Coil audio transformers?

[Moby]

wow... "incoherent blurb"... hmm...

1:10 is the ratio of windings from the primary side to the secondary side. 150:60k is the "nominal impedance" or "reflected impedance", has more to do with inductance, and is not necessarily relevant to the turns ratio, or DCR readings. This describes the two loads the transformer is meant to see on the primary and secondary windings to operate at proper bandwidth. That's why an old school 50ohm mic sounds so weird and mid rangey  when plugged into a modern mic pre meant to see 150ohms or higher.

Hi Coilaudio. Excuse my ignorance, but I still don't understand relation between 1:10 ratio and impedance 150:60k.  Maybe I missed something, but my knowledge tells me that 150:60k should be 1:20. Can you please show us the formula you used to calculate 1:10=150:60k?

 

[abbey road d enfer]

wow... "incoherent blurb"... hmm...

1:10 is the ratio of windings from the primary side to the secondary side. 150:60k is the "nominal impedance" or "reflected impedance", has more to do with inductance, and is not necessarily relevant to the turns ratio, or DCR readings. 

Nominal impedance is directly related to inductance and expected LF extension. Reflected impedance has to do with turns ratio. These terms are not interchangeable.

  This describes the two loads the transformer is meant to see on the primary and secondary windings to operate at proper bandwidth.

How do you reconcile that with your earlier statement "Our transformers are designed to go straight to the tube". Are there any tubes that present 60k at the grid?

I want you no harm, but please allow us the fact that some of us are actually scientists.

 

[PRR]

"60K" may be the notional/nominal impedance of the expected tube grid capacitance at the top of the frquency range. 100pFd (Miller on high-gain triode) is 60K impedance at 27KHz.

If you truly had 60K on the secondary of a 1:10, then you have 600r at the primary. *Most* lo-Z mikes expect a load greater than their own impedance. 600r is nominally higher than 100-200r.

What we really have, without fix-up networks, over the top of the audio range, is a primary load which declines from very-high down to about 600r (100pFd*(10^20) at 27KHz-- i.e. around 1,200r in the top octave, a happy mis-match to a 100-200r mike.

This IS conventional loading for tube inputs. Ratios do vary from 1:7 to 1:15 to trade-off voltage gain and hiss against bandwidth and cost. 1:10 is a sweet-spot for 100-200r mikes. The "Ohms" you put against that are, of course, reflections of what is connected TO the transformer, not properties of the transformer.

 

[coilaudio]

Hi Coilaudio. Excuse my ignorance, but I still don't understand relation between 1:10 ratio and impedance 150:60k.  Maybe I missed something, but my knowledge tells me that 150:60k should be 1:20. Can you please show us the formula you used to calculate 1:10=150:60k?

Sure, they are actually 2 different specs, as opposed to one spec written two different ways.

1:10 is the "turns ratio" and has to do with the gain, or loss of gain you get when putting a transformer in circuit. On a 1:10 step up transformer, if you put 1v on the primary, you get 10v on the secondary. Same example for a step down transformer with a 4:1 turns ratio; Put 4v on the primary, you get 1v on the secondary.

The 150:60K is the separate spec that relates to the "nominal impedance" and "reflected impedance" of the transformer when it is in an ideal circuit. In other words, with a 150ohm mic plugged into the primary, 50k would be reflected on the secondary to the input grid of the first tube.

 

[coilaudio]

Nominal impedance is directly related to inductance and expected LF extension. Reflected impedance has to do with turns ratio. These terms are not interchangeable.

  How do you reconcile that with your earlier statement "Our transformers are designed to go straight to the tube". Are there any tubes that present 60k at the grid?

I want you no harm, but please allow us the fact that some of us are actually scientists.

ef86, 6j7, 5879... Sensitive tube grids like looking at a high impedance secondary. Always wondered why UTC went through all that trouble.

UTC LS10 and LS10x:  50-600ohm primary,  50-60k secondary
UTC A10, A11, A12:  50-600ohm primary, 50-80k secondary
UTC HA100/HA100X and HA101/HA101X: 50-600ohm primary, 60k, 80k 120k secondary
UTC o-1 and o-2:  50-600ohm primary,  50-60k secondary

While I'm a huge  fan of science and smart people in general, I think we generally prefer to do design work with our ears instead of oscilloscopes. What looks good on paper doesn't always sound good to the ears. You can check out my credits at Allmusic.com (Jim Vollentine), or check out our Instagram to see what kind of gear we hang out with, repair,  work on, and use every day.

 

[Moby]

Sure, they are actually 2 different specs, as opposed to one spec written two different ways.

