Redbox to Headphone amp

Hi,

I've been a long time reader of this forum. I have finally purchased a GM40 a few weeks ago and I am getting to know it and like it very much.

When I use it at home I connect it to my Scarlett. I was looking for a laptop free solution and purchased a headphone amp, the Behringer MA400.

But when I connect the Redbox XLR out (mic level) to the headphone amp mic input, it clips terribly unless I turn the master volume knob down to 1 or less - which is detrimental to the overall tone of the amp ( know I can't get a great tone on headphones but...still).

I do not understand why would the mic level redbox out be too hot for a mic input. Is the headphone amp faulty or am I simply missing something?

Hi Rico.  First thing to ask here is, do you have the Mic level selected on the back?  I know you say "(Mic level)" but I wanted to be sure you weren't just assuming it would be that sig level.  It's also easy to mix up the switches if you are looking over the top of the amp.  Make sure it isn't the bottom On/Off switch you are looking at.  When you have the Redbox set On that will be in the Mic position and the Line/Mic text is sitting directly above it.  If you have it set to line then it will overload badly.

Next, what is the Mic input level set to on the Behringer?  Again that question is only to make sure that you have that dialled down rather than relying solely on the H&K Volume to do the job.  Very basic stuff I know but it pays to double check this first.





EDIT:

Just had a quick look at the specs of the Redbox out and the MA400 mic in.  The Redbox is quoted as +4dBV max and the MA400 as -18dBU.  When you convert those to the same reference point, (that's the U and V), that basically means 1.6V from the Redbox to 100mV max required at the MA400.  Not looking good if you don't balance that up.  Oddly the Redbox spec in the GM40D manual is only for that single level, it doesn't give a separate level for the lower Mic setting.  If you have your GM40D dimed (never a good thing despite "common knowledge") you will be pumping close to that 1.6V out from the output transformer at the input to the Redbox circuitry.

(EDIT EDIT: Duuurrrrhhhhhh! Moron alert! I should have written "...you will be pumping close to that 1.6V out from the output of the Redbox circuitry.")

I have all of the GM36 circuitry simulated in Spice.  (That's an industry piece of simulation software if you don't know it.)  That shows that the GM36 Redbox drops the level from amp output to Redbox output by 28dB, that's dropping the signal level by about 25X as it is shaped.  The max output should be around 22V for 40W into 12ohms (midway between 8-16ohms as the H&K amps are specd) so that should be dropped to just under 1V.  That's in the ballpark for line level stuff, remember it is the max available.  The GM40D has the Mic/Line selection which the GM36 doesn't have and that should take care of dropping it down to the 100mV the Behringer can cope with.

Hi Bordonbert. Mic level selected (as opposed to line) on the back.

I have tried several mic input levels on the Behringer but I have mainly used it as a volume knob, not as potential gain knob. I have not tried to dial it really low and turn up the master volume know on the amp. I will give it another try today and keep you posted!

Edit: Just read your edit, thanks for the extra info.

Apologies for the need to correct the last post.  I appear to have been having a "senior moment".  It isn't vital but check out the EDIT EDIT.


I have just asked H&K for the figure for the attenuation from Line to Mic level at the Redbox out.  That isn't included in the amp's specs and it really should be.  I reckon the specs were inherited as a starting point from the older GM36 version and they just didn't notice it wasn't in there.  The GM36 has no Mic/Line selection of course.  I'll keep you posted when they get back to me.

Also, just a thought.  Have you tried running the signal into the MA400 via the monitor input?  I know that is a different single ended 1/4" cable but if you have one handy...

So I tried dialing down the mic input knob - with no luck. It really acts like a volume knob. Even when close to zero I can hear the headphone amp clip.

I contacted Andertons to let then know the product wasn't what I expected it to be and they've offered a refund (love their customer service). So everything is ok, I haven't lost any money. I'd still like to understand what's going on though, not to repeat the same mistake!

I can not run the Redbox to the 1.4 input as I do not have the correct connector. I tried connecting my guitar straight to the 1.4 and it sounded as expected (volume was ok, sound was dog shit - no cab sim).

If I understand what you wrote correctly, a maxed out redbox line output should deliver around 1V when the Behringer can not cope with anything above 100mV. Meaning that either the Mic output does not drop the signal 10X (compared to the line output) or the Behringer XLR input is faulty.

