Tech: Preamp (PA/recording) feedback

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 31 Oct 06 - 01:34 PM

... or better still the THAT1510
Anahata

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 31 Oct 06 - 01:29 PM

Erm, for SSM7021 read SSM2017...

A.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 31 Oct 06 - 01:08 PM

The TL07x range are horrid for audio.
5532/4 aren't too bad. Neve used them all over the place.

Just to match a typical mixer mic input stage, you should be looking at the intrumentation amplifier style chips like the SSM7021 or (much better) INA217.

For top level performance, you still need discrete components and you're looking at things like triple paralleled matched transistor pairs. If you're planning an 80dB gain setting you need to to pull out all the stops for low noise. You don't usually need more than 60 dB.

Anahata.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 30 Oct 06 - 03:49 PM

My previous mixer had unbalanced inputs, because it was really a keyboard mixer. In my defence, it was what I could afford at the time! :-/ So I found out all about power supply noise and why balanced inputs are a Good Thing...

Anahata, the first-draft version is just using ye olde TL074, I'm afraid. Cheap, easy to get hold of, and reasonably low THD if I make sure it's not driving low-impedance inputs. Nowhere near good enough for production though, of course, but fine for experimentation! A more serious version will be using 5532s or 5534s as a minimum, or maybe some more exotic variants if testing shows real improvement, and I'm going to replace the TL074s with 5532s (on adaptors boards) anyway on general principles, now I know that things work. For the final version, improved THD over the 5532/5534 isn't as important to me as improved bias current, so a fancy new JFET one that hits all three major food groups (low bias current, low noise, low THD) would be worth it. I know Burr-Brown for one keep throwing out ads about their new designs, but I've not kept up-to-date with what's actually around right now.

Graham

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 29 Oct 06 - 02:35 PM

They've never done that to me - yet. Of course, if they do it's easy enough to swap them for a thing that just earths the cold.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 29 Oct 06 - 02:30 PM

Duh - no, the mic lead didn't have a jack, of course it had an XLR - it was the amp that had a jack input. Might have been the TA50.

A.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 29 Oct 06 - 02:28 PM

My PA amp has balanced inputs (XLR and TRS jack). Much simpler!

I tried using one of those XLR-jack adapters once, on a mic input (I forget why the mic lead had a jack on the end, can't have been one of mine). The result: the transformer picked up masses of hum from a nearby power supply. Never again!

[complete change of subject] Graham, what ICs are you using in the front end?

Anahata

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 29 Oct 06 - 01:20 PM

Hah! The 'oxygen-free copper' thing!! The reality is oxygen free copper is only an advantage at the manufacturing stage, as it allows them to draw the strands much finer, making the finished cable more flexible. It makes no different to the sound! Oh, and that's scientific fact, not opinion... ;0)

I like the idea of the oxygen-free listening area, though - but it would be more interesting if they used helium rather than nitrogen!

The beer mats and gaffa tape must have taken years of research to perfect... not to mention the beer needed to sharpen up the mind!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 29 Oct 06 - 07:11 AM

To avoid the problem of unbalancing at the amp end I always carry with my rig 3 or 4 giant jack plug adapters - they reduce and XLR to a sack plug, but there is a little transformer in there ro combine the signals. They are switched to offer impedance matching too. The only real problem with them is that if the jack socket you plug them into is not in perfect order the weight of the adapter wiggles the plug out of line and you get an unreliable contact. This can be solved by carefully wedging 3 beer mats underneath and fixing them together with Gaffa tape (a truly professional solution)!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 29 Oct 06 - 06:47 AM

I realised my last post about cables wasn't as clear as it could have been. Sure, balanced or unbalanced matters, and proper twisted pairs matter for balanced common-mode noise rejection, and quantity of copper (ie. resistance) matters, and flexibility matters, and shielding matters, and even braiding matters for robustness. But all those prats talking about cables sounding "dark" or "sweet" or "trebly" - it is, as they say, to laugh...

And that's why I'd rather talk to you guys who I know actually *use* the stuff for real, instead of just collecting it as a kind of twisted hobby.

