I’m giving it a crack... fully DIY built phono preamp

[BinaryMike]

Looking at transformers I’m a little confused.

I think you're looking for something more like: http://www.antekinc.com/as-05t280-50va-280v-transformer/

The 50 VA rating can be thought of as a roughly 50 watt maximum total load capacity, which is excessive, but smaller AnTek products don't have the other features you want. It has two primaries that can be series-connected for 230~240 VAC line voltage, one 90 mA HV secondary with a tap for adjustment, and two nominally 6.3 VAC heater windings. With 240V on the primary and a light load on the 2A-rated heater windings in series, you should have enough headroom for DC heater regulation. The Faraday shield is an important feature for preamp service.

 

[chinacave]

Watching with interest- great + clear explanations.

 

[neevo]

I’m forging ahead at a snails pace. To validate to myself I’m going ahead with the research and interest to build the amp, I’ve purchased some tube sockets as the initial step.

Next up I’ll be working out a layout and seeing what other parts I’ll need. A Mouser order is in my future, plus I’m thinking I’m going to go down the terminal strip route.

I’m still not 100% clear on the transformer and regulated DC supply so lots more reading to do there but I’m going to build a separate chassis for the power supplies so maybe that’s something that I can consider once I’m comfortable with the main amp layout.

The bit I’m trying to get my head around at the moment is how to have the tubes down the middle and then each channel either side. Obviously this means a few wires crossing over the HT feed, so I’m thinking about twisting wires, how to terminate them and which wires need to cross at right angles.

Not there yet. Lot more comprehension required.

 

[kward]

Just a thought--if you go with a separate chassis approach for the power supply, then you gotta worry about how to get the power needed to run the circuits over to the audio circuit chassis. That takes the form of an "umbilical" of sorts to do that. People tend to use Jones plugs for this sort of thing with stranded wire (for flexibility) making up the numerous runs you will need in the umbilical itself.

I did a build this way once (using two chassis and a hefty power/ground umbilical). It worked, but it wasn't the easiest thing I've ever done, and required quite a bit of think-through in the design stage to make sure I had all the right voltages and grounds I needed.

 

[neevo]

I did it on my Little Bear build too. Used an aviation plug but the cable was heavy duty and overkill and it’s not the most flexible thing.

 

[neevo]

I think you're looking for something more like: http://www.antekinc.com/as-05t280-50va-280v-transformer/

The 50 VA rating can be thought of as a roughly 50 watt maximum total load capacity, which is excessive, but smaller AnTek products don't have the other features you want. It has two primaries that can be series-connected for 230~240 VAC line voltage, one 90 mA HV secondary with a tap for adjustment, and two nominally 6.3 VAC heater windings. With 240V on the primary and a light load on the 2A-rated heater windings in series, you should have enough headroom for DC heater regulation. The Faraday shield is an important feature for preamp service.

Gotcha. That’s a lot cheaper than I thought it would be.

 

[triode17]

I’m a competent desolderer and solderer. I’ve spent a lot of time and effort rebuilding a few parts including amps and a couple of preamps based off the Little Bear preamp.

My Little Bear looks beautiful and I’ve had such fun making it, but I long for something quieter and I want the satisfaction of knowing I built it myself. I put a lot of time in to my preamp and even relocated the transformer into a separate chassis but I’m still cursed with hum at a level that’s simply not enough for the resolution of my system, especially with a HOMC and some quieter records where I want to crank the volume a bit. Generally it’s fine, but my system is working so well and sounds so good that “fine” is not good enough anymore.

View attachment 1155798

Rothwell mentioned having a go at point to point wiring in my LB thread so I reached out to him and he was incredibly gracious with his time and knowledge. He mentioned an RCA design (I think that’s what it’s called) from Valve Wizard. They used to make PCB’s but they’re no longer available and to be honest I want to have a crack at the wiring point to point instead.

I’m expecting a huge amount of learning but I’m hoping I can make something reasonable and quiet, especially with some help from the experts here.

I’ve been doing a tonne of reading, checking the schematic and parts list and I think I’m mostly comfortable with it, at least enough to think about how I’d start the process.

