Is my LT44 transformer suitable for audio (de)coupling?

ohms.

Yes - I have a box of old MAT-01s from PMI. They are strictly reserved for such projects. I don't know if they are still available.

( a

-126dBu

surprised to

The 5534 is not bad, but I wouldn't say it is the quietest way of doing things. I had to make a very small preamp (just one op amp) for a high impedance (50k) microphone. I searched for ages for quiet op amp before I realised that an OP27 is optimized pretty well perfectly at this impedance, with an excess noise of only about 1dB. Amazing!

long

I really wish noise was expressed as a noise figure, rather than a level. That way it wouldn't matter what impedance you were using, you would simply have a figure of merit that told you how much worse the pre was than theoretically perfect.

d

--
Pearce Consulting
http://www.pearce.uk.com

Actually, as a follow-up, following some discussion in r.a.p , my most recent mic pre featured a modest increase in input Z to 2.5k. I note from the links posted here that Rupert has gone as far as going to 10k now, which did surprise me a little. I'd like to spend some time listening to the effect of loading on various mics actually.

Graham

ohms.

I recall the beast.

Somewhere I think I have some of those similar Nat Semi parts that featured multiple devices on-die. Forget the part number now. Oh no - I *was* right - the LM394 - just checked in case. Though that would be an IC but the M just means monolithic. They're not even insanely expensive now !

Also took a look at some esoteric fet data a while back. Noise somewhere down in the

500pV/sqrt Hz region. Interfet is the company.

consoles (

using a

-126dBu

surprised to

Indeed not. They could have used something from AD or PMI and instantly improved the noise figure.

They're good op-amps. Never had the budget to design them into anything though. :-(

as long

Hmmm, I wonder how that would go down with those who 'cheat' by using a 150 ohm source instead of 200. I note that Mackie ( I think ) is now quoting noise with the input

*shorted* too.

Graham

multiple

just

They're not

Yes that is very similar. The great thing about using these discretes as front ends is that the knee frequency for 1/f noise is way lower than the transistors in the average op-amp. So not only don't they hiss - they don't rumble either. Three of them in parallel is about right for a mic at about 150 to 200 ohms.

in the

But what about the current noise? You need to multiply that by the source impedance to add in its effect. I've looked at a few FET input op amps with amazingly low voltage noise, and this always more than makes up the difference.

source

input

Yup, you really do have to read specs with a cynical eye these days.

d

--
Pearce Consulting
http://www.pearce.uk.com

Heh heh. But I'll guarantee which one sounds better...

--
*OK, who stopped payment on my reality check?

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

Last time I played with this idea I found it very sensitive to RF interference.

--
*Always borrow money from pessimists - they don't expect it back *

    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I wouldn't put money on that if I were you. I've measured the performance of the Behringer, and I can find no fault with it. It has problems, of course, but they are limitations imposed by cheap implementation - certainly not in the sound department.

d

--
Pearce Consulting
http://www.pearce.uk.com

Well, there should have been nothing inherently RF sensitive about it. Like any other sensitive system, it needs all the usual RF-proofing tweaks to keep it sane.

d

--
Pearce Consulting
http://www.pearce.uk.com

Even if that were correct it is immaterial in most situations where the gain is not set to maximum (and that is the ONLY place a mic amp achieves an equivalent input figure this good). As gain is reduced, output noise begins to dominate and the old Neve mixers still beat most 'modern' designs in that respect.

Put simply, equivalent input noise is only part of the story.

ian

Not that is really matters. An improvement in noise figure of 2.5dB will only improve the signal to noise at very high gains by the same amount. Unless you are using dynamics on very quiet sources, the self noise of your condensor mic is the limiting factor not the noise figure of the mic pre.

Ian

Its a lot easier to reduce the noise figure at a narrow bandwidth at microwave frequencies.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And asitappens, I can achieve these sorts of figure at audio as well.

d

--
Pearce Consulting
http://www.pearce.uk.com

2 GHz at 12 GHz is 16.67% which is narrow. Don't tell me that there is no filtering at all. A circulator or isolator has a usable bandwidth which keeps out of band noise out of the amp. One of my KU band receivers is aboard the ISS.

Audio is true broadband, from DC to whatever the upper limit is set at by the design and limitations of the components if it is DC coupled, and from less than 100 Hz if its AC coupled.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Don't these things tend to be measured in octaves, as it were?

--

*A snooze button is a poor substitute for no alarm clock at all *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW To e-mail, change noise into sound.

Not only that, but at low to DC freqs., the noise increases dramatically of most devices. When looking at op-amps I have to often look at DC drifts, which is the largest noise.

greg

Of course there is filtering. The waveguide coupler as a bandwidth, as does the mixer. The image filter then has a bandwidth, and finally the IF amplifier. Despite all this filtering - none of which is lossless - the noise figure still happens. It really isn't easy even compared to audio. Well done for having a receiver aboard ISS, but ground transmitters tend not to be particularly power-limited, so the noise figure probably isn't quite the issue it is with a terrestrial receiver. In fact there would be no point because it is pointing at the earth, which is hot.

But achieving low noise at audio - however you want to describe its bandwidth - is actually pretty trivial. The fact is that manufacturers choose not to implement it because they can make something that is "reasonable" for a lot less money. The business of achieving low noise at audio doesn't rely on being able to juggle S parameters, physical layout, board materials and mechanical design like microwave.

d

--
Pearce Consulting
http://www.pearce.uk.com

Yes, but remember we are talking about scraping away at the last vestiges of the state of the art here. Optimum noise match varies over that range, and designing matching structures that keep the complex impedance of the waveguide transition at the exact conjugate of the input transistor noise match over such a bandwidth really is far from simple. By comparison audio is a breeze - I promise.

d

--
Pearce Consulting
http://www.pearce.uk.com

Tell me about it. I've had to design baffling into a system before now to control air currents that were causing cyclic temperature drifts. Generally in an audio amp, though, there is sufficient DC feedback to render this a non-problem.

d

--
Pearce Consulting
http://www.pearce.uk.com

In our analog receivers the DC offset was measured, and a 18 bit ADC was used to trim the offset to keep it to about a millivolt from wherever it was set. Some external decoding required a fixed DC offset, and this was available from the front panel. We also had a 63 dB range in the output level control, digital with accurate .1 dB steps. This was on the Microdyne 700 and 1620/1670 series telemetry receiving equipment.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida