescent current of the EL84? I don't really need, to drive such low impedance. If the amp. could drive 1kOhm or 2kOhm, that would be perfect for me, I think.

 

[SY]

What's up with that?

I mean, 600 ohm loads were commonplace in the telecommunication, broadcast and recording industries long before transistors came on the scene. Yet today, if you mention even a 10k ohm load, let alone 600 ohms, bottleheads tend to shriek in horror.

Why is that? Was there some "ancient wisdom" that was somehow lost along the way?

se

No, you just have to pick a tube and circuit that's capable of driving that load. The commonly-used preamp tubes and circuits aren't. If you've got a tube like a 12AX7 that will swing (at best) 0.1mA of signal current without severe distortion, Ohm's Law tells you that your output will be limited to about 60 millivolts. A heftier tube like the ECC88 will get you up to half a volt, which is still not a great thing. The ECC88 is perfectly capable of driving 10K without inducing shrieks from bottleheads.

The great broadcast equipment that drove 600 ohm lines usually did so through step-down output transformers, and that's a can of worms. An expensive can of worms. It could also be done with a power tube, again at great expense and complication.

 

[pedroskova]

What's up with that?

I mean, 600 ohm loads were commonplace in the telecommunication, broadcast and recording industries long before transistors came on the scene. Yet today, if you mention even a 10k ohm load, let alone 600 ohms, bottleheads tend to shriek in horror.

Why is that? Was there some "ancient wisdom" that was somehow lost along the way?

se

It wasn't until the mid to late 50's that 20kHz started becoming a standard for "high fidelity". Langford-Smith in RDH4(1953(?)) used 40Hz - 15kHz as a design goal for hi-fi audio, and 50Hz-10,000kHz was deemed good quality for radio receivers.

 

[Steve Eddy]

No, you just have to pick a tube and circuit that's capable of driving that load. The commonly-used preamp tubes and circuits aren't. If you've got a tube like a 12AX7 that will swing (at best) 0.1mA of signal current without severe distortion, Ohm's Law tells you that your output will be limited to about 60 millivolts. A heftier tube like the ECC88 will get you up to half a volt, which is still not a great thing. The ECC88 is perfectly capable of driving 10K without inducing shrieks from bottleheads.

The great broadcast equipment that drove 600 ohm lines usually did so through step-down output transformers, and that's a can of worms. An expensive can of worms. It could also be done with a power tube, again at great expense and complication.

Gotcha. Thanks!

Just kinda sad that it takes such great expense for tubes to do what a 25 cent opamp can do. Though admittedly the opamp isn't going to give you the same result.

se

 

[Steve Eddy]

It wasn't until the mid to late 50's that 20kHz started becoming a standard for "high fidelity". Langford-Smith in RDH4(1953(?)) used 40Hz - 15kHz as a design goal for hi-fi audio, and 50Hz-10,000kHz was deemed good quality for radio receivers.

I think you misread my post. I was referring to a 10k ohm load, not a 10kHz response.

se

 

[pedroskova]

I think you misread my post. I was referring to a 10k ohm load, not a 10kHz response.

se

I was, in a most vague way, referring to the roll off that would occur with something like a 12ax7 driving a 10k, or worse, a 600R load, with stray capcitances etc.. This may have been deemed passable in broadcasting in another era, but not at later dates.

 

[Steve Eddy]

I was, in a most vague way, referring to the roll off that would occur with something like a 12ax7 driving a 10k, or worse, a 600R load, with stray capcitances etc.. This may have been deemed passable in broadcasting in another era, but not at later dates.

Ah, gotcha. Yeah, it was most vague alright.

Makes sense now. Thanks for the clarification.

se

 

[SY]

Just kinda sad that it takes such great expense for tubes to do what a 25 cent opamp can do.

se

I look at it the opposite way- it's a wonderful thing that a 25 cent opamp can do the same thing as $20 worth of tube circuitry and with a miniscule fraction of the energy consumption.

 

[Danko]

After you understand each other, could you take a look at my post?

=-> http://www.diyaudio.com/forums/showthread.php?postid=656844#post656844