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PCM1798 ECC83/EL84 PP UL balanced combo

Hi all, i have build PP UL EL84 amp Dynaco A-410 clone. I'm now searching the best combination of connecting my dac lampucera pcm1798 to it. I thought that a good idea would be to connect dac with El84's in balanced mode without phase inverter on ecc83 and use ecc83 as anode followers to gain signal from dac (+ and -) and put on EL84's. Maybe someone made such combination and would like to tell me how does it work or what should i change in this schematic. I didn't make correct calculations for resistor in ecc83 power supply, i wrote ~200k but i will check it when it will be working (correct to make 200VDC for ECC83). Please tell me what You think about this conception. Best regards
link to schematic: http://www.imagebam.com/image/936d01254980485
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I am unfortunately unable to give good advice regarding your question, but I would like to point to a small error in your EL84 circuit; perhaps accidentally omitted.

You must have resistors from the EL84 grids to common, say 560Ks in your case. They usually go from the junctions of the 0,1µf coupling caps and the 1K grid resistors to common, but may also go directly fron g1 to common (short leads.)

I hope you also get good advice regarding the coupling to the DAC.
It is an interesting idea you've come up with. McGyver suggests some sort of volume control - and I would definitely go for a dual precision attenuator. I have two concerns though: Distortion may be a problem since there's no feed-back loop. I would also add some sort of adjustment to balance out inevitable differences between the + and - signal. This is also the reason I suggest a precision attenuator to adjust the playback level. You could (perhaps) apply the volume control to the grids of the EL84s - but that means that the ECC83 will operate at full amplification all the time - and thus distortion becomes even more of a concern. Perhaps it would be a good idea to add a cathodefollwer as the first stage, then apply the volume control and then the ECC83 amplification stage. Just ideas. - Good luck with the project.
That is really a good question. How could i forget about volume control. Most of the time this combo will work with computer but of course i want to have a possibility to change the volume when i connect it to cd player.
Thank You McGyver for a notice.
And to You Klarskov for Your suggestions. What do You think about such version: connect volume control to the grids of el84 and decrease the gain of ecc83 so it could operate in optimal gain? Is this a good idea You think? Can i only put a resistor in the grid of ecc83 to do this?
About the adjustment to balance out inevitable differences between the + and - signal i think i could use a common cathode resistor (bypassed by a capacitor) and a potentiometer to make such adjustments. In the cathodes of el84 of course i will do the same. I will paint it to schematic in a few minutes
I am not sure that a common cathode resistor would do the trick of balancing the + and - signal. It is sometimes done in phasesplitters in power amplifiers, but most of these designs also allow for individual adjustment of the gain of each branch. But one variable and one fixed cathode resistor might do it. Usually this balance is anly adjusted in one place, so there souldn't be a need for the same arrangement around the EL84s' cathodes.
I am not sure I understand the volume arrangement you suggest around the grids of EL84 - because you'll (probabaly) need all the gain sometimes.

Having thought a little more about it, I definitely think I would go for the cathode follower as the first stage to make sure that the I2U conversion circuit loads the DAC predictably at all levels.


2011-01-23 10:57 pm
why not simple cathode ccs?
should yield best cmrr and gain, and allowing DC coupling to dac - taking advantage of balanced dac in a best way. its a bad idea to have POT after anode follower - high output impedance and low value pot(any pot) dont mix.
perhaps some other tube instead of ecc83(lower mu and rp). also jfet(bf862 or toshiba 2sk...)diff amp works well as i/v, and if matched correctly they will be at same DC potential -> dc couple to tube amp. oh and after jfet iv, stick stereo pot between plus and minus outputs (low value pot). that way, no dc flow thru pot=no crackling :)
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To next 667: thanks for such idea. I haven't got any experience with cathode ccs. It is sth new to me and maybe in future i'll give this method a try. But for today i would like to stay with 'vintage' methods.
McGyver, how would You solve my problem? I have lampucera pcm1798 dac and el84 pp ul amp and i want to make them work with eachother as good as it is possible and using parts i already own if possible.
McGyver: I must admit i'm surprised what You wrote. I thought omitting phase inverter will give me only advantages. I will make comparison this week. Changing topic a little, please tell me which in Your opinion tube analog stage should i use (with resistor i/v conversion) in dac to work properly with my amp?
Yes, you omit phase splitter, but in expense of what? You don't applied a volume control. And what to do in order to apply a volume control? In the end it will be needed additional tube and nobody knows how will it sound.

Eventually you may try this:


You may apply a pot to the grid of V1b instead of grounding it (I wonder if V2a/V2b could be omitted with EL84 instead of 6550). And you still need some gainstage and maybe some filtering for PCM1798 with resistive I/V.
Exactly, i still need gain stage in dac. Klarskov wrote about cathode follower before anode follower and between them volume control, but i wonder if cathode follower is the best match for pcm1798.
No, it isn't. With resistive I/V you need gain about 15-50 (depending on I/V resistor value) to obtain proper signal level at out.
By the way, is it possible to control volume of digital signal delivered to dac?
I'm afraid that digital volume control affects quality by much.
If i stay with dynaco a-410 circuit i will have to increase gain in it, because only source that make it play loud is the output of my sound card in computer. None of my dac output stages could make it. I tried nitro 6n16b circuit from tubeclinic.com and anode follower on 6h2n. How should i do this?