The redbox's mic out works just fine with my Scarlett. When set on Line out and gain all the way down (Scarlett solo, no pad), I encounter some clipping issues, but not when set on Mic out. I do not think my Redbox is faulty.

I am considering purchasing a cheap mixer for the purpose of silent recording but I'd like to make sure I won't repeat the same mistake again.

Forgive my improper use of terms, I am far from knowledgeable (and I am foreign) and thanks for your insight!

Don't worry about your English, it's fine!  I'm a Geordie from Tyneside so I have the same problems you do dealing with Southerners.

I'm still waiting to hear back from H&K, though I think you have pretty much proved that the H&K is not the problem here.  It does seem odd that the Behringer is reacting like this but the Scarlett is ok.  I'll have a look at what I can find about the Scarlett and let you know what that turns up.  Keep in touch here, there may be something to learn, (for both of us hopefully).

Oh, and incidentally, is that Andertons Guildford? If so we are not too far away from each other with me being down the A3 in Bordon. Small world!

The Scarlett series seem to be pretty much the same for their general purpose inputs. Mic: 9dBu, Line inputs (variable gain): 22dBU. That's an impressive 2.2V and 9.75V. No wonder you weren't overloading them. Which model Scarlett is it you actually have? I'm beginning to wonder if you actually have a duff Behringer unit. That is not likely in today's electronic world but it is possible. It may be worth taking your GM40D along when you take it back and run it up to show them what is happening. They should let you try another unit to see if it is a faulty item problem or a design weakness. I know they have a sketchy reputation with guitar gods and gurus but I have always found Behringer to be reasonable quality for a budget range.

Andertons Guilford indeed, but I live in London. I am not going to go to the actual store, but I'll take the unit with me next time I practice with the band and ask the singer to connect his xlr lead to the headphone amp. This way I should know whether the unit is faulty or not. I've owned several Behringer items in the past and never had any issue.

I have a Scarlett Solo 3d gen. The line output tends to overload it but not the mic output. I've also connected the Redbox straight to the PA a few times without any clipping issue.

Let me know when you hear back from H&K. Thanks for your efforts btw!

I've heard back from H&K support now.  As I said they are always super helpful if you treat them with a little respect.  The Mic/Line switch drops the level by only 6dB.  This is a lot less than I was expecting.  They must have their reasons to design this way.  All design decisions are a compromise in some way or other to achieve definite goals.

There is another 3dB attenuation available by simply switching the Power Soak from 40W to 20W and that should not affect tone.  The single output valve pair left working will be driven exactly the same as with all 4 valves live.  It just means less drive into the speaker with less voltage (-3dB) at the input of the Redbox circuitry.  So that should give an overall total of 9dB reduction.  This could explain why you have the problem.

We know from the spec that the max output of the Redbox in Line mode is +4dBV.  With 9dB attenuation, (6 from the Mic select and 3 from the 20W Power Soak mode), that means you now have a max output of -5dBV.  This translates to 560mV RMS (which is the equivalent of close to 800mV peak).  That is high in my experience and definitely is not ideal for the MA400's input of 100mV RMS, (141mV peak).  It would often be taken care of by a sizable switched reduction at the input of many large professional mixers/preamps but that is beyond the features of simple units like the Behringer. The best the Behringer could do is to reduce it severely via its Gain knob. There would be no loss of tone in using that even if you do have to get pretty savage with it. Cutting down the voltage to the Mic preamp circuit does not compromise any overload characteristics at all as that is what we are trying to avoid here.

You could maybe make up a resistive attenuator to drop the Redbox level down by any factor you choose.  The actual circuitry is easy for this, (it's only a pair of pairs of resistors), and I can give you working values if it helps.  This would rely on your soldering experience though and if you have none then you are stumped.  And of course Andertons have already agreed to take the unit back which is by far the easier option.  Just look for a higher level of overload or even switchable attenuation on the input of the next unit you look at.

The little experiment I condicted on my side has been consistent with your findings - the Behringer unit is not faulty, it worked flawlessly with a mic.

When at home, I use the amp on power soak mode, I reckon it cancels the 40w to 20w solution.