I’ll also build a cabinet from scratch too as I’ve done with my friends chassis as I want that level of satisfaction too. 3mm Alu and hardwood ends (the plastic still has to be pulled off the Alu):

View attachment 1155799

If I can get it quiet enough with some general tubes, I’m toying with the idea of buying some really nice Telefunkens

I’ll build the amp with 12AX7’s and an AU7 for the cathode, also based on Rothwells recommendation.

The only part that confuses me is the power supplies. I’m pretty much ok with the HT supply and I’ll hunt down a good quality 300v transformer that I will try and shield with a metal divider in the chassis.

The one that really has me stumped is the heater supply. I’m pretty sure I want to go down the rectified DC path for the heaters to eliminate any potential hum but I’m not clear on the parts required for the DC rectifier and also what sort of transformer is required to properly supply the rectifier, 2x AX7’s and an AU7.

If you are going to isolate the transformer in a separate box, do all the AC to DC conversions there and pipe out the DC to the preamp. This will keep all the AC out of the preamp enclosure. But be sure to add a small electrolytic at the preamp end. I'd use about 1/10th of the final power supply output cap value. i.e. - a 22uF cap on the output, use a 2.2uF and a 0.1 film across it to bolster the impedance and filter any high freq. noise coming across the lines. I am about to do the same with my Steve Bench "12AY7 cascade with 6DJ8 Phono pre." It works well but is too close to the power trans. in my Frankenstein preamp.

 

[Lavane]

I have a phono that uses 12ay7's and their variants. It has a warmer sound compared to ax7's or au7's.

 

[neevo]

@triode17 id assumed that the HT feed would still be in the amp obviously. That’s AC isn’t it? Why is that not a noise risk?

I’d been planning on how to twist those feeds as much as possible and run signal wires across them at right angles as much as possible.

 

[neevo]

Getting my head around the transformer secondaries and heater wiring.

I understand that I can series the 2 x 6.3v secondaries on the transformer to get 12.6v

So with that in mind I’m trying to understand the wiring for the heaters. 12.6v means I need to wire the heaters in series eg hot to pin 4 and ground to pin 5. That’s for a heater for a single tube though. In order to heat all 3 tubes do I daisy chain them all together?

Transformer hot in pin 4, out pin 5 to tube 2 pin 4, out pin 5 to tube 3 pin 4, out pin 5 to transformer ground.

 

[kward]

If they are 12AX7's, you'd wire them in parallel. Meaning all the pin 4s to pos and all the pin 5s to ground.

 

[neevo]

2 x 12AX7 and a 12AU7 cathode follower.

So hot to all pin 4’s, ground to pin 5’s.

Thanks!

BTW @kward love the avatar. I’m a huge fan of Supertramp and it’s also one of my 3 year olds favourites too.

 

[Celadon]

"Psst.. Pass me the popcorn"--
said the Cheshire cat...

 

[neevo]

Yes, because you won't get better performance with using separate high voltage and low voltage transformers. Getting a custom wound EI type to your exact voltage needs with external Faraday shield is not that expensive from Heyboer transformers, although shipping to Australia might be a concern.

A power transformer with a Faraday shield isn't strictly necessary if you are skilled at layout and building. But for the lesser skilled, I think it will greatly assist in getting a quiet phono preamp.



If you use 12V filament tubes, I'd use a 12V three terminal regulator. Like the one AA posted above, or here's a schematic of one I built, as well as a picture of the actual working unit:

View attachment 1156267

For your build, replace those two series wired 6U8 filaments with two 12AX7 filaments wired in parallel. I used a 2940 three terminal regulator because they offer a low dropout voltage, but other types would probably work just as well. For example, Analog Addict depicted essentially the same circuit but with a 7812 regulator.

Front:
View attachment 1156269

Back:
View attachment 1156288

I've been pulling a parts list together for the preamp to get my head around it all. Pulling together the regulated DC parts from your list I have it all together but have a couple of questions:

1. With the setup you recommended, is a twin 6.3v secondary (wired in series) going to be enough to power the DC regulator and heaters?
2. Your circuit connects to ground (beyond the transformer power and ground and heater power and ground). Where is this on your actual part? Is this connected as a chassis ground or am I missing something obvious?