1:10 is the "turns ratio" and has to do with the gain, or loss of gain you get when putting a transformer in circuit. On a 1:10 step up transformer, if you put 1v on the primary, you get 10v on the secondary. Same example for a step down transformer with a 4:1 turns ratio; Put 4v on the primary, you get 1v on the secondary.

The 150:60K is the separate spec that relates to the "nominal impedance" and "reflected impedance" of the transformer when it is in an ideal circuit. In other words, with a 150ohm mic plugged into the primary, 50k would be reflected on the secondary to the input grid of the first tube.

Thanks. So, let me elaborate. You want to say that your transformers are designed that way so turns ratio isn't conneced directly with the reflected impedance?

 

[iampoor1]

The 150:60K is the separate spec that relates to the "nominal impedance" and "reflected impedance" of the transformer when it is in an ideal circuit. In other words, with a 150ohm mic plugged into the primary, 50k would be reflected on the secondary to the input grid of the first tube.

I dont understand this. If a 150ohm mic is plugged in to a 1:10 input transformer, with a 60k secondary loading, isnt the mic mismatched to the transformer, because the transformer is presenting a 600ohm impedance?

 

[emrr]

No.  Books have been written on this site and elsewhere about reflected Z. 

 

[abbey road d enfer]

I dont understand this. If a 150ohm mic is plugged in to a 1:10 input transformer, with a 60k secondary loading, isnt the mic mismatched to the transformer, because the transformer is presenting a 600ohm impedance?

It is indeed mismatched, in the strictest sense of the word, which implies optimum power transfer, which occurs with a loss of 50%. Microphones are supposed to be bridged, i.e. optimized for voltage transfer. There is no strict definition for bridging, although it is commonly admitted that the load impedance should be about 10x the source impedance, resulting in an input impedance of 1.5-2k for a mic. 600r reflected impedance clearly does not satisfy the predicament, which doesn't mean the performance will be necessarily degraded significantly. Mic designers optimize their products in view of the 10x rule, however, there's a recent tendancy to increase the input impedance of mic inputs, as in some cases they would measure at 10-20k. The audible benefits are not always obvious nor decisive. However, IMO, reducing the input impedance never gives interesting results, sonically.
I should mention that in most cases, variable impedance is achieved by shunting the signal with a potentiometer, which is an objective degradation of performance. Academically, variable impedance should be accompanied with simultaneous adjustment of the operational parameters of the circuit. One can consider the Neve 300/1200r switching as a better alternative, since it changes the input xfmr ratio at the same time, but it doesn't change the input transistor's quiescent in order to optimize the OSI (Optimum Source Impedance); the tolerance to source Z is vast enough to make that unnecessary in practice, though.
Tube mic pres present a specific case, as the secondary is supposed to be loaded only with the grid of the input tube. As a result, the impedance presented to the mic is variable, since it depends very much on the raising reactance of the primary's inductance, resulting generally in a gentle attenuation of LFand upper HF resonance, until it hits the limit due to the Miller capacitance of the input tube, which can be as high as 100pF, as PRR mentioned, for a typical triode, or much lower, for a pentode, or a cascode stage.
The gentle brightening of the frequency response is a fundamental design point in RCA ribbon microphones, where the load is specified as transformer with unloaded secondary connected to the grid.

 

[abbey road d enfer]

Googles refected impedance
Is that anything like lowering impedance for a dynamic mic to preamps that have an input impedance higher than the mic wants to see?

That would be a transformer; I would think this could introduce effects worse than underloading the mic.

I saw a Youtube video about this where someone from Shure mentions how it'll affect the resonant frequency response.

Beware of Youtube! Yes, loading a mic affects the frequency response; in several ways. Reducing the load Z decreases the level, first, then it decreases treble, because of the inductive nature of dynamic mics. Inductive source into a resistive load=> low-pass filter.
It also reduces the amplitude of the resonance peak due to the mechanical resonance of the diaphragm, by dampening it, but the effect is limited, due to the very low Qe ( electrical quality coefficient), and the intrinsic high Qm (mechanical quality coefficient) of the moving system.

 

[Gold]

There was a large Langevin install at the UN when it was built, in fact at least one Langevin catalog piece was designed specifically for that install.

There were a lot of people drooling when they remodeled the UN. Apparently they spared no expense on the original audio install.

