The Compactron Line Preamplifier - part 1

Hi!

A Compactron Line preamplifier is in the making. Here some initial fotos.

This is a line only version of the previously introduced Compactron Preamplifier. It uses the same chassis style with brushed aluminum front and backplates and sandblasted top plate with my name embossed.

This time anodised in black, which was chosen by the customer who commissioned this preamplifier. He also decided the arrangement of the knobs on the front, features like the phase switch and of course the number and type of inputs.

The preamp will have one balanced and three RCA inputs.

Outputs are present in both balanced and SE fashion.

The chassis plates arrived and construction can begin.

The circuit is transformer coupled and volume control is done via a 24 step AVCs (volume control transformer) for low output impedance. This could also be used to drive high impedance headphones (300Ohm or more) but the customer did not require a headphone jack.

Black anodised knobs have been made for this preamp. But other colors can be chosen as well for some contrast:

Or stainless steel:

The internal sub modules are already assembled:

 

Power supply with two 6BE3 rectifier tubes and the signal tubes are two 6FJ7. The latter have been chosen for the required gain. Such a preamp can be ordered with different gains and also with different features like head phone jack, tape loop, etc.

Contact me if you want your own Compactron preamplifier customised to your needs.

Stay tuned for an update when the assembly of this preamp is finished.

Best regards

Thomas

The Compactron Preamplifier

Hi!

In February I showed the prototype build of my new Compactron preamplifier concept. I am happy to report that it is finished in the series production chassis.

The prototype measured a whopping 65cm in depth. Clearly too much for most racks or side boards. So a lot of effort, layout and planning was needed to cut that number down. While the result still is not exactly small, it measures 434mm by 434mm now. Height is 100mm plus feet. Here is the final product:

Let me give some background about the motivation of this preamplifier which is the first of a series of different preamps I am going to offer. Many of my customers are vinyl lovers who have multiple turntables or a turntable with multiple tonearms and cartridges. So I have often been confronted with the request to allow multiple inputs and possibility to use both MM and MC cartridges. My D3a phono preamplifiers are simplistic designs optimised for shortest possible signal path. For that reason the MC step up is always integrated in those. In addition the D3a has rather high input capacitance which is no problem for MC cartridges but could be too much for certain MMs. 

So I decided to design a new circuit based on different tubes which offers the possibility to use MMs as well. At the same time I addressed the possibility to adjust input impedance and capacitance via switches on the front. Another criticism I receive often is the sheer size of my 10Y/D3a preamplifier combo which consists of 4 chassis total. So the result is a full function preamplifier with line and phono inputs. And the power supply is integrated as well. All in one single chassis. Here a peek into the chassis when it was fully assembled:

My biggest concern was to get it hum free with a sensitive phono section and power supply all in one chassis. To address that the signal tubes are all located at the back side close to the inputs. All switches are located close to the tubes for shortest signal path. Only the power switch and the volume control switch are directly behind the front plate. For the latter this is not critical because I decided to build this unit with a maxed out line section with transformer output and transformer volume control. The volume control receives a very low impedance signal which is not prone to hum pick up. The power supply is located at the front and I could keep tube rectification and choke input filtering. the latter even for the heater voltage.

I am happy to report that the unit is not just low hum but hum free. Even without using lesser quality toroidal transformers which I already got made just in case. I could use the same high quality power transformer as in my other preamps. Above a photo half in-between construction. Here you can see the horizontally mounted tubes which give the preamplifier its name: All tubes in this unit are 12-pin Compactron tubes. The tube lineup is: 6FM7 and 6FJ7 in the signal section and 6BE3 in the power supply.

Another highlight: I managed to avoid electrolytic caps in the high voltage supply alltogether. The only electrolytics are 2 EPCOS caps in the heater supply. And yet another feature for users of many different cartridges: The gain of the phono section is selectable in 4 steps. These are the gain numbers (total gain at line out):

low gain setting: MM 42dB, MC 60dB

medium gain: MM 48dB, MC 66dB

high gain: MM 54dB, MC 72dB

maximum gain: MM 60dB, MC 78dB

You asked for all the flexibility and adjustability, here you have it.

The gain switch was a late addition and it is a bit crammed into a corner on the backside. It could also be located at the front if preferred. This switch is not just selecting different voltage dividers but changes the actual gain of different stages by adding bypass caps to the cathode resistors.

Above the back panel. Gain selector on the left, then outputs (XLR and RCA). Next a Balanced (XLR) input followed by single ended inputs. A ground lift switch in case of a ground loop in the system. Then 2 MM inputs and 2 MC inputs selectable via a front knob. The MC impedance selector has a setting marked custom. This can be used if a more specific load resistance is desired. In that case the input can be loaded with RCA plugs with resistors soldered in. These plugs go into the right most jacks. Next the ground post, mains inlet and fuse holders.

Nowadays I see many amplifiers and preamplifiers with a lot of gimmicks on the front, with nicks, crevices or waves. But I wanted something simple and discreet. Just function. A bit of a reminiscence to classic gear from the 1980ies. The result is this preamp 25kg of pure analog delight. No PCBs, no microcontrollers, no LCD display, no relays. All hand wired.