Graham.

PS. I remember reading a pisstake design for a "perfect" listening environment once. It included a vacuum pump to suck the air out (replaced by nitrogen or something) and space suits for the listeners, because if oxygen-free copper is so much better, obviously we need to keep it oxygen-free. ;-)

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 28 Oct 06 - 10:53 AM

Yes, but when connecting a balanced source to an unbalance input you have to unbalance it at the input end... that was what I was saying, in essence.

The question was really relating to a so-called 'polarised' guitar lead, which is really just a fancy unbalanced cable which offers slightly better noise cancellation than simple co-axial cable by using a twisted pair.

;0)

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 28 Oct 06 - 08:35 AM

If you connect earth to "cold" at the amp end, you have not got balanced line operation, and you do not get hum cancellation.

Hum cancellation occurs when the "phase" signal and "antiphase" signal are added at the transformer or summing amplifier and because they are added out of phase with each other interference signals cancel each other out.

Humbucking guitar pickups do use this effect, creating two out of phase signals by having the magnet pairs of reversed polarity and therefore two signals of different phase that can be added not subtracted (ie added out of phase with each other). This is what makes playing with the wiring of the little switches that can be used to make a humbucking pair into (1) parallel humbuckers (2) series humbuckers (3) North single phase (4) South single phase (5) Additive single phase and (5) out of phase. It is real brainache stuff to figure out which three versions you want and how to wire the funny special DPDT switches used for the purpose to get the combinations you need. I Always have to go and look it up and compare several disagreeing cribsheets to get the answer and if you have a south/south or north/north humbucking pair instead of a south/north pair it is a nightmare!

Oh, also some high end acoustic guitars with internal preamps do have a mini-XLR output, so presumably do run a balanced output.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 28 Oct 06 - 06:30 AM

Ooops! That should read 'common MODE rejection'!!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 28 Oct 06 - 06:28 AM

No - that's not 'polarised', but 'balanced' to 'unbalanced' - a different thing altogether.

It is normal practice to connect the screen to the 'cold' conductor at only one end to prevent earth loop hum.

A balanced line has a twisted pair carrying the 'hot' (phase) and 'cold' (antiphase) signal, and the screen is the reference point for the 'balanced' signal.

The twisted pair reduces induced noise through 'common mod rejection' - the noise gets into the signal, but is rejected because it is 180° out of phase with itself, thus effectively being cancelled out. The technology is borrowed from telephony, which is why we still refer to a signal as 'line level' (0.775v at 0dB).

For some strange reason, guitars are rarely balanced even though it is now the norm with microphones and mixers.

I'm sorry if my attempt to simplify this into 'lay terms' offends purists, but it's nearly right!!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: GUEST,jim m
Date: 28 Oct 06 - 04:01 AM

As I understand it, polarised guitar cables have the +ve and -ve signal running down two cores and the outer screen is connected to ground at only one end. That end should be at the amp. It makes sense to me, but I only use this stuff...

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 27 Oct 06 - 07:30 PM

When Klotz co-ax speaker cable first came out, the rep came to visit us with a sample of the 2.5sq mm version for us to try, as it would fit in XLRs more easily . We noticed that the thickness of the 'screen' (outer conductor) seemed rather less than the inner when soldering up the connectors, so we asked him about it. His response was hysterical!!

I quote: 'Well, the power gets used up by the loudspeaker, so you don't need as thick a wire for the return'...!!

The poor fellow was serious!!! That was around 25 years ago, but it still gives us a laugh! We did get it replaced with the correct stuff, though.

Interestingly, it was once we started using the co-ax speaker cable that some of our Bose 1800 amplifiers (Mk I) began to be randomly unstable. They'd overheat and trip out when in a quiescent state for no apparent reason... we checked the DC offset bias, nowt wrong there... perhaps the capacitance of the cable was a problem.

We didn't get to the bottom of it, but the amps were due for retirement and we've never had such problems with any others.

