Passive Mic Pre?

[deanp920]

Hi,

I'm curious about having the option of using hot condensers without an active preamp on loud sources like drums and guitar amps.

I need to get phantom power in there somehow.

I was thinking about using a nice 1:4(?) mic input transformer with phantom power on the front end, direct coupled to a 4(?):1 step-down line output transformer.

My output transformer needs a 3K source impedance for best frequency response, and I'm not sure how to wire an input transformer directly to it and still end up with the right source impedance to the OPT primary. Will the microphone's source impedance be reflected to the input trafo's secondary as a square of the turns ratio or what??

Might need an attenuator in there somewhere, too. I dunno.

The impedance issue is really where I would appreciate some advice, but any thoughts are welcome.

Maybe there is an easier way, or is this just a lazy, stupid idea?

Dean

 

[NewYorkDave]

There wouldn't be any point in using a 1:4 coupled directly to a 4:1. If you just want isolation, use a 1:1.

Typical "modern" line inputs present an impedance of 10K to 20K, sometimes higher. If you used a 1:2 mic input transformer, that would transform the ~200-ohm source impedance of the mic to 800 ohms, which is still low enough that it would not be noticeably shunted by 10K or higher. And you'd get 6dB of gain in the process. Assuming a 10K input, the impedance seen by the mic would be 2500 ohms, which is pretty close to your target. With a 20K input, the mic would see 5K.

The output level of a "hot" condenser on a loud source can approach and sometimes exceed 0dBU. With a 1:2 transformer, you'd have peaks of about +6dBU, sometimes a little higher, which is a level that most balanced line inputs can handle with no problem.

 

[Guest]

I have an idea to make mic preamplifier
only from transformers and buffers.
And we can do buffer effectivelly (diamond OPA660 like, or discrete better)
transilinear and without resistors.
This can be good neoclassic mic preamp.

... And without resistors, devices what make only heat and noise...

xvlk

 

[Guest]

Very interesting stuff... I've pondered this a bit, but I don't know enough yet to put it all together. I'd love to hear your idea xvlk.

 

[dayvel]

Sounds to me like all you want is an external phantom supply. Cheap to buy commercially and no biggie to DIY. I like NYD's idea; transformers have some adavantages over caps for phantom-blocking, but a couple electrolytics would work pretty well, too. No reason to make this complicated.

 

[Gus]

Someone made something like the posts here at the old place.

 

[stuartpre]

Yep, old thread...

but i was thinking of how to make 6 cheap and good mic pres for hot signals like close mic'ed drums. For dynamic/self powered mics so no phantom power required.

What about 6 input transformers 1:2? Maybe, no pad required, maybe just a phase switch on the output of the transformer.

Any builders/designers have any advice on this...or the best transformers for this application?

 

[ruffrecords]

Like most things, something similar has been done before. Back in the day at Abbey Road Studios, they used to have a three channel passive mixer that combined three mic inputs into one - used often on Ringo's drums apparently.

That's the reason vintage mic pres only have about 34dB of gain - with hot tubecondenser mics you did not need any more and with drums you would probably need a pad. Dynamics are a lot less sensitive than condenser mics so you might struggle to get enough level to achieve a decent signal to noise into a line input.

Cheers

Ian

 

[PRR]

Seems to me the original BBC Broadcasting House was all passive faders mixed before any amplification.

http://www.orbem.co.uk/bh32/bh32_t.htm

Note the first actual amplifier is upstairs in Control Room. This sufficed for the higher output carbon units and the WE mikes which had head-amps internally.

A 3-pot passive mixer sure was a common upgrade for US "remote broadcast" boxes, which fed a mike at ball-park, church, or City Hall to a phone line to the radio station. Sooner or later a need for more than one mike arose, a passive pre-mixer needed few new parts.

 

[emrr]

I have recorded a lot of loud bands with old fixed gain 40dB preamps.  They always need a 20dB pad.  They are really 20 dB preamps in that case.  A loud drummer can still overload the AD converter with that and a dynamic mic, especially a hot modern n/dym type. 

Let's examine this.  The input transformer on anything like that is 20dB gain in itself.  Then we have -20/+20/+20 as the simple equation.  You could simply pass a 20dB input transformer to a cathode follower with no output transformer and drive any modern input just fine.  You need it balanced?  Probably not really, you can always use the old resistive balance trick just fine.  Just in case you want it whole hog though, run it push-pull cathode follower and actively drive both lines.

Wait, the request was for 'cheap', so never mind.  If you want to try the transformer thing mentioned, it has to have mic transformer grade shielding, at least 30dB case shield, or better.  Those won't be cheap. 

Something like the THAT solutions will be the cheapest answer.  Rochey has these available which also fit the bill:

https://groupdiy.com/index.php?topic=35660.0

The single best transformer 1:2 approach would be the Jensen 990 / John Hardy type, again, not a cheap approach but one that fits the bill. 

 

[stuartpre]

Thanks for all the replies, some good thoughts there.

 

[alexc]

Fascinating stuff  and definately 'food for thought' 

 

[PRR]

> something similar has been done before. ...used to have a three channel passive mixer

This is how most early talkie movies were made. This is how most early PA was done. Radio also.

When going to early film-sound, 78, or AM, the medium's noise floor does not require ultimate low hiss in the mike system.

The suggestion below is from Thordarson 1934, and is transformer-heavy. Lighter implementations of the concept are possible.

The double-dot thing with 4.5V battery is a carbon mike, so fairly high level. They suggest buying 200:100K iron for each mike and stacking them. "Series mixing". In today's world, we would note that the bottom secondary may be 50K above ground, and the upper secondary has hundreds of pFd (uuFd) to ground, so response is falling at 10KC (10KHz). But that was acceptable then.

Fig 3 shows many possible sources. Carbon mike, preamp tube/transformer, what appears to be a phono pickup plus transformer, and 500/125r transmission line. Here they are scaled to 200r, padded, and stacked to 800r, then transformed up to grid impedance.

I have mixed electret capsules with 1K pots to a plain mike input. Unbalanced works if the mixer is close to the mikes. That was for dramatic speech; you can get cruder for rock/pop drum mixers. OTOH the rise of $99 Phantom mikes (also $99 mini-mixers) has made such rude affairs too complicated to mess with.