#1 Dieselpunk Preamp of Doom and the Beast with Two Backs
Posted: Sat Dec 10, 2016 6:51 pm
So the only DIY I've ever done up to this point has been building gear from kits, the first of which was the WAD 2A3PSE over a decade ago. After this came two Transcendent OTL kits (T16 and Son of Beast, the latter of which is still in use) and I think there was a Bottlehead phono stage in there somewhere as well.
This new project moves on from that, the layout and chassis are my own design, but in some ways it's still a 'kit' build, in that the circuit designs are not my own. However, I'm hoping the build log will still be of interest.
For the preamp, I wanted to go balanced along with full remote control, so I have decided on a solid state design from Ti Kan of AMB labs. His 'kits' are very well regarded but also quite demanding, in that he pretty much only supplies the circuit boards and the 'Volumaster' software chip for running the relay-based attenuator and input selector - the upside of that is that the boards are very flexible and configurable to what you want. I will be building the α24 fully-differential line amplifier, to be fed by the LCDuino-1 / δ1 / δ2 relay based attenuator and source selection system. This will give a true balanced output to the power amps to be located near the speakers, in addition it will supply a 12V 'turn on' voltage to power relays in the power amps, enabling the whole system to be powered on and off remotely.
I've designed the preamp with quite a 'retro' style to match my much-prized 301 turntable. Here are a couple of CAD pics of the planned layout:
This one shows the front panel:
And this shows the internal layout plan:
Top right is the α24 preamp circuit boards, to the left of those are the stack of δ1 / δ2 attenuator and source selection boards for balanced operation. There are 3 toroids along the left - one to supply 5V 'always on' to the control system and solid state relay (orange), one switched 12V to supply the VU meter driver circuit, backlights and power amp switching relays (blue) and one to supply +/-10V regulated to the preamp boards. The associated power supply boards are along the front, rightmost near the pot is the buffer for the VU meters, fed by the unbalanced output from the preamp. The idea being to align the 0dB point on the meters with 30W or so output, so that we can easily see how close we are to clipping / speaker overdrive.
Sifam VU meters and RCA control knob have been purchased:
The VU meter buffer comes from JLM Audio in Oz:
And here are the AMB circuit boards that arrived this week, beautifully made:
Moving on to the power amps, a hell of a lot of thought has been taking place around where I wanted to go with these, but in the end I have decided to build the Transcendent 'Beast' amps, although with 12 EL509's per side rather than 16. After having heard my 'Son of Beast' give a pretty good account of itself at Owston, and later demonstrate a wonderful synergy with Steve's Metronomes (we're planning to get our own pair of those as well), I decided that I didn't want to lose the unique clarity and transparency that OTL's have (plus it removes the requirement to get all spendy on OPT's). The EL509 output stage of the full on 'Beast' is very similar to the 'Son', just with more valves in parallel, and a meatier driver stage. Bruce sells the PCB's and plans for a very reasonable price, and I now have them - again, very well made:
The full 16 valver occupies a chassis of 17" by 17" and that was too deep to fit on a bookshelf. However there is the option to run it with fewer output valves, and I worked out that if I rotated Bruce's layout around by 90 degrees and limited the build to 12 valves (still good for 75 watts which is plenty), the chassis depth could be reduced to a shade over a foot. So they'll look like this:
Meredith quite rightly pointed out that they should be mirror-imaged, and it took me some time to work out how to do that when the circuit boards, and therefore the placement of the B9A input/driver stage valves, were *not* mirror imaged. It then occurred to me that I could cover the (pretty ugly) power toroids, B9A valves and fuse holders / status LED's to achieve the desired effect. The curve on the top was inspired by the Atma-Sphere amps. Here's a picture with the covers transparent:
And the internal layout showing the smallest toriod inside the case:
There will need to be an input / isolating transformer to handle the balanced line feed, and a power relay for remote on/off switching.
Currently I have a spreadsheet with all the parts listed out (there are lots), which I'll use to build orders from various suppliers based on the best price. When will the actual build start? Dunno, but soon-ish I hope!
This new project moves on from that, the layout and chassis are my own design, but in some ways it's still a 'kit' build, in that the circuit designs are not my own. However, I'm hoping the build log will still be of interest.
For the preamp, I wanted to go balanced along with full remote control, so I have decided on a solid state design from Ti Kan of AMB labs. His 'kits' are very well regarded but also quite demanding, in that he pretty much only supplies the circuit boards and the 'Volumaster' software chip for running the relay-based attenuator and input selector - the upside of that is that the boards are very flexible and configurable to what you want. I will be building the α24 fully-differential line amplifier, to be fed by the LCDuino-1 / δ1 / δ2 relay based attenuator and source selection system. This will give a true balanced output to the power amps to be located near the speakers, in addition it will supply a 12V 'turn on' voltage to power relays in the power amps, enabling the whole system to be powered on and off remotely.
I've designed the preamp with quite a 'retro' style to match my much-prized 301 turntable. Here are a couple of CAD pics of the planned layout:
This one shows the front panel:
And this shows the internal layout plan:
Top right is the α24 preamp circuit boards, to the left of those are the stack of δ1 / δ2 attenuator and source selection boards for balanced operation. There are 3 toroids along the left - one to supply 5V 'always on' to the control system and solid state relay (orange), one switched 12V to supply the VU meter driver circuit, backlights and power amp switching relays (blue) and one to supply +/-10V regulated to the preamp boards. The associated power supply boards are along the front, rightmost near the pot is the buffer for the VU meters, fed by the unbalanced output from the preamp. The idea being to align the 0dB point on the meters with 30W or so output, so that we can easily see how close we are to clipping / speaker overdrive.
Sifam VU meters and RCA control knob have been purchased:
The VU meter buffer comes from JLM Audio in Oz:
And here are the AMB circuit boards that arrived this week, beautifully made:
Moving on to the power amps, a hell of a lot of thought has been taking place around where I wanted to go with these, but in the end I have decided to build the Transcendent 'Beast' amps, although with 12 EL509's per side rather than 16. After having heard my 'Son of Beast' give a pretty good account of itself at Owston, and later demonstrate a wonderful synergy with Steve's Metronomes (we're planning to get our own pair of those as well), I decided that I didn't want to lose the unique clarity and transparency that OTL's have (plus it removes the requirement to get all spendy on OPT's). The EL509 output stage of the full on 'Beast' is very similar to the 'Son', just with more valves in parallel, and a meatier driver stage. Bruce sells the PCB's and plans for a very reasonable price, and I now have them - again, very well made:
The full 16 valver occupies a chassis of 17" by 17" and that was too deep to fit on a bookshelf. However there is the option to run it with fewer output valves, and I worked out that if I rotated Bruce's layout around by 90 degrees and limited the build to 12 valves (still good for 75 watts which is plenty), the chassis depth could be reduced to a shade over a foot. So they'll look like this:
Meredith quite rightly pointed out that they should be mirror-imaged, and it took me some time to work out how to do that when the circuit boards, and therefore the placement of the B9A input/driver stage valves, were *not* mirror imaged. It then occurred to me that I could cover the (pretty ugly) power toroids, B9A valves and fuse holders / status LED's to achieve the desired effect. The curve on the top was inspired by the Atma-Sphere amps. Here's a picture with the covers transparent:
And the internal layout showing the smallest toriod inside the case:
There will need to be an input / isolating transformer to handle the balanced line feed, and a power relay for remote on/off switching.
Currently I have a spreadsheet with all the parts listed out (there are lots), which I'll use to build orders from various suppliers based on the best price. When will the actual build start? Dunno, but soon-ish I hope!