Treble rolloff in active preamp's volume

Discussion in 'Pickups & Electronics [BG]' started by mbrancalion, May 10, 2012.

  1. Dear Talkbassers,

    after listening .... and recording... and analyzing the way my new electric bass (4 months) sounds, i got the conclusion that the onboard Glockenklang 3-bands preamp rolls of the high frequencies when the volume pot is not full maxed. Even if i take it back just a bit, i hear a difference. And i mean in active mode.
    It's not something like "fletcher-munson curve", etc...... I analyzed several time the spectrum of recordings i made with 100% volume and then with not 100% volume, at 24bit @ 96 Khz.

    The long story is that months ago i had several units sent back to the Glockenklang factory because these units suffered from electromagnetical interferences and the volume pots where not precise.

    This last unit they sent me had a different push-pull volume pot, and they told it was an upgrade, besides the upgrade in an integrated circuit which picked up every sort of interference nearby (before). So, i've already had my dose of communicating with these folks... and honestly i'm fed up with this......

    The fact is: i rarely max out the volume in my basses. I'm used to older Bartolini preamps (especially the TBIBT, which i cannot find anywhere for this new bass, else i'd bought it in a second), and these don't rolloff highs, anywhere, even at 5% of volume.
    I use, for the 90% of time, the volume at 3/4 of its course, because i need adjustment. With the Glockenklang i cannot do it anymore, because it sounds different.
    I don't care if old Glock units sounded different: now they do it this way. And i don't like it. And i didn't find anywhere in the web were it was said, otherwise i'd have never bought it.

    So, my question is: are you aware if other current onboard preamps in the market now suffer from the same rolloff? In particular, i'm interested in the Aguilar OBP3.
    Does it rolloff highs? .....

    And i absolutely don't care to have a passive mode. But i don't want to drill or fill the holes in the bass.
    I want something with 5 pots, and with a good hi-fi sound.

    Another question: as the Glockenklang preamp has a 1:1 ratio regarding active/passive output level, and i really would benefit from a boost in volume in active mode (i never go passive), in your experience has the OBP-3 a sufficiently hotter output, more than if we were going in passive with the PUs ?

    Thank you for your eventual replies. And thank you once more if someone has a TBIBT unit he wants to sell .... :)
  2. colcifer

    colcifer Supporting Member

    Feb 10, 2010
    I'm not familiar with the preamp but if the volume control itself was passive, that would explain it.
  3. AFAIK, every pot is passive. I've never seen a powered up pot....... :)

    Or did i not get your reply in the way you intended? .......
  4. To clarify, a volume pot can go one of two places in an active design. It can be after the preamp or before the preamp.

    After is simple, it just controls the level before the output. Usually these are a lower value, and there should be no effect on tone as volume is rolled off; it's like the master on your amp.

    Before has more complicated effect. Since, for all intents and purposes, the pot is directly connected to the pickup it affects impedance. This is an issue with passive basses, as the volume pot's loading on the pickup will attenuate highs by varying impedance.

    With an active/passive setup things get more complicated. I would assume that the volume pot is between the pickups and the pre input. There are ways around it but it takes weird parts, like dual-ganged pots with different values on each wafer and such.

    I would suggest checking to see where the pickup plugs into to be sure. If it plugs straight into the volume pot, then that's the cause. It's a less effective design, but if you want volume control on both active and passive it's the easiest and cheapest way to do it.

    The TBIBT had the input to the pre/buffers first, so that is why there was no HF rolloff. Downside to that is reduced function in active mode.

    EDIT: Looking at the pictures, the Glock appears to have the volume pot first, meaning it is not active (fed by an active device and therefore isolated from the pickups). So that's the cause of the high frequency rolloff.

    As to the Aguilar, the input appears to be at the blend pot, before preamp, so it most likely will.

    Rule of thumb is: if the volume pot is first, there will be HF attenuattion when you turn down the volume.
  5. Thanks for the reply.

    In the Glockenklang preamp the single outputs from the pickups go into a module with a pan pot, as i would expect.
    After all the other modules and circuit, then comes last the volume pot, and its output goes into the output.

    It's not like this. But i wouldn't expect a case as you mentioned with an onboard preamp made for blending 2 pickups with a pan pot ........

    Yes (as the Glockenklang case), but also the TBIBT has the volume pot as the last thing in the chain, and from there the signal goes to the output jack (as the Glock).
    But in the TBIBT the volume pot impedence is 25 KOhm.

