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A Passive and Active HiFi PreAmplifier Discussion & Archive.
Click Here to view the Setup and PreAmp Discussion Page.

I was commissioned - back in August 2015 to design, build and supply a Passive PreAmplifier built in a lovely mini Chinese Enclosure to match my Power Amplifiers

9 months in development, this unit is now a superb example of a fully featured old school PASSIVE & ACTIVE PreAmplifier. Lots of features no one else has!!

A Passive PreAmplifier is basically a simple switching system to allow a number of signal sources such as CD Players, Tuners, Cassette Decks etc to feed into a Power Amplifier. The volume going to the loudspeakers to be controlled with an attenuator within the PreAmplifier.
The theory is to have as little as possible capacitance or resistance within the PreAmp so that the purity of sound is not affected.

To match my Power Amplifier Cases I decided to house the PreAmplifier in a lovely Chinese Enclosure:
External dimensions are 210mm(8.25") Wide x 58mm(2.25") High x 180mm(7.25") Deep.
A MINI PreAmplifier that packs a punch!

2205

The 2205 Chinese Case Style

2205

The 2205 Chinese Case Anodised Black Style

Designing a Passive PreAmplifier

Let's think - what do we need in a Passive PreAmplifier?
Well we need say - 4 stereo inputs via RCA phono sockets.

How can we switch these inputs?
2 choices - one of those rotary switch systems - or let's go old school and have a 4 way self cancelling push button switching system.

How are we going to control the volume level?
Lots of choices - one of those 150 - 48 position stepped clicky attenuators - no they are too expensive and VERY posey!
Let's pick a relatively inexpensive yet good quality ganged potentiometer. I defy anyone to tell me (by listening to the music or the signal to noise) which is the 150 CLICKY stepped attenuator and which is the noiseless carbon Pot.

Would it be good to have a Balance control?
Yes of course - you might have your favourite listening chair to one side of the room - so a Balance control is a must.

Well - is it worth making like this? I've got similar control systems in the workshop!
There is so much room left in the Chinese Case as well!!

What is required?

Here it is - a Passive PreAmplifier! Complicated Eh?

OK - as this unit is especially for Hi-Fi Buffs - let's add a TAPE IN and OUT. The Tape function needs another switch to allow SOURCE and TAPE monitoring.

What is required?

Good facility but still lots of room in the Case though!


Designing a Passive PreAmplifier (Continued)

Trying out a prototype - I fell over the usual sensitivity problems associated with HI-FI units and modern Digital Music Sources.
An example is that an old Hi Fi Tuner will output around 180mV per channel and a CD Player will output around 1.2Volts.

Switching between these 2 musical sources nearly broke the speakers as the CD is VERY loud for the same Pre-Amp volume level compared to the Tuner.

Back to the design of a competant PreAmplifier.

Passive PreAmplifiers are renowned for supposedly giving no colouration to the signals running through from input to output.

Lots of VERY expensive units have the 150 stepped Attenuator which is, in practice, a bank of resistors built into a fancy casing with some Gold contact wipers and the 48 heavy duty Gold contacts connecting the resistors to the wipers in whatever position it is in. A Clicky Expensive thing indeed!!

In the same veign - the simple high quality carbon potentiomer produces the same result of attenuating the signal to the level you want to feed to the Power Amplifier. We have already decided to use the relatively inexpensive Carbon Potentiometer - so why don't we give the user a facility to tune each input of the PreAmplifier to the signal source in use?

Some extra control pots inside the unit will allow each channel of each input to be changed so that all inputs are at the same level.
Hence the prototype and future production now has customer adjustable input trimmers for all channels.

Input Trimmers

Resistive colouration to the music? I think NOT! Just some trimmers to change the individual input volumes - Just like the 150 attenuator.

Passive & Active?

There is still lots of room in the Chinese Casing - the prototype Passive PreAmplifier is working very well - sounds GREAT - yet it needs more!

Trawling through the Internet I could not find many Passive and Active PreAmps. In fact NONE of note!

Why not use the space available inside the enclosure to include an Active PreAmplifier as a switched option?

