Cats out of the bag...

[machineghost]

I have been working on this project since July 2010. I did not want to make any mention of it on the forum with reason.

It has now reached a stage where its becoming a tangible reality and I think I can dare yapping about it here.

This is a dual channel vacuum tube microphone pre amplifier that has been designed (and busy being built) by request from a few of my sound-engineer friends. I guess you can sum it as a "dare".

The circuit is based around plain old common cathode triode stages with transformer input and output - floating balanced in and out as will as high-impedance instrument input (i.e. electric guitar). Zero feedback and isolated power supplies on all the feeds.

Gain attenuation is done with a shunt-type stepped system in 12 steps.

48 VDC power is supplied on the mic lines for solid-state condenser microphones.

The unit features over 200 VA of toroid transformers (custom wound).

Chassis was manufactured at a renowned facility in Pretoria.

All wiring (will) is done point-to-point - no PCBs.

Gloria in excelsis Deo

 

[moresound]

Alright so more info please. How much?

 

[machineghost]

Alright so more info please. How much?

I prefer to discuss pricing offline or via private messaging. I dont want to be accused to using this fine forum to sell my product. At the moment this is the only unit being produced and I doubt there will be any mass production happing in the near future.

This is fruit of my passion for music and the recording thereof.

One thing I can say is that this one unit is currently costing me an arm and a leg!

 

[mshilarious]

Why does a preamp, even a tube preamp, need 200W? That's enough for 100 12A_7s . . . are there power tubes involved? How many tubes this thing got?

I guess I don't see why a garden-variety 12V toroid couldn't be used, with the plate supply from one side of the dual primary, and the phantom supply multiplied off the 12V. But then I am a cheap man who likes to conserve power . . .

Food for thought if you publish a DIY version, it will save at least $100 on the transformer. Probably more, I would guess.

edit: oh, South Africa, 230V. Well, there goes the dual primary trick . . .

 

[machineghost]

Why does a preamp, even a tube preamp, need 200W? That's enough for 100 12A_7s . . . are there power tubes involved? How many tubes this thing got?

I guess I don't see why a garden-variety 12V toroid couldn't be used, with the plate supply from one side of the dual primary, and the phantom supply multiplied off the 12V. But then I am a cheap man who likes to conserve power . . .

Food for thought if you publish a DIY version, it will save at least $100 on the transformer. Probably more, I would guess.

edit: oh, South Africa, 230V. Well, there goes the dual primary trick . . .

The transformers are slightly over-sized for a reason. There is two tubes per channel.

Remember - the power consumption of the unit will be equal to the systems actual performance. When its not conducting any signal its power consumption is very low.

I prefer to use custom made transformers as I have some pride and dont want to employ "ghetto" fixes in the unit.

Publishing of a DIY version... mmm... I'll think about it.

 

[mshilarious]

Power consumption of the heaters is the major draw in a tube preamp, and that doesn't vary at all. I suppose common cathode means single ended, which means class A, so that doesn't vary power much with signal either. Maxium plate dissipation for four 12A_7 tubes (eight plates) is 8W--that would be extraordinarily uncommon, and certainly you couldn't have 8W RMS as that would mean no signal.

Two tubes per channel--again I don't know, but assuming 12A_7 types or similar, that's 8W to the heaters.

Add phantom power (2W), and allow for all circuit losses on the way, I get maybe 50W maximum. Slightly oversized? What is the reason? If you never share any details, how will we know?

I wouldn't call standard power circuits (the voltage multiplier, etc.) "ghetto" fixes. If it supplies 10mA per mic and never sags below 44V, and is suitably quiet (<10uV is nice), it works. The mics will never know the difference . . .

 

[stevieb]

This promises to be a very interesting thread- on one side, we have a proud papa who is almost ready to pass out the cigars and introduce his creation to the world. On the other, we have a Knowledgable skeptic who is questioning the over-built quality of the thing. Both defensible positions, from the onset. Some good stuff can come from this- perhaps a new, high-quality pre-amp has been spawned, perhaps some cost-cutting can be done without any compromise of quality. As I said, interesting.

Let's just hope that the discussion remains respectful and constructive. Now, return to your corners, and when the bell rings, come out swinging!

DING!

 

[mshilarious]

Well I see a huge chassis and a huge figure for power consumption that doesn't square with four preamp tubes in a standard topology, if that is indeed what we have. So there's either a lot more than meets the eye, or a lot less. If it's a lot more, then hopefully OP will share, otherwise this is not a very interesting thread. I mean if somebody wants 200W capacity where a fraction is required, OK, but is that really the case? We don't know, we don't even know what tubes are used. It looks to me as if the thing is sized for power tubes, that's all.

There are some other potentially interesting items, like the ground lift on the rear panel, I wonder what that does. I can't read the second switch from the right on each channel, wonder what that does too . . .

