Bass amp fault diagnosis help please (became merits/downsides of XLR DI outputs on amps)

DartmoorHedgehog · November 2023

My daughter's bass amp has gone wrong. Apparently it's making a buzz and a squeal, affected by master volume but not by gain knob. No sound from instrument input. Headphones sound same as sound from speaker. She's worried she's damaged it by accidentally putting phantom power on the di cable, but I thought that was unlikely with modern kit. It's an Eden EC15 combo (new one, since g4m took over the brand).

Any suggestions? Could phantom power on the di socket have broken something in the preamp?

ecc83 · November 2023

Depending on where and how in the signal path the DI (XLR?) output is taken then yes, 48 volts could well have blown a chip and killed the signal.

We could argue that due to spook juice being much more common on home kit than even ten years ago, mfctrs SHOULD protect every mortal inny and outy but that really is too much to expect on anything but THE most upmarket gear.

Time to see an amp tech I am afraid.

Dave.

Danny1969 · November 2023

A lot of bass amps have a transformer for the DI so are protected as there's no DC path through to anything. Other circuits use a couple of opamps to create a balanced output and although the output may well be capacitor coupled the higher phantom power voltage can reverse bia the caps and thus render it useless in protecting the DI circuit from the 48V

I'm not familiar with that brand but if it's an analog non modelling bass amp then it shouldn't be too hard for a general electronics engineer to fix

DartmoorHedgehog · November 2023

Thanks folks. Sounds like the phantom power could have done some damage then (I'm surprised designers don't allow for that considering how common it is to have power on mixers and audio interfaces).

Yes @Danny1969 I think it's a fairly normal analogue preamp.

I'll suggest she finds a local tech (she's away living the student life now so I can't look at it myself).

ecc83 · November 2023

"(I'm surprised designers don't allow for that considering how common it is to have power on mixers and audio interfaces)." Not that simple or cheap. Inputs should have two reversed biased diodes across the input but these can only be biased to the extent of the normal supply rails, usually a maximum of +&- 18V though most 'prosumer' kit is +&- 15V or less. 48v can overpower that and destroy a chip. Outputs are not normally protected as YOU SHOULD NOT CONNECT THEM TO BLOODY MIC INPUTS! A transformer as mentioned, will save the drive chip's A but traffs are expensive and bulky compared to modern SMT construction.

Dave.

DartmoorHedgehog · November 2023

ecc83 said:

Outputs are not normally protected as YOU SHOULD NOT CONNECT THEM TO BLOODY MIC INPUTS!.

Fair cop

Easy mistake to make though with a general computer audio interface. Depends if her interface will accept instrument-level signal into XLR I suppose - if it'll only take mic-level (pretty sure mine assumes mic if it's xlr to be honest) she'd be better off using an xlr to 1/4" cable or a separate DI box. That would also prevent the accidental phantom power thing happening again.

ecc83 · November 2023

DartmoorHedgehog said:

ecc83 said:

Outputs are not normally protected as YOU SHOULD NOT CONNECT THEM TO BLOODY MIC INPUTS!.

Fair cop Easy mistake to make though with a general computer audio interface. Depends if her interface will accept instrument-level signal into XLR I suppose - if it'll only take mic-level (pretty sure mine assumes mic if it's xlr to be honest) she'd be better off using an xlr to 1/4" cable or a separate DI box. That would also prevent the accidental phantom power thing happening again.

Ah, I see but you would normally not connect a DI amp output to a MIC input? The DI would likely be at at least -10dBV (316mV) level and be close to overloading most AI mic pres'. Might even be at +4dBu and that would certainly overload it.

And yes, a cheapo passive DI box solves everything!

Just like to add that I am not at all a fan of XLR skts on guitar amps UNLESS they are mic inputs, e.g. acoustic amps. DI outs, EM outs, keep them on jacks PLEASE! And before anyone starts! Yes! I KNOW, "They" fit one...Daft I calls it.

Dave.

ICBM · November 2023

I agree with Dave - fitting XLR outputs on amps (bass as well as guitar) is a mistake, because firstly it makes using a better external DI box difficult unless the amp also has it duplicated with a 1/4" unbalanced line output, secondly because unless done properly (and more expensively) with a transformer it runs the risk of trouble with phantom power (and in my opinion non-transformer DIs don't sound as good with electric instrument amps anyway), and thirdly because good luck getting any professional sound engineer to actually agree to use it instead of one of their own DI boxes...

