An Archival Phono Preamplifier - the power supply

Gary Galo

Editor’s note: this online article is an addendum to Gary’s archival preamp design article in Linear AudioVolume 5

Transformers and connectorsI am a firm believer in external power transformers and rectifiers, particularly for high-gain circuitssuch as phono preamps. The external supply for the Archival Phono Preamp is shown in Figure S-1and Photo S-1, and is very similar to the one I described in a follow-up to my Adcom GFP-565 modi-fication series [1].

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An Archival Phono Preamplifier

POWER SUPPLY CABLE:NEUTRIK NL4FC CONNECTORS MOGAMI W2921 - 4 X 14 AWG

2-

2+

1-

1+

YELLOW

ORANGE

BROWN

RED

IEC POWER CORD 14 AWG

- +

Ext SupplyChassisGND

- +

- +

2

5

6

7

8

11

12

1

20

VAC

Ext SupplyChassisGND

EXTERNAL POWER SUPPLY CHASSIS

120VAC 60Hz

20

VAC

13

VAC

J801NEUTRIK NL4MP

J1120VAC

QUALTEC FUSED LINE FILTERLF801

860-04/003

Fast-Blow

F801 1.5A

C8180.47uF/63V

+ C8171000uF

D810-D813 PB-61

C813-C816 0.47uF/100V

1A

T80212V AC

D805-D808 HFA08TB60

C805-C808 0.47uF/100V

D801-D804 HFA08TB60

C801-C804 0.47uF/100V

VPS36-3600T801

18+18V AC L80156mH

L80256mH

+

C810

2200uF/100V

+

C809

2200uF/100V

C8111uF/63V

C8121uF/63V

2mAD809

LED+25VDC

-25VDC

GROUND

+18VDC

2mAD814

LED

R805-R808 47.5 1/2W

R801-R804 47.5 1/2W

R80933.2k

R81012.1k

Gary GaloLinear Audio

Archival Phono PreamplifierExternal Power Supply

B Figure S-1

Date: 11/4/2012 Page: 1 of 1

Rev ID

Figure S-1: The external power supply houses a 130VA dual-bobbin transformer for the main power supply and atoroidal transformer for the relay supply, plus all rectifiers and raw filtering.

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It should be obvious that no wall-wart can meetmost of the requirements for this supply. Goodpreamplifier dynamics begin with a high-currentpower transformer. I used a 130VA dual-bobbintransformer with separate secondary windings,which allows separate full-wave bridge rectifiersfor the positive and negative rails. In my view,

dual-bobbin transformers are preferable to toroidal types since their bandwidth is considerably morelimited, which helps attenuate noise on the AC power line. Rick Miller supplied measurements illus-trating this for an article I authored back in 1995 [2]. An Avel-Lindberg D-3022 toroidal transformerhe measured was virtually flat to 200kHz, whereas a Magnetek FD7-36 dual-bobbin type was 34dBdown at this frequency (such characteristics are not unique to these manufacturers, but are typicalof these transformer types, regardless of brand). The disadvantage of dual-bobbin transformers isthat they radiate a greater hum field than toroidals, making an external supply mandatory. To en-sure the lowest noise levels on the raw DC rails, the rectifiers are high-speed, soft-recovery HexFredtypes with R/C snubbers, and common-mode chokes are placed between bridges and the raw filtercapacitors. The relay power supply has its own transformer, rectifiers and raw filtering, also housedin the external chassis.

The Neutrik powerCON connectors would be an obvious choice for the interconnectbetween the external supply and the preamp chassis. Unfortunately the powerCONs have only threeconductors – I needed four because of the separate relay supply. Neutrik also manufactures a con-nector for loudspeakers called the speakON, made with four conductors, which seemed a logical so-lution to my problem. However, Neutrik notes the following on their web site: “speakON is NOT to beused as an AC mains or power supply connector!” This seems very strange, since the construction,voltage and current ratings of the speakONs are similar to the powerCONs, The speakONs are ratedat 30A RMS continuous, 250V, with a peak dielectric strength of 4kV DC. Puzzled about their warn-ing about power supply usage, I contacted Neutrik America to inquire about using the speakON con-nectors for supply connections, and received the following response from Fred Morgenstern, ProductManager: “Power connectors require an extensive, lengthy, and costly recognition process by the ULin the United States and similar safety rating agencies in other countries. Neutrik has never submit-ted speakON for such recognition for power applications. Thus, it is not recognized by these agen-cies for use in power applications. A further consequence is that products that use speakON for theirpower connections cannot be UL approved.” So, it really boils down to the fact the Neutrik did not

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Gary Galo

Photo S-1: Inside view of the external power supply. Themain 130VA dual bobbin transformer is in the center,with rectifiers and raw filter capacitors on the left. Thetoroidal relay power transformer and rectifier is on theright.

