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IC AD 845 preamplifier DIY...notice some of the documentation from 1998 taken from the cellphone
A preamplifier with crystal clear sound based on the IC 845 from Analog Devices
After completing two “do it yourself” tube preamplifiers that went really well, it was time for a preamplifier with silicon transistors. After the tube preamplifiers The “Röh-1” & “The Aurora”, both of which reproduce hifi sound with warm and pleasant overtones, the development in 1998 came to construct a preamplifier with silicon transistors.... or more precisely a collection of transistors in a chip that is referred to as an IC.
Basic
The transistor was invented on December 16, 1947 by, among others, John Bardeen, Walter Brattain and William Shockley at Bell Laboratories, USA. So the transistor is not a new invention.
The first integrated amplifier IC was the μA702, invented by Bob Widlar of Fairchild Semiconductor and released in 1963. Although early op-amps worked, it was not until the μA741, released in 1968, that ICs became widely used.
Development then proceeded rapidly, and in 1975 the TDA1034 chip from Philips was introduced, which was renamed the NE5534 in 1979 from Texas Instruments. The NE5534 quickly became a favorite in professional audio equipment due to its extremely low noise and high bandwidth, which still makes it relevant in audio circuits today.
Before renovation some photos from the 1998 version of the IC 845 preamplifier in connection with the removal of cables and more....
Note that several psu capacitors has been changed after 28 years
After 28 years, the IC AD845 chip is still at 0.2 millivolts DC-out...but the wiring is a bit of a mess.
It could be done better.....
Note the green/red dual LED...green is on...amplifier is "ON"...
First attempt with a flip-flop channel selector ...it worked, because the preamp is completely silent
Everything made by hand which also includes a few small errors which were corrected on the shown A4 pcb after the print was ready
The preamplifier with the 8-pin IC AD 845 has been loaned out for a number of years, but was returned because the amplifier was no longer used.
The IC AD 845 preamp needed cleaning and an update after 28 years, and I was amazed at the transparency that this old preamplifier could deliver. As a designer, I had completely forgotten the sound quality from this IC preamplifier.
First a few pictures to give an overview
Channel selektor and the circuit below
Then follows my own idea to activate the amplifier.
On the back side of the preamp is the mains switch including a Demco approved chassis fuse. When you connect the amplifier with the switch, you only turn on the input transformer which is a 15 - 0 - 15 VAC that provides the +- 15VDC supply to the channel selector.
Now there is light in the red "ON" LED on the left on the amplifier front. If you then press one of the channel selector's 4 push settings, the amplifier's +- 15 VDC supply is activated. This means that there are two supplies in this amplifier.
The amplifier's +- 15 VDC supply is completely independent of the channel selector's psu. It should be noted that no. 2 transformer for the amplifier is an 18 - 0 - 18 VAC which is regulated down to the desired +- 15 VDC supply for the AD 845.
By pressing the channel selector e.g. "CD" turns off the red "Stand by" LED and the next "Warm-up" red LED lights up. After 25 seconds the "Warm-up" LED changes from red to green and the amplifier is ready to play.
It took a while to get everything "playing", and from a cost-saving point of view, it is perhaps not entirely in the spirit of the times to have the first "Stand-by" mains transformer on all day...
But this how the preamp turn-on works
This is part of the idea where point "3" gets signal from the selector, and "4" and "5" is the "ON" led. "1" and "2" disconnect and shuts down the preamp.
The Omron 12 V relay open and closes the preamp power-supply with a push on "1" and "2"
Note that the - 15 VDC supply rail is not used, but this has no effekt on the preamp.
Solder side of the relay delay +- 15 VDC psu for the channel selector
My idea for the power supply for this IC AD 845 preamplifier works great as the "noise level" at the last distortion test was well below 90 dB.
The only thing wrong with this picture is the output voltage.
The 10K trimpots output is +- 15 VDC and not the shown +- 18 VDC
The “new” IC preamplifier with AD845 from 1998 for update in 2026
My “new” construction, in contrast to the “bottles” in the two previous tube projects, was to be an amplifier with a very low output impedance, so the choice fell on the AD845 from Analog, introduced around 1988.
Although the AD845 is designed for general precision and high speed (with a slew rate of 100 V/µs), the AD845 is known in Hi-Fi circles as a high-end upgrade of older circuits.
Very low output impedance means that the typical open-loop output impedance (Output Resistance) on pin 6 of an AD845 from Analog Devices is only 5 Ω.
When short-circuiting the current, the AD845 can typically deliver 50 mA.
Regarding load, the AD845 is specified to drive a parallel load of down to 500 Ω and 100 pF with full performance.
The low output impedance and ability to deliver high current make the AD845 very well suited for driving fast A/D converters and capacitive loads in audio equipment.
How much voltage can the AD845 deliver?
At full output (output voltage swing), an AD845 can deliver a voltage of ±13 V when supplied with the standard operating voltage of ±15 V.
Specifically, the voltage swing on pin 6 of the AD845 at a supply of ±15 V can swing out to approximately 12.5 V to 13.5 V in both the positive and negative directions.