Recently (since Speakons became the norm) we've reverted to more conventional cable. It coils more easily!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 27 Oct 06 - 03:14 PM

Rather than spend silly money on speaker cable, use domestic 30Amp housing cable (twin and earth). Plenty of copper, no wound strands to have capacitance between them. Just don't expect it to flex without fatiguing.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 27 Oct 06 - 02:13 PM

Yeah... 'grain oriented cable' that has to be correctly polarised on a loudspeaker circuit...

But it's AC... so how come no-one has sussed that it can't matter?!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 27 Oct 06 - 02:02 PM

My problem with gearslutz and the like is probably my attitude, which can be reasonably summarised as "if you can't measure it, it ain't there"... :-)

I know for sure that I know very little about serious audio. But I know a fair amount about electronics. For that reason, a lot of the high-end-audio willy-waving gets right up my nose. A classic example - the "high-end" forum of gearslutz has a thread devoted to "how do particular cables sound?" Well unless said cable is incredibly poor quality, or is damaged in some way, or is very long, then as an electronic engineer I'm in a good position to say that at 20kHz it doesn't make a blind bit of difference.

That's why I'd actually rather ask here. Less people, sure, but more chance of getting an opinion from people who aren't using "placebo" opinions to reinforce their bad judgement in blowing thousands on something that doesn't make a difference. Or in other words, people whose opinions I value.

Graham.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 27 Oct 06 - 09:05 AM

I'm on gearslutz already :-)

You could try your luck on news:rec.audio.pro too. Expect a rough ride, but there are professional audio designers there, as well as users.

Transformers can be good but the good ones are very expensive. Forget about £350 for 8 channels...

Anahata

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 27 Oct 06 - 08:57 AM

Too late, Jim, I'm sulking now!! ;0)

Subject: RE: Tech: Preamp (PA/recording) feedback
From: GUEST,jim m
Date: 27 Oct 06 - 08:47 AM

>As regulars on here may agree, mudcat isn't the best for knowledgable advice on high end audio...

just read this back and it sounds a bit not-like-i-meant-it. All the advice in this thread so far has been good :) but at times mudcat can be the worst place for technical advice.

The two websites I gave links to are for people REALLY into audio and I'd recommend anyone to check them out.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: GUEST,jim m
Date: 27 Oct 06 - 08:44 AM

A piece of advice - ask this question on the tape op and gearslutz messageboards - http://www.tapeop.com/ and http://www.gearslutz.com/board/ - both have a 'diy gear' section and have world respected designers happy to answer questions to newbies. As regulars on here may agree, mudcat isn't the best for knowledgable advice on high end audio...

For that price I (for one) would buy one if it truly is as transparent as you say. Don't bother with the switchable impedance - it makes a huge difference to DI'd guitars and high impedance lines (guitar amps for example) but hardly any noticable difference to low impedance mic signals. Make the gain high enough for ribbon mics - eg the Coles 4038 as a benchmark for the amount of gain needed (lots!). I know its a tall order for someone doing this on thier own, but add a digital output card (a third party one?) and I'm sold on what you've described.

Why do you say transformers are bad for linearity though? I was reading this last night - http://www.prosoundweb.com/chat_psw/transcripts/rupert.shtml - admittedly Neve used his own designs of transformers but still...

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 27 Oct 06 - 08:12 AM

Ah, I see what you mean. Yes, that's an option. I'm not entirely convinced, though - you'd need lots of extra supporting pillars round that area of the PCB so that it wasn't taking too much load. If it's a small toroid then that's OK, but larger ones for power supplies, it's probably asking a bit too much of the PCB. They're surprisingly tough, but I'd rather not push my luck.

Graham.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: The Fooles Troupe
Date: 27 Oct 06 - 07:42 AM

"using self-adhesive pads and cable-ties to keep the toroids in place."

Many construction projects in hobbyist magazines nowadays use plastic cable ties - the locking type - thru appropriate holes in the PCB to lock the toroids (and many other similar components, chokes, small power transformers, etc) down solidly to the PCB - NO STICKY PADS!. You can also tie things to the case - but it's preferable if the ties don't actually come outside - that way they can't be damaged and let loose heavy bits to bounce around inside - especially if they have uninsulated wiring!

