ACM 200 - transformer

[antichef]

I'm pumped to have these two ACM 200 mics - I had ordered a 200 and a 300, and after the orders closed I was kicking myself for not getting a pair, but due to the swap of the 200 for the 300, I got one anyway

They sound pretty good, but a little lacking in the high end - I haven't done much testing yet. I immediately found myself thinking of the transformer swap I did on my other ACM ribbons, and how much improvement there was. I cracked open one of the 200s.

I saw an internal fiberglass board with what looks just like the transformers that I took out of the ACM2 and ACM3 mounted in it -- also on the board is what looks like a little SMT circuit, but it's been painted over with black lacquer, so you can't really tell what it is. It's "south" of the transformer, so to speak, and so my guess is that it's a phantom powered preamp, a-la a condenser mic, that takes the regular old ribbon mic signal from the regular old ribbon mic transformer and juices it up and sends it on its way.

If that's right, then I *should* be able to put a Lundahl or Edcor ribbon mic transformer in place of the existing one, right (provided I can make it fit)? I don't have time now to pull one of the ACM200 transformers, but Marik taught me how to measure the ratio, so I can do that and compare it to the one I pulled out of the ACM3. If they're the "same" (with some tolerance, of course), then I'm probably right?

 

[mshilarious]

If they are the same ratio, then yes. But with a powered ribbon, you really hope they aren't the same ratio, because if they are, then there is no benefit to having a circuit in the mic. In fact, if they aren't higher ratio, then I would look at swapping in a tranny that is.

The reason is the noise performance--there is no particular reason that a preamp inside a mic is any quieter than a regular ol' preamp (although it could be for a particular unit). However, if the mic transformer is high ratio (and the mic amp has low current and voltage noise), then you can materially improve the signal-to-noise ratio that you'd get over a regular ol' mic preamp.

The reason why you wouldn't use such a transformer without an amp in the mic is the mic's output impedance would be too high.

Before you go chopping it up, it would be nice to measure the mic's self-noise so you have an idea what you are starting with.

 

[antichef]

Before you go chopping it up, it would be nice to measure the mic's self-noise so you have an idea what you are starting with.

What's a good approach to doing that? I don't have an oscilloscope (not that I know if one of those is even used for this).

 

[ausrock]

Anti,

Just wondering if you could supply some info on the pres and ACM 200 mics for me.............I realise you have your pres racked but do you have any idea of what an ACMP 73 might weigh, also the weight of the ACM 200.

I'm waiting on two of each when the shippers eventually get to my order but as the pres are for someone else, I want to try and split the shipping costs up according to the approx., weight of my mics and the other guys pres.

BTW, have you done anything with the capsules yet or are they waiting for the right mic to come along.

Cheers,

ChrisO

 

[antichef]

Just wondering if you could supply some info on the pres and ACM 200 mics for me.............I realise you have your pres racked but do you have any idea of what an ACMP 73 might weigh, also the weight of the ACM 200.

I'll weigh one tonight (USA time) - I'm thinking around 10-15 pounds for the 73. The ACM-200 itself ain't heavy, but it comes in a little flight case, along with a shock mount and stuff - probably 5 pounds altogether -- I'm guessing. I'll weigh one of those too.

BTW, have you done anything with the capsules yet or are they waiting for the right mic to come along.

Two days ago, I put them in the pair of ACM-583s that I got - they sound great!!! thanks again!!!

 

[mshilarious]

What's a good approach to doing that? I don't have an oscilloscope (not that I know if one of those is even used for this).

No, you don't need a scope. What you do need is a test environment that is quieter than the gear under test. That means a reasonably quiet room--get it as quiet as possible, turn off all fans, furnace, etc. Do the test late at night to minimize outside noises like cars, etc. Turn off your monitors and anything else in your room/house you don't absolutely need for the test!

And then it means a known quiet preamp to plug into. Finally, you need a quiet reference microphone; any reasonably quiet LDC should do, although watch some of the cheapies because they seem to have inexplicably high self-noise ratings--you want something 12dBA or less.

Then you need a reference signal level. This should be a 94dBSPL 1kHz tone, but anything in that ballpark is OK, as long as it is the same for all tests. A tuner's A=440 should work fine.

Finally, you need to set enough gain on your preamp such that its output noise exceeds your converter's input noise. +40dB should do.

You should now have a rig capable of resolving 18dBA self-noise or so, maybe better, but if the mic is that good, that's good enough.

First, record the noise of the preamp + converter. The classic way to do this is with a 150 ohm resistor between pins 2 and 3. An inline pad should work well if you have one of those. If not, just plug in a dynamic mic and record your quiet room noise. This is just a reference level; hopefully you won't actually need this. This noise should appear "white" - basically flat on an FFT (you can ignore below 1kHz for this purpose, although that should also be relatively flat here unless you have to use the dynamic mic for a termination).

