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Modest recording setup, preamp trouble.

Discussion in 'Preamps & Processing' started by esthar, Apr 13, 2007.

  1. esthar

    esthar Guest


    I'd like to start out by saying I'm new to recording and to these forums, and am in need of aid w/ my current recording setup.

    I've recently started recording a rock and roll album, but have ran into some issues. I have a very limited budget but would like to have the best quality possible with my pre-existing equipment. I currently using a Boss BR-8 studio-in-a-box, and it's pretty okay, not good but okay. I have four microphones, including a Shure SM57(X2), SM58, and a Rode NTK Tube Condenser(this one is at my brothers so have not been able to test as of yet on it). The problem w/ the BR-8 is that recordings that come from it tend to be run-of-the-mill and sometimes lackluster, compared to most pro recordings. But I recently came in possession of a 1940's preamp/mixer, no manufacturer or plates labelled, but it has the whole vintage look, big Bakelite knobs etc. The unit has 4 channel 3pin XLR female outputs and a fifth 4 pin XLR output I assume to hook up to a recording console(I do not have one of these patches so it has yet to be tested), or any other external piece of equipment. The front panel has two 1/4 inch input jacks for phones or for output. Now using the preamp as a stand alone there is a crisp, bright tone, fat, no hum or transistor like hiss. But when I attempt to hook it up to my Boss recording console, using the 1/4 patch in the front of the unit(as I have no XLR outputs on this vintage piece) I get a very distinct hiss between the devices this can be somewhat eliminated by touching the patch at the preamp end, But still get that transistor-esque radio wave sound. This abundantly obvious on the tracks, I've tried equalization on my Boss, and the noise filter, which seems to do okay but the threshold is really high and every time something other then white noise enters, it becomes very obvious that there is an effect applied. Could this be just a lousy patch cable? I've tried multiple but none are high end patches and although the preamp is old it's been kept in immaculate condition by the previous owner. But this problem only occurs when going from one device to another. My intention to record live 2-3 tracks at time, and by all means try and get the sound of late 50's and 60's. But this is not just noise but very apparent distinctive hiss. What could I do to eliminate these problems? Is there a relatively inexpensive device that I can purchase that has four XLR outputs that I could connect to the preamp, and not have to worry about 1/4 inch patching? Would it be worth modding? Or would an external mixer be a better idea to hook up to? Would a noise gate correct this issue without effecting my sound?

    I think part of my problem comes from the Boss digital preamp and my preamp conflicting, but the booklet for the BR-8 actually suggests a preamp connection for low impedance microphones. Am I thinking about this backwards? What are my alternatives at this point?

    I'm a novice at recording, but have a good ear, sometimes the engineering aspects go way over my head. Any advice you can give I would be quite obliged.
  2. esthar

    esthar Guest

    I've been doing some research and I am wondering if it simply could be a low impendence signal conflicting with a high impendence signal, would a DI box work in this scenerio, do they act as a isolation transformer? Could this reduce the noise?
  3. RemyRAD

    RemyRAD Guest

    Sep 26, 2005
    esthar, while it would be really helpful to know the manufacturer of this "vintage tube microphone mixer", I can safely say that the 4 pin XLR type connector you are describing is generally NOT AN AUDIO INPUT NOR OUTPUT but generally are used for +12 volts of DC power! Be very careful! Very few inputs and/or outputs were ever designed with 4 pin XLR's, to be used in an audio application and certainly no old tube equipment that I know of.

    Now the hiss issue that you are getting says to me that there is a gross gain staging level mismatch. That is to say, the output of your tube mixer, even without a signal, is a higher level line output source, going into a rather lower level consumer line input. This could be as much as a 14 to 20 DB difference in level match and with that, an equivalent noise boost, like you described. And you should NOT be taking the tube preamp output into ANY microphone input. It doesn't matter what kind of connectors you're looking at as many of these connectors are used interchangeably at different levels of signal, these days, more so than in the past.

    If you're vintage tube mixer utilizes only 1/4" outputs and your BOSS recording thingy only has 3 pin XLR microphone inputs then, yes, a reasonably priced "transformer" (not active) Direct Injection (DI) box would be able to take the 1/4" output from your tube mixer and with the help of an "XLR" male to male "sex change reverser" adapter, you'll be able to plug into the 3 pin XLR microphone inputs. The direct box and/or Radio Shaft high impedance to low impedance adapters are typically transformers with a 10:1 windings ratio. What this does, since a transformer can be used bidirectionally, can step up or step down impedance and while doing so, actually turns up or, turns down the level in DB's, reflected by the amount of turns ratios on the transformer. So a typical impedance adapter from Radio Shaft and/or a quality direct box such as a Whirlwind Director will generally provide up to 10 DB of boost in one direction to 10 DB of cut in the other direction. So these DI impedance matching boxes also serve to help match levels.