 

[kward]

A twin 6.3V secondary will be enough voltage to allow regulation and power the heaters because the rectified DC voltage as measured at the first filter cap averages about 15V (with some ripple), which allows 3V drop across the regulator, and if using a 2940 or similar low drop out type, will still regulate (i.e., remove ripple) and get you 12V output. In testing I've done with that exact circuit board, there is enough headroom to regulate even if the wall voltage drops to 91% of nominal value. I deemed that pretty safe in my part of the world since I've never seen the voltage delivered by the power company in my area go that low, even on the hottest days when everyone's AC is on full blast. It is always (in my area) no more and no less than about 2% of nominal value. So this was a safe bet for me. But if you are concerned about this, it's better to give yourself a little bit more voltage margin, say a 14 VAC secondary (but not too much more than that since the extra voltage drop is just spilled off as heat inside the regulator chip itself which will require more robust heat sinking).

Additionally, 3x filaments of the 12A*7 variety will draw 0.45A DC total, so the transformer used should have a secondary current rating of at least 1 Amp AC. But I'd up that to a 2 amp AC rated secondary to provide a bit better margin and run the transfomer a bit cooler.

The ground on the circuit board is connected to one prong of one of those two prong green terminal blocks. There are two terminal blocks on that board, with two prongs on each--one terminal block accepts the 12.6V AC input, and one prong of the other terminal block outputs regulated 12 VDC, while the remaining prong is for ground. You need to connect the ground of the power supplies to the audio circuit ground so that all grounds circuits are forced to the same voltage potential (which we call "ground"). You do this by connecting a wire from the ground prong of that terminal block to the circuit ground of the rest of the amp. You can hard wire in the leads if you want and forego the use of terminal blocks. I just decided it would be easier to use terminal blocks so I could take the board out for maintenance or repair w/o having to unsolder stuff.

In a typical build I would do, there would be only one connection to the chassis from the circuit ground.

 

[neevo]

I was going to put the transformer in a separate chassis, with a shielded transformer do you think that’s unnecessary?

If it was in a separate transformer I’d need to think about how I connected it to the audio ground given it would be in a separate chassis.

Edit: transformer I’m looking at is 2A 6.3v secondaries.

 

[rothwellaudio]

A big mistake (in my opinion) with DIY phonostages is to think of the whole thing as two entirely separate parts: the audio circuit and the power supply. This thinking leads to people building a power supply in one box and the audio circuit in another - not a bad thing in itself - but they place the voltage regulation in the box with the power supplies. That's a mistake because the 0V reference point will be at the 0V terminal of the regulator, and inside the power supply box is not the best place for it.
My recommendation would be to have the transformer(s) in the power supply box along with the rectifiers and first smoothing capacitors, then feed the unregulated DC to the audio box. Do the voltage regulation and/or any additional smoothing in the audio box and have the 0V reference point inside the audio box.

 

[neevo]

I’m happy to build a metal enclosure inside the main amp if that’s a simpler solution?

 

[rothwellaudio]

^ No, that doesn't simplify anything, it gives you little of the benefits of a separate box but still leaves you with the uncertainty as to where the 0V reference is.

 

[neevo]

Ok, sounds like I need to refresh my understanding. Let me rethink this so I can be clear about what’s happening.

Thinking of the amp schematic as 1 circuit makes sense so I'll stop thinking of them as discrete sections.

Can you help me understand the 0V reference? I tried to google it but didn't come up with anything. I am trying to find the 0V reference on the amplifier schematic but I obviously don't understand whats going on with this. On the Little Bear its easy as one of the transformer windings has a centre tap at 0V, but with a transformer that doesn't have centre taps I am a little lost.

Also looking at the schematic the HT+ feed is highlighted but with no return. I would assume the ground items highlighted would be the return (after C9 and C10), do I have that correct? Also there is also a common ground for the audio path with a chassis earth on it too. Does the ground tie in to the return for HT+ too along with a chassis ground? I'd assume this ground would be at a different potential to the ones around C9 & 10 given its attached to the chassis?

Sorry for the (probably) very simple questions. The last thing I want to be doing is trying to make an amp if I don't understand whats going on enough.