Interstage Transformer Nomenclature

[TerribleT]

Hello,
I posted this question in other forums that I thought could help but no one responded so here I am. If you've seen this already excuse my multiple posts. But I really do need to know.
Could someone please explain the nomenclature used to describe an interstage transformer (IT). I've seen various designations like: 1:1, 1:2, 1+1:2+2, 5k:5K, etc... I understand even less about how one would choose an IT for a particular driver tube/power stage tube. I know I'm asking a lot, but please, if you have any insight in this matter, help me out. Regards,
David

[Bas Horneman]

I've seen all your posts at the other forums....as Sidewinder...and I did not reply because I thought someone who knows more than I do would answer.

So I'll try...
1:1 is that the turns ratio is the same on primary and secondary.

thus 2:1 means that there are 2 turns on the primary for every turn on the secondary...this in turn means that if you have a load of 5k at the output side...... the reflected load for the tube on the "primary" side will be 10k. etc. etc. etc.

1+1:2+2 means the same but there are 2 sections on both sides. This can be handy ...because if you connect the 1+1 side in parallel and the 2+2 side in series you would have 1:4 ratios...

I hope this is correct and if it is I hope it has helped...




Cheers,
Bas

[SY]

Just to add on, when impedances are quoted, they're assuming the secondary loaded with the specified resistance. If it's terminated with a different resistance, the impedance presented by the primary will change accordingly. So, if you have a 10K to 10K transformer and load it with 20K, the primary will present a 20K load.

[rdf]

When a manufacturer quotes an impedance, isn't that the optimum operating point for widest, flattest bandwidth and lowest LF distortion? For example, a Hammond 804 (we use often at work) can be strapped one-to-one as either 600:600 or 150:150. Terminating the latter's output with 10K will reflect a higher impedance to the primary but the low end still has to contend with the 804's low number of turns and DCR. I ask because after a couple of decades using line level trannies in a broadcast setting the simple turns ratio ratings used by most interstage manufacturers are still unclear to me.

[Sch3mat1c]

Originally posted by rdf
When a manufacturer quotes an impedance, isn't that the optimum operating point for widest, flattest bandwidth and lowest LF distortion?

And/or power efficiency. Which isn't of concern here, but is in OPTs which are just beefy ISTs with a low-Z secondary. Unless you're driving a zero bias class B output and have hair-thin margins for drive capability. But when does that ever happen...

For example, a Hammond 804 (we use often at work) can be strapped one-to-one as either 600:600 or 150:150. Terminating the latter's output with 10K will reflect a higher impedance to the primary but the low end still has to contend with the 804's low number of turns and DCR. I ask because after a couple of decades using line level trannies in a broadcast setting the simple turns ratio ratings used by most interstage manufacturers are still unclear to me.

Correct; you also have parasitic (inter-winding and inter-layer) capacitance which becomes a greater concern to higher impedances. Any reactance really, inductive or capacitive, narrowing in the bottom and top end responses respectively.

In a perfect world, you could use 1 turn coupled to three turns for any frequency...sigh...

Tim

[TerribleT]

Hello,
Thanks for the replies. Could you indulge me further? If a schematic shows a 1:1 for an IT, what would the actual impedances be for the primary? 5k or something else?
Regards,
David

[SY]

For a 1:1, the primary impedance will look like whatever impedance is loading the secondary.

[TerribleT]

Originally posted by SY
For a 1:1, the primary impedance will look like whatever impedance is loading the secondary.

I apologize for all these basic questions, but I can't seem to get a firm intuitive understanding of ITs. Thanks for the patience. I kinda understand the concept of impedance reflection, but in an article by George Sanguinetti in Sound Practices Issue 15 he had used a Sowter 8423 IT with a 5k pri 1:1+1 between a 7788 driver and a vv52B that was choke load and parfed to a MQ2004opt. Later after he measured the plate resistance he and Brian Sowter decided that a 2k pri IT would be better. So the question is how are the 2 ITs different? Both are I assume are connected 1:1+1. Thanks
Regards,
David

[SY]

The ratio of the reflected impedances is a function of the ratio of the number of turns. BUT.... this is something determined at midband. At the frequency extremes (for a given transformer), non-ideal elements like primary inductance and interwinding capacitance come into play, changing the frequency response and distortion characteristics of the transformer.

For example, I'm using some Jensen 1:1 input trannies that are optimized for 10Kohm loads. At 1kHz with a 10Kohm load on the secondary, the primary looks like a 10K resistor, the -3dB point at the high end is about 85kHz, and the rolloff is well-damped. Pass a 1kHz square wave through it and it looks pretty good.

Now load that same transformer's secondary with a 100Kohm resistor. At 1kHz, as expected, the primary reflects that load faithfully and looks like a 100Kohm load. But now the frequency response has a big bump at 50kHz and that same 1kHz square wave shows ringing on its leading edge.

[rdf]

Exactly, but for me that still leaves the question how to approach a transformer when the specifications are listed as simple turns ratios. Is normal practice in this case to contact the manufacturer for an opinion on the specific applicationin mind?