One particular project not only had a choke tied down this way, but also used a tie placed on the ferrite core in such a way as to ensure a permanent air-gap, so the ferrite core didn't saturate when the ferrite core was closed with another cable tie.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 27 Oct 06 - 06:20 AM

Bet I can - and bet he didn't do it twice... ;-)

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 26 Oct 06 - 02:18 PM

;0)

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 26 Oct 06 - 11:16 AM

He'll use thermosetting next time!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 26 Oct 06 - 11:08 AM

I know someone who used hot melt glue to fix a toroid... bet you can't guess what happened!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 26 Oct 06 - 05:47 AM

Thanks Bernard - I know about that one. Luckily not from personal experience, but I know a man who did! I considered using plastic bolts for safety, but they're just not meaty enough for the job. I'll probably end up using a self-threaded fibreglass stand-off instead of a metal bolt, to make sure there's no chance of someone opening it up and shorting it out accidentally.

Richard, that makes a lot of sense. I *could* do a proper hi-Z option in the megohms, but it's all more hassle. OK, may be better to do one thing well. DI boxes aren't incredibly expensive anyway. And as you say, it gives me something to do for my next project. ;-)

Foolestroupe, that's exactly what I'm doing in my prototype. If you're bolting it down, the bolt head sticks out a ways unless you've got a dimple pressed in the base, and that'd get in the way of other rack-mount gear. So I'm using self-adhesive pads and cable-ties to keep the toroids in place. I don't think it's a great solution for commercial stuff though, especially if it needs to stand up to heavy treatment. The sticky pads are eventually going to give up the fight, and then you've got a heavy transformer (and attached mains cables) whanging around loose in the box! Not good news at all.

Graham.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: GUEST,nob
Date: 26 Oct 06 - 04:49 AM

Lots and lots of assignable aux(illiary) channels.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: The Fooles Troupe
Date: 25 Oct 06 - 09:52 PM

It's not unusual to use plastic "cable clips" for this nowadays.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 25 Oct 06 - 08:11 PM

Just a thought... probably teaching granny to suck eggs, but worth mentioning...

Be careful mounting a torroidal transformer in a 1U box - the mounting screw mustn't make contact with both the top and bottom of the box (assuming it's metal), or you'll end up with a 'shorted turn'... bad news! Things get very hot!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 25 Oct 06 - 07:32 AM

I hate to say it but I think the Hi-Z option could well be an optional extra in a separate box, because IMHO ordinary Hi-Zs are just not high enough most of the time. Likewise ordinary DI boxes. I really did mean megohms when I said it. 220K is not going to cut it.

Anyway the ultra-Hi-Z inputs would want to be jack, which would bugger up the Cannon-ins, and the combined XLR/Jack is just another thing to go wrong.

You were right first time (apart from circuit-board mounted sockets and controls) - if it's a mic preamp, make it a dedicated one. Flexibility is the mother of abortion.....

What would be neat as an extra would be a mini-DI-box, jack in XLR out taking its power off the phantom. Then I imagine you could have a switchable impedance, about 33k for people with preamps in their guitars, 3.3meg for those without

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 25 Oct 06 - 05:10 AM

Hmm, I see what you mean, Bernard, and looks like Richard's in agreement too. OK, I'll think seriously about that then - it's stuff like that from "power users" that's good to pick up. Extra assembly time will be a pain, but if it's something that'll definitely give some benefit then it's worth adding it to the list. Re my mate from Tektronix, returns under warranty were his big problem, but they had maintenance contracts as well - still, I guess it's likely that you're right and most of the problems came down to manufacturing problems revealing themselves sooner or later.

Transformer-wise, yes, the transformer will be right at the back of the box. All the interesting bits will be happening at the front, as close to the inputs as possible.

For instruments, I was kind of expecting people to be using a separate DI box. But if you reckon people are likely to want to jack instruments in directly, maybe a hi-Z option is worth it then. That'll mean axing a low-impedance option though (500R, say, per Anahata's suggestion) but it doesn't sound like that's a major priority. I've only really got room for one extra pushbutton switch - there isn't space to add a pot or a big old three-position slide switch - so it's whatever gets the most value from that pushbutton.

