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Summing differential output to single ended with tubes?
Summing differential output to single ended with tubes?
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Old 12th February 2018, 09:21 PM   #11
MagicBus is offline MagicBus  Greece
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My experience with the I/V resistor starting from 100 ohm is the lower the better. Perhaps a max 56 ohm. Personally, I feel very good with 2 ohm.
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Old 12th February 2018, 09:27 PM   #12
Lingwendil is offline Lingwendil  United States
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Summing differential output to single ended with tubes?
I'm pencilling in 47R as the max I'd like to use for now, back when had access to nice audio test equipment I found that once you go much higher you start to get a smidge more (But still pretty negligible) THD, but it was still exceedingly low, and not much worse than an opamp implementation would run on my eval board at the time, but all my testing was single chip stereo, so it should be even better going dual mono.
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Old 14th February 2018, 05:08 AM   #13
PRR is offline PRR  United States
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Quote:
Originally Posted by Lingwendil View Post
Speaking of grid hiss, how would I be able to ballpark a realistic noise floor for this setup?
Pencil 2 microVolts per tube over the audio band. (RMS summing: two tubes makes 2.8uV.)

It varies A LOT with the tube. You may find that in a crate of 100, a couple touch 1.5uV, most run 1.6-2uV, and a dozen run 3uV to 100uV.

It varies with current, except high current reduces hiss in the KHz range but typically increases 1/f hiss (random rumble) below (even above) 1KHz.

There is data saying that a "good" tube at well selected operating point can approach 0.5uV over the audio band.

The rule in recording is "don't bet on it!". Even if a preamp shows 0.5uV on the bench, tubes drift, sometimes quite a bit. Tube mike preamps aimed for low hiss almost invariably take a 1:5 to 1:10 transformer from 100r mike to grid.
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Old 14th February 2018, 12:04 PM   #14
Lingwendil is offline Lingwendil  United States
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Summing differential output to single ended with tubes?
I thought of trying out some microphone transformers, since this seems to be a similar situation where they may work well... Looks like Edcor has some affordable options that may be suitable for much less than all the Gucci types that you see the Golden Ears pushing.

Using such a low mu tube it looks like the noise won't be too much to worry about, especially if I err on the higher side for my I/V resistance to get a bit higher voltage to swamp it out.



Well, started up on the boards. Doing each pair of LTP/CCS circuits on a single board, and a pair of cathode follower/CCS each on single boards as well, for two boards per channel in addition to the DAC and PSU boards.

Summing differential output to single ended with tubes?-img_20180213_204623-jpg

Gotta order some more caps and put in headers for off-board connections (PSU, signal) but the buffer modules are pretty much done.
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Old 14th February 2018, 11:18 PM   #15
PRR is offline PRR  United States
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Quote:
Originally Posted by Lingwendil View Post
I thought of trying out some microphone transformers.....
The signal level from the DAC runs higher than the hottest microphones.

I *have* seen modest iron take much higher than mike level without gross distortion; but I did not drive <50Hz or put a THD meter on it.

150:10K "line" transformers may be as good a bet.
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