    So if you take that direct box and need the 1/4" output from the tube mixer to the 1/4" input on the direct box, you can then take the 3 pin XLR output into your XLR microphone input on your BOSS studio in a box. You will have to in all probability, reduce the microphone preamp gain setting and/or switch in the "pad", in order to use your vintage tube mixer as an input source to your studio in a box.

    So the entire problem was just one of level mismatch or improper gain staging.

    Nothing to be gained up or down
    Ms. Remy Ann David
  4. moonbaby

    moonbaby Mmmmmm Moderator

    Feb 23, 2005
    Hi, and welcome to RO!
    Ah, a lovely old museum piece you have there. Before we get into the nitty-gritty, I'm going to warn you : there are lethal voltages in that puppy!!! Don't open it up and probe around the circuitry, even with no power applied because it can KILL you! OK, here we go, a couple of questions are in order:
    1)Is there a 2-prong or a 3-prong plug on the Frankenbox's AC power cord? I suspect a 2-prong, not good...
    2)Your description of the XLR sockets is confusing. If these are female
    (3 holes in a rubber or plastic center) these are your inputs, not outputs.
    This is where you should be plugging the mic...3pins are the male XLR, used as an output.
    3)When you say "transistor-esque radio wave sound", are you picking up AM radio broadcasts from afar? Or do you mean that it sounds like a tinny little POC radio? I suspect the former on this one...
    4)How are you monitoring the output of the Frankenbox when it sounds "bright, clear, and fat"? Is it through headphones plugged into one of those 1/4" outputs you described or some type of amp hooked to it?When you run the box to the Boss, what input on the Boss are you using?
    5) I suspect that as a combination of the crappy, unsafe power cord, and an internal grounding issue at the 1/4" output, you are getting this RFI(Radio Frequency Interference) and noise.
    I don't believe that a DI box will solve your issues here, because I think that you have a chassis grounding issue, but I may be wrong. I think that this thing needs to be properly wired to use a modern 3-prong AC cord. I've had to do that with all of my vintage tube amps from the 50s-60s. It isn't "rocket science", but you do have to know what you're doing, or you can DIE!!!! Then the 1/4" outs on the box have to be checked for a grounding issue. At this time, a short (hopefully less than 20feet), shielded guitar-type cable can be acting as a radio antenna.
    I have to run now, I will be mixing at the Jazz festival in town this weekend, but check out what I said, reply with any answers you have to the questions, I'll check in later this weekend.
  5. esthar

    esthar Guest

    Awesome, I'll heed that advice and try a few of your suggestions. You are right about that 4 pin adapter, upon receiving the preamp I plugged it in and my pinky finger grazed that output and I received a bit of a shock! :(

    The back of my preamp/mixer has 4 XLR outputs for mics, and in the front has two 1/4" inputs, and the Boss has no XLR just 1/4" as well. I spoke to a local tech and he believes that the transistor buzz might be due to the preamp not being grounded. Which could be as well. I'll let you know the results.

    I could take pictures of the unit if that would help as well.
  6. Davedog

    Davedog Distinguished Member

    Dec 10, 2001
    Pacific NW
    Yeah....take those pictures.

    Heres a bit of a suggestion and some of it involves the nomenclature youre using to describe your situation.

    There can be a lot more help if we all get on the same page.

    Inputs/outputs. First....a mic doesnt use an OUTPUT they use the INPUTS and these are the panel mounted FEMALE of the XLR species. You say there are four??!! Cool. Hopefully there are four level controls for these. There has to be an output somewhere. It could be the 1/4" jacks you keep calling inputs. This piece of gear is sounding more like an old 4 into 1or2 mixer all the time. Using the outputs of this into that standalone recorder means you are probably sending a line level signal into inputs (on the recorder) intended for mic level signal. This would explain the hiss.

    The four pin connector could be speaker level as a lot of old mixers had a power amp in them for on-board monitoring. Could be something else too.

    Take the pictures. Do several angles. Open the top and there are a bunch of folks on here that know what they would be seeing without a tester.

    I have no doubt you are having a mismatch of both voltage and purpose as well as a grounding issue.

    BTW. A 40's vintage mixer would NOT have XLR's. So a picture is worth a thousand words.

  7. esthar

    esthar Guest

    Yeah sorry for mixing up my terminology, today is a bit hectic. But this weekend I'll make an attempt to take some indepth pictures to show you all.