Like Bernard says, it's the old story - everyone wants different things. Ask two people and get three opinions... ;-)

Graham.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 25 Oct 06 - 03:45 AM

Oh yes - I meant to add the bit about Hi-Z inputs for guitars. Yes, that's important to some people. You don't need it on all channels, though.

Apart from piezos, standard electric guitars with magnetic pickups also require high impedances, for no good reason except tradition and history because the first amplifiers used bottles.

I've discovered another failing of PCB mounted connectors - a jack input on the mixer I use for the band has broken (the plastic surround has cracked off). The entire mixer is one big PCB and it's going to be a very tedious job to remove all the knobs and fixing screws to replace the socket - and then I don't know how easily a replacement can be found.

Anahata

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 25 Oct 06 - 02:46 AM

Grab - the bits that waggle are the pin sockets within the connector, and simply plugging and unplugging an XLR in will cause stresses that cannot be prevented... which is why we have problems with our hire gear that uses PCB mount sockets, but far fewer problems with those that don't.

Perhaps your pal who worked at Tektronix was only looking at the short-term (under warranty), and the problems he was referring to were really caused by manufacturing errors... certainly, all modular desks with ribbons between the modules have reliability problems at one time or another, so eliminating that is good.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Richard Bridge
Date: 24 Oct 06 - 06:12 PM

PCB mounted ANYTHING that gets handled will sooner or later be a dry joint. It also means that repairability demands EXACTLY the same mounting points for the component.

Some users are going to want to run guitars straight in, and some of those will want to use piezo transducers, and those need an input impedance in the megohms. Or maybe you use a separate DI box for them.

Otherwise Treewind is on the money

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 24 Oct 06 - 05:51 PM

My Soundcraft mixer has all the circuitry on one big PCB.
It also has an external power supply. Make what you will of that...

Even a toroidal transformer is trouble when you have sensitive mic inputs close to it. OTOH the best preamp in the world manages to have the PSU on the board - but that's only two channels and I'll bet the sensitive end is as far away as possible from the transformer. Also the transformer is potted and clearly overall screened.

Anahata

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 24 Oct 06 - 05:41 PM

Thanks guys.

Not a bad design spec. but you'll never make any kind of a living selling them at that price.

Story of my life! :-) Yeah, I just fancy knocking out a small batch and seeing how they go. My initial figures has box+PCB+bits coming to about £150 each for a batch of 10, and add maybe 3 hours of my time soldering and screwing per board (I'm pretty nippy with a soldering iron). So £250-300 is around break-even point for being worth my time to do it (if I include some payback from the time spent designing it in the first place). I doubt it's ever going to take off in a big way, but if I can get it working and flog a few, I'll be quite happy to see people using kit that I've made. It's a bit more rewarding than just building a one-off for yourself.

For what it'd be used for, the only reason I started building my "first draft" design in the first place was as a front-end to my PC recording gear - I thought I'd rather have a crack at it myself and see how it turned out. So if you guys think that's where it's most likely to see action, I guess I'm not a million miles away.

Thanks for the hint on dynamic mic loading, Anahata. I might put that in or leave it out - obviously things get easier if I leave it out! I don't know a lot of this stuff, so thanks for that pointer for something to read up on.

I think I'll stick with a single set of outputs - saves having a whole extra set of connectors on the back (cost and hassle). I suspect the level warning may be redundant anyway, bcos most people would probably have separate VU meters on whatever this is feeding - don't know whether to keep this or just ditch it altogether. For the input sockets, I guess go with the doubles then if you both agree on that. I wasn't planning on having extra TRS inputs as well, just kind of an idea for improvement - those dual-purpose sockets are a neat way of adding features without changing too much.

