Fender FR-12 preamp mod : The Hissterectomy

WKSmith said:
Excellent point!
Humbuster outs are not balanced.

Some folks are running cool stuff like the Ethos into the FR though ..or HX stomps, iridium's, etc with TRS outs.
Click to expand...
Yes. And it's funny, I didn't comprehend what was going on with my Ethos until you mentioned about the out of phase thing.
 
WKSmith said:
Yeah, there is no documentation about input levels or input impedance or what happens if you plug the xlr out into a pa with phantom power turned on.
Click to expand...

I’m assuming the output impedance of line level products are ~10k ?
We do not know the input impedance of the FR10/12s ?
WRT the impedance matching transformer…we need to know these figures.
1 = gnd/shield, 2 = +, 3 = - , for an xlr(info only)
In addition, the turns ratio is important….assuming 2:1…or if the trans is a 1:1…connect the unbalanced signal(T) to the center tap…
This sound about right? :)

I need a potentiometer and a signal generator to measure the input impedance of the FR10/12s….can’t find my decade box. :(
 
200man said:
I’m assuming the output impedance of line level products are ~10k ?
We do not know the input impedance of the FR10/12s ?
WRT the impedance matching transformer…we need to know these figures.
1 = gnd/shield, 2 = +, 3 = - , for an xlr(info only)
In addition, the turns ratio is important….assuming 2:1…or if the trans is a 1:1…connect the unbalanced signal(T) to the center tap…
This sound about right? :)

I need a potentiometer and a signal generator to measure the input impedance of the FR10/12s….can’t find my decade box. :(
Click to expand...
The stock pre has 2 10k resistors to ground on each balanced line.. which should make the 1st amp stage see 5k input impedance .

The V2 is much lower. It expects a typical 600ohm balanced xlr (anything higher than 1K will affect freq response) this is intentional to tune the first stage flat.

So yes a 1:1 600 ohm tranny would be ideal.
 
WKSmith said:
The stock pre has 2 10k resistors to ground on each balanced line.. which should make the 1st amp stage see 5k input impedance .

The V2 is much lower. It expects a typical 600ohm balanced xlr (anything higher than 1K will affect freq response) this is intentional to tune the first stage flat.

So yes a 1:1 600 ohm tranny would be ideal.
Click to expand...
Okay, thanks…so a 600 ohm to 600 ohm(1:1) tranny and connecting the unbalanced signal line to the center tap.
Secondly, I’m not sure that connecting the shield to the ct of the balanced side is necessary?
 
200man said:
Okay, thanks…so a 600 ohm to 600 ohm(1:1) tranny and connecting the unbalanced signal line to the center tap.
Secondly, I’m not sure that connecting the shield to the ct of the balanced side is necessary?
Click to expand...
I would leave the unbalanced input side center tap open.

The center tap of the balanced out is the 0v reference between the +V of XLR pin 2 (or Tip) And -V of XLR pin 3 (or Ring).

The FR will lose its mind without a 0V reference on XLR pin 1 (or Sleeve).

🙂
 
  • Like
Reactions: Rex
WKSmith said:
I would leave the unbalanced input side center tap open.

The center tap of the balanced out is the 0v reference between the +V of XLR pin 2 (or Tip) And -V of XLR pin 3 (or Ring).

The FR will lose its mind without a 0V reference on XLR pin 1 (or Sleeve).

🙂
Click to expand...
Then how do you get a higher level signal if it is 1:1 and you dont use the center tap…I think Im about to learn something? 😀
 
The FR balanced differential input 1st stage sums the two after inverting the - signal

the 1/2+ signal Voltage plus the inverted 1/2- signal voltage equals 1:1 signal referenced to 0v

If you float the input center tap and feed just the outside terminals of the input side of the tranny with 4.77v (+4dB)
..it will output +2.385v and -2.385v referenced to the output side center tap as a 0v reference or virtual ground.

The differential summing amp in the FR will invert the -2.385v to get ..
+2.385v + +2.385v = +4.77
 
Last edited:
IMG_2237.jpegIMG_2235.jpegSo I just sold my the original FENDER preamp I removed because of the hiss. I listed it as a joke and someone bought it on Reverb. I mean I thought it was pretty clear that it was not worth anything.

Why would anybody buy this thing?

Buyer is in San Ramon California
IMG_2236.jpeg
 
WKSmith said:
The FR balanced differential input 1st stage sums the two after inverting the - signal

the 1/2+ signal Voltage plus the inverted 1/2- signal voltage equals 1:1 signal referenced to 0v

If you feed the outside terminal of the input side of the tranny with 4.77v (+4dB) it will output +2.385v and -2.385v referenced to the center tap.
The differential summing amp in the FR will invert the -2.385v to get ..
+2.385v + +2.385v = +4.77v
Click to expand...
I understand the balanced inversion of the negative side…
But I was thinking we are sending a line level signal unbalanced which is much less I think than 4.77v ? -10dBv
 
200man said:
I understand the balanced inversion of the negative side…
But I was thinking we are sending a line level signal unbalanced which is much less I think than 4.77v ? -10dBv
Click to expand...
I would send it +4 from a humbuster 1/4" out using a TS cable. 😎 Which defeats the humbuster function.
 
Remind me; the XLR out level is dependent on how loud you have the FR turned up, correct? Sucks for the FM3 user using Out 1 to FOH and Out 2 1/4" to the FR.
 
JTI said:
Remind me; the XLR out level is dependent on how loud you have the FR turned up, correct? Sucks for the FM3 user using Out 1 to FOH and Out 2 1/4" to the FR.
Click to expand...
The xlr out on the FR?
It mirrors the xlr or trs input on the FRs

And yes it sucks for FM3 folks. Which is the reason for the transformer talk.
 
  • Like
Reactions: JTI
WKSmith said:
The xlr out on the FR?
It mirrors the xlr or trs input on the FRs

And yes it sucks for FM3 folks. Which is the reason for the transformer talk.
Click to expand...
I can deal. It's just that the FR is juuuuuuust loud enough. If you are at an outdoor gig with minimal sound reinforcement and not mic'ing guitars; it's a close call imo.
 
Hi Liam, just a quick question, please:

If I use a regular guitar TS cable to connect the balanced TRS output from the HX Stomp to the balanced TRS input of the FR-12 would I still loose volume?
 
davi_ssilva said:
Hi Liam, just a quick question, please:

If I use a regular guitar TS cable to connect the balanced TRS output from the HX Stomp to the balanced TRS input of the FR-12 would I still loose volume?
Click to expand...
Yes. It would be much better to use a TRS cable. A TRS from the HX to an XLR in on the FR would be the way to go.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom