D-AOC PCBs - the building and help Thread

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L091c

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Feb 12, 2014
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Ah good call, checked, they are 47K and 10K resp, like the BOM :/
Just in case, I will trace back the values using the schematics, double check any divider that is in play.
Hmm all resistor values look ok.
Could it be because the board is not (yet) connected to any potmeter?
 
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radiance

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How's the plate resistance tolerance on these ECC81/12AT7 tubes?
If everything else checks out fine the difference in voltage could be due to difference in plate resistance between tubes....
 

L091c

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How's the plate resistance tolerance on these ECC81/12AT7 tubes?
If everything else checks out fine the difference in voltage could be due to difference in plate resistance between tubes....
Can I measure the plate resistance somehow sensibly? (This is my first tube project, I might ask silly questions)
 

audiophreak

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Ah good call, checked, they are 47K and 10K resp, like the BOM :/
Just in case, I will trace back the values using the schematics, double check any divider that is in play.
Hmm all resistor values look ok.
Could it be because the board is not (yet) connected to any potmeter?
I seem to remember I got antsy to check voltages and found discrepancies , posted here ( not sure when ) and made the same statement about not having the pots in yet , when I put the pots in the circuit and retested - all voltages checked out good.
 

audiophreak

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Ah good call, checked, they are 47K and 10K resp, like the BOM :/
Just in case, I will trace back the values using the schematics, double check any divider that is in play.
Hmm all resistor values look ok.
Could it be because the board is not (yet) connected to any potmeter?
Yes, I posted on pageb69 of this thread , look at my voltages , that was without pots , after I installed pots , all voltages were good.
 

L091c

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Feb 12, 2014
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I seem to remember I got antsy to check voltages and found discrepancies , posted here ( not sure when ) and made the same statement about not having the pots in yet , when I put the pots in the circuit and retested - all voltages checked out good.
Thanks! When I searched for "voltages" (plural) your post didn't match, my bad.

I went into the rabbit hole and followed the advice to try other tubes, so I bought a matched pair of Miniwatts. The voltages on pin 1 of these EEC81s are now: 225V (L) and 218V (R). Doesn't measure matched, just not sure if this difference (hear-ably) affects the performance, the previous set of Siemens measured both the same voltage. What would your preference be?
 

L091c

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Are pots installed?
Thanks for your comment Scott, no not yet, we found out that the missing pots are the cause of our 130V difference from spec. Voltages will drop when the pots are installed.

But my next question was: how important is having equal plate voltages on each side? With the new tubes they're 3% off. Radiance mentioned plate resistance tolerances so hence my question. With my limited knowledge: when Mu and Gm are matched, I don't think it matters that one plate runs hotter than the other. Is that an acceptable assumption?
 

Script

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Planning on adding Lundahl LL1540s to input. Is it as simple as implementing what the datasheeet says (schematic on right with 2 x 12K) or is it even simpler and could leave out the resistors ?
 

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MisterCMRR

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Those resistors are necessary to avoid a resonant peak in frequency response (flatten HF response) and minimize overshoot and ringing for square-wave input signals. See the section under "self-resonance". Such networks are generally required for any input transformer that contains a true Faraday shield.
 

Script

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Thank you. I understand Zobel networks and that the datasheet says that RC is not required with R<25K. So it just the two 12Ks for CMRR of >60dB, which seems very good.. Had just wondered and wanted to confirm as I recall it being mentioned somewhere above that the LL1540 could be implemented without any additional resistors. It'll be hooked to input of circuitry which looks like this with a 15K.

1632525439047.jpeg

In general, I think this circuit doesn't really call for input transformers at all. Can feed unbalanced as is or -- and this has crossed my mind -- add IC debalancing instead, which might be sacrilege for some, but definitely cheaper and probably also 'cleaner'.
 

L091c

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In general, I think this circuit doesn't really call for input transformers at all. Can feed unbalanced as is or -- and this has crossed my mind -- add IC debalancing instead, which might be sacrilege for some, but definitely cheaper and probably also 'cleaner'.
Ohayo! Yes, since most of us do not use it in live concert situation with cable runs of >100m, it can run unbalanced fine (see: What are you using your D-AOC on? Switchable transformers?).

The point of using 'iron' is its sound when it saturates. I am building mine not only for its compression function but also for the sound & color it brings into my chain. I had Lundahls planned for it, but after thinking hard I actually will put the Edcors and Lundahls in a separate transformer box with some Triads as well for switchable flavours.
 

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