For the PCB thing, I've heard that theory, Bernard. But I've also got a friend who worked at Tektronix and was part of the team that revamped their range back in the 70s/80s. His word on it was that as soon as they axed the independent connectors and wired-together modular setup and replaced them with PCBs everywhere, their reliability problems simply vanished overnight. I can see the point about "if it's going to waggle then let it waggle a bit". But there's also the angle of "stop it waggling in the first place". Once the socket in question is soldered to the PCB in four points, has two extra mounting pins, and is screwed to the front panel, it's pretty well held! And of course the other thing from my POV is that soldering and assembly is significantly faster, like maybe 2-3 minutes per connector - multiply by 8 and by number of boxes, and that's significant! I guess I'll see how my first-draft version holds up with my use (I'm a little heavy-handed with my gear ;-) and make up my mind from that.

I thought about an external PSU too, but it doesn't really look practical. Extra cost for an extra box for starters, plus you've got more hassle on protecting the outputs from the PSU and the inputs to your kit. It needs a balanced +/- supply for driving the main stuff and a higher input for the phantom power, so it's not like I could use a standard wall-wart to drive it anyway (or not without step-up converters inside, which are the kiss of death for audio). And toroidal transformers are as near no-noise as makes no difference, so there shouldn't be any noticeable effect from that. Box-wise it'll all go in a 1U rack-mount - I've found a place (Holt Broadcast Supplies) who do some nice sturdy boxes and make small production runs of custom boxes (drill/punch/engrave/silkscreen).

OK, thanks for the feedback there - some very positive ideas on what to be looking at. I wouldn't hold your breath on anything happening soon though! ;-) But if I ever do get this going, I'll sling you both a couple to have a play with...

Graham.

PS. I could do you a picture of my "Mark I", but it wouldn't be at all representative and I'd only get embarrassed at all the things it doesn't do. :-/

Subject: RE: Tech: Preamp (PA/recording) feedback
From: jeffp
Date: 24 Oct 06 - 04:50 PM

Phantom power is almost a necessity. Maybe switchable in banks of 4 inputs. Otherwise, anyone using condensor mikes will need a separate phantom power unit.

Subject: RE: Tech: Preamp (PA/recording) feedback
From: Bernard
Date: 24 Oct 06 - 04:24 PM

It's always difficult deciding what to include, and what to leave out. Two people will always give you quite different preferences, according to what they do. The modular approach is one answer to this, but clearly beyond your planned budget.

Yes to the XLR/TRS sockets, they were an ingenious solution to a problem.

BUT! Don't PCB mount, as that is inherently unreliable. There needs to be a little flexibility between the socket and the board. I work in audio hire, so believe me!

I'd say, though, that keeping the PSU external to the device is preferable... it solves so many problems and creates so few. Most manufacturers do it, and not always for reasons of cost.

I have an Audio Developments AD145 'Pico Mixer' (with PPMs!), which has an internal re-chargeable power pack, and the power supply interfaces through a four pin XLR. A little dodgy, as talkback headsets use four pin XLRs, too...!

I also have an M-Audio Delta 1010, so I agree wholeheartedly with Anahata!

Balanced outputs are desirable... not so sure about 'orrible phonos (RCA jacks across the pond!).

Are you planning for this to be in a 1U 19" rackmount?

A picture would be nice... hope it takes off!

Subject: RE: Tech: Preamp (PA/recording) feedback
From: treewind
Date: 24 Oct 06 - 03:47 PM

Not a bad design spec. but you'll never make any kind of a living selling them at that price.

If people are going to buy outboard preamps, they are usually going to want real quality. It's not a mixing desk (no EQ, no mixer) so no use to anybody who needs one of those. If it's a front end for a mutitrack hard disk recorder (Yamaha/Zoom/Aiwa/Roland/Korg etc.) it's got to be better than the mic preamps built in. It might sell as a front end to something like the M-Audio Delta 1010.

I think you'll find the time it takes to build it and the details will make it uneconomical, but if you do make one put me down for one to try out!

Details like switches for p48 on each channel, even just designing a good low noise P48 supply with soft switchon/switchoff, will soon soak up your budget.

Incidentally, would anyone consider those dual XLR/TRS jacks worth having
They save space, if you're going to have jacks on each input too.