 

[crazydoc]

Just my opinion, but I think if you're going to post something in this forum, it behooves you to give the particulars of design and construction (schematics, methods, pictures of the innards, etc.) Otherwise, post it in the appropriate equipment forum.

Frankly, I don't give a shit that you built something, but I'm interested how you built something.

 

[stevieb]

This is "the appropriate equipment forum" for a home-built piece. And I think the OP has given us enough info for now- nothing about the rules here that says he can't decide how much info is to be released. If not knowing the whole deal offends you, don't view this thread. If you want to know exactly how it was made, buy one and reverse-engineer it, just like everybody else!

I dunno about transformer application theory, but perhaps the way-overbuilt power transformer is so far past the point of diminishing returns, because going there gets the device well into the "sweet spot?" I recall, decades ago, replacing filter caps on a bass amplifier in high-school electronics class. My instructor pointed out that each time I doubled the cap value, I would see a 50% improvement in noise filtering, but to do so more than once or twice was probably not cost-effective. If I were building or modding an amp for critical studio recording, however, those "diminishing returns" might be worth it, especially in today's pristine digital world of diminishing noise floors. Again, I don't know the theory, but if it holds true for transformers, that might explain the "need" for such a huge transformer.

"Not interesting?" Hell, this thread is fascinating, I would think, for an electronics theory piker like myself, or a shining star like mshilarious.

 

[crazydoc]

This is "the appropriate equipment forum" for a home-built piece. And I think the OP has given us enough info for now- nothing about the rules here that says he can't decide how much info is to be released. If not knowing the whole deal offends you, don't view this thread. If you want to know exactly how it was made, buy one and reverse-engineer it, just like everybody else!
...
"Not interesting?" Hell, this thread is fascinating, I would think, for an electronics theory piker like myself, or a shining star like mshilarious.

Just my opinion, like I said - nothing to do with "rules". Your right, if he doesn't show how he's building it and the thinking behind it, I won't be following this thread. I don't want to build one - I want to see how he builds it. Without this, it holds no interest for me.

And he hasn't even built it yet.

Au revoire.

 

[machineghost]

Goodness me...

The transformers has been over-sized for this single unit so that I can prototype a variety of circuits. Using large transformers also ensure a healthy supply of current to the system. Nothing wrong with that.

mshilarious - questioning is good but accusation is offensive. I dont see the point of interrogating me for using a 200 watt power supply in this creation of mine...? Interrogate the large corporations in the cities that run enormous air-conditioners 24/7.

Oversized is under-rated...

I will get to the specifics as I go along. Today I am only busy with the chassis dry fit.

Lets see, on the front panel we have the following controls (from left to right):
POWER | HT Stand-by | Phantom | Input Attn | Inter-stage drive | Output Attn |
Large holes are where the VU meters go, small holes are for the 12-step Gain attenuation switches. - same on both channels.

Ground-lift... come on... you must be kidding...

The hole below the ground lift is for a grounding post.

Large hole on the right is for a integrated fuse/IEC inlet plug.

Circuit is indeed Single-Ended Class A - triode tubes. 12AX7 and 12AU7

 

[stevieb]

I wasn't intending to call anyone out or be rude, sorry if I was. Only wanted to suggest that we have a little faith while we see what develops, at least for a while...

 

[mshilarious]

Not kidding about the ground lift, it could have been on the chassis or with respect to the DI input. I'm not super-keen on the chassis lift, even with the post, but I don't know how things are in SA. In the US, a technical ground has to be bonded to the main service ground via #6 copper on the exterior of the building. So if you've gone to the trouble of a technical ground, there is no reason not to use it for all of the safety grounds at the outlets in the studio, unless you are trying to make the best of old and not very good wiring without rewiring.

The danger is an uninformed user decides the ground lift is a good idea to solve hum on a guitar that is actually from EMI at the guitar, hasn't used the grounding post, the chassis isn't double insulated (as would be required for UL listing in the US for an appliance without a safety ground), there is a fault to chassis, and then . . . well, crazydoc is an MD, he can pick it up from there . . .

Another trick to a floating audio circuit is phantom power. Ideally, pin 1 is bonded to chassis at the XLR inputs. Obviously there needs to be a circuit for phantom to flow, and if you have input transformers you could indeed isolate the phantom side from the rest of the circuit. But you can't isolate pin 1 from chassis without using isolated XLRs (which you may have), because the cable connector might have pin 1 bonded to shell.

Now you could have a situation where the XLR cable shield becomes a bonding point between a technical ground and a safety ground. Hmmmm. If the technical ground isn't bonded as required . . . not good. In fact, we used to have a fellow here who will go unmentioned (VictoryPete) who, even though he was an electrician, swore up and down that the US National Electrical Code (NEC) allowed unbonded technical grounds, because that was the way he had wired his studio. Ouch.

These are considerations that many people reading this thread might not be familiar with, so they need explanation. Otherwise they get a bad idea from a picture, and things go downhill quickly . . .