    Then maybe we can locate the source of the issue. Thanks for taking the time to help me. I'll follow-up in a bit. Gotta get back to work. :(
  8. esthar

    esthar Guest

    Okay, sorry about the massive delay. It was my Birthday this weekend so some well deserved fun was in order, most of my weekend was spent with some friends. But aside from all that I uploaded a series of photos for you guys to view and some discriptions...


    Tell me what you think. I connect with a standard patch cord from the front panel 1/4" inch outputs to my boss digital recorder Which also uses 1/4" inch connections. Being TRS or not does not seem to make a difference. Although I tried a DI box and it did seem to quiet down the level of noise but it's still very apparent.

    Hope this helps.
  9. RemyRAD

    RemyRAD Guest

    Sep 26, 2005
    OK! I think I have a better handle on what is going on here.

    You are quite right, this was a mixer designed for on location broadcasts, for radio. The two 1/4" connectors that you are using for outputs, were in fact, designed for a pair of high impedance mono headphones and were not intended to be used as the outputs. These were not intended to be the recording outputs, which may explain the problem you are having?

    If you look at to the right of the meter, the two things under the rotary switch are probably screw type "binding posts". It makes perfect sense to have put them underneath the rotary switch. This made it easier for the engineer to connect to the telephone lines and is the actual output that was connected to the POTS (plain old telephone system) telephone lines. The binding posts are probably coming from the secondary of that UTC transformer output, as seen in the picture you posted. You should use the output from the binding posts, not the 1/4" connectors on the front panel. It doesn't matter which binding post goes to the tip of the 1/4" plug that will be connected to your "studio in a box" 1/4" input. The other binding post should be connected to the sleeve of the 1/4" TS connector. If you still hear the buzzing hum, try connecting a chassis ground wire from the chassis of the mixer, to the sleeve connections of the 1/4" TS plug. So what you would have then is one of those binding posts shorted to the chassis of the mixer and connected to the sleeve of the 1/4" TS connector.

    I believe this should clear up the grounding problem you are experiencing. That rotary switch above the binding posts you may think controls output level but most likely it was designed to place a 600 ohm or other value load across the binding posts to be able to "hold" the telephone line on, otherwise it would disconnect. Yes, it would appear that the level changes when you rotate that switch, since it is connecting a load across the secondary of the UTC output transformer, which will in turn knock down the output level, as the output of the transformer is loaded down by the resisters connected to the rotary switch.

    Now that output is going to be a hefty + 4 dbm (1.25 volts), when the meter indicates zero. And, you should absolutely have that rotary switch that is above the binding posts, probably rotated clockwise, to properly load the output of the transformer. This will also ensure that the output of the mixer will have a flat response. Without the termination switch engaged, the output frequency response is likely to be affected in a sonically unpleasant way, since the transformer does not see a proper output load. And you will want to have your gain trim level on your studio in a box turned all the way down, so as not to overload its internal electronics.

    You should also make sure that any unused inputs on the mixer, do not have their volume controls up.

    The 6J7's, are the microphone preamp first stage amplifier and are the same "metal envelope" (that's right, they're not made out of glass) vacuum tubes used as the first stage playback head preamp in the original Ampex 300's & 350's analog tape recorders (not to be confused with the 351 which used the new miniature 12AX7 and similar, dual triodes). They had metal instead of glass envelopes to better shield these tubes from extraneous interference, used for small signal high gain amplification and as you can see, popular for the microphone preamp. They were however notoriously "microphonic" and were frequently mounted with a rubber grommet vibration preventive mount. Ampex actually used special hand selected 6J7's for these very small voltage high gain applications. So they were all checked for noise and only the best were installed that were lowest in noise.

    I hope this solves your problem and gives you some good tube sounds.
    Ms. Remy Ann David
  10. esthar

    esthar Guest

    Thanks for all the information Remy You have been very insightful, I tried what you said to see if it would make any difference, but alas nothing. I do get a signal but it is equal to the amount I get through the 1/4" connections. then I started to try some different things and when you mentioned connecting the a 1/4" inch from chassis to chassis, I started to see a noticeable difference and when I connected a patch from one binding post to the xlr cord on the back of the mixer(just the tips touching) it resolved the issue. I assume this means there is a ground loop somewhere, how can I properly correct this issue, could a local electronics tech do anything? I've also added some for pictures for a more indepth look if that helps.
  11. RemyRAD

    RemyRAD Guest

    Sep 26, 2005
    Actually, it's not a ground loop. A ground loop creates hum. You have actually resolved a ground issue. If by your description, the hum and buzz ceased and the signal finally got fuller sounding, by Jove, I think you got it? Now just find a neater way to create that same connection and you should be rocking and rolling in no time.

    Old stuff. Gotta' love it.
    Ms. Remy Ann David

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