JohnRoberts

Re: Phantom power with no blocking caps in transformerless circuit?
« Reply #20 on: June 16, 2020, 05:09:17 PM »
Please enlighten me JR. Like I said, I qualified my post with the clause that we're not talking about cost. Otherwise, my understanding has always been that (cost aside) a transformer is superior in every metric.
and I disagree...
Quote
Even if noise of a high-perf transistor is less than a transformer under whatever confluence of circumstances you're citing,
a preamp
Quote
it seems to me you could use a 1:1. But another benefit of the transformer is that you can match the OSI of the transistors behind it. So if my understanding is incorrect, please explain the details under which an active circuit can beat a transformer in any metric, noise, cmrr, whatever. A transformer is a passive device. So where is this noise coming from that is being spanked?
It isn't just noise but frequency response and linearity too....

A transformer involves converting voltage into a magnetic flux, couples that flux from primary to secondary, then converts it back to voltage. A very good transformer can be very good, but not even close to a straight wire with gain... Of course solid state preamps are not perfect either but the design decision I made almost half a century ago was that the cost of premium transformers was just not justified wrt to comparable performance.

If cost was no object I still wouldn't use a transformer (personal choice, I'm frugal), but as I already shared they have their benefits for live sound dealing with questionable grounds, in a "show must go on" time pressure environment. In a permanent studio application we have the luxury of time to sort out ground problems.

JR   

PS: We used a good transformer inside the Peavey tube mic preamp, but that was a homage to classic design, not a best numbers exercise. Tubes don't work well transformer-less.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


squarewave

Re: Phantom power with no blocking caps in transformerless circuit?
« Reply #21 on: June 16, 2020, 06:42:00 PM »
Obviously I'm not talking about replacing a straight wire with gain with just a transformer. Most of the gain would be from the pre that followed it. If we agree that a CFIA circuit is about as good as it gets, then putting a transformer in front of that is going to be hard to beat. It eliminates a number of parts, has superior CMRR and matches impedances more closely which all equates to lower noise. Aside from low-frequency distortion (which would be at levels nobody cares about), it's not entirely clear to me that a transformer circuit would not be superior in every way. At least nothing about your answer has really cleared that up for me.

Re: Phantom power with no blocking caps in transformerless circuit?
« Reply #22 on: June 16, 2020, 07:35:01 PM »
The thing is, and I get what JR is trying to say, that by using a much cheaper and much cleaner part like a transistor there is really not enough justification to use a transformer, sure you can get rid of the input capacitors and they have great CMRR, but is that high CMRR really needed in most applications? perhaps if you are on the road and don't want any surprises then a transformer is justified. As for the THAT 1512, that chip was designed to get rid of the transformers! it has a 1.3dB Noise Figure with a 200 ohm load, that is almost as good as it gets, using a transformer will yield marginal noise figure performance and you'll degrade the 0.001% THD specification, plus you'll be spending around $100 more, when a $7 part is enough.
« Last Edit: June 16, 2020, 07:38:05 PM by Dualflip »

squarewave

Re: Phantom power with no blocking caps in transformerless circuit?
« Reply #23 on: June 16, 2020, 08:45:05 PM »
As for the THAT 1512, that chip was designed to get rid of the transformers!
I think perhaps you're thinking of THAT 1200 "InGenius" line inputs.

A transformer eliminates the coupling caps and protection diodes (although I would probably keep the RF filter because it should be integrated into the connector). That's like 8 parts that don't have unshielded PCB traces running all over the place. And you don't have to worry about "The Phantom Menace".

And you don't need to spend $100. A good shielded mic input can be had for much less.

You can rationalize all you want but personally, if I'm going to take the time to design and build some one-off gear, I'm not going to skimp and use some weird circuit that jumps through hula-hoops just to eliminate blocking caps. Electrolytic are good enough these days that you just don't need to worry about that anyway. Whoever came up with that circuit was probably just doing it to show people how clever they are. It's one of those theoretical things you will never / should never actually build.

Re: Phantom power with no blocking caps in transformerless circuit?
« Reply #24 on: June 16, 2020, 10:23:37 PM »

You can rationalize all you want but personally, if I'm going to take the time to design and build some one-off gear, I'm not going to skimp and use some weird circuit that jumps through hula-hoops just to eliminate blocking caps. Electrolytic are good enough these days that you just don't need to worry about that anyway. Whoever came up with that circuit was probably just doing it to show people how clever they are. It's one of those theoretical things you will never / should never actually build.

I totally agree with you on that, however, I do not agree that an IC such a 1512 needs a transformer, its just fine as it is, I don't mind the "extra" parts since they are dirt cheap compared to a good transformer


 

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