Pultec Input Stage

[daskew81]

Hey folks, I hope this is a relatively simple question. I'm designing a Pultec style circuit, it's almost finished except I'm not sure what to do with the input transformers.

I've seen many different configurations for input transformer selections here. Some use a 1:4, some use 1:1. I'm assuming the 1:4 transformers are just trying to make up for some signal loss and give the gain stage a bit more signal?

In the 1:1 cases, some use 600:600, others use 10k:10k. Does the 1:1 impedance selection even matter considering how impedance reflects through a transformer? As long as the secondary is properly loaded can you get the same result? I see Jakobs design loads the secondary with a 10k resistor.

Is the input transformer only serving as a way to unbalance a balanced input, or is there something I'm missing in regards to impedance matching with the passive filter stage?

I apologize if this is rudimentary, I'm just having trouble wrapping my head around the purpose of the input transformer besides unbalancing the input, or providing a bit more signal for the gain stage.

 

[gyraf]

you'll need a 1:1 or around that for pultec input, higher ratios and your load impedance gets to be very-very low

Already at 1:1 your input driving circuicy is looking into some 75-100R worst-case at resonant frequency and full high boost / max Q.

With a 1:4 input transformer this gets to be 4.68 to 6.25 Ohms - not a decent load for anyone

/Jakob E.

 

[emrr]

You could go with a step-down to lighten the input load, but then you’ve created the problem of needing even more make-up gain.

You can also do what I did with my ioaudio mid freq pultec clones, and use an active buffer, with or without a transformer front end.  THAT 1246/1646 combo in my case, works/measures perfectly. 

https://groupdiy.com/index.php?topic=26790.msg508063#msg508063

 

[daskew81]

Thank you for the replies! I've been on the road so haven't had a chance to dig into it yet.

you'll need a 1:1 or around that for pultec input, higher ratios and your load impedance gets to be very-very low

Already at 1:1 your input driving circuicy is looking into some 75-100R worst-case at resonant frequency and full high boost / max Q.

With a 1:4 input transformer this gets to be 4.68 to 6.25 Ohms - not a decent load for anyone

/Jakob E.

I can understand how the transformer with different winding ratios would affect input impedance now.

Am I understanding also that the only purpose the 1:1 is serving is to unbalance the signal? It doesn't matter if its a 600:600 or 10k:10k, as long as it's 1:1 the input impedance should remain the same?

 

[daskew81]

You could go with a step-down to lighten the input load, but then you’ve created the problem of needing even more make-up gain.

You can also do what I did with my ioaudio mid freq pultec clones, and use an active buffer, with or without a transformer front end.  THAT 1246/1646 combo in my case, works/measures perfectly. 

https://groupdiy.com/index.php?topic=26790.msg508063#msg508063

This is great Doug! I haven't ruled out the possibility of using a solid state input, it just might take some redesigning of the power supply.

 

[gyraf]

Am I understanding also that the only purpose the 1:1 is serving is to unbalance the signal? It doesn't matter if its a 600:600 or 10k:10k, as long as it's 1:1 the input impedance should remain the same?

technically yes, but copper resistance of transformer gets to be an issue too - a 600:600 transformer usually is <50 Ohms DC R, where a 10K:10K is more like 500-1K Ohms. And as this copper/DC resistance turns up in series with the signal from the transformer, you will get a quite different range of boost available at the high-boost pot if going with a 10K trafo.

/Jakob E.

 

[daskew81]

Thank you Jakob!