PCM1798 ECC83/EL84 PP UL balanced combo - diyAudio
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Old 17th May 2013, 02:33 PM   #1
dankal6 is offline dankal6  Poland
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Default 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

Last edited by dankal6; 17th May 2013 at 08:07 PM.
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Old 18th May 2013, 09:44 AM   #2
dankal6 is offline dankal6  Poland
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Anybody?
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Old 18th May 2013, 07:59 PM   #3
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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 0,1Ķf 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.
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Old 19th May 2013, 10:03 AM   #4
dankal6 is offline dankal6  Poland
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Thank You very much, i forgot about them, i will paint them on schematic. I wonder if the anode follower with ecc83 is the wright way to gain signal from dac to el84's?
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Old 19th May 2013, 10:23 AM   #5
dankal6 is offline dankal6  Poland
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corrected version:
ImageBam
Maybe someone had noticed some other errors?
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Old 19th May 2013, 11:46 AM   #6
McGyver is offline McGyver  Poland
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Are you going to listen to it at full power all the time?
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Old 19th May 2013, 12:19 PM   #7
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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.
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Old 19th May 2013, 01:03 PM   #8
dankal6 is offline dankal6  Poland
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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?
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Old 19th May 2013, 01:13 PM   #9
dankal6 is offline dankal6  Poland
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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
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Old 19th May 2013, 02:01 PM   #10
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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.
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