Transformer DI off of Amplifier Speaker Tap
« on: July 10, 2019, 01:00:53 PM »
Hi everyone -

I once saw mention of using a transformer to create an isolated direct out from a speaker tap, and it's something I'd like to try building. If I were to tap off of the speaker connections on a guitar amp and connect those to a transformer input, what winding ratio would be the most desirable if I wanted to send the transformer output to a line input? Would I want an impedance ratio that roughly matches the amp output (8 Ohms?) to the impedance of the line input (10K Ohms?), or would I want a ratio that adheres to the "10x" output-to-input concept (step up the 8 Ohms to ~1K Ohms)? Would it be better to do this into a mic pre instead of line input?

If I'm right in thinking I need to step up the impedance, would I need to pad the output of the transformer because of the resulting voltage step up? If it's not clear already, I have only a very rudimentary understanding of these topics, so please let me know if I'm way off on anything.

Thanks!

DK



abbey road d enfer

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #1 on: July 10, 2019, 01:49:06 PM »
Hi everyone -

I once saw mention of using a transformer to create an isolated direct out from a speaker tap, and it's something I'd like to try building. If I were to tap off of the speaker connections on a guitar amp and connect those to a transformer input, what winding ratio would be the most desirable if I wanted to send the transformer output to a line input? Would I want an impedance ratio that roughly matches the amp output (8 Ohms?) to the impedance of the line input (10K Ohms?),
That would definitely be a 30dB step-up, resulting in injecting 300+ volts into your line input! You don't want that.

Quote
or would I want a ratio that adheres to the "10x" output-to-input concept (step up the 8 Ohms to ~1K Ohms)? 
Don't think in terms of impedance; you want level. The amp's output delivers 10-40V rms, you need to pad it down. You don't want a big transformer (cause costs money), so you pad down 20dB so you have about 1-4 Vrms, then you need to pad it down again somewhat. You may use a 20dB step-down xfmr or a 20dB resistive pad.  You end up with 100-400mV.
If you disconnect the loudspeaker, you need a dummy load, same Z as the speaker, same power handling.

Quote
Would it be better to do this into a mic pre instead of line input?
Whatever suits you, but if you go mic level, you add noise.

Quote
If I'm right in thinking I need to step up the impedance, would I need to pad the output of the transformer because of the resulting voltage step up? If it's not clear already, I have only a very rudimentary understanding of these topics, so please let me know if I'm way off on anything.
As I said, don't worry too much about impedance. Impedance is important when power transfer is important. The amp's xfmr needs to be impedance matched because the designer wants to produce as much useful power as he can, but at line or mic level, we don't care!
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #2 on: July 10, 2019, 02:15:51 PM »
Don't think in terms of impedance; you want level. The amp's output delivers 10-40V rms, you need to pad it down.

I already have a big vintage UTC B-9812 transformer that I'm hoping to use for this, and I measured one of the voltage ratio options at about 16.8:1, which would step the 10-40Vrms down to about 0.6-2.4Vrms. There's also an option that has a 33.6:1 voltage ratio, which would output about 0.3-1.2Vrms. Either of those worth a try?

DK
« Last Edit: July 10, 2019, 02:35:19 PM by declankennedy »

CJ

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #3 on: July 10, 2019, 03:37:17 PM »
they do that on the Mesa D-180,

here you go>

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squarewave

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #4 on: July 10, 2019, 03:38:48 PM »
A transformer is a great way to tap into a speaker for a variety of reasons. But you don't want to load the speaker any more than it already is by the speaker. So the primary needs to be at least 600 ohms or so. And you need to step down so in practice the primary could be quite high (maybe 10K or so). How much to step down is hard to say because it depends on how much voltage is going across the speaker. If the speaker is 16 ohms and you're really cranking the amp, you might need to step down a LOT. So you either need a transformer with multiple taps or a transfomer with a pot after it but the later is a little sloppy because it adds series resistance. You could put a 1K pot across the speaker and then step down from there to get low impedance again. But then if there's any DC on the power amp out (there shouldn't be) the pot could make scratchy noises. Probably the simplest, cleanest (and incidentally cheapest) solution would be to use a line matching transformer like you might get from any number of AV parts stores. These have taps for various power levels which is of course pointless in this case but as you switch taps you'll get different levels. Just put a switch on the taps and that's it.

However, there is one potentially serious pitfall. If you switch taps while signal is coming out of the amp, it could leave the primary disconnected momentarily and cause an inductive spike that could destroy your line input. Probably not since you're stepping down but still - don't switch taps with any signal going through the amp.

squarewave

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #5 on: July 10, 2019, 03:42:52 PM »
here you go>
This doesn't make sense. Is that primary is really 8 ohms?

abbey road d enfer

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #6 on: July 10, 2019, 06:21:40 PM »
I already have a big vintage UTC B-9812 transformer that I'm hoping to use for this, and I measured one of the voltage ratio options at about 16.8:1, which would step the 10-40Vrms down to about 0.6-2.4Vrms. There's also an option that has a 33.6:1 voltage ratio, which would output about 0.3-1.2Vrms. Either of those worth a try?

DK
Probably, but are you positive the xfmr can handle the large voltage?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

Re: Transformer DI off of Amplifier Speaker Tap
« Reply #7 on: July 10, 2019, 06:44:59 PM »
they do that on the Mesa D-180,

here you go>
Actually this xfmr should be characterized as a 1-2k 1:1 with center-tapped sec. *But it is driven by a 6kohm source, which completely f...s up the response, which ends up being very peaky at about 250Hz and skirting at 6dB/octave each side.

* 3.4H at 80Hz (lowest note on 6-string gtr) computes at 1.7kohm
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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