fully differential transformer balanced mic pre?

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bigugly

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Jun 27, 2004
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I've been tinkering with the idea of making this. http://groupdiy.twin-x.com/displayimage.php?pos=-441 Does anybody see anything wrong with it before I go and make a PCB. thanks

James
 
As Jakob said, it is arguable if there's any advantage for balancing. I once asked Bill Whitlock about this - I hope it is OK to post his answer for your reference:

Q:
I had the pleasure to read an article ("Practical Circuits For Quiet Audio Transmission") by Walter Jung; in this article, the author shows that a JT-11P-1 has better CMRR if it looks into a balanced load, in this case an instrumentation amplifier in the usual three-opamp-configuration. However, in your application notes I do not see balanced loading of input transformers. Is there a distinct disadvantage of doing so (except higher cost etc.)? Will every input transformer benefit from a balanced interface? Is a simple one-opamp-instrumentation amplifier sufficent to improve the CMRR?

My questions are related to a microphone preamp design I'm working on and which will use a JT-13K7-A, but I would be glad if you could answer them in a more general fashion.

Jung's article may be downloaded from his site
(http://home.comcast.net/~walt-jung/wsb/html/view.cgi-home.html-.html).


A:
Thanks for writing Jensen. We do not generally recommend balanced loading of our input transformers. This is especially true for microphone input transformers. In order to properly balance the distributed capacitances of the secondary, the necessary design tradeoffs would compromise bandwidth and time-domain response. Since noise is a prime consideration for mic preamps, and single-ended preamp topologies will always have lower noise than balanced ones, we intend the secondary to be (ac) grounded at the indicated end.

In the case of the single JT-11P-1 that Mr. Jung's experimented with, a significant CMR improvement was indeed noted, but I hasten to add that he did not measure frequency or phase response in that configuration. I believe it would be compromised somewhat. I also believe, because of the lower impedances involved, that a balanced secondary load will have far less effect on the JT-13K7-A. I don't know if a simple differential amplifier (single op-amp and 4 resistors) would give comparable results. It would probably depend largely on the input impedances of the circuit.


Note that his answer is related to Jensen transformers. There are other transformer manufacturer that recommend balanced loading (Lundahl comes to my mind) - it all depends on the specific transformer.

Basically I see two advantages for balanced configurations:
* higher output level
* very high CMRR both for the input- and outputtransformer (may depend on type)

Both are prone to be completely irrelevant in practical use.

Specific thoughts on the given circuit:
* I would distribut the ICs differently - make U1A and U2A to U1A and U1B etc. This way, the output offset of the line driving pair tends to match and thus you can skip the output coupling caps (C9/C10).
* The input transformer needs a very high turns ratio for best noise performance. The OPA604 is a voltage-noisy coveal and wants a high source impedance, which is doubled due to the balanced nature of the circuit. Without taking the pocket calculator out, I'd say a 1:10 is the lowest reasonable turns ratio if you care about noise.
* Where does the first stage pair get their + input bias current from? It is low with JFET inputs, but not zero. Relying on leake is not good. Replace R5 with two grounded 11k (you'll need much higher values for the suggested 1:10 transformer).
* At low gains, the first opamp pair has to drive a rather hefty load - if you use the 1:10 transformer, I would double the impedance level of the feedback network (i.e. R1=440, R3=860 etc.).
* If you intend to drive 600 ohm loads, you should use a 2:1 output transformer, as with a 1:1 each opamp will see 300 ohms which is too low. You'll loose the high output level with this.
* Just to make sure: you know that the dual pot should be of linear law? A big advantage of this topology.

Sorry for the extended writing - I hope it is for any use!
Samuel
 
The JT-MB-CPCA was made exactly for the purpose of decoupling balanced mic input circuits although more for retrofitting purposes. Then again, there is no compelling reason to do it as Samual pointed it out already.
 
thanks for the fast replies!

Jakob, I don't have a very deep un derstanding of electronics so when I drew this up it started as a normal trans I/O dual opamp pre. Also, I had seen a differential pre on the market (ViPre) so I thought, "what the heck, build it and listen". So no, there was no particular reason for this.

Samuel, that is very informative. It seems that there is really no benefit to my design so I'll just eliminate the bottom half, ground the lower lead of the secondaries and build it as you and Jakob suggest.