Extra switch on the front to say ACTIVE or PASSIVE, from the old school a transistorised Mullard circuit with Bass and Treble Controls born in 1953. A lovely sounding analogue PreAmplifier!
Tried and tested - this is how PreAmps were in the 60's, 70's and onwards. I built loads of them during the 70's.

Generating the Gerber Files via Cad Software for the PreAmp circuit board, the MKI now has the Mullard Active PreAmp embedded - yet still more room on the board - shall we put another Mullard (1953) tried and tested transistorised RIAA equalised magnetic cartridge input option in as well - WHY NOT?

The difference in the design of the ReelAudio RIAA facility is that all the main inputs can be fed by a Magnetic Cartridge Turntable - the switch controlling RIAA or Normal is connected after the 4 input push button controls - thus a Turntable 4 way comparitor if you need one!

In Active mode the PreAmp needs a Power Supply. Design a PSU Board that will also fit in the Chinese Casing. Send away for the boards to be made.

MK1 Boards MK1 Boards MK1 Boards MK1 Boards
Note the RIAA EQ ON/OFF switch on the rear of the MK1 board. Not user friendly at all!!!

The MK1 also had expensive Gold Phono Connectors which had to be wired to the main board - bit of a mess!

Back to the CAD software and redesign the board layout - let's also include the PSU on the main board (much larger) with PCB mount Phono input and output connectors to save noise. The MK2 is Born!


The MK2 & MK3 Circuit Boards

The MK2 Main Board now has the PCB mount Phono connectors, an inclusive Power Supply, the RIAA switch on the front panel - lovely.

After construction of the MK2 Boards - I discovered a tracking fault on the Tape In/Out circuitry. DOH!
Re-Design and then get the Circuit Board Company to make some MK3s!!

The MK1 and MK2 Boards now have sticky rubber feet on the base and are used as give away mouse mats and coasters!

The MK3 Main Boards arrive - and everything works! Passive is great - Active just has a slight hum niggle if the BASS control is full on.

The integral PSU is at fault - too close to the rest of the PreAmp tracks. Screening and all sorts of trials could not fix the hum.

MK3 Boards MK3 Boards MK3 Boards MK3 Boards

The MK4 & MK5 Change Version Circuit Board

With the MK3 working properly - yet with a hum issue in ACTIVE mode - a suggestion for a headphone facility in Active mode side-track the development slightly. Another Circuit Board can just fit in - so the MK4 change version was born with a Headphone drive.

You will notice now that the old Mains transformer has gone - an external DC PSU connected via a 2.5mm ROCO plug connector has replaced it!
ALL HUMs GONE!! ............HURRAH!!

MK4 Boards
RIAA and Headphone Boards now shown
RIAA Board
RIAA EQ Plug In Board shown

Oh, by the way, the later boards have a very important additional set of trimmer controls [b] (shown below) - these adjust the sensitivity of the ACTIVE side of the PreAmplifier so you can match the PASSIVE levels you have already set.
Therefore once you set the input sensitivities to match the input sources in PASSIVE mode [a] you can then adjust the ACTIVE left and right hand input trimmers so the the ACTIVE and PASSIVE Volume levels are the same! Good Stuff EH?

General Layout
The Pots shown are (in order) VOLUME, BALANCE, TREBLE, BASS. Tone controls operate in Active mode only.
Active Input Adjusters [b]
Active Input Sensitivity Adjusters [b]
Headphone Amplifier Board
Notice those extra wires at the rear? See below.
Almost There
It is almost there!!!

The extra wires on the rear:
Another suggestion from a potential client is to have a BI-Amp option!! Use the Tape Output for either TAPE or via a switch - Feed another Power Amplifier from the standard Variable output.
So a mini toggle switch is now on the rear which gives this facility.
Using the current stocks of main circuit boards this facility will be wired.
The MK6 issue of the circuit board will have this and the ROCO DC connector installed directly (no more extra wires).

On Board Bi-Amp/Tape switch
The MK6 Board with On Board Bi-Amp/Tape switch and Roco DC Connector. Oh and the Headphone Amp is now plugin too!!

