Real Mic from a Chinese Toy

[clintrubber]

[quote author="jdbakker"][quote author="Rossi"]@ clintrubber: Are you sure your P48 supply is as powerful as it should be? The 47.3 V unloaded could suggest that it may not be fully compliant to P48. Dead short, each of the legs should supply about 7 mA.[/quote]
Last I checked P48 was defined as 48V +/- 4V , so 47.3V looks perfectly legal.

JDB.
[couldn't easily find an online copy of IEC/DIN EN 61938 (the Phantom spec), so no link, sorry][/quote]

Hi JD,

FWIW, I recall that that +/-4V is even added on the leaflet that's added to the MCA SP-1 package, so by doing so I figure they realized they should make a mic that does reasonably well over that complete 8V range.

I understand Rossi's wondering about the dropped unloaded P48, might have indeed been a sign of a 'soft' supply. But the reason will be otherwise, since that desk should in principle be able to work with 24 channels worth of phantom. Dunno if they really went that far; in the fine Beh. tradition they might have skimped a bit on supply-muscle :wink:

Let's see what happened here the last few days:
Rossi posted various info, from which I picked one sentence and we all got pretty sidetracked in a not too unrelevant way. (I'm not unguilt)
But back to the actual mics again ? (if there's still interesting stuff to be discussed)

Bye,

Peter

 

[burdij]

The spec (DIN 45 596) quotes 48VDC +-4 V. The DIN spec quotes the maximum current at 2ma per input. The newer IEC spec quotes a maximum current of 10ma. In order to deliver 2ma. through a pair of 6800 Ohm resistors and produce 48V at the connector, you would need to set the supply at 55VDC. I have noticed in the Neumann phantom supplies, the voltage is often set to 52VDC (the value I use also). This will keep you inside the +-4V limit at 2ma.

The oscillator DC/DC converter will only draw enough power from the phantom to supply the losses in the circuit and any power output delivered to the "load". Since the load essentially draws no current (being a capacitor), the oscillator only has to make up for power losses within its own circuit (such things as transistor bias current networks and core losses). These circuits draw very little current in a microphone.

 

[Guest]

[quote author="Rossi"]

@ wavebourn - Most real world preamps have input impedances of about 2k (not 200 ohms), the absolute maximum would be 13.6k due to phantom resistors. If you lowered the value to increase the max current, you'd also lower the input impedance limit.
[/quote]

It is not necessary: I could make a box to supply 48V by a twin constant current source with a servo, in such case I could get as high input impedance as needed to make that poor emitter followers to live a happy life. But I decided instead to modify mics using better amp with needed roll-off on highs and good output transformer, now I can connect them to any equipment and get nice sound as the result.

However, building such boxes is an option, since it is easier to add it to the setup than to modify stock mics. I can even use some tubes to get nice +4 dB output.

...the next thread will be called "A tube pre for a Chinese FET mic"

 

[zapnspark]

The January 2002 issue of Recording magazine had an article by Scott Dorsey that included a schematic of a Schoeps-like microphone circuit.
Unfortunately, an important component of that circuit was left out.
It appears to me that the right hand part of the schematic was truncated.
Repetitions and variations of that circuit are all over the internet.
For reference, look at the schematic for a real Schoeps CMC microphone.
You can see that a 47 uF capacitor (C9) is connected to the junction of collectors of T2 and T3 and to ground. This capacitor creates an output impedance of the emitter followers in the neighborhood of 45 ohms.
Without it, the impedance rises to over 150 ohms and drive is compromised. The lack of this capacitor also causes an early onset of distortion at high SPLs. Those emitter follower collectors need a low AC impedance path to ground and that's exactly what C9 provides.
I think that some of the simulations done on this circuit have been done WITHOUT that capacitor. If so, performance appears far less than optimum.
This topic has been covered in more detail over at the Yahoo Micbuilders group.
This is a "neglected topic" that I thought I should bring up here.

ZAP

 

[Rossi]

[quote author="jdbakker"][quote author="Rossi"]@ clintrubber: Are you sure your P48 supply is as powerful as it should be? The 47.3 V unloaded could suggest that it may not be fully compliant to P48. Dead short, each of the legs should supply about 7 mA.[/quote]
Last I checked P48 was defined as 48V +/- 4V , so 47.3V looks perfectly legal.

