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DIY reamp box
Old 10th September 2013
  #1
Lives for gear
 
🎧 15 years
DIY reamp box

well I have a project that I have to do and have one of these things. I know many good things with electronic. In my box of project transformers (I build tube stuff off and on), I have some triad A-10j's and some utc o-1's. I am planing on using a Triad A-10J.

My question is should I put a switchable pad on the input side (150 ohm) as well as a fixed t attenuator on the out (56kto sec, then 6k to ground, 1M out).

I also notice that none of the DIY designs I have seen had inpedence matching in mind. Isn't this an issue? even the jenson design is 10K:10K. I wonder if there goal was generic compatibility as some boards have hi-z than low-z output. My board's outs are 51 ohm impedence so the 150 ohm would be a closer match IMO. It also seems like the jenson design doesn't count for the input impedence of the tube amp. (1M according to the mesa boogie double rectifier schematic)

I noticed it is different than the patenent are they trying to get around that?
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Old 10th September 2013
  #2
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tINY's Avatar
 
1 Review written
🎧 15 years

The impedance ratios quoted on transformer specs are related to parasitics and optimized design. Most people aren't slamming them or using super low-level signals, so it's less important than ratios.

I'd shy away from the output pad. You're post suggests that the output impedance (to the amp input) will be below 6k ohms. Most guitar impedances are higher. I think you want to aim for about 10-14k ohms output impedance from the transformer.

With a turns ratio of 1:10.5, 12k would require about 110 ohms output impedance from the source.

So, add a series 50 ohm resistor and design your pad as a 110 ohm L-pad. But. you should really just lower the output from your DAC.






-tINY

Old 10th September 2013
  #3
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1 Review written
🎧 15 years

Whitlock's design is probably even better. You can dial in the impedance...



-tINY

Old 13th September 2013
  #4
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🎧 5 years
If you're re-amping from a line level to a mike input, you don't need or want to 'match', or even get close to the 50 ohm output impedance of the line level output. In the old days, gear looked like a 600 ohm source, and required a 600 ohm load. Aside from gear like that, which also usually had a switch to attach the necessary load, you do not want to use a low impedance load whatsoever. 150 ohms is far, far too low to do any good. 2K ohms is a good minimum, and 10K ohms is generally a lot more polite, and it usually won't make things noisier in the grand scheme of things.

Another good concept to understand is that you can arrange for the primary or the secondary of a transformer to have the rated load, in order to handle the parasitic capacitances and inductances etc., but you can use the transformer in a bridging mode and leave the other coil unloaded.

Specifically, in this re-amp case, you could build a line attenuator, perhaps a pi attenuator with a variable 'middle ' resistor to provide variable attenuation, and connect that to the high impedance coil of a transformer, probably a 10K to 150 ohm 'line to grid' transformer, and then connect the 150 ohm coil without any attenuator to an XLR connector, straight to a mike amp. In this way, the transformer will be loaded properly to prevent capacitive peaking and such, but you're not torturing the line level device, asking it to drive 150 ohms.

Re-amping is a great thing, and you can do it with only one attenuator and one transformer - it's simpler than you think. Reply if you have any questions!!
Old 13th September 2013
  #5
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1 Review written
🎧 15 years

I didn't think that anybody was re-amping by using a mic input.

Most people want to record a dry guitar feed straight from the instrument and play it back through an guitar amp later. That way, they can get the performance they want and try various amp/cabinet/mic combinations for the finished track.



-tINY

Old 13th September 2013
  #6
Registered User
 
🎧 10 years
D.I.Y. Recording Equipment | DIY Microphones, Preamps, Compressors, etc.

LINE2AMP PCB/Transformer Bundle

I did that one with a few jacks a plastic box and a switch.
Old 13th September 2013
  #7
Registered User
 
🎧 5 years
DOH! Sorry for the brain fart - replace the XLR with a 1/4" connector and the circuit is the same. The 10K to 150 transformer gives you some attenuation that you want, and the pad can give you more if you need it. I've used that to drive a SansAmp as an effect loop with a workstation, and also the standard 'send to an amp and mike it'.
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