Spent quite some time last night messing with the bias. IIRC Tascam told me that "LPR35 needs twice the bias". The bias amps only go up to about 200mV output max and I messed with the tuning and the traps (non-ideal) to see if I could tweak that at all but I can tell you it isn't worth doing that...keep the tuning and traps as spec-ed in the manual (minimize the DC and AC voltage respectively). I was doing what was suggested by tascam which was to scope the output of a recorded 10k tone and adjust the bias amp output for minimum distortion, but this is difficult with the narrow format as there is more "noise" in the recorded signal anyway and its hard to decipher the noise and the distortion on the scope.
I decided to try using the LF modulation technique instead to try and find the optimum bias point. This is the "listening for 'rocks'" technique. I know this also isn't as effective with the narrow format, but I decided to try it anyway. I dialed up a 7Hz tone on my oscillator setup and tracked that to 4 tracks with a track in between each (i.e. tracks 2, 4, 6 and 8) and to try to avoid any tape saturation confusion I tracked it at about -3VU. I set the bias amps for those tracks 5mV apart so that I could listen back to the result across a 20mV spread. I started with the lowest output at 70mV (so track 2 at 70mV, track 4 at 75mV and so on). I repeated the process until I could play back a minute or so of the 7Hz tone on a track without hearing any "rocks" and then noted what the bias amp output was set to on that track...its difficult because there is a lot of HF modulation noise...you don't actually hear the 7Hz tone of course but you REALLY hear the modulation noise puff-puff-puff-puff-puff-puff with the "rocks" (kind of a pop-pop-pop-pop-pop to varying degrees) and once the rocks go away then you start to hear more HF noise...the distortion actually seems to start becoming more hissy...excuse the verbage here but the best way to describe it is piss-piss-piss-piss-piss instead of "puff"...I thought that was the "noise" factor I was looking for to balance the distortion (rocks) and noise (hiss) to find the optimum bias point (in between those too) but things were getting more piss-piss-piss (I'm starting to sound like I have tourette's disorder) and I was STILL hearing some rocks. Then I realized the "noise" I was listening for wasn't in rythym with the 7Hz tone, the puff-puff-puff but rather is a more constant hahhhhhhhhh. When I realized that was the upper limit I was looking for, and the disapperance of the rocks was the lower limit, that's when I found what I think to be the optimum point. That was at about 120mV bias amp output. This is totally contrary to what Tascam told me, but I know what I'm hearing. I setup all tracks to 120mV, recorded 2 minutes of 7Hz tone and listened back to all the tracks...no rocks, and little "hahhhhhh". To test I dialed one track up to 150mV bias amp output and while the modulation distortion continues to get more "pissy" (pisss-pisss-pisss) the "hahhhhhhh" also gets louder. So I'm pretty comfortable with the 120mV figure for LPR35.
The only caviat here is that I'm not done with the reproduce frequency response. I can't even HEAR the 16k tone reproduce which is supposed to be within +/-3dB of 0dB at 500Hz. My HF response strarts dropping by 4k. I haven't messed with the record eq yet. I was worried about how it was going to sound because of all the noise artifacts I'm hearing with the tone ladder I'm recording to check the record eq and I was thinking "MAN! If I dial up the record eq to get the response in spec there's going to be too much noise!" because it sounded like there was too much already. I decided to do a test of program material...a selection of snippets of pre-recorded stereo consumer material of varying styles bussed in pairs to all 8 tracks, now with the dbx on of course.
It sounds great. Man I love tape. It is very quiet in terms of noise with the dbx on and everything just sounds really sweet and the 388 really does an amazing job with the low-end thanks in part to the 7.5ips transport speed IMO...puts a little head bump at around 60Hz for those of you that haven't noticed and the bump is often something to contend with in the 100Hz range at 15ips...but even though my HF response is off at this point the highs sounded really good when reproducing the material...smooth and present. I'll tweak the record eq tonight and that may mean tweaking the bias a bit which will mean redoing the record level...and there may be a reproduce eq tweak to do as well. I had to guess at where to start rolling the HF reproduce response because I'm using a 15ips cal tape that only goes up to 20k (so 10k at 7.5ips) so I started rolling at 8k...that may be too late? In which case the HF response may still be too high at 16k to balance with the bias and record eq. So I'll mess with it a bit more and see.
I will tell you though that doing all this testing and tweaking makes me REALLY appreciate a 3-head machine. Once this is set it'll be set and good. Under normal circumstances its not such a big deal but with all the bias experimentation I'm getting REALLY familiar with the RTZ button.
The 388 sounds really nice, fellas. Oh, and the transport is operating flawlessly...tensions are enabling nice crisp tape handling and nice fast-wind performance.