From: peterca@ento.csiro.au
Newsgroups: rec.audio.high-end
Subject: CD tweaks
Date: Mon, 21 Dec 92 13:17:46 EST
Organization: University of Wisconsin - Milwaukee
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

I have been doing a series of modifications to the analogue stage of my
marantz CD60SE recently.  This is a standard 16*4 Phillips chip set but is
the "Special Edition" version of the CD60.  I also listened to the standard
version and they were very different with the standard version being thin
and harsh sounding with less detail.  I bought the workshop manual for the
CD60 and as far as I can tell the differences between the two is the
extensive use of Elna cerafine electrolytics for power supply decoupling
next to most chips and a pair used back to back for output coupling.  The
analogue filter stage has a curious mixture of discrete and surface mount
resistors but the caps are Phillips polypropylenes.  Deemphasis is switched
with a fet.  
My first change was very conventional.  I removed the NE5534 dual opamps
and replaced them with AD712s (A$2.30 ea).  This change was assessed double
blind by changing one channel (but I didn't know which) then getting a
friend to decided which channel was better and why.  We had exchanged
impressions and were in fair agreement.  Then the circuit was traced and we
found the changed channel was the prefered one.  Essentially the change
could be described as more detail but slightly darker sound and less tizzy.

The second change was to replace the AD712s with AD827s (~A$9.50 ea).  This
gave a smaller, further improvement in detail and restored the original
tonal balance. At this point I was convinced that any noticable improvement
was good value at under ~A$20  (~US$15).  For convienience the original
opamp was replaced by a turned pin gold plated socket which made trying
different opamps easy.  For further opamp improvement I expect you would
have to use single opamps with extra decoupling caps for each but this
could still be mounted on a component header to make for easy and
reversible changes.  I haven't tried this however.  
All the above is fairly conventional and not at all original.  The third
change was more radical and was an original idea (though someone else most
likely has thought of it before me).  Basically my thoughts were as
follows:  Current output DACs like the TDA1541A don't like to see a voltage
on their output and ideally would like to see a very low impedance to
ground.  The ideal current to voltage converter would otherwise be just a
resistor which would produce a voltage across it proportional to the
current.  (I read that in an application note for an AD opamp recommended
for current to voltage conversion.)  I then wondered what impedance opamp
I/V converters present to DACs in usual applications and was told by a
friend that it is generally 50ohms to several kiloohms.  [however I have
also been told an opamp set up for I/V conversion presents a virtual earth
to the DAC as its output fedback works to keep its inverting input (DAC
connection) at earth potential -so I'm not sure which is correct] These
presumably result in a voltage appearing on the output of the DAC so I
thought what if I use a resistor which is smaller than 50 ohms.  This
should be better but for one problem: as the resistor gets smaller the
voltage across it for a given current gets smaller but the DAC is happier. 
The small voltage will need to be amplified and if it is too small noise
becomes a problem.  My friend did a calculation for the noise figures of
the AD797: one could use a 10 ohm resistor for a 2mA peak current (-> 20mV)
amplified to 2V out and still get 100dB SNR.  If the resistor is bypassed
with a capacitor the voltage across will be deterined at low frequency by
the resistor but at high frequencies will be less and less with increasing
frequency.  This appealed to me because the ultrasonic output of the DAC
need not even enter the first analogue device and so its performance at
>audio frequencies would be of less consequence.  Also I could shunt the sampling frequency to earth with a series inductor-capacitor.  
The result of this is a 27 ohm resitor across the output of the DAC
conveniently mounted on a component header.  The value of the resistor was
obtained by trial and error to match the output volume (with the gain stage
below) to that of my phono stage.  Across this is about 133nF of
capacitance made of a mix of polyesters and polystyrenes I happened to
have.  My calculations say this gives the same cutoff frequency as was in
the player originaly and a 6db/octave slope.  I also have 1mH-820pF series
circuit across the resistor for the notch at 4x44.1kHz  (the inductor has
3ohm DC resistance so it is not a deep notch).  Also across the resistor is
a flying lead out of the player to a separate gain stage.  This gain stage
is followed by an RC filter to give a steeper slope to the filter at higher
frequencies and the notch again, this time much deeper.  This then sees the
100Kohm vol control of my preamp.  I was able to leave the original circuit
of the player intact with this arrangement which will be useful for later
sale of the player or return to the original while doing further mods to
the outboard gain stage.  
The gain stage is a single 12AT7 valve/per channel used as a shunt
regulated push pull (SRPP) circuit which is class A, linear and uses no
conventional feedback.  Listening tests indicate that the noise of this
circuit is low enough to not cause any problem.  It is also satisfying that
this stage is essentially the same as the second gain stage of my phono amp
which makes any CD/LP comparisons a bit fairer.  Subjectively the sound has
moved closer to my turntable but still not as good.  It seems more open and
detailed, less tizzy and generally more emotionally involving with this
circuit compared with any of the opamps in the original circuit.  The tonal
balance is much the same as original -only the AD712 seemed to change this.
 It also is nice that it compliments the rest of my system which is a SRPP
valve phono amp and line amp and a pair of Leak TL12 valve power amps (much
modified) driving much modifed Allison One speakers.  (The turntable is a
LinnLP12/Ittok/ortophon MC3000 with T3000 transformer to stepup to MM
level)  For the listening tests I also used a large home made mosfet amp
(AEM6000) and a pair of Magnepan Typanis which were on loan.  
The circuit lacks either the deemphasis circuit or output muting.  So far I
havn't encountered a disc with an obvious need for deemphasis and I
understand they are rare but I haven't looked far yet.  Changing tracks etc
does not produce any clicks or thumps so I may get away without the muting
-perhaps it helps for damaged discs?
This experiment was done from a position of semi-ignorance and I would
enthusiastically welcome any comments or suggestions for further
improvements.  

----------------------------------------
Peter Campbell
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601
Australia  (peterca@ento.csiro.au)
Ph.61-6-2464158 (w), 61-6-2516213 (h),
61-6-2464173 (fax)
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