Next, plug in your quiet LDC and record the test tone. Yeah, just put the thing on a stand and be quiet! You don't need to shove it under a pillow if you room is quiet enough.

You will see a fair amount of activity below 1kHz. This is a combination of the capsule's noise, which is pink, but mostly the residual noise of your room--real quiet is very hard to achieve at low frequencies. That's OK, we are really more concerned with >1kHz.

Do the same thing with your ribbon. Using the relative signal levels of the test tone (NOT the noise!), normalize the two files. Now plot both on an FFT and compare. The ribbon will almost certainly be noisier--it's at a big physical disadvantage to the LDC. The question is how much noisier, and what color--at some point, the ribbon noise will switch over from pink to white noise, and the lower the frequency that occurs, generally the worse it is. So have a look at the 1kHz to 10kHz range, and see how far above the ribbon is from the LDC, and that will give you a pretty good idea of the difference in noise ratings.

If it's 20dB, the ribbon mic/circuit has issues. 10dB is OK. Less than that would be ideal, and is probably a sign of an overwound transformer, or maybe just a hot ribbon motor

Oh yeah, the preamp noise test? If any of the mic noise samples hit that curve (before normalization), then your preamp/converter combo is a limiting factor in the test. Try increasing the gain on your preamp and see if that helps. If not, well, I guess you don't need to worry about mic noise!

 

[ausrock]

I'll weigh one tonight (USA time) - I'm thinking around 10-15 pounds for the 73. The ACM-200 itself ain't heavy, but it comes in a little flight case, along with a shock mount and stuff - probably 5 pounds altogether -- I'm guessing. I'll weigh one of those too.

Two days ago, I put them in the pair of ACM-583s that I got - they sound great!!! thanks again!!!

Fantastic!!! Much appreciated and I'm happy to hear about the capsules.

I'll be interested to know what develops with the trannies for the 200............if you find a suitable upgrade, I may have to ask a favour to get a couple across The Pacific .

Cheers mate,

ChrisO

 

[drBill]

I realise you have your pres racked but do you have any idea of what an ACMP 73 might weigh

6.5 kG per the box.

 

[Marik]

I'm pumped to have these two ACM 200 mics - I had ordered a 200 and a 300, and after the orders closed I was kicking myself for not getting a pair, but due to the swap of the 200 for the 300, I got one anyway

They sound pretty good, but a little lacking in the high end - I haven't done much testing yet. I immediately found myself thinking of the transformer swap I did on my other ACM ribbons, and how much improvement there was. I cracked open one of the 200s.

I saw an internal fiberglass board with what looks just like the transformers that I took out of the ACM2 and ACM3 mounted in it -- also on the board is what looks like a little SMT circuit, but it's been painted over with black lacquer, so you can't really tell what it is. It's "south" of the transformer, so to speak, and so my guess is that it's a phantom powered preamp, a-la a condenser mic, that takes the regular old ribbon mic signal from the regular old ribbon mic transformer and juices it up and sends it on its way.

If that's right, then I *should* be able to put a Lundahl or Edcor ribbon mic transformer in place of the existing one, right (provided I can make it fit)? I don't have time now to pull one of the ACM200 transformers, but Marik taught me how to measure the ratio, so I can do that and compare it to the one I pulled out of the ACM3. If they're the "same" (with some tolerance, of course), then I'm probably right?

As Mshilarious has suggested, most likely the ratio of the transformer is much higher than with no phantom powered circuit. It however does not necessarily mean a lower noise and in fact, can be contrary to what one could expect.

I saw two commercial phantom powered ribbons. Both had unacceptable noise level. The transformer was only part of the reason.

Have you measured xformer ratio, yet?
Any chance to see the guts?

Best, M

 

[mshilarious]

As Mshilarious has suggested, most likely the ratio of the transformer is much higher than with no phantom powered circuit. It however does not necessarily mean a lower noise and in fact, can be contrary to what one could expect.

Well, you have to hope. I mean, an amp with low current noise is easy enough. That just leaves the transformer design, and I'll leave that to you since you know quite a lot more than I do!

But yeah, if it's just a marketing feature, that is bad news . . . there is no reason why you can do a quieter amp run off phantom power then you can with all the current you want . . . I guess you get complete protection of the ribbon from phantom power woes, but that's about it.

 

[antichef]

I'm falling a bit behind on research - I may get some time in the wee hours over the next few days (if I can... stay.... awake.....) - the camera's back in town, in any event. At least subjectively, I can say the ACM-200 doesn't seem noisy.

 

[Marik]

Well, you have to hope. I mean, an amp with low current noise is easy enough.