Setup Instructions (Inside the Lid)

Setup Instructions

Production MK5 Units pre-ordered
Here we have some MK5 units in Silver Fronts with the new Toggle Switch BIAmp/Tape Output option installed.
Use the Tape Output for either TAPE or via a switch - Feed another Power Amplifier from the standard Variable output.
Silver Front MK5
Silver Front MK5
Silver Front MK5
Silver Front MK5
Silver Front MK5
Production MK5 Black Anodised Units pre-ordered
Black Front MK5
Black  Front MK5
CNC work etched on the rear (shown) is no longer an option as it takes 2.5 hours per panel.
The Black Tape with white print is olde worlde and quite effective.
Latest MK5 Production Rear

Special Build for a client with the Headphone Socket on the rear.

Black Front MK5
Black  Front MK5

Latest Production Units MK5 with Turntable Earthing Post and Bi-Amp option included.

Note the latest CNC'd front panels have my Logo Ident - "Baldwin PA2205"

Black Front MK5
Black Rear MK5

Warranty:

The PreAmplifier is guaranteed for a period of 12 months from the date of purchase.
Normal Usage is implied by the word 'Guarantee' and 'Warranty'.

Click Here to view the Products & Pricing Page.

All Warranty repairs are undertaken on a return to base principle.
Any carriage costs are the responsibility of the Client

REELAUDIO MAKES NO WARRANTY, EITHER EXPRESS OR IMPLIED WITH RESPECT TO ANY PRODUCT, AND SPECIFICALLY DISCLAIMS ALL OTHER WARRANTIES, INCLUDING, WITHOUT LIMITATION, WARRANTIES FOR MERCHANTABILITY, NON-INFRINGEMENT AND FITNESS FOR ANY PARTICULAR PURPOSE. REELAUDIOS' SOLE OBLIGATION AND LIABILITY FOR PRODUCT DEFECTS SHALL BE, AT REELAUDIOS' OPTION, TO REPLACE SUCH DEFECTIVE PRODUCT OR REFUND TO BUYER THE AMOUNT PAID BY BUYER THEREFOR. IN NO EVENT SHALL REELAUDIOS' LIABILITY EXCEED THE BUYER'S PURCHASE PRICE. THE FOREGOING REMEDY SHALL BE SUBJECT TO BUYER'S WRITTEN NOTIFICATION OF DEFECT AND RETURN OF THE DEFECTIVE PRODUCT WITHIN NINETY (90) DAYS OF PURCHASE. THE FOREGOING REMEDY DOES NOT APPLY TO PRODUCTS THAT HAVE BEEN SUBJECTED TO MISUSE (INCLUDING WITHOUT LIMITATION STATIC DISCHARGE), NEGLECT, ACCIDENT OR MODIFICATION, OR TO PRODUCTS THAT HAVE BEEN SOLDERED OR ALTERED DURING ASSEMBLY, OR ARE OTHERWISE NOT CAPABLE OF BEING TESTED, OR IF DAMAGE OCCURS AS A RESULT OF THE FAILURE OF BUYER TO FOLLOW SPECIFIC INSTRUCTIONS.

IN NO EVENT SHALL REELAUDIO BE LIABLE TO THE BUYER OR TO ANY THIRD PARTY FOR ANY INDIRECT, INCIDENTAL, SPECIAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES (INCLUDING WITHOUT LIMITATION LOST PROFITS, LOST SAVINGS, OR LOSS OF BUSINESS OPPORTUNITY) ARISING OUT OF OR RELATING TO ANY PRODUCT OR SERVICE PROVIDED OR TO BE PROVIDED BY REELAUDIO OR THE USE OR INABILITY TO USE THE SAME, EVEN IF REELAUDIO HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

Units are Designed and Hand Built by:
Paul Baldwin
www.ReelAudio.Co.UK
12 Tack Lee Road
Yapton
Arundel
West Sussex
BN18 0HB
neverfinished0@hotmail.com
Tel: 01243 553231
Mob: 07858 089421

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