Jun 21, 2018

LCR Phono preamp Part 3

Power supply

A good sounding preamp or amp always means a well calculated power supply, this is the circuit heart. Too often neglected or subject to approximations because calculation is time consuming and most hobbyist do not evaluate the real impact on music restitution. A supply is NOT an oversized transformer plus a huge amount of filtering capacitors.

Tech corner with useful reminders about time constant and ripple attenuation.

1- Time constant
Picture from the Net. Courtesy of Landr Journal
To understand the great importance of time constant we first need basic information about audio signals. These are totally different of the sin waves we use to measure power, distortion or phase in our amp and preamp.
An audio signal is a very complex association of a fundamental and many harmonics of different amplitude and duration. For our supply it is a succession of non-symmetrical transients that form the signal envelope or ADSR of music. To make it short we must deal with attack transients that are in the 15 to 20 milliseconds for the fastest instruments (piano, guitar, drums), which means the supply must recover in that interval of time before the next attack.
The first LC or RC cell time constant should not exceed that value.
In the real life it can't be that precise due to the components tolerance, especially electrolytic capacitors, thus it is a safe practice to calculate a first TC not more than 10 milliseconds (but not under 5 milliseconds or sound becomes harsh).
Note: For proper transient response, TC should increase after each decoupling cell.

2- Ripple
For this preamp I choose a CLC filter.
Ripple with an input capacitor depends upon C, total current in circuit and mains frequency. It can be appreciated by the relation

                                                          Vr = Itot / C x f

Itot Total current in circuit (Ampere), C input capacitor (Farad), f ripple frequency (twice the mains with a full wave rectifier)

3- Smoothing cells
LC and RC networks are voltage dividers for the AC current that comes from the rectifier. They are necessary to smooth out the ripple.

Let's calculate the preamp supply step by step.

I have a 240V / 320V_0_320V power transformer followed by a GZ32 rectifier and a 24µF C1 capacitor in a circuit drawing 60mA @ 375V.

                                Vr1 will be 60 10-3 / 24 10-6 x 100 = 25V or 6,6%

Time constant depends upon C and Rs the total series resistance. To calculate Rs we must:
1- measure primary and half secondary (for a full wave rectifier) transformer winding resistance (respectively 12Ω and 120Ω) to determine the total equivalent resistance Req
2- calculate transformer voltage ratio n = Vs/Ve = 320/240 = 1,333
3- calculate Req = Rsec + n2 Rprim = 120 + (1,777 x 12) = 140Ω
4- determine rectifier internal resistance Ri (~260Ω)            O.H. Schade method
5- calculate Rs = Req + Ri = 400Ω
6- calculate TC = Rs x C1 = 400 x 24 10-6 = 10 milliseconds

Note: a GZ34 rectifier with its lower internal resistance (~100Ω) will give a
6 milliseconds TC, on the other hand a GZ37 (~500Ω) something around 18 milliseconds. This to say that changing the rectifier will affect both TC ratio and DC voltage, reason why sound also changes (dry and bright with a GZ34, warm and mellow with a GZ37). Too many people just make a plug, play and listen test without considering this factor and recalculating the whole supply as it should be done.

a first cell (L = 10H/160Ω and C2 = 100µF) makes a voltage divider where

                                             Vr2 = Vr1 ZC2 / ZL + ZC2

      ZC2 = 1 / 2 π f C2 (~ 16Ω @ 100Hz)             ZL = 2 π f L (~ 6,3KΩ @ 100Hz)

      Vr2 = 25 x 16 / 6300 = 63,5mV / -75dB and TC = 16 milliseconds

and so on... until I reach at least -150dB ripple rejection and proper TC to feed my preamp.

PSU schematic

Parts and construction

Quality parts are chosen for their reliability and sound qualities. In addition to the MP/MKV and F&T electrolytic I usually favor, the filtering bank will be fitted with excellent vintage SAFCO-TREVOUX 15µF PIO.

These military caps are incredibly rugged, well made and heavy. They are new and fresh as if they just came out of the factory, still in their original 1959 packing. These caps have one of the lowest loss tangent I measured on paper in oil capacitor and give a smooth and clear sound. This upgrade is at the price of a cumbersome supply weighing about 20 lbs (the hermetically sealed transformer could power a 6V6 push pull), hence the two handles to ease manipulation.
Be very careful during wiring, especially pins selection, ground returns and heaters. Remember that input signal is in the same magnitude than heater/cathode leakage noise and it can be a nightmare to get rid of a low frequency buzz.

most of the wiring is solid

Power supply with sub chassis to ease construction

Tubes I used

I have Siemens (Germany) and TFK (Philips Heerlen plant) on hands. See part 2
Tests were conducted with Philips E86C as output tubes.
To say the truth both 88's are great sounding tubes with very detailed music, lot of life and great sound stage. Maybe I will keep the Philips Heerlen that appear to be the quietest tubes... by a very slight margin.

Replaced the Philips E86C (Mullard made) by TFK EC806S and I entered in a different world. The Telefunken's are audibly better by any means. Deep, wide, alive. They are THE tubes to go! Incredible punch with sweetness. Accuracy is astounding. With the Philips the overall sound is very pleasant but by comparison music appears slightly out of focus, like behind a curtain. Amazingly had the same feeling with the E80CC Philips vs Tungsram in the SRPP preamp.
So, I made another test with the 88's and the best association is E88C Heerlen + EC806S TFK, no contest. Perfect balance, articulation and clarity.

Some vinyls for this test, out of my 5000+ LP's collection, mostly jazz