Well, yes, and... no .
Indeed, it is quite easy to make low-noise amp for a condenser mic. However, the situation with the ribbon mic is not as easy. The whole problem is that JFETs work very nicely with high source impedances, but not so well with low. In fact, there are only very few devices (most of them BTW, are whether obsolete, or very expensive) which would suit the application. Here the common gate connection would make more sense, but opens the whole can of different worms, so only some neat tricks would do...
BJT's would work better, but there are not very many readily available with low enough Rbb to fit the bill. Besides again, they have other problems and the sound mojo is not the last.

That just leaves the transformer design...

The main problem here is copper resistance. Say, we have primary DCR 0.1 Ohm eek:, but not unusual to see). With 1:40 xformer the preamp sees it as 0.1X40^2=160 Ohm of resistance noise. Say, our transformer is 1:100, then with the same pri DCR we have already 1KOhm of PURE NOISE .
On top of that take shunting effect of the sec. copper resistance and other losses.

As an example, my own ribbon transformer primary has only 0.0002-0.0004 Ohm, which with 1:100 ratio would translate into just a few Ohm of noise, together with the secondary.

IMO, with a small core EI the Chinese ribbons use it is impossible to make a low noise high ratio transformer.

I guess you get complete protection of the ribbon from phantom power woes, but that's about it.

IMO, the main feature of phantom power here whould be presenting a stable and uniform load seeing by the ribbon, which would make it work in the most optimized way for given construction.

Best, M

 

[antichef]

OK - one step at a time - first the picture:

there's no circuitry on the other side of the board - just the other side of the transformer.

 

[antichef]

OK, so I'm trying to get sane measurements off of the cheap ribbon transformers that I took out of my group buy ribbons. Marik at one point successfully coached me to test a tube mic transformer, and everything worked as expected - but he also gave me explicit (color of each wire) instructions as to what was the primary and what was the secondary. Additionally, he warned that because of the higher ratios and lower current handling capacity of ribbon mic transformers, that I should apply a small voltage to the secondary and see how it gets reduced on the primary. Apparently, it's easy to fry these things if you mess up.

What I'm doing is to run a "test oscillator" plugin in my DAW, and output to a TRS cable -- to start, I output a 1khz sine wave and put one test lead on the sleeve and one on the tip, and measure AC voltage - then adjust the gain to get to where I want to be on the voltage.

That much is consistent - I went for 0.015 volts at that point. It's kind of downhill from there. The transformer I'm working with has a red and white lead that went to the ribbon motor, and a black lead (underneath the red lead) and a bare copper lead (underneath the white lead) that went to the XLR pins. Extrapolating from what Marik told me, I want to say that the red and black leads are the primary, and the white and bare copper leads are the secondary. Make sense? But when I do my measurement, I tend to come up with a ratio of 1:1, which doesn't make sense. Unless... I've already messed up and shorted the transformer?

 

[mshilarious]

I don't think the trafo is THAT fragile . . . I mean, 15mV? Anyway, if red and white go to motor, I would think those are the primary? So apply your voltage to black and bare, and see what happens . . .

I've done much more hideous things to ribbon trafos . . . like 30VAC, or 48VDC . . . they should only melt if the load is excessive or the DC through the winding is excessive (you should keep that really small, <1mA), but from the sound of it, you have the transformer unloaded and only an AC signal applied?

 

[dgatwood]

I doubt that even an ohmmeter would put out enough power to be an issue... by several orders of magnitude.... Speaking of which, assuming the primary and secondary windings are made of similar thickness wire, couldn't you just measure the resistance of the two windings? Or is this just an issue of insufficient accuracy?

 

[antichef]

Yes - the transformer has been removed from the mic and is completely isolated -- the voltage is coming from my audio interface, which can put out several volts if I turn up the gain high enough, but as I mentioned, I've turned it down instead of up.

Probably right that I'm confused about the primary and secondary - not sure why I have so much trouble with that. When I take everything back out tonight, I'll measure the resistance, too.

BTW, I did take the measurements as mshilarious suggested for the self noise rating, but haven't yet done the analysis.

 

[mshilarious]

assuming the primary and secondary windings are made of similar thickness wire, couldn't you just measure the resistance of the two windings?

I think that's a dangerous assumption with a ribbon trafo--the goal is to get the primary DCR to a super-low figure.

 

[rodabod]

Try using a multimeter to identify the primary and secondary. Primary will have very low DCR, secondary will be relatively high DCR. Obviously both should be isolated from each other.

 

[antichef]

Try using a multimeter to identify the primary and secondary. Primary will have very low DCR, secondary will be relatively high DCR. Obviously both should be isolated from each other.

make sense! (except that I wouldn't have know about the difference in resistance). Also a good way to be sure I haven't shorted the little guy!