Transformer DI

[MoonBassAlpha]

Hi all
Has anyone added a transformer balanced DI to the speaker output of a small valve amp? I imagine some dropper resistors would be in order, or is it more involved than that?
Cheers
MBA

[Beer of the Bass]

The lower diagram on this page would work, although I haven't tried it myself.

http://www.tube-town.net/cms/?DIY/Needful_Things/Di_%26amp%3B_Line

[MoonBassAlpha]

Nice one beer, looks just the ticket! Just got to find that transformer from about 30 years ago now...

[bertbass]

Or, you could just buy one. https://www.studiospares.com/Microphones/DI-Boxes/Palmer-PAN-01-Passive-DI-Box_602860.htm?CAWELAID=120173110000146033&CAGPSPN=pla&catargetid=120173110000154407&cadevice=c&gclid=CjwKEAiA3Ou1BRDso5XyhduuwFASJABP3PED2MF679pRXvdSk2t7gjHPJoVhcvg_9FJcJs0dCyo9XhoChkrw_wcB#rtabs1

[BigRedX]

[quote name='bertbass' timestamp='1455112739' post='2976011']
Or, you could just buy one. [url="https://www.studiospares.com/Microphones/DI-Boxes/Palmer-PAN-01-Passive-DI-Box_602860.htm?CAWELAID=120173110000146033&CAGPSPN=pla&catargetid=120173110000154407&cadevice=c&gclid=CjwKEAiA3Ou1BRDso5XyhduuwFASJABP3PED2MF679pRXvdSk2t7gjHPJoVhcvg_9FJcJs0dCyo9XhoChkrw_wcB#rtabs1"]https://www.studiosp...hkrw_wcB#rtabs1[/url]
[/quote]

Does that have the correct attenuation for feeding a speaker level output?

[url=http://www.emosystems.co.uk/Products/Passive_Direct_Injection_Boxes.html]These from EMO definite do[/url].

[MoonBassAlpha]

Yes, the Palmer one does too:

Passive DI box. Jack input with paralell "THRU" output, transformer balanced XLR output. Attenuable input sensitivity, can handle both line level and speaker level signals.

Thanks for the suggestions guys, I'd like to build something in, but this looks like a good starting point.

[MoonBassAlpha]

Just spotted this one on sale today with a tenner off, and it's a dual, which might be handy at some point. It says it takes speaker level signals, and has 3 levels of attenuation rather than 2 that the Palmer box has:
https://www.studiospares.com/Sale/2016/Flash-Sale/Studiospares-Dual-Passive-DI-Box_449300.htm#rtabs3

Offer finished 2nd March, 3pm.
I think I might just have to @ <£20

[Passinwind]

Take a look at how Fender taps a line level signal off the speaker out signal in many of their old tube amps. All you need then is any old decent isolation (1:1 ratio) trafo.

[MoonBassAlpha]

[quote name='Passinwind' timestamp='1456860581' post='2993013']
Take a look at how Fender taps a line level signal off the speaker out signal in many of their old tube amps. All you need then is any old decent isolation (1:1 ratio) trafo.
[/quote]
Thanks, any links?
I've bought the passive box now, but always interested to see how others have done it. Are the Fender ones balanced though?

[Chienmortbb]

If the Fender trick uses an isolating transformer, the output will be balanced. It will only become unbalanced if you connect to an unbalanced input.

[Passinwind]

[quote name='MoonBassAlpha' timestamp='1456910687' post='2993367']
Thanks, any links?
I've bought the passive box now, but always interested to see how others have done it. Are the Fender ones balanced though?
[/quote]

Here's how they did it in my guitar amp: http://ampwares.com/schematics/fender_30.pdf

As I said, balancing it would just be a matter of adding a line level 1:1 output transformer. I have one in a separate box, comes in handy for all sorts of things.

[Beer of the Bass]

[quote name='Passinwind' timestamp='1457337399' post='2997386']


Here's how they did it in my guitar amp: http://ampwares.com/schematics/fender_30.pdf

As I said, balancing it would just be a matter of adding a line level 1:1 output transformer. I have one in a separate box, comes in handy for all sorts of things.
[/quote]

That's very similar to the method used in the tubetown link above, though one uses 47K/5K for the divider and the other uses 100K/10K. I'm not sure what the purpose of the capacitor in parallel with the pot on the first example is - it looks like it could bleed some highs to ground, but I haven't tried calculating the corner frequency.