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wish to go through the process of obtaining approval for the speakONs for power supply connec-tions, which is understandable. Given the voltage and current ratings of the speakONs, and the factthe UL approval for my hand-built prototype was not an issue, I had no reservations about usingthem in this low-voltage, low-current application (I emphatically do not recommend, under any cir-cumstances, using the speakON as an AC mains connector!). For my power supply interconnect, I usethe Neutrik speakON connectors with Mogami W2921, a 4-conductor, 14 AWG cable.

The external supply has no power switch. I control power with an AC power line conditioner, but anAC switch could certainly be added to the supply. I do not recommend leaving this preamp, or anyother piece of audio equipment, powered all the time.

RegulatorsI am a firm believer in regulated power supplies in high-performance audio equipment. For the pro-totype Archival Preamplifier, I chose three-terminal, adjustable regulators with low-ESR capacitors,based on a supply I described in Audio Amateur back in 1990 [3]. The regulators are shown in FigureS-2 and Photo S-2. I set the rail voltages at ±16VDC, a little over 10% below the absolute maximumratings of most of the devices used in the preamp, to ensure long component life.

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An Archival Phono Preamplifier

TO LINE STAGE REGULATOR BOARD (IDENTICAL TO ABOVE)

PHONO PREAMP/EQ REGULATOR BOARD (USE IDENTICAL REGULATOR FOR LINE AND OUTPUT STAGES)

IC901 / 902 / 903 PIN-OUT

O

1 2 3

NEUTRIK NL4MP

RELAY SUPPLY REGULATOR BOARD

3

2

1

32

1 2W

25V

(ADJ) 63V

3 2

1

2W

25V

(ADJ)

63V

IC901 & 902 REQUIRE HEAT SINKS

IC903 REQUIRES HEAT SINKD921 1N4002

D9221N4002

D923LED2mA

+12VDC

C9241uF

+ C923470uF

C9221uF

+ C921470uF

IN

COM

OUTor L7812CV

IC903LM340T-12

-25VDC GROUND +25VDC

IN

COM

OUT

IC901LT1085CT

+

63v

C9014700uF + C905

120uF

D9011N4002

D9051N4002

+ C9094700uF

D9031N4002

+16.0VDC

D907LED2mA

C9031uF C907

1uF

C9111uF

+

63V

C9024700uF

IN

COM

OUT

IC902LT1033CT

+ C906120uF

D9021N4002

D9061N4002

+ C9104700uF

D9041N4002

-16.0VDC

C9041uF C912

1uF

C9081uF

+18VDC

-25VDC

GROUND

+25VDC

R9218.06k

R90311.3k

R9011k

R9051.2k

R90411.3k

R9021k

R9061.2k

R90722.1K

Figure S-2: The main supply regulators are built around Linear Technology LT1085CT and LT1033CT 3A, 3-terminaladjustable regulators. Separate regulators are used for the phono preamp/equalizer, and the line stage and output

line amplifier. The relay supply has its own regulator based on an LM340T-12.

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For the best possible performance, Walt Jung’s Im-proved Positive/Negative Regulators still set the stan-dard for preamplifier supply regulation [4]. I usedseparate regulators for the phono preamp/equal-izer and the line stage and line amplifier.

The relay supply should have its own regulator. A fixed LM340T-12 or 7812A type will be fine. If therelay supply is left unregulated, sawtooth ripple will appear on all of the relay supply lines, with boththe ripple and DC level changing with loading, as relays are switched on and off. To keep hum levelsas low as possible, the only AC that should appear on the audio boards is the audio itself – all supplyrails should be ripple-free. The audio boards also have separate relay ground buses that connect tothe main system ground only at the star, near the raw DC supply input.

References1. Galo, Gary. “GFP-565 Preamp Follow-Up Mod” in audioXpress, Dec. 2004, pp. 26-33.

2. Galo, Gary. “Regulators for High-Performance Audio,” Part 4 in Audio Amateur, 4/1995, pp. 34-43. Downloadable at: http://waltsblog.waltjung.org/?page_id=533 , along with the remainingparts by Walt Jung and Jan Didden.

3. Galo, Gary. “Preamp Power Supply” in Audio Amateur, Issue 4/90, pp. 47-48.

4. Jung, Walt. “Improved Positive/Negative Regulators” in Audio Electronics, Issue 4, 2000, pp. 8-19.Downloadable at: http://waltsblog.waltjung.org/?page_id=533

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Gary Galo

Photo S-2: One of two main power supply regulatorsbased on LT1085CT and LT1033CT 3-terminal adjustableIC regulators.

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