PR99 repro amp LC filters

[Cyrano]

Hi,

For those in EE field, is there any reason to stick only with ceramics caps in LC filters (2.2mH/560pF).  Caps are rated at 20% with no requirements for temp stability other than mica standard. 
I tried RTE and they are sounding a bit plasticky/wooly in comparison to micas, but those micas have grain in midrange plus they sound less HF extended to start with, LF are very good.  When adding tiny styrene (20pF) on top of existing mica 560pF it creates unprecedented transparency and it makes mid-low frequencies sound 'gone' they are so rounded that almost unreal, vocals are hanging in the air effect without a trace of any color, detached from distortions and with hights just impossible to match.  And the same time low-midrange is less developed to my liking  SO I have got hooked again :-) 
I am looking for some suggestions.  Is there any alternative worth exploring?  I will experiment with different bypass ratios as this might be a key for proper balance but where to get those pesky old micas of good quality from?  Any particular brand Studer liked and used? Thanks again for sharing your experience.  It all started with adding Murdorf silver-oil to power supply, the usual atomic fall out, it was supposed to be just to see how they would do, now I need to do better.

Andrew     

[Tim]

If you are using this deck for listening to TP tapes then it would sound best just to remove the circuit altogether.  As you know, it's just there for filtering out the record bias frequency.   Even if you occasionally do recordings, you can still get by without it.  Just don't tape-monitor in record mode with your speakers cranked-up. Use headphones if you really need to record-monitor. You do need the filter when testing or adjusting the unit's record mode, though. Pull the circuit and put it at the input of your test rig.  Or put by-pass switches across it.  In playback mode, the best-sounding LC filter is no LC filter.

Tim     

[Cyrano]

Tim,

I would have to calculate created by removal of the LC parts low pass filter (1k/3n3 & 470R/10n) in both positions, this might proof to be unnecessary hustle, designers tend to cut the cost after all for us.  This output is driving phono amp, output amp and Monitor out being about anything and does it at the same time, there must be many reasons for this filter. 
Anyway have you got good results?

Andrew 

[Tim]

I'm not sure what your trying to say (your first sentence).

 As far as your claim of "there must be many reasons for this filter".   There is only one reason: to filter out the 150kHz record bias tone when in record mode. If you generate a plot of the 2.2mH/560pF filter, you will see it does precisely just that.   If this bias tone were to get into your power amp and speakers in sufficient quantity, it theoretically could cause damage. In pure playback mode, though, it has zero function and could be removed if you want.  Or you can just leave it in.  Having it or not having it won't make huge difference in your final sound quality.

[Cyrano]

Tim,

That part I got, there is for that reason to cut off possible leaks from generator.  I was thinking about LC removal impact on the remaining parts on follow up circuits nothing else.  They could put it in front of that last stage too.

The other good news is that RTE I had on hand was used but most likely not for long enough so after 3 days it settled in and it is a keeper, it is worthwhile upgrade.   
Thank you for clear description of what LC was used for.  Mundorf Silver in Oil in PS opens up sound so much that mica in question became annoying limitation.  I will definitely come up with more questions!   

Best
Andrew
   

[Cyrano]

Tim,

I found the answer to my concerns.  I have got curious and had to look no further than PR99 MKII prior PCB version .82. 
They had omitted LC filter  and in addition  RC low pass filter 10n with load resistor change from 470 to 390 ohm.  Your suggestion might be more than to try, it will work straight.

Regards
ANdrew