* 12-way switchable gain
I like that. It's good enough for the DAV BG-1, it's good enough for anyone.

* 3K3 input impedance
That's fine. Some are lower. You don't usually need to go lower than 1k, except Paul Stamler did some tests that showed a SM57 sounded best loaded with about 500R. A switch would be interesting - I mean to try the loading thing with some other dynamics (pointless with condensers).

* +4dBu/-10dBV switch - doesn't change outputs, but alters warning/overload warning points.
Not sure about that. Why not have -10 coming out on phonos, and +4 coming out on TRS/XLR (impedance balanced, compatible with everything) at the same time?

Good luck with it. Some very fine audio products and companies have started out with ideas like yours, but commercially you have far east mass production for competition, or you have to get into the super quality market where some very talented engineers have thought very hard about their designs.

Anahata

Subject: Tech: Preamp (PA/recording) feedback
From: Grab
Date: 24 Oct 06 - 01:02 PM

Hopefully this gets the attention of a few resident experts on recording and PA stuff.

As a bit of an electronics geek, I've been building my own 8-channel mic preamp. A first cut seems reasonably good, but I've realised two things. First thing is that I can do a better job, knowing what I know now. And the second thing is that this is potentially something I could sell!

I've had a bit of a look into small production runs on cases and PCBs, and this looks like it could be workable. Before I commit myself to anything unworkable though, please can some resident experts check my assumptions on what features I should be including?

Main features:-

* 8 channels.
* XLR in, TRS balanced/unbalanced out. Gold-plated connections, naturally. Incidentally, would anyone consider those dual XLR/TRS jacks worth having, or should I save a few quid by just using regular XLRs?
* 12-way switchable gain. 0dB (in case of high-output DI use), 20dB, then up to 80dB in 6dB steps. More than 12 ways on a switch makes them incredibly expensive, sadly. Pots could be used, but (a) you can't easily match stereo channels, and (b) the preamp characteristics mean that most pots aren't accurate enough at the ends of their travel.
* 48V phantom power, individually switchable for each input.
* 3K3 input impedance. Will this be OK for most mics? (Checking on the web suggests it would be, but maybe someone with more experience could say for sure.) Also, would switchable input impedance be worth having, say between 3K3 and 150R or so?
* Basic level indication LED - green for signal present, yellow for warning, red for overload.
* +4dBu/-10dBV switch - doesn't change outputs, but alters warning/overload warning points.
* Switchable warning threshold for setting desired headroom (-6dB or -20dB) depending on application.

Other design features:-

* First off, no EQ onboard. 8 channels doesn't give us enough front-panel space anyway! But my thinking is that a preamp should be entirely transparent, or as near to as possible, so if you want EQ then you do it downstream afterwards, because you can't get it back when you've taken it out.
* Which leads me to transparency. My measure is THD+N. "Golden ears" ain't in it - I don't want "warm" sounds or anything like that. I'm a believer in Douglas Self's comment that adding distortion for "niceness" should come from a "niceness knob" somewhere, and isn't a desirable characteristic of the system as a whole. Once again, you can't get it back when you've taken it out. Sadly I don't have a meter for measuring THD+N - I plan on borrowing/hiring one later to get an accurate measure, but in the meantime I'm designing everything to get this as good as I can. So we're looking at op-amps, not tubes.
* True balanced design - two balanced paths from input to output.
* No electrolytic capacitors in the signal path. Polarised electrolytic caps are OK but you can't guarantee the polarity of input voltages, and bipolar electrolytics are dodgy, so it'll be polyester caps for input DC blocking. Outputs will use DC servos to remove any offset that creeps through.
* No audio transformers - if capacitors are bad for linearity, transformers are worse!
* Everything on one PCB. Trailing wires are the enemy when it comes to noise and reliability.
* Toroidal transformer(s) for mains input, so no noise from that.

Cost-wise, I'm aiming for around £250-300. Maybe start off lower and see if they sell, and then check how long it takes me to put them together and cost it from there.

So, what do people think?

Graham.