mic preamp modification

I read in sci.electronics.design that rex wrote (in ) about 'mic preamp modification', on Fri, 8 Apr 2005:

0402 is almost certainly a date code for April 2002.

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It had occured to me that they might not be transistors. If there was a marking on the board, I didn't record it. It's a non-trivial task to open this thing up and I won't get back to it for at least a day or two, but that's definately something I should check for.

I almost took a picture while I had it open, but I had no charged batteries in my camera.

Hello Phil,

Yes.

This wasn't in a studio but pretty close to a large collection of commercial AM transmitters. It can be the an issue in other locations as well. For example, if someone turns on the huge array of fluorescents in our church that lets off a good dose of grief right into the audio closet, into the wireless links, everywhere.

Yes, you can get 100dB+ if nothing from the outside reaches the BE junction. But when it does, oh boy.

Regards, Joerg

That's rather different from the dual 4x1 that's in this, though. There isn't some other drop-in that would be better is there?

After a little googling, MAX4618 looks like it might be better and Digikey stocks it. Any comments on that or others?

Hello Rex,

No idea, I use the 4000 series muxes only where linearity isn't critical. But this chip is unlikely the cause of noise, just maybe a wee bit on non-linearity.

Regards, Joerg

Yes, I suppose that's true. Guess I need to use your diagnostic bypassing technique on the running circuit and find out what is really important.

Hello Rex,

At the end I'd also touch the power supply rail. When I did that once on a phantom rail it blew me away. I never thought that could have been the cause.

Regards, Joerg

You don't get any isolation at those frequencies so interchannel interference was obviously not a consideration.

The 4052 is most probably switching all those inputs XLR and LINE into the preamplifier chain. The JRC2068 is a low noise wideband amplifier in wide use in many designs, like a lot of Yamaha stuff- so they did not skimp there. The 62Z SMD marking code most likely belongs to Zetex, and if it's a simple 3-lead single transistor, it is probably a simple emitter follower used to present a high (6.2K ohm they say) impedance to the XLR input and a low impedance source for the JRC2068 for lowest noise-and this does not preclude linearizing feedback from the 2068. From your description, the problem sounds like it is not that the preamplifier noise floor is so high, but that your microphone signal amplitude is so low- at least when loaded by the preamplifier- this is borne out by your observation that the low-level dynamic mics are coming in at lower amplitude than you are used to..

These are definately electrolytic caps on the input. They have a stripe down one side for polarity. I was reading the information by looking through various gaps between frame and other devices with a binocular microscope. I thought I found all the information, but now I agree I must have missed the value/voltage. Naturally, the side of these things that I could easily see had no useful information.

Most of the rest of the board is SMT.

I haven't really tried yet. I kind of assumed that they weren't likely to give me one if I wasn't an authorized service person. I'll ask them now though.

Thanks for the offer. If I get lucky I'll contact you.

to

Ahhhh - good old Philips ! They make good caps - always did. They sold off the passive side and it became Beyschlag-Centralab for a while - but Vishay have subsequently bought it.

Ahhh - that ! Been there - done that !

I'm fairly fussy about the electrolytics that our subcontractors use. PSU caps

*need* to have ESR / ripple current specified of course for example. AC coupling is something else. We've had good results with some of the Asian brands that the Chinese want to use of course. Just need to keep an eye on what they buy !

Graham

Well I did turn off the phantom voltage, so I assumed that essentially turned the mic into a resistor. I just compared the inactive mic to nothing connected and sounds about the same to me. I can try the resistor later, but I expect about the same.

From browsing the web on this product, it seems to be well recognised that the mic preamp is pretty lame. That's why Doug Oade is doing some business by offering mods on the one's he sells. The published S/N spec is 65 dB. He measured 80 dB after his changes. I'd be happy with that.

I have seen comments that if you use an external preamp through the line inputs, it records fine, but that means one more thing to carry and eat batteries.

You *do* realise that some condensor mics produce quite a high level of 'self noise' ? Enough to degrade the mic amp spec for sure.

Try it with a 200 ohm resistor across pins 2 and 3 of the XLR and see what happens.

Graham

I think he already said it used SMT ! Maybe use a pin as a probe ?

Graham

I reckon Feb 2004 personally.

V.likely. See my 'correction' post. If it's as you describe it's certainly an electrolytic cap.

Do you have any way of getting a schematic ? Mic amps are one of my specialties. I'll give you my advice for free.

Graham

sold to consumers.

Certainly in the 'audiophool' category !

Just look at the renaissance of valves / toobs ! As non-linear as you could imagine ( esp with the craze for so-called Zero NFB circuitry ) but they look cool - LOL ! So they

*must* be good ! I guess those heaters are responsible for that *warm* sound.

Oh dear I could go on and on.

A couple of decades ago ppl were raving about DC coupled power amps. Certainly gives great bass damping factor. Now consider how the output transformer of a valve output stage behaves. BTW, the current vogue with the loonies is the single ended triode output. Certainly rich in harmonics. I wonder if the fools realise that what they like about the sound of such is actually its

*inaccuracy* ?

Graham

I just remembered that this thing has a 20 dB mic attenuation switch. That must be one of the functions that the 4052 is doing. There is also a mono recording mode where the left mic input is recorded in the left channel but a 15 dB down version of the left mic is recorded on the right channel. I forgot about all these complexities.

I just tried flipping the 20 dB atten on and it reduces the hiss. Maybe that is telling me that something in the cap/transistor/switch region is the source of the noise.

The line inputs come into the board away from where this 4052 is. I think there may have been another 4052 closer to the line inputs but I'm not sure and don't want to dismantle it agan now.

I saw a discussion somewhere on the net about audio circuits where someone replaced 2086 with AD8066 or some other amps. No one in this thread seems to think the 2086's are a problem, so I'll accept they are probably ok.

I have no reason to doubt that speculation on function.

I had a quick browse around the Zetex pages just now and didn't find any SOT23 NPNs that match the marking code.

Well as I said in another thread message, the specs aren't that good as published and that one guy is improving S/N by 15 db with some kind of mod. There are lots of posts various places around the net that say the main flaw in this recorder is the mic preamps.

I do appreciate everone's comments here. It's helping me think about how to look at it when I get to opening it up again.

transistors.

Ok, this is sounding good (no pun, of course). I was just reading your reply to Fred's message too.

The only large devices in the input area are the 4 (2 per channel I assume) electrolytics. There are then two transistors per channel and they are both SOT-23 form. No TO-92 sized stuff anywhere that I noticed.

Next in line after the transistors seems to be the 4052 then the 2068. I mentioned in the other message some possible functions for the 4052 that just occured to me.

So I await further comments on the transistors.

God - I hope not. Even the Asians aren't normally that dumb !

It's actually *very* similar to the ubiquitous NE5532. Low noise audio specified op-amp ( dual ). Totally fine.

I hope not. Although that could be part of the problem if so. I might expect it to be the second transistor of a compound pair though - such as I use myself.

Low noise input stages require large geometry devices to get intrinsic semiconductor resistance to a low level ( for low thermal noise ). Such devices are pretty much invariably still in TO-92 packages, such as the still available 2SA1084.

There is a simple standard for establishing mic pre-amp noise. It's called EIN

- equivalent input noise. OK you need the test gear to check it - but there is an established standard.

A good mic amp will have an EIN at max gain of around -128 ~ 129 dBu ( that's within a few dB of thermal ) with a 200 ohm source.

0dBu = 0.775V ( historically related to the long abandoned 600 ohm dBm - audio no longer uses 'matched impedance' circuits )

Graham