I am a Middle School teacher and there's a tech/design lab in my school, I may ask my colleague whether he could teach me to build up an attenuator as I'd love to learn it!

Just to make sure I will not face the same issue again, what specs should I look at when purchasing my future mixer/headphone amp? Max input peak? Gain range? Max voltage gain?

Thanks a lot mate by the way!

Something like this is what you need.  None of the values are exact in any way, your tech guy might use different ones if he feels they need to be trimmed.  These would do for most applications.  I'm not sure how much you already understand of this stuff so if I am teaching my granny to suck eggs I apologise.  It may help others coming around and seeing it.

As your Redbox output is balanced, (that means it has both a normal and an inverted version of the signal on the two signal lines), you will need to attenuate both lines equally to preserve the balance which is important for lowest noise.  You can see there are 2 similar pairs of resistors.  The horizontal ones are 5k6 (5,600ohms), and the vertical ones are 680ohms.  The Redbox has a 680R resistor in each of the output lines to protect its output opamps and to set its output impedance to 1360R balanced.  In practice these are added to the 5k6 resistors to make them effectively 6k28.  Then adding the 680R vertical resistors brings each whole chain to 6k96.  This means that you will drop the H&K's output to 680/6960 = 0.098x.  That's close enough to 0.1x to not even think about it.  That means a 20dB attenuation from 1.6V to 160mV.

If you wanted more attenuation you could increase the two 5k6 resistors.  Just do it equally for both.  Increasing them to 10k would drop the 1.6V to give you 96mV, or 9k1 would give 104mV, again both close enough to 100mV to not worry about it.

The colours for the resistors are accurate in the pic.  There are two systems you may come across, 4 band and 5 band.  The last band is a tolerance indicator so the others are what denotes the value.  For your resistors they will be as follows:  680R = 4 band - [Blue, Grey, Brown] or 5 band - [Blue, Grey, Black, Black];  5k6 = [Green, Blue, Red] or [Green, Blue, Black, Brown];  9k1 = [White, Brown, Red] or [White, Brown, Black, Brown];  10k = [Brown, Black, Orange] or [Brown, Black, Black, Red].  There is no minimum size/wattage for these, any tiny ones will do, as there is no power for these signal levels.

One thing that could be useful here is that XLR connectors can be daisy chained.  You can build a short attenuator cable with a new pair of connectors which fits directly between your normal cable and the amp.  It is very possible to build your resistors into the Redbox end directly on the terminals in the plug which makes for a simple "add it into the line" type of alternative when it is needed.

The only thing to look for in a different headphone unit would be for the maximum input level for overload to be significantly higher than the Behringer.  The gain control can always be set low to suit the following circuitry but it is overload of the input circuitry which comes before it that is the problem here.  The only way to deal with that would be to choose a unit where they have designed for much larger voltages or where they include a switchable attenuation.  Or you could use your own attenuator as above of course.

Incidentally, you may find other layouts online which use only 3 resistors in place of the above 4.  They do work but I don't recommend doing that for your application.  They lead to very bad, (sometimes even damagingly bad), what is called common mode rejection.  That isn't an issue for a microphone but it could be for someone linking to amplifier outputs where they may even be unbalanced.

Hope this is clear enough and not so complicated it puts you off.

Final update:

I went to the tech guy at my work place. He told me the simplest thing to do was to buy a XLR extension/20db attenuator such as this one.

It is working well so far. It does not sound great, but it is definitely usable for home practice and it is a -great- budget option (20 quids for the Behringer headphone amp + 10 quids 20db attenuator).

At that price it's as cheap as my cable option and a lot less effort. Glad that worked out for you, that was good advice from the tech. I do tend to be a bit DIY in my approach I'm afraid. The "just buy it stupid" option is always the last one to be considered. It comes from 50 years of designing and building commercial electronic gear of all sorts, and from seeing how badly designed and implemented some of the guitar gear on the market is. You wouldn't believe some of the really poor pieces of design there are out there hiding behind really big names, especially in the rip off world of pedals where a few awful designs just keep on getting recycled with a higher price tag each time. And people queue up to throw their money at them.