DartmoorHedgehog · November 2023

So, @ICBM and @ecc83 - having never had an amp with an xlr di output myself so not really thought about it before... If she just wants to connect the amp to an interface for recording, would an xlr to 1/4" jack lead work (into an instrument or line-in input on the ai)? I had a look at the amp spec and there isn't a 1/4" output apart from the effects send (and speaker - she knows not to use that!)

Danny1969 · November 2023

I wouldn't bother using the amp to record ... just plug the bass directly into the interface. There's loads of bass plugins like Sansamp which get a great bass tone to track.

ICBM · November 2023

DartmoorHedgehog said:

So, @ICBM and @ecc83 - having never had an amp with an xlr di output myself so not really thought about it before... If she just wants to connect the amp to an interface for recording, would an xlr to 1/4" jack lead work (into an instrument or line-in input on the ai)? I had a look at the amp spec and there isn't a 1/4" output apart from the effects send (and speaker - she knows not to use that!)

XLR to 1/4” is a bad idea, it's always a bodge - it will probably cause a ground loop depending on which two of the three wires are grounded. She could use the FX send, although it may well also have the same problem.

A DI box is the best solution by far, they aren't expensive and have more uses than just connecting an amp to an interface. Connect it to the FX send.

Danny1969 said:

I wouldn't bother using the amp to record ... just plug the bass directly into the interface. There's loads of bass plugins like Sansamp which get a great bass tone to track.

That might not work well if it's a passive bass, since the input impedance of the interface is probably too low.

DartmoorHedgehog · November 2023

Thanks - I've put guitars with passive pickups straight into my AI before and got decent results so that's not a bad idea to try. I've got an xlr to jack lead that I use with mics occasionally when I need to connect to something without a balanced input - it's worked for me, don't remember any problem with hum - perhaps I've been lucky though.

As for connecting a di box to the send, I think this situation started because she wanted to be able to hear the speaker on the combo while she was recording it rather than using headphones. I assume plugging into the send would cut the signal to the power amp unless it's a parallel loop. Same problem with if she was using the combo as a monitor and di for FOH. If an xlr di output is weird then what do people usually do with it? Actually I think my band's bassist's amp has an xlr di output that gets used a lot at gigs - I'll ask him what he connects it to next practice, doesn't seem to freak out sound techs at gigs.

EDIT: Just had a look at some DI boxes - seems like they have both 1/4" and XLR inputs. So I'll suggest for home recording either software amp as Danny suggested or get a cheap DI box if she really doesn't want to use headphones. Once she gets her amp fixed...

ecc83 · November 2023

" If an xlr di output is weird then what do people usually do with it? " Tifano!

I only have an early Blackstar S1 schematic with XLR out but it is a duplication of the emulated TRS jack. Both are balanced from a proper, twin amp differential circuit and multiple 'clamp' diodes are fitted (maybe Bruce knew of the cack handers?!) The level is -10dBV or 0dBV i.e. WELL over mic level and therein lies the main problem I have with XLRs in this position.

Musicians are mainly 'jack' people. With the exception of power amp jacks, you can do little harm connection one jack to another and gitists pretty much know power amps can bugger things (and themselves).

But! A male XLR3 really has only one destination? XLR cable to a mic input.

Would not be so bad if the XLR OP was smashed down to -40dBV* and even better as ICBM said, if transformer fed but unless amp makers, you are doing either or both of these things. Stick to effin jacks PLEASE!

*Such attenuation would also make it pretty spook juice proof.

Dave.

ecc83 · November 2023

Err? Not seen many DI boxes with XLR INPUTS? The two jacks are almost always just linked to allow an amp to be used as well. The XLR is the output to a mic input. I strongly suggest a passive box. They will not have the 'magic meg' input Z (likely about 150k Ohms) but this is rarely a problem with guitars and certainly not for bass.

Keep the guitar VC at max then the lower input Z will have least effect.

Dave.

DartmoorHedgehog · November 2023

ecc83 said:

Err? Not seen many DI boxes with XLR INPUTS?.

At least some do (the Behringer one below calls it XLR unbalanced input). She's not wanting to DI the bass as such in this case, she wants to take a signal from the amp as well as using the amp speaker.