Thanks again!

James
 
Agree with the comments. A few more:

If one were to pursue the fully balanced mode (which I agree is not necessary and worse for random noise) the tracking of the potentiometer sections will be important, so that spec should be given attention.

It's tempting to consolidate C1 and C2 into a single bipolar 'lytic and lift the ground referencing at that point. This makes the input stage have unity gain for common-mode signals intrinsically, independent of the resistor and potentiometer matching.

C3 and C4 aren?t doing much across the 430 ohm R?s---they should probably connect between the first amps? inputs and outputs.

Although the offset voltages and bias currents are small, the electrolytic coupling caps should have symmetrical polarity. Right now they alternate between circuit halves.

As shown, assuming that the transformers have the same winding sense vs. pinout, and that input and output connectors are wired similarly, the preamp will invert absolute polarity, which you will not want. Flipping the output pin connections will fix this.

Further (unnecessary) enhancements might change the second stage configuration so that it too has common-mode rejection. Probably the most promising would be the ?SuperBal? (see Fig. 12 of this reference: http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm ) which uses two op amps with the second as an inverter with feedback from its output to the n.i. input of the first amp, to synthesize two summing nodes at the first op amp. This configuration can be a bit tricky to phase-compensate though. Note that although Self doesn?t indicate it, the output of A2 is the complementary polarity to the one shown, so this is a differential output stage as well.

Again, the improved common-mode-rejection enhancements are most likely unnecessary, although it would be a different situation for transformerless configurations.
 
I agree with the answers.... there's no point.
That form of mic amp (working unbalanced) certainly works well; the advantages are distributed gain between the stages, huge possible gain swing, and sensible law for a lin pot.
I met a guy a couple of years ago who claimed to have invented it... (I had been using it on and off for at least 20 years).
There are a number of ICs with good enough noise performance to make it practical with say 1:4 ratio input transformer... and that makes the CMRR easier to handle and helps with overload margin.
 
[quote author="TedF"]I met a guy a couple of years ago who claimed to have invented it... (I had been using it on and off for at least 20 years).
[/quote]

That happens so much. I try to be encouraging to people who really want to invent something but it's tough to do.

The "SuperBal" stage mentioned above has been attributed to more than a few people; Self referenced Steve Dove who I believe told me he invented it, although I think it preceded that. Steve does not claim invention in the famous chapter on consoles iirc.

Another pronounced effect I've noticed is how an idea will suddently be "in the air" and thought about by several people at about the same time. The ones who rush to publish and/or patent are the "winners" in a sense---if it's a good idea anyway :razz: .

Feynman was talking to a bunch of brainy high schoolers at a summer program and one of them asked him "Can you tell us about the theory that a positron is an electron travelling backwards in time?" Feynman: "Well, yeah! That's my theory!"
 
I didn't mention it in my post, but actually, I invented the 'superbal' including the name, in 1973 in Windsor UK, (38 Alexandra Road) and it forms the basis for a lot of my current (!) work. (Self aught to know this; it's common knowledge)
It was an idea scrawled on my 'whiteboard' one morning... and we used it in the form of a balanced mixer in a matter of days. (Alice mixers)
Steve Dove was actually working for me at the time, and if he looks in on these forums, pls come in on this one!
I didn't think it was relevant to the subject of the thread.
 
Ted, perhaps on a previous forum, perhaps on this, there was some discussion on the wording of the balanced current system and IIRC, a little puzzlement.

Can you give us all a brief rundown on how and why these terms are applied, or might that be like asking the chef for the recipe? :wink:

Yours most respectfully,

Keith
 
[quote author="TedF"]I didn't mention it in my post, but actually, I invented the 'superbal' including the name, in 1973 in Windsor UK, (38 Alexandra Road) and it forms the basis for a lot of my current (!) work. (Self aught to know this; it's common knowledge)
It was an idea scrawled on my 'whiteboard' one morning... and we used it in the form of a balanced mixer in a matter of days. (Alice mixers)
Steve Dove was actually working for me at the time, and if he looks in on these forums, pls come in on this one!
I didn't think it was relevant to the subject of the thread.[/quote]

Thanks! That is great to know. I will credit it appropriately from now on :grin:
 
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