[R-390] OT: Transistor Amplifier Question

[Barry]

These are across some of the 2SC1000s (HT310002As) as well as some of the 2SC458s (HT304580Rs).  I'm using KSC1845FTAs to replace both (although I think the KSC1845FTAs may have a higher gain than the 2SC458s and hoping that doesn't have an adverse effect.  These are on the phono preamp board (the lower case "s" is for plurality and not part of the P/N).  Perhaps those are used more on the phono board to help with hf noise from the needle/cartridge (or, possibly, to conform to RIAA standards?).

The other 2SC1000s are on the normal preamp board.  The schematic doesn't show those caps on that board but they're were installed there - at least for the first transistor in signal path.  For each channel, that board consists of a pair of complimentary transistors which are coupled to the tone controls.  The output of the tone controls are coupled to the second complimentary pair before being fed to the PA.

Unfortunately, the schematics do not entirely agree with the boards (particularly on the phono board) so I'm having to go by what's there instead of entirely what's on the schematics.  Apparently Marantz made several versions of similar boards that were used on several different models of amps and receivers but finding a matching schematic is pretty difficult now.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Alan Victor" <amvictor at ncsu.edu>
> To: "Barry" <n4buq at knology.net>
> Cc: "Roger Ruszkowski" <flowertime01 at wmconnect.com>, "390 list" <r-390 at mailman.qth.net>
> Sent: Monday, May 18, 2020 12:46:53 AM
> Subject: Re: [R-390] OT: Transistor Amplifier Question
> 
> Adding C from base to emitter will significantly reduce the unity current
> gain crossover frequency or F_tau.
> 
> So, yes, if the desire is to roll off high frequency gain for whatever
> reason, this added C in that location will do it. Curious if you can read
> the device p/n or is it an internal number and not a 2N or MPS, etc...
> 
> Alan
> 
> On Sun, May 17, 2020 at 6:45 PM Barry <n4buq at knology.net> wrote:
> 
> > Okay, Roger.  Given that Marantz had/has a reputation for sounding great
> > (and I tend to agree with that), if there was a way to enhance the sound,
> > especially with such a small part like that, I'm sure they didn't hesitate
> > to include it and I will restore it the same way it was.
> >
> > Thanks!
> > Barry - N4BUQ
> >
> > ----- Original Message -----
> > > From: "Roger Ruszkowski" <flowertime01 at wmconnect.com>
> > > To: r-390 at mailman.qth.net
> > > Sent: Sunday, May 17, 2020 7:38:34 PM
> > > Subject: Re: [R-390] OT: Transistor Amplifier Question
> > >
> > > Barry,
> > > The problem is complex and common.It is a bias stabilization method that
> > > works.In an audio pre amp any thing better is OK.
> > > We were looking at some 7805 or 7905 or 7812 voltage regulators.The
> > comment
> > > was why the pf cps on the input and outputs of theregulators. What is +
> > / -
> > > 50 pf going to do for the DC voltage andcircuit bias.
> > > It is not the exact value.At different device sizes, different pop corn
> > > noises get trapped insidethe specific device not the specific capacitor
> > > value.The science of what is going on in the circuit is complexand why
> > RF is
> > > real squirrely compared to DC and Audio.Different cap types, silver mica,
> > > foil, ceramic, different dielectrics, and packaging all have different
> > > characteristics relevant to audio amplifiers.
> > > However the discussion of why the extra caps are includedwith three
> > terminal
> > > voltage regulators explains it all the circuit science in nerd.
> > > The 50 pf caps do change the linearity of the amplifier stageeven if the
> > > improved nuisance misses your ears.The idea is that neutralizing the
> > > hysteresis on the transmission linebetween active amplification
> > junctions is
> > > to retain signal fidelity.
> > > The caps self resonate and this effect extends or boosts the high
> > frequency
> > > response in the amplifier stage giving better detail to lower audio
> > > frequencies. (Richer harmonic content)Those little caps just let the
> > circuit
> > > sound better when the signal is more than just a simple sine wave.
> > > Roger AI4NI
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