Fx Loop Cut box / unboost

matt1973

Thinking about putting one of these Loop Cut boxes together, just a very simple circuit containing a pot, jacks & switch. Can anyone ITK suggest a potentiometer value?

ecc83

Always a compromise.

If the FX output is from a valve it might be a highish source impedance so you would not want to load that lower than 100k.

Trouble is that makes the output resistance of the box high and can lead to HF loss (aka "tone suck"!) so keep the output lead as short as possible, say 2mtrs max.

But most FX outs are an op amp these days and so go for 10k log and have cables pretty much as long as you like!

And! Fit two jack in parallel so that you have a "clean feed" from the FX out. Costs beans, can be handy.

Dave.

timhulio

I'd chuck a simple buffer in there before the volume pot. Otherwise it'll sound the same as turning the volume down on your guitar - wooly as hell.

ICBM

timhulio said:

I'd chuck a simple buffer in there before the volume pot. Otherwise it'll sound the same as turning the volume down on your guitar - wooly as hell.

Not if the amp has a proper IC-driven FX loop, as Dave says most amps do these days. Even a lot of valve ones are cathode-follower buffered. You can always add a treble pass cap anyway, far less hassle than a buffer in the box and doesn't require power.

I like the idea of a parallel jack for splitting the signal too.

timhulio

Good point. I missed the fact that this is for an FX loop. Effects loops are alien to me, as I've only ever used vintage amps.

matt1973

I think, this should be a an IC driven loop - 1988 Marshall jubilee. There's also pedals in the loop that have buffers so that shouldn't be an issue.

I've got say though, the more I look into this, a Graphic EQ might be the way so as to keep the bass in check.

matt1973

@ecc83 can you let me how to do the parallel jack wiring? Google isn't being very kind to me on that search term

ICBM

matt1973 said:

I think, this should be a an IC driven loop - 1988 Marshall jubilee.

No, unfortunately that's an amp with a very poorly-designed loop - driven from the plate of the previous valve stage and then via a 100K resistor, with no buffer. The tone suck is fairly serious on these unless you have a buffered pedal first in the loop and preferably very close to the amp with a short cable, if you can.

matt1973 said:

@ecc83 can you let me how to do the parallel jack wiring? Google isn't being very kind to me on that search term

Simply give it two 'input' jacks, connected to each other tip-tip and ground-ground.

not_the_dj

@ecc83 any predictions on how one of these http://www.ehx.com/products/signal-pad would work in the FX loop of a HT-50?

matt1973

ICBM said:

matt1973 said:

I think, this should be a an IC driven loop - 1988 Marshall jubilee.

No, unfortunately that's an amp with a very poorly-designed loop - driven from the plate of the previous valve stage and then via a 100K resistor, with no buffer. The tone suck is fairly serious on these unless you have a buffered pedal first in the loop and preferably very close to the amp with a short cable, if you can.

matt1973 said:

@ecc83 can you let me how to do the parallel jack wiring? Google isn't being very kind to me on that search term

Simply give it two 'input' jacks, connected to each other tip-tip and ground-ground.

Would a buffered pedal into the front of the amp suffice? 

 So the two jacks... 

You with the hot of the input to the hot of the output and put the pot in parallel?

matt1973

Brain fart above... The parallel bit is just an extra output for slaving, speaker emulators, etc?

ICBM

matt1973 said:

Would a buffered pedal into the front of the amp suffice?

No, not at all - the problem is with the output from the FX send.

matt1973 said:

The parallel bit is just an extra output for slaving, speaker emulators, etc?

Yes - no need for it if you'll never use that, I just thought it was a neat idea that I hadn't come across on any of these boxes yet.

It might be less useful on this amp anyway because the loop comes before the tone stack… which is also not ideal.

It was complaints about the loops on these amps and the JCM800 Split-Channels that led them to do the much superior IC-buffered loops on the JCM900s… although they went and partly buggered that up by running the signal through the switches in *both* jacks in series, thus worsening the well-known problem with dirty jack contacts.

ecc83

Wish I 'adn't said nuffin now!

The extra plleled jack is just one of my foibles! I tend to think that I might need a test feed or a recording output.

I would draw it up and post an attachment but I cannot see how here.

IC, never had a dirty contact problem. You must get some real ***t to fix! What I HAVE had is a couple of A30s which had higher than normal hum levels when the VC was advanced sans jack. You guessed it! The input jack was not shorting. This turned out to be a damaged socket which had obviously been strained with a lead and the contact bent.

Bit of a fiddle to fix because the 6dB pad resistors are all built around the two sockets..But! We can't have £1000+ amps out of specc' can we? Even if the owner never noticed!

Dave.

ICBM

ecc83 said:

IC, never had a dirty contact problem. You must get some real ***t to fix!

It's one of the most common problems - noticeably so on amps which are otherwise very reliable, like Peavey solid-state combos which are almost bombproof apart from blown speakers and this. Generally it happens because they never get used, so a tiny bit of corrosion gets into the contact point.

I really don't like using jack switching contacts for critical signal path points for this reason. It's bad enough for FX loops, but speaker/headphone jacks are even more of a pain when they're done like that since the contact point tends to arc once it's gone iffy, so you often have to replace the jack instead of just cleaning it.

ecc83 said:

This turned out to be a damaged socket which had obviously been strained with a lead and the contact bent.

Also a very common problem with any amp with PCB-mounted jacks - and/or if they don't crack the pin joints!

ecc83

"Also a very common problem with any amp with PCB-mounted jacks - and/or if they don't crack the pin joints!"

That should not happen if the jacks are well secured to the panel and the soldering is good. I have certainly never had a problem in any amp or pedal with this.

I HAVE had the issue with USB ports on the front of PCs and the DIN and USB port on my Evolution MIDI keyboard.

There are of course millions of jack contacts in use (and dormant for months) in jack patch bays. I spend some 2/3rds of my forum time at music recording places and have not read of any such problems. I reckon the jack makers save all the ***T for geetarists!

Dave.

ICBM

ecc83 said:

"Also a very common problem with any amp with PCB-mounted jacks - and/or if they don't crack the pin joints!"

That should not happen if the jacks are well secured to the panel and the soldering is good. I have certainly never had a problem in any amp or pedal with this.

Have you seen the force that some musicians apply to them?!

There's no way you can stop those plastic Cliff/Re-An jacks bending upwards if someone pulls the lead tight between the guitar and the amp and then stands on it in the middle. The only way to guarantee the pins don't pull through the board is to rivet them over on the underside, and then sometimes they just snap at the top side of the board - which makes fault-finding tricky because at first it looks like the pin is still soldered… but when you've seen it once you look for it the next time. I'm really not joking…

Actually the best way to stop the problem is to design (or modify) the amp so that the jack is the other way up or mounted sideways on a small separate PCB which can flex independently of the main board, so a downward tug on the jack doesn't pull it away from the PCB. Like a certain modern amp I know of .

You should be proud that the ID60 has resisted all attempts I know of by rehearsal studio users to kill it, so far .