If most DI boxes don't have xlr inputs, what do sound techs plug the xlr "DI" outputs of amps into when we play live?

Image: https://d1aeri3ty3izns.cloudfront.net/media/20/207873/1200/preview.jpg

ecc83 · November 2023

DartmoorHedgehog said:

ecc83 said:

Err? Not seen many DI boxes with XLR INPUTS?.

At least some do (the Behringer one below calls it XLR unbalanced input). She's not wanting to DI the bass as such in this case, she wants to take a signal from the amp as well as using the amp speaker.

If most DI boxes don't have xlr inputs, what do sound techs plug the xlr "DI" outputs of amps into when we play live?

OK, you got me there! But that ain't yer typical rough ass DI! I do not know how good it is? Behringer kit can be good (have some) or ***t. I shall look at the spec and price of that box and maybe get one to pull apart before passing it on to son. Yes, the XLR would take a feed from the XLR DI out we have been on about but being, AFAIK a passive box, 48V can't hurt it.

Dave.

ecc83 · November 2023

I stand corrected, the DI100 is an active box and takes a PP3, might also be phantom powered. In any case the AMP's XLR will be going nowhere near phantom power. I would STILL suggest a basic, passive DI. Will have a varder for thee.

https://www.bax-shop.co.uk/di-box/art-pdb-passive-di-box?utm_source=google&utm_medium=organic&utm_campaign=surfaces&utm_source=google&utm_medium=cpc&utm_campaign=17858053119&utm_term=&adgroup=&creative=&gad_source=1&gclid=CjwKCAiAgeeqBhBAEiwAoDDhn2Vj4pucgjY7Wh3gnqWMV2dnKscjy31wrC2DlpPs--n675V_eo1xUxoCdgQQAvD_BwE

Roughly the same price as the Behrry. Art have a very good name for all kinds of audio transformer products.

Dave.

DartmoorHedgehog · November 2023

OK thanks Dave, but with that passive DI you linked to she'd still have the problem of what to connect the amp to (seeing as the output from the amp is XLR). If an XLR to jack cable would work then couldn't that go straight into the AI as line level rather than via the box? The AI will take instrument or line level input via 1/4" jack, so the issue is that the output of the amp is XLR so using a normal XLR cable makes the interface think it's a mic.

I know Behringer can be a bit variable (and they're usually rip-offs of somebody else's kit) but some of it's pretty decent. That was just the first one I found a picture of for illustration purposes though.

ICBM · November 2023

DartmoorHedgehog said:

OK thanks Dave, but with that passive DI you linked to she'd still have the problem of what to connect the amp to (seeing as the output from the amp is XLR). If an XLR to jack cable would work then couldn't that go straight into the AI as line level rather than via the box? The AI will take instrument or line level input via 1/4" jack, so the issue is that the output of the amp is XLR so using a normal XLR cable makes the interface think it's a mic.

You would be better connecting the FX send of the amp to the 1/4" input of the DI box. I would try the FX send directly to the interface first though, and see if it causes hum. If it doesn't, you don't need the DI box really.

DartmoorHedgehog · November 2023

ICBM said:

You would be better connecting the FX send of the amp to the 1/4" input of the DI box. I would try the FX send directly to the interface first though, and see if it causes hum. If it doesn't, you don't need the DI box really.

But I think that would cut the signal to the power amp wouldn't it? I think she wants to be able to record and still hear the sound through the combo. Personally I would just use headphones for recording, but I think this all started because she wants the sound from the amp speaker. When I've recorded bass I've never used an amp, just a pedal. I have recorded guitar at the same time as being able to hear the amp, but that was a Blackstar with 1/4" emulated out. I think she's over-complicating things trying to use the amp and should just go straight into the AI and use software processing (and headphones), or use a mic, but if that's not how she wants to do it...

ICBM · November 2023

DartmoorHedgehog said:

But I think that would cut the signal to the power amp wouldn't it?

No, usually only plugging into the FX return cuts the signal path.

The only issue will be if it's one of the daft amps that have the FX loop before the EQ.

Howdy, Stranger!

Categories

In this Discussion

Become a Subscriber!

Bass amp fault diagnosis help please (became merits/downsides of XLR DI outputs on amps)

Comments