Unfortunately it seems 0dBV is not 3%. If I use 0dBV I get:
[CODE]R/P Inp THD SNR
rec 0dBV 0.02% 88.7dB
ply 0dBV 10.2% 42.6dB
ply -10dBV 11.1% 41.8dB
[/CODE]
The first row is 0dBV output on the QA400 and 234 input level adjusted to yield 0dBV on the QA400. This is recorded to tape but of course what the QA400 is seeing is the "pass through" signal.
The second row is the playback of that recording with the output level again adjusted to read 0dBV on the QA400. This shows that 0dBV gives much higher THD than the target 3%. I *think* 0dBV into the 234 is supposed to be 3% though.
The third row is the 234 output adjusted for -10dBV on the QA400. This shows that the output level does not affect the results much.
So after some trial and error I found that if I adjusted the 234 input level to -5dBV, that would get me vauely close to 3% when played back. Here is the plot and corresponding table using -5dBV:
[CODE]R/P Inp THD SNR
rec -5dBV 0.02% 83.8dB
ply -5dBV 2.8% 44.3dB[/CODE]
The first row (blue trace in plot) is 0dBV output on the QA400 and 234 input level adjusted to yield -5dBV on the QA400. Again, this "pass through" signal is also recorded to tape.
The second row (orange trace in plot) is the playback of said signal with the output level adjusted to read -10dBV on the QA400. I used -10dBV because that is the nominal level for the 234 (at 0dBV the meter needle is violently thrashing).
The grey trace is the same recording but with the QA400 stimulus stopped. Meaning no signal.
Unfortunately it's not crystal clear to me how to go about determining the SNR in this case. It's not even clear that this is a valid experimental process since the QA400 might be using the signal being output to calculate SNR (which is not used during playback from tape). In fact, sometimes I have to stop/start the QA400 to yield a sensical spectrum. But when the level is stable it does seem to work.
One part of the circuit that catches my eye is the recovery amplifier which is U102 in the schematic:
[CODE]U102 RC4558P
Q101 2SK270A GR[/CODE]
According to the block diagram in the manual it's making almost 50dB of gain. I wonder if replacing C109 would help get be closer to the spec'd 52dB SNR that clearly I'm not getting right now. Then maybe replacing the recovery amp altogether might be worthwhile.