I think a lot of your low sound quality could simply be down to the fact that you are using headphones. They are never terribly good for guitar in my experience. Even good modelling units never seem to be able to really put the life back into a silent signal. But as long as you are happy to work with it and accept its limitations then you have a great solution there.

Hi there,
I'm looking into buying H&K GM40 Deluxe mainly to use for recording and portable silent practice with headphones, so have been wondering about an easy way to connect headphones without dragging my interface/laptop with me all the time. Basically, what topic starter asked

I'm wondering if there are any other not too expensive alternatives to the solution provided? After doing some googling it seems like LD HPA 1 headphones amp (cannot post a link as I'm a newbie) can handle both Line/Mic outputs from the H&K? But as I'm novice into the topic I'm afraid I might be missing something. Could someone confirm if it looks reasonable, please?


And if someone know any other small headphones amps / mixer that might work here, please let me know! I'm not good enough for DIY, so I'd rather prefer to buy something that would work OOTB.

Thanks!

That seems to be a pretty reasonable unit to use Thinline, if a little expensive. It does offer you XLR connections and portable battery usage which is good for what you want. Signal level is always a bit of an unknown quantity with this sort of unit. It never ceases to amaze me that so many manufacturers find so many ways to NOT stick to the same standards. The Redbox Max Level Output is quoted as 4dBV and the LD HPA1 maximum input is quoted as 16dBU. dBU and dBV are using two different reference standards however it is easy to convert and you should find that those levels are pretty much in the ballpark to work together. The impedance matching between units should drop that a little, it isn't a precise piece of calculation, but it won't be at all noticeable.

I hear what you are saying about the diy approach, (which I know I am often keen to get people to explore), and in your case that isn't an option just as you say. There is no reason to go for an expensive low distortion hifi type of unit. All of your tonal adjustments will be made in the GM40D so the headphone amp can be pretty much plain and simple. Remember, guitar use is not the same as hifi. The low distortion standards are nowhere near as demanding. You will already be deliberately introducing a good few % distortion into the signal as you play, even on Clean settings, so an additional fraction of a % from a basic headphone unit is neither here nor there in real terms. There is also no "stereo imaging" aspect to worry about which requires low distortion.

If you can get a reasonable price on that unit I would say fine, it will do the job. It would help if you could actually try one out before committing to buy but that isn't always possible I know. Maybe make sure you could return it if you don't like what it is doing? Have you seen this unit online or is it available from an outlet somewhere near you?

Hi Bordonbert,
Thanks a lot for the quick reply!

Yes, I've done those dBV and dBU conversions, but as I was afraid I'm missing something I've decided to ask just in case!

And yes, I'm not looking into hifi type unit. After reading this very helpful thread (thanks!) I was just searching for a small unit (headphones amp or mixer) under 100 pounds that can handle GM40D output. Ideally, Line output as it seems to be what they recommend to use in such cases. And yes, that was quite a challenge as all specs for products are always in different units or missing some info etc.

I think Behringer + XLR extension/20db attenuator is a gread setup too. I was just wondering if there is anything else out there that can handle it as it is and maybe provide a bit better quality (Rico mentioned that it doesn't sound great). That's how I found LD HPA1.

Reading your comment though it looks like I won't be able to get a better quality even if I use Line output, is it right? Or am I missing something here?

I found it online but most of the shops support at least 14-days return back policy, so I'm not worried to try it out. But as I mentioned in the first comment, I don't have GM40D yet so cannot try. I'm very interested to buy GM40D so I'm exploring how it might fit my needs and what's needed for that.

You are on the right line with using the Redbox for this. Using the Line Out socket you will have only the flat preamp section signal going into your other device, (presumably the headphone amp in your case). The Redbox adds to that the contribution of the entire power amp under drive using the power soak, an emulated 4x12" speaker setup including both drivers and the cabinet, and also the compression of the speakers driving the air, not often considered, and room ambience too. That is why the Redbox is so useful. Without needing a mic in the room or even speakers being driven you get the full sound contribution of the amplification chain right from guitar to air in your headphones or mixer! There are other emulators out there which you can buy as separate items but you have that built in with the GM40D. It's definitely the right way to go for headphones if you decide to pull the trigger on one.

Thanks for the answers with detailed explanations! If I decide to pull the trigger, I'll provide a feedback here.