Analog Devices guarantees in the datasheet a minimum swing of ±12 V under load at 500 Ohms
And peak-to-peak this corresponds to a total voltage difference of approximately 26 V p-p at full output without clipping.
At full power bandwidth, the AD845 can deliver a swing of 20 V p-p all the way up to a frequency of 1.75 MHz.
The AD845 op-amp has limitations. This means that it will always lose approximately 1.5 to 2.5 V relative to the supply rails due to the saturation voltage of the internal transistors. If you increase the supply voltage to its maximum limit of ±18 V, the output swing will increase accordingly to approximately ±15.5 V.
However, as I believe, components kept within specifications will cause less errors in the circuit. So for long-term durability, I recommend running the AD845 at a maximum of ±15 VDC. The AD845 has plenty of voltage swing to drive and equip most output amplifiers at ±15 VDC.
As proof of great reliability, we measured a noise level that is 90dB down on an amplifier built in 1998. As proof, this means that the AD845 amplifier chip is particularly durable when used at factory specifications.
Enough about the data
Here is a drawing of the IC AD 845 preamp in the 1998 version which is not quite ready...update to follow
After having built two preamplifiers with tubes which were both very quiet when idle without "hiss" 50-60Hz hum, the idea was to design the entire amplifier in the same way...i.e. one PCB where the complete power supply would also be integrated. But as with all other "do it yourself" equipment, changes are made along the way as the construction progresses.
After the "complete" +- 18 VDC supply and the amplifier with riaa, tape buffer and line stage were laid out, the idea came to start up the amplifier and activate the channel selector with on-off pressure switch (4013BE as bistable flip-flop). This required an extra +- 15VDC supply and since there were more than one mains transformers in stock, the solution was an extra independent mains transformer for the various flip-flop functions.
By "borrowing" a circuit from other previous preamplifier design, I found a simple 4-way switch function based on 4071BE CMOS and 4013BE flip-flop
Without saying too much, various functions in amplifiers are something that I like to have as part of the project, such as red/green LEDs, on-off push button functions or start-up delay function.
With the selector circuit, the push button system was working and the channel selector worked, but the actual start function of the amplifier was missing, and for this purpose, a relay-controlled start-up was added to the external PCB for +-15 VDC using another 4013BE flip-flop.
As can be seen in the complete amplifier diagram, the start-up is delayed via a relay function. This means that the upstart delay is about 25 seconds after the red ”warm-up” LED changes to green ”ready to play".
This is the 25 seconds relay delay circuit
When +15VDC is connected, C1 (47uf) is slowly charged by the resistor R1 (2M2).
Until C1 has a sufficiently high voltage to the base of T1, IC1's (AD741) + plus input is close to 0V.
When +15VDC is connected, a positive voltage is created between R6 (6K8) and R7 (10K) on IC1's – input (minus), and therefore a negative voltage is obtained at IC1's output compared to the voltage at the point between R8 (390R) and R9 (390R).
The current therefore flows through LED D7, which lights up red.
After charging C1, T1 (BC560 PNP) opens after +- 25 sec., which in turn opens T2 (BC550 NPN) and the 24VDC relay is connected.
At the same time as the relay is connected, the positive voltage across IC1's + input (positive) increases. The positive voltage on IC1's + input is now more positive than the positive voltage on IC1's – input.
Therefore, the voltage on IC1's output quickly becomes more positive compared to the point between R8 (390R) and R9 (390R).
D7 (red) turns off, and the current changes direction and turns on D6 (green) which lights up. By changing R1 (2M2) the connection time can be varied. Similarly, changing C1 will give a shorter or longer delay.
If +18 VDC is used the delay time will bee approx. 10 sec.
The red "ON" LED indicates "stand-by" The red "Warm-up" LED show preamp startup
The "Warm-up" LED is now green and ready for play The flat "Feme" relay opens for audio signal
The sound of the IC AD 845 preamplifier
The IC 845 amplifier stands in sonically contrast to my existing tube preamplifiers, that deliver a warm sound character across a wide frequency range with many finely resolved instruments, where this 1998 IC construction is distinguished by a very fine crystal clear detailed sound-neutral reproduction.
The clarity and transparency of the sound image from the IC AD 845 preamplifier can be quite an experience, if you have a pair of muted speakers with not so fast midrange and treble units. The IC AD 845 can also feel like an upgrade of the entire sound system if you own a power amp not so fast .
In any case, it has been a pleasure to recall and listen to this preamplifier again that delivers a fantastically transparent and well-defined sound image.
KJH
After the review of my IC AD845 project from 1998, a series of photos follows that can describe the renovation in 2026 better than words.
Everything in this IC AD 845 preamplifier is handmade....this also applies to the 5 mm thick aluminum front plate that you can see in the various photos. As you can also see, there is gold lettering in old-fashioned italics on the front plate. It says, among other things, "CD", "Tuner" and "Phono", "ON" and more.
35 years ago there was a shop in Copenhagen that sold art supplies such as easels, paints and brushes, but they also had transfer letters on plastic sheets from a company in Japan called "Letraset".
Unfortunately, you can no longer buy transfer letters from "Letraset"
In 2026, you will therefore have to settle for looking at my transfer letters from 1990 with gold printing in italic script.
The End :-))
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