 

[iampoor1]

It is indeed mismatched, in the strictest sense of the word, which implies optimum power transfer, which occurs with a loss of 50%. Microphones are supposed to be bridged, i.e. optimized for voltage transfer. There is no strict definition for bridging, although it is commonly admitted that the load impedance should be about 10x the source impedance, resulting in an input impedance of 1.5-2k for a mic. 600r reflected impedance clearly does not satisfy the predicament, which doesn't mean the performance will be necessarily degraded significantly. Mic designers optimize their products in view of the 10x rule, however, there's a recent tendancy to increase the input impedance of mic inputs, as in some cases they would measure at 10-20k. The audible benefits are not always obvious nor decisive. However, IMO, reducing the input impedance never gives interesting results, sonically.
I should mention that in most cases, variable impedance is achieved by shunting the signal with a potentiometer, which is an objective degradation of performance. Academically, variable impedance should be accompanied with simultaneous adjustment of the operational parameters of the circuit. One can consider the Neve 300/1200r switching as a better alternative, since it changes the input xfmr ratio at the same time, but it doesn't change the input transistor's quiescent in order to optimize the OSI (Optimum Source Impedance); the tolerance to source Z is vast enough to make that unnecessary in practice, though.
Tube mic pres present a specific case, as the secondary is supposed to be loaded only with the grid of the input tube. As a result, the impedance presented to the mic is variable, since it depends very much on the raising reactance of the primary's inductance, resulting generally in a gentle attenuation of LFand upper HF resonance, until it hits the limit due to the Miller capacitance of the input tube, which can be as high as 100pF, as PRR mentioned, for a typical triode, or much lower, for a pentode, or a cascode stage.
The gentle brightening of the frequency response is a fundamental design point in RCA ribbon microphones, where the load is specified as transformer with unloaded secondary connected to the grid.

Thank you very much. Makes sense, and clears up some other questions I also had. I really appreciate your explanation.

 

[rackmonkey]

Hey Jim. Welcome to the forum.  Steve sent me the CT-41 and very kindly threw in a CT-41HN to sample. Appreciate you guys taking the time to work with a small fry DIYer, especially given that the intended use for these is in Coil Audio gear. 

I've got a few repairs to do before trying these in-circuit, but I did some quick measurements when I got them and based on the results am anticipating that these will perform very well.  The ears are the ultimate test, so I'll report back once I get the preamps running,

For those interested, I did a lazy man's frequency "sweep" (using white noise) with my usual setup of function/noise gen->DC signal amplifier->series resistor (7.5k, 10k, 15k on respective tests)->DUT->load decade box (600 ohms)->Pete Millett sound card interface (new!)->sound card->REQW. Results are on Google Drive, linked in the "a bunch of transformer freq response tests" post in The Chamber.  Flat response 20Hz-24kHz w/slight rise (about 1dB, iirc) above 13k or so (again, iirc) on the CT-41, similar results w/CT-41HN but with slightly higher rise in the high frequencies. Can't measure bandwidth beyond that because my sound card doesn't support it.

I've always used a Leader LCR-740 LCR bridge for inductance measurements until last week, when I picked up one of those fancy digital meters (DE-5000). The new meter allows for 100, 120, 1k, 10k and 100kHz inductance measurements. The Leader allows an external generator to be connected and supports frequencies down to 50Hz. I used these transformers (and a few others) to test the accuracy of the new meter and did some comparisons of the bridged readings and the digital meter.  With the exception of one transformer (an old UTC custom output), 100 Hz measurements on the digital were within 10% of the bridge. At 100 Hz the primary inductances were:

CT-41: 288 H digital, 295 H bridge
CT-41HN: 1330 H digital, 1380 H bridge

I'll bridge them at 50Hz too, but there's really no need as these inductances indicate no worse than 1dB down at 20Hz with source impedances up to 15k.

Anyway, thanks again to you and Steve, Jim. Pretty jazzed about these and looking forward to hearing them. Really like what you guys are doing and best of luck with it in the future!

BT

ef86, 6j7, 5879... Sensitive tube grids like looking at a high impedance secondary. Always wondered why UTC went through all that trouble.

UTC LS10 and LS10x:  50-600ohm primary,  50-60k secondary
UTC A10, A11, A12:  50-600ohm primary, 50-80k secondary
UTC HA100/HA100X and HA101/HA101X: 50-600ohm primary, 60k, 80k 120k secondary
UTC o-1 and o-2:  50-600ohm primary,  50-60k secondary

While I'm a huge  fan of science and smart people in general, I think we generally prefer to do design work with our ears instead of oscilloscopes. What looks good on paper doesn't always sound good to the ears. You can check out my credits at Allmusic.com (Jim Vollentine), or check out our Instagram to see what kind of gear we hang out with, repair,  work on, and use every day.