Back to the nitty-gritty, if no schematic, how about a block diagram? What are the four triodes per channel doing? If only two stages, is the first stage cascode, the output push-pull? Or is there something going on with compression? I can keep wild guessing for a while . . .

Also, the mere fact that the transformers are oversized doesn't mean that power consumption is too high, if you're drawing 10W the fact that the transformers can supply 200W doesn't waste power. It's a lot of money in copper and iron though.

 

[machineghost]

The chassis will be grounded according to accepted laws. The signal ground will be made switchable. About cables being wired incorrectly - I cant help for that.

My inspiration for this comes from working in a Tom Hidley designed and built studio. There is was chief technical and serviced gear ranging from Focusrite, SSL, Neve, Manley, Anthony DeMaria, AMS, Marshall, AKG, van den Hul, Lexicon, Avalon, MeyerSound, Yamaha... etc - you name it. I am amazed at what some companies got away with by selling ultra expensive gear.

Please humor me - I have a good 12 years of electronic experience behind me and by now I know what I am getting into by doing this project. If I did not then I would not have attempted it.

Regarding the stages - it is 3 common cathode stages cascaded exiting in a cathode follower driving a coupling transformer. Input is via a transformer with floating signal elements (except when Phantom is engaged).

No feedback is employed. Gain is around 65 dB depending what tubes (brand) are used.

More info later... currently I am battling a nasty bout of flu.

 

[mshilarious]

Yes, you have 12 years, but this is the DIY board. Many people here have little to no experience, if they see something like a chassis ground lift and a ground post, they might think hey that's a neato keen idea, try to implement that, and end up dead. There are zillions of threads here (not so much in DIY, but elsewhere) where someone has hum, immediately suspects a ground loop (even when only a single piece of gear is involved), then somebody starts encouraging them to use two-prong adaptors without consideration of why the adaptor has a ground tab on it. So you give them a chassis ground lift in a box that has 300V inside, and it's like giving a loaded shotgun to a five year old.

I would be surprised if a commercial US manufacturer implemented that feature because of potential liability (if you gear doesn't pass UL, it's tough to get products liability insurance). At a minimum, there would be a big giant warning label on the chassis next to the switch.

A couple other suggestions if you go fully commercial; the chassis looks pretty deep, maybe >16". That will be too deep for quite a few racks. It's borderline, so if you could trim a couple of inches it would be more compatible.

Second, the DI inputs on the back suggest that a lead will be left hanging, perhaps into a patchbay. I don't know if the DI inputs are normalling jacks, obviously that would be a problem with a full-time lead, and I don't see a switch, so the unterminated lead could inject interference into the circuit. That, and inappropriate cabling might be erroneously selected by the user, resulting in an additional capacitive load on the instrument. This is why DI inputs are typically front panel, since ease of rear panel access is not always possible.

Finally, connector shells terminated to pin 1 is not inappropriate cabling (just about every microphone on the market has a chassis connected to pin 1; I hope they all do . . .) I'm in the habit of doing that to all connectors, because sometimes connectors get daisy-chained, so you can't rely on ground termination at a rack chassis.

I'm sure you are familiar with this article, but I post it for others' benefit:

Sound System Interconnection

And even if a shell grounded to chassis was incorrect wiring (it's not), you can't assume that users don't have cables like that. Because when an incompatibility arises that only affects your gear (their cable/mic combo works fine elsewhere), they will blame your gear. If they ask you, you can correct their cable wiring to your preference, but they won't ask you, they will just blab on the internet that your gear is crap. Voice of experience here, trust me on that one . . . or if you don't believe me, read some of the Manley-bashing threads on GS ("I opened my Massive Passive and saw Chinese capacitors, this gear must be total crap", etc.)

 

[machineghost]

A couple other suggestions if you go fully commercial; the chassis looks pretty deep, maybe >16". That will be too deep for quite a few racks. It's borderline, so if you could trim a couple of inches it would be more compatible.

Second, the DI inputs on the back suggest that a lead will be left hanging, perhaps into a patchbay. I don't know if the DI inputs are normalling jacks, obviously that would be a problem with a full-time lead, and I don't see a switch, so the unterminated lead could inject interference into the circuit. That, and inappropriate cabling might be erroneously selected by the user, resulting in an additional capacitive load on the instrument.

The case was designed within limits of rack-mount standards.

Regarding the DI... I got it all under control. The DI is only activated when a jack is inserted. I never really designed the unit to act as a DI so its merely there *if someone decides to us it*. This is first and foremost a microphone pre amplifier.

No offense intended but it always amazes me how people can voice their opinion on projects like this in terms of design suggestion etc. Recommendations is always welcome but never underestimate the designers skills and project vision.

It was maybe not such a good idea to post this here after all. Its not a DIY effort but I doubt if it will turn over into a commercial effort.