JDB.
[couldn't easily find an online copy of IEC/DIN EN 61938 (the Phantom spec), so no link, sorry][/quote]

Yeah I know, that's why I said it could suggest that. I was referring to the current requirement, not the voltage requirement. Many p48 supplies crap out if the mic draws more than say 3 mA.

Anyway, my ancient Behringer MX8000 was okay, in that respect.

 

[Rossi]

[quote author="zapnspark"]
Unfortunately, an important component of that circuit was left out.
It appears to me that the right hand part of the schematic was truncated.
Repetitions and variations of that circuit are all over the internet.
For reference, look at the schematic for a real Schoeps CMC microphone.
You can see that a 47 uF capacitor (C9) is connected to the junction of collectors of T2 and T3 and to ground. This capacitor creates an output impedance of the emitter followers in the neighborhood of 45 ohms.
Without it, the impedance rises to over 150 ohms and drive is compromised. The lack of this capacitor also causes an early onset of distortion at high SPLs. Those emitter follower collectors need a low AC impedance path to ground and that's exactly what C9 provides.
I think that some of the simulations done on this circuit have been done WITHOUT that capacitor. If so, performance appears far less than optimum.
This topic has been covered in more detail over at the Yahoo Micbuilders group.
This is a "neglected topic" that I thought I should bring up here.[/quote]

Yes, you're absolutely right. I hadn't noticed before. I wonder if it was a mistake or if SD left that capacitor out for some reason. His circuit doesn't follow the Schoeps original in all respects, after all. He left out other things as well, such as the bandwidth limiting capacitors and of course the oscillator. Also, he uses a different FET at a considerably higher voltage.

 

[Gus]

Wavebourn

I have read most of this thread and I don't understand what your issue with the real Schoeps circuit is.

For vocal microphones I tend not to like it BUT this is a TASTE issue. If one looks close at the Schoeps circuit on the web the output pair is matched and the bias resistors are selected.

For fun work out the 12 and 48 volt operating points of the output pair.

There is something small I would change in the design but I don't want to post because it seems people are missing it or if they see it they don't post.

This is more a taste and cost issue IMO.

 

[Guest]

[quote author="Gus"]Wavebourn

I have read most of this thread and I don't understand what your issue with the real Schoeps circuit is.

[/quote]

The issue is usage of emitter followers in hope they will perform like a transformer. I am going to build a box for such widely available output to make it happy, i.e. supply enough of current and provide high enough input impedance.
About 25 years ago I've designed a railway communications system that consisted of power amp (600W output), many horn speakers on poles, and microphones with buttons on that poles. Originally, it had mic transformers, but I decided to go electronically and implemented something similar to Shoeps' mic amp. I've ended up with distortions and parasitics, but the idea was to save costs for better sound quality without expensive transformers. Supplied by enough of current and loaded by high impedance it may give a decent sound, but a need for such symmetrical output to drive a box on a short cable is questionable.

Did you consider open collector (or open destination tyhat is simplier) outputs powered by a phantom power?
For example, couple of JFETs with a Zener in sources, the rest of amp (and capsule's CD/DC converter) is powered from this Zener?

Speaking of tastes, they may be good, bad, and horrible...

 

[Marik]

[quote author="Wavebourn"][quote author="Gus"]Wavebourn

I have read most of this thread and I don't understand what your issue with the real Schoeps circuit is.

[/quote]

The issue is usage of emitter followers in hope they will perform like a transformer...[/quote]


I was unaware of that and always thought that in fact, they were designed to perform and sound like a transformerless :?.

The bottom line, could you in a blind test identify Schoeps, vs. let's say tube, ribbon, or FET source follower mics?

 

[Guest]

[quote author="Marik"][quote author="Wavebourn"][quote author="Gus"]Wavebourn

I have read most of this thread and I don't understand what your issue with the real Schoeps circuit is.

[/quote]

The issue is usage of emitter followers in hope they will perform like a transformer...[/quote]


I was unaware of that and always thought that in fact, they were designed to perform and sound like a transformerless :?.

The bottom line, could you in a blind test identify Schoeps, vs. let's say tube, ribbon, or FET source follower mics?[/quote]

Happy New Year, Marik! :thumb:
Mr. Ohm is waiting for you... :green: