mic preamp modification

Hello Rex,

FWIW, the 4052 is a dual 4:1 multiplexer. Sometimes also used to switch resistors in and out for gain control. I don't know about this particular opamp but check it's data sheet for noise performance. If it is considerably worse than others a swap might help.

Anyways, you might want to print all the data sheets of the parts and draw a schematic to understand how it works.

Phantom power sources can be a noise contributor but if so that could be filtered.

Regards, Joerg

I'll guess it's a design issue. ;-)

Tuh ! Silly wanker.

NO. The 'front end' is what matters.

That's a typical SMD code. There isn't enough space to fit the full part number. Sorry !

NO.

So close as next to nil that you might as well forget it.

105 means 105 degrees C rating - that's fine. 0402 is normally an SMD footprint size.

Nope.

I do !

Unlikely. Once the circuit topology is fixed there's usually bugger all that you can do to improve it.

The Eastern guys don't have a clue how to make a good mic amp - apart from Yamaha maybe ?

Graham

Erk ! Thinks *non linear resistance with applied voltage*.

It's actually quite good.

It won't.

Damn well shouldn't be a noise source unless the designer was mentally defective !

Graham

In one project I worked on, one of the most competent and conscientious engineers I've known used the 8:1 mux from that family for a wide, RF (2 to 13 MHz), barrel switch where noise and distortion were both critical parameters. (The signal at that point was boosted enough that thermal noise from the switch was not too harmful, and judicious use of carefully biased followers eliminate most distortion.)

I expect you were more charitable than to do that.

electrolytics.

Often, I think, appearance is the primary design goal in expensive audio gear sold to consumers.

Best regards,

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

This is too easy it's not even fair, but I love Monster Cables and how they can even manage to take a regular old phone cord for dial-up Internet connections and turn it into "Ultra-High Speed Internet Phone Cable"

... Oh yeah, 3kHz and 30dB SNR clearly needs $2/ft cables!

I've seen designs where cable TV power splitters were actually disassembled and their parts then removed and soldered down to a different board because the commercial alternatives (e.g., those from MiniCircuits) were actually more expensive overall!

---Joel Kolstad

Hello Graham,

Yes. The true audio freak would frown and at least use an SD5400 here.

Then I guess I have diagnosed mental defects in two designers life-to-date...

Both systems looked very expensive and "pro", BTW. Needed ferrites and better electrolytics.

Regards, Joerg

Filtering phantom power is a great application for a capacitance multiplier, because you don't care much about losing 0.6 volt. One resistor, one capacitor, one 2n3904--sayonara to pickup. Much cheaper than LCs, too.

Cheers,

Phil Hobbs

Hello Larry,

Being a cheapskate when designing stuff I often use PIN diodes with a long enough carrier lifetime for RF muxing. The SD5400 is usually reserved for more valuable tasks such as servoed gain or phase control.

Yes, one of my rules is not to place blame on a person. I just corrected the problem and went on with life.

electrolytics.

Oh yeah. I have seen gear in polished boxes with gold plated connectors that cost hundreds yet contained less than 50 Cents in parts. The best was a "compressor". It contained two 1N4148 diodes.

You can also see that effect in some lab gear. I remember the first time I opened a "power splitter" that retailed in the four-digit range. It contained a plain old hybrid with three resistors. Ok, they were laser trimmed. But I have designed a lot of laser trimmed circuitry and it doesn't cost an arm and a leg.

Regards, Joerg

Have you actually seen a cap multiplier demodulate AM? That's some noisy studio you have there! I've seen well over 100 dB isolation of

100-kHz switching junk with a two-pole cap multiplier--50 mV knocked down to < 0.2 uV. (That upper limit was derived from the reduction in offset voltage of a single-phase lock-in detector that had some ripple problems.)

Cheers,

Phil Hobbs

Hello Joel,

Done that, for lab experiments. It's just so tough to crack these splitters open if you don't want the dreaded F-connectors.

Regards, Joerg

Joerg wrote: > For example, if someone turns on the huge array of fluorescents in

Luckily we don't have that problem, but as soon as we have budget to do so (hah! aka as soon as I get sick enough of it and buy them myself) we're finally going to enter the age of dimmed house lights. Also, say bu-bye to the flourescents anyway.

Main thing I'm worried about there is whether the more inexpensive dimmer packs are going to inject RF noise all the time. AFAICT there are at least 3 major classes of dimmer topologies, and I'm assuming that the cheaper models use the least effective/efficient/quiet. I know the spotlights themselves buzz with the NSI D2DMX/D4DMX packs, though I haven't heard any injected noise in the board and receivers that're ~20ft away. Maybe masked by the acoustic noise of the bulbs themselves...

Well, he won't work on mine since I didn't buy it from him, so I though he might be willing to give me a general idea.

Yes, I know. There are some lists online and I have been successful with matching some in the past, but not this time.

These are not smd parts. They are radial lead, about 5mm dia x 10 mm long. It was hard to read them in the circuit but I didn't see anything other than the 0402 that could be a value, so I assume it might mean

4000 somethings. 4 uF maybe? Didn't see any voltage rating though.

But the Oade guy I mentioned above seems to have found a way to get S/N from 65 to 80 dB and distortion from 0.08 to 0.06. Getting the unit apart is a pain, I did it to look at the board, so I think he can't be doing too much to the circuit for what he is charging.

Is Marantz design done in Asia? I though they were based in Europe.

Phantom power is not the problem, or at least not the main problem. I connected a condenser mic, turned phantom off to disable it and cranked up the volume. That's where I realized that there is more hiss from this thing than I would have liked.

Hello Graham,

In these cases it was proximity to switchers and the phantom supply picked up noise. The electrolytics were, well, bottom-of-the-line garden varieties. After all, had they used the Philips caps which I put in or another good brand the profit margin would probably have plummeted from

98.5% to 98.4%.

When I talked to the (stunned) audio people about what else had been done and I mentioned ferrites all I got were those deer in the headlights looks.

Regards, Joerg

Hello Rex,

If you really want to figure out where that hiss comes from there is a trick: Take a cap and ground one side with a short braid. Hold the cap at the inputs of the various amps in there. But turn down the speakers since this can cause a loud pop if there is DC on a node. Discharge the cap everytime before probing a new spot or you could fry things.

When, for example, you capacitively short the input and nothing changes but you short the next input and the hiss stops then you know the first stage is the culprit.

Regards, Joerg

Ah, yes, never thought of that. The important stuff must have been somewhere else on the thing that I couldn't see.

Thanks, that's a good tip.

LOL !

Noisy 48 volts ? I like to keep the noise on my phantom power supplies down to a few hundred microvolts ( audio band ).

Graham

Hello Phil,

Just be careful when there are transmitters around, especially AM. Transistors need more than a lone cap in that situation or they might act as demodulators. 80dB of rejection ain't always easy.

Regards, Joerg