As mentioned this is just the first of a series of Compactron preamps. If you are interested in something like this, you can specify the features you want depending on your need and budget. There will also be phono and line only versions. Above the layout for a simple phono without any adjustability. Or a phono only with the same functionality as above minus the line section:

Here shown with the gain switch moved to the front. Possible options for a phono unit are transformer coupled output or just RC coupled output. Also more inputs can be added and even several different pairs of selectable built in MC step ups. Or variable EQ to allow proper playback of historic records:

Or MM only:

Want it all? Can do, just need to double the height:

Or just a simple line stage only with input selector and volume control:

The linestage can be with transformer coupled outputs with volume control or a simpler RC coupled unit.

Or need more line features like Mode select, phase switch and tape loop?

As you can see the possibilities to customise are endless. Also headphone amps can be built. Hope you understand that it is impossible to list all features that could be done and calculate their price. If interested drop me an email with a list of your requirements and I will send a quote. As an indication prices for simple phono or line only versions will start at around 5000 Euros (EU price including VAT, 4200 export).

Best regards

Thomas

The New D3a Phono Stage - Part 4

Hi!

The D3a LCR Phono Preamplifier in the new chassis is now completed with it's power supply.

 

The power supply chassis is basically the same as the actual preamp, 434mm wide, 434mm deep and 100mm high plus feet.

The new chassis are designed and laid our such that the phono can be stacked directly on the power supply.

In part 3 I showed some of the construction steps and inside photos of the preamp itself. Hear some views of the power supply.

The inside almost complete:

The rectifier tubes are on  a separate plate:

In  this case I chose 6BE3 TV damper tubes for their lower height.

The plate with the rectifier tubes is placed under the opening .

This allows the user to easily access the rectifier tubes by removing the square lid.

This is done in the same way as for the signal tubes.

The chassis shown is still a prototype. New metal plates are made in brushed aluminum for a more refined look. Different woods can be chosen for the sides and also a full metal option is available. 

On the photos there are simple rubber feet mounted. The feet are easily replaceable and can be exchanged for some pointe feet which I got made in stainless steel.

The existing portrait style chassis with tubes and transformer covers on top will continue to be offered. Drop me an email if you are interested in a D3a phono in this new look. A matching 10Y line stage is in the works too.

Best regards

Thomas

The 6KN6 Mono Amplifiers - Part 2

Hi!

The 6KN6 Mono Amps are finished. In this post I will share the selection process for the driver tube and show photos of the finished amps.

To reach an output power of about 10W I selected an operating point at the maximum plate dissipation of the 6KN6 with about -100V grid bias. That requires a voltage swing of 200V peak to peak from the driver. As usual I wanted the driver to be interstage transformer coupled. This requires a tube with low enough plate resistance. But that usually comes with rather low gain. Especially since I want my driver to have large headroom which excludes choices like the D3a or 417A. Also just for consistency and style I wanted all tubes to use the 12 pin compactron base.

I first looked at the 6FM7 which has a low mu section with very low plate resistance and a high mu section. However the amplification factor of only 5.5 is very low and would mean a 3 stage circuit is necessary to get enough gain. I wanted the option to stay with 2 stages only. The 6AK10 would fit the bill especially with all triodes wired in parallel. However the 6AK10 is not linear enough for such large voltage swings to my taste.

I finally ended up with the 6FJ7, a compactron equivalent of the 6DN7. The 6FJ7 is a dissimilar double triode. The low plate resistance section has an amplification factor of about 15 at 2kOhm plate resistance. It is very linear and can deliver large voltage swings. But a gain of 15x is still not quite enough to get a usable input sensitivity. There are two possible choices to solve this. Add an input transformer with a step up to reach the desired sensitivity or add another gain stage in front of it. The other triode section in the 6FJ7 has an amplification factor of 22. As such a bit much but RC coupled and with unbypassed cathode resistor, about half of that would remain.

After tossing both variants back and forth in my mind, I decided to use both and a switchable set up. A direct input which drives the lower mu section through a step up transformer, making this a pure power amplifier and the higher mu section RC coupled with volume control potentiometer in front of it to make this an integrated amplifier.

This offers the possibility to start with the amp as integrated and later upgrade it with a preamplifier like my 6AH4 linestage or even a 10Y line stage. This would also give an easy way to compare the sound of such a line preamplifier with transformer volume control against a simple resistive potentiometer and RC coupled stage. Just with the throw of a switch.

With the use of a 1:4 step up input transformer the power amp direct input needs about 1V RMS to reach full output power. The input section measured at about 12x gain, close to the predicted half of the amplification factor. So about 300mV sensitivity for the high gain input which gives enough gain headroom for low volume sources.

Since the amps are built as monos the separate volume controls provide an easy way to adjust balance as well if needed. Since I used stepped attenuators with defined positions it is easy to adjust the left and right volume at the same levels.

So here they are, the finished 6KN6 integrated mono amplifiers easily switchable to power amp configuration. Since I often get requests for integrated amplifiers I decided to build them this way also to have the possibility to demo the sonic benefits of a separate linestage.

To complete the discussion of the tube line up, the choice for the rectifiers was easy since there are plenty 12 pin compactron TV dampers. I decided to use the 6BE3.

In part 1 about these amps I showed the initial construction steps. Here the top plate completely wired:

All chokes and the interstage transformer are mounted on a separate plate:

The completely assembled amplifier:

And with the bottom cover mounted:

Some pictures of the amp in operation:

Some more views:

This was a fun project which shows how many possibilities there are to build an amplifier with uncommon tubes.

I will probably bring these amplifiers to the High End show in Munich next year. Come to the show to listen to these amps or visit me in Lindau for a demo.

Best regards

Thomas