Adcom GFP-555 User Manual

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AdcomGFP-555PreampUpgrade

In the best tradition of DIY audio, this author’s modifications significantly

upgrade a proven preamp design.				By Charles Hansen
	modified my Adcom GFP-555 in	does not have the dual-composite op		supply, I did upgrade the power supply,
	a manner similar to Gary Galo’s	amps that the -565 has. Victor Cam-		changing to a 36V CT 2A secondary,
	GFP-565 project in his series of	pos recalled that the -555 was probably		with a copper flux band and steel end
	articles in audioXpress1-5. I did not	designed by Nelson Pass before Victor		plates. I fit in the larger transformer by
do a “purist” upgrade like Gary—I kept		arrived at Adcom.		removing the line voltage selector switch
the tone controls, which now supply			While I did not use an external power	and its extra wiring.
aInew set of output jacks, and all the
			FIGURE 1: Block diagram comparing original with modification.
available inputs but with an upgrade to
gold jacks. I changed the tone control
op amp from the original NJM2041 to
the OPA2604. The LAB and NORM
outputs are now both DC-coupled. I
also removed the headphone amplifier,
which was a mediocre design with an
NJM4556 op amp, so I decided to save
the available watts for the new AD744/
AD810 output stage, which draws a
lot of current. The GFP-555 has even
higher gain (22.5dB) than the 565, so
the volume control wiper is well down
at normal listening levels.
	The lower 14dB gain of Gary’s
line stage was a welcome change. The
AD744/AD811 line stage was a bit
more difficult to install because the -555				AE-adcom-01
	FIGURE 2: Power supply schematic.

AE-adcom-01

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I kept two of the existing plastic dualdual phono jack modules, which make a desirable mechanical connection between the PC board and the rear panel. These are the Tuner In/Tape 1 In pair and the Signal Processor In/Out pair. The former two will be active, while the Processor pair will be strictly mechanical and bypassed with jumpers.

Parts cost for this upgrade was about $115, based on the parts list in Table 1.

Ihappened to have the GFP-555, but

Irecommend using the –565 if you are considering this type of upgrade. Follow Gary’s articles for the purist version, but for a “less-than-purist”modification that retains the original I/O flexibility of the Adcom design, this article will provide guidelines.

I strongly suggest you obtain the service manual, for the model of preamp you are modifying (readily available as reprints on eBay). I mapped all the PC

board jumpers onto the schematic so I could replace them with discrete components where necessary. I must thank Gary Galo for his comments on my proposed modifications; he was very helpful and provided some interesting insight into the Adcom preamp designs of that era.

MODIFICATIONS

Prior to making any modifications, I verified all the DC voltages shown on the schematic (it had sat unused for a few years), checked that all the controls and input/outputs were functional, gave the Adcom a listening session,and made a few measurements.

Figure 1 shows block diagrams for the original GFP-555 and for my modifications. In both cases the CD, Tuner, Tape 1, Tape 2, and Video/Aux inputs are processed through the Listening input selector switch, with the phono

preamp section providing one of the inputs. There was also a separate Recording selector switch that coupled to the Tape 1 andTape 2 tape loop jacks.

Audio signals proceed from the input switch to the Signal Processor I/O jacks, which are furnished with removable metal links.The next step is the Balance control, the Mono/Stereo switch, and the Volume control, which is equipped with a Contour switch to the tapped volume control that could select the loudness contour function for low listening levels. Finally, the selected audio signal was sent on to the active line stage.

The output of the line stage can then take a number of series paths.When the tone controls and filters are bypassed,the line stage is direct-coupled to the LAB output and capacitively-coupled to the NORM output. The headphone amp input receives this same signal. Three additional switches in the line output

PHOTO 1: GFP-555 original power supply rear.		PHOTO 3: GFP-555 mod power supply rear.

PHOTO 2: GFP-555 original power supply front.

PHOTO 4: GFP-555 mod power supply front with new transformer.

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signal path allow selection of the tone			space between the transformer and the		removed the components for the head-
controls—a Lo filter and/or a Hi filter.			front panel.		phone amplifier.
		The original phono preamp and line	POWERSUPPLY		My modified power supply is shown
stage ICs had a house number, “5E,”					in Fig. 2. After mapping all the jump-
which Victor thought was an OP37.			Gary used a separate power supply chas-		ers onto the schematic, I replaced all the
This would make sense, because the			sis for his GFP-565 modification,which		power supply jumpers with larger wire
OP37 is one of the few op amps whose			has the distinct advantage of lower		gauges. Next, I removed all the power
maximum supply rail voltage was ±22V,			phono stage hum and noise. However, I		supply components from the PC board
and the Adcom schematic shows the			wanted to retain a one-box preamp.The		and I replaced the Jamicon bulk filter
5E operating on ±21.5V rails. On the			original power supply regulators were		caps with Nichicon KZ and Panason-
other hand, Gary observed that with the			composed of discrete transistors Q901		ic HFQ types. I used LT1085CT and
phono gain switch set to the lower gain			and Q902 that used ±32.5V raw DC		LT1033CT linear regulator ICs for the
position, the circuit does not satisfy the			rails to derive the ±21.5V DC op amp		new ±16.2V DC op amp power supply
gain-of-5 stability requirement of the			rails.Another set of transistor regulators,		rails.
OP37,so maybe the “5E”was an OP27.			Q905 and Q906, further stepped this		Because the delay relay has a 24V
		The modified block diagram at the	down to ±15.6V DC for the headphone		coil, there was no need for 31.2V DC.
bottom of Fig. 1 is essentially the same			op amp,and provided 31.2V DC for the		The original design used a 274Ω series
up to the output of the line stage.At this			output delay relay circuit.		resistor to drop the coil voltage to 24V.
point the amplified signal takes two dis-				Photos 1 and 2 show the front and	I removed the resistor and dedicated
tinct paths.The first is direct-coupled to			rear views of the original supply. In the		Q905 and Q906 to a reduced ±10V DC,
both the LAB and NORM output jacks.			foreground of Photo 1 I have already		which saved some power dissipation in
The second path uses all the switched
				FIGURE 3: Phono preamp schematic.
tone control and filter circuitry of the
original unit, but goes to another set of
output jacks that I added called Tone
Out.
		You can also see that I changed all
the ICs to reflect improvements made
over the intervening years. The phono
preamp went from the 5E to an AD845.
I replaced the line stage 5E with the
aforementioned AD744/AD811 com-
posite amplifier. The NJM2041 tone
control IC became an OPA2604.Finally,
I removed the 4556 headphone ampli-
fier and all its passive parts, although I
left the jack in place on the front panel.
I tried unsuccessfully to fit my Head-
room headphone module board6 in the					AE-adcom-03
		FIGURE 4: Line stage schematic.

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