Jan 26, 2017

E80CC SRPP Line preamplifier Part 1

This project was initiated by a request of a line preamp with selectable input sensitivity to match sources with different voltage outputs. This preamp should also have a low output impedance to drive long connecting cables without any loss.
With such goals in mind it became evident that a simple 6J5 preamp would not fit these requirements.

1- The use of two gain control stacked together would greatly impact music transcription.
2- The gain of this preamp would have been too low to accommodate an input voltage divider up to -6dB.

A high gain stage with a voltage divider at input and a volume attenuator at output (which is the best way to control the gain from my humble opinion) was the way to go.

For that purpose there are a lot of good circuits with good linearity and gain, cascode, µ follower, totem pole, push pull. Push pull circuit, seldom used in preamplifier, is very interesting because it cancels all even harmonics. With such a circuit you can expect very low distortion using triodes but a phase splitter (transformer or tube) add complexity to the circuit, increases final cost and is hard to implement in a standard chassis.

One circuit shares this low distortion, high gain capability without the necessity of a splitter stage, the SRPP.

I have been working with for a long time and I know how it works (well), its limitations and drawbacks (very few when correctly understood and used).
Some would argue that it have a special sonic signature, especially on voices, with a kind of emphasis that could be appealing but do not reflects reality.
Believe me, when correctly used and adjusted (a FFT analyzer is of great help) it can be very neutral without being clinical or cold with huge dynamic and stunning resolution and accuracy on complex music.

SRPP (very) short history and use

Originally intended for video use (1943) it was widely used to drive low impedance capacitive loads with high gain and wide bandwidth.

SRPP as it should be set up.

As indicated by its name, the SRPP is a push pull even if not evident at first glance.
I won't bother you with any mathematical demonstration because it is not the main topic and there are very good publications that help to understand this circuit, Audio Xpress The Optimized SRPP and Tube Cad SRPP deconstructed.
Never forget it works like a push pull only when loaded and gain, distortion, bandwidth greatly depend upon this load. Moreover its linearity is a direct relation with the flowing current which itself depends upon load.
The good thing is if you have a well defined load that won't vary with the amplifier connected behind, you can fine tune the circuit to get the lowest distortion playing with only 3 resistors. Reason why I choose to load my SRPP with transformer.

Finding a good one was easy because I had some Tamura on hand.
The TKS20 used in this preamp is a 600/10K line input transformer but can be reversed used as there is no current flowing through primary.
Tamura are among the best transformers I had to work with. Unfortunately most are discontinued and prices skyrocket on the second hand market.

Previously I said that the best way to control gain was the use of an attenuator behind the transformer. I was lucky enough to have some Daven 600 ohm dual T pad, perfect for the purpose.
These pieces of engineering are unbelievably well made with plain silver contacts and sorted resistors for perfect tracking. They were built to last a lifetime. The only disadvantage is a 6dB insertion loss that have to be taken in account when calculating overall gain.
These attenuators are very hard to find today, especially NOS, and it is interesting to build its own T or bridged T attenuator. See Pots & Attenuators tutorial.

Tube choice

To reach my goals I needed a tube that gives a minimum overall gain of 4 (attenuator will divide it by 2). This means a SRPP gain of 16,30 (transformer voltage ratio seen by tubes is 4,08) and a tube with a µ of 20/30, a ρ of 7/12 K to correctly match the 10K dynamic (AC) load.
Double triodes like 12BH7A,13D3,E80CC would be good candidates. In this study it is stated that V1 tubes are identicals.
I finally used the E80CC because I had excellent results in the E140 amplifier and it is easy to source compared to the British 13D3. I made some simulations with the 12BH7A but gain was too low.

More to come...

Jan 11, 2017

Pots and attenuators

In every audio equipment there is a gain or volume control to set music reproduction to an enjoyable level . This control is usually devoted to a simple potentiometer mostly for cost reasons. On pricey materials you can find more elaborated and expensive devices like L pad attenuators but if there are just 2 or 3 resistors in signal path to minimize sonic alteration the reflected impedance constantly vary like in basic potentiometers.

In professional gears, like mixing consoles, pots are very seldom used and signal attenuation is provided by more elaborated units. T pad and bridged T pad are the most widely used.
If they essentially remain voltage dividers these particulars setups are intended to keep a constant impedance whatever the attenuation. Very important when used in 600 ohm lines they can be of real benefits to prevent bandwidth alteration in amateur constructions.

Such attenuators are by far the best way to control signal. Unfortunately good NOS units by Daven, Tech Lab or Langevin are scarce and expensive but it is possible to the average DIY'er to build its own one with the great advantage of choosing the impedance to meet particular loads (I have on hand some excellent transformers with uncommon 900 ohm loading impedance).

They are quite easy to build, you just need a good soldering iron, some quality pliers, a bunch of resistors and plenty of time. I personally use Dale RN60D, Dale RLR7C or Philips/Vishay MRS25 (excellent for the task, easy to find and affordable). I also prefer the bridged T attenuator over the more classic T pad because it is fully symmetric.

pic taken from allaboutcircuits.com

Bridged T calculator greatly eases resistors calculation.

A good rotary switch is essential. Must be of the shorting type (make before brake) and have at least 20 contacts. Some old Siemens are excellent but hard to source.
Fortunately very high quality equivalents have been made in the former East Germany by RFT and are easy to source. They have 24 contacts, silver or palladium on copper. With such a switch you can build a 0 to -40 dB & off / 2 dB by step attenuator.

Step by step construction

First edit a file with impedance load, desired attenuation and resistors values.
For a 600 ohm one with a 2dB step the resistors values are:

RFT rotary switches are the best available choice for the price. Cold war time material, they where intended to work under adverse conditions. Very well constructed they are easy to disassemble and reorganize to suit our purpose.

1- Fully disassemble switch (except rotary mechanism) to access the phenolic wafers.
The silvered ones need some gentle cleaning with a piece of fabric (Never ever use chemicals !)

2- From a 3 wafers unit, set 2 wafers back to back for the R1 series resistors using the small spacers.
These will be populated later. I choose to make the common loop first because it is the trickiest part to do.

3- Make a common (Ground) loop from a silvered copper wire and insert a short fiberglass (or any other insulation material) sleeve to prevent any unwanted contact

4- Common loop hold in place and soldered on contact 1, this is the off position

5- Shunt resistors R2 bent and cut to proper length prior to solder. It helps a lot to solder first the resistor on the opposite contact to the common loop. It makes this one stiffer.

6- Common loop fully populated

7- Now it is very easy to feed the R1 series resistors wafers

All R1 resistors soldered...

8- Reassemble the shunt resistors wafer, add the two Z0 impedance matching resistors and you are done. Takes 8/10 hours for a complete unit, it's worth the effort for a good attenuator.

Any question, feel free to write