    I suspect that the impedence in the Glock's pot is not 25 Kohm, but something like 250: i can't read the label. The pot is assembled in a block module.
    Anycase, i would expect 250 because the circuit is made for working in totally passive mode too......

    I don't know what schematics you read, but it's not like you described. The volume comes last .....

    Also in the Glock the PU outputs go into the blend pot, but it's not the blend pot which causes the treble loss, but the volume (that comes last) .........

    Thank you.
  6. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    The Glock has an active blend circuit, it is one of the few onboard preamps that has this feature. So in most other cases the blend pot could be an issue, but in this case the blend pot both isn't and shouldn't be any issue.

    As far as why the volume pot is a problem here, I'm baffled. Unless they included a tone rolloff circuit in order to emulate the tone behavior of the volume knob on a passive bass?
  7. I exchanged several email with the guys at Glockenklang, and one of the arguments was the buffering of blend, and they wrote that the blend is not buffered: the PU inputs go straight to the pot, even if the wires are soldered to different terminal in that part of the circuit.

    This preamp has a passive mode, whereas the treble pot work as a low-pass filter from middle-course to full-closed.
    But in active mode (the case i considered) this pot works as a normal treble control.
    So, it doesn't enter in my equation ......
  8. SGD Lutherie

    SGD Lutherie Inactive Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    The higher value volume pot might be causing the problem, because as you turn down you are placing a larger series resistance between the output of the preamp. It shouldn't make a difference, but I don't know what their circuit looks like.

    This is why preamps with bypass usually have the volume at the input. That will load your pickups slightly, but the buffered input usually make the effect fairly small.

    Is this happening when you plug into our amp, or an audio interface your computer? If the input impedance is too low the volume pot might be causing this problem.

    As far as the interference on the older model, is your control cavity shielded?

    Personally I never turn the volume down on my bass unless I'm muting it between songs. It's best to run it on full.
  9. father of fires

    father of fires Commercial User

    Nov 29, 2006
    Chief of Medicine at Damnation Audio

    Why would you want a volume boost just to run the volume knob at half mast. Run them full on and call it a day.

    If there isn't a buffer between the volume and the output then you'll also be dealing with high frequency roll off from the interaction between the pot and the inherent capacitance in your guitar cable and the input impedance of the following circuit. It may be a slight roll off but it is surely noticeable.
  10. It happens in every situation. BUT it's much more evident when i plug directly in a mixer or audio card (so, not so high input impedence).
    When i plug in my Markbass combo or my rack Demeter preamp the rolloff effect is much more subtle (anyway, i'd like it didn't happen......)

    Yes, at full.

    The older model had a problem itself (certified by the Glockenklang's guys). It picked a cell phone 10 meters far away .....
    This new unit is the quietest i've ever used (as far as interferences).

    Eh.... unluckily i use to play in a different way .... :)

    Thank you for your reply.
  11. Because i play best when i have remote control upon my final volume, without going to my amp or asking the sound engineer.

    Never noticed with the Bartolini TBIBT preamp, which has the volume pot in the same place, in the chain ..........

    And i mean: noticed with my "ears", not with eyes ..... :)
  12. SGD Lutherie

    SGD Lutherie Inactive Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Markbass amps actually have pretty low input impedances. They list them as 500 Kohm (500,000). Most amps have input impedances in the range of 1Megohm (1,000,000). So this sounds like an impedance issue.

    Demeter on the other hand list the input impedance as "> 1megohm" so the effect should be a lot less there.

    When using an audio interface or plugging into a mixer, you should not turn down your volume. Leave it all the way up and adjust at the mixer. You can induce noise into the signal with the volume lower.

    There's a few ways to deal with this. Obviously passive volume controls suffer from treble loss. This is one of the advantages of active basses. A common fix for passive instruments is to implement a "treble bleed" circuit. This is a small value cap, often parked with a parallel resistor to pass some high frequencies as you turn down the volume pot.

    Another idea would be to move the volume control before the preamp and after the blend. This is how it's set up with some preamps. You may still some a slight tonal change when you turn down the volume.

    If you want to get fancy, use a dual volume pot. One deck being 250k and the other 25k. Then when you switched into passive mode you would also switch volume pots. I don't know what kind of switch you are using. if it's a push/pull, that might not work because you might need more than 2 poles. I'd have to sit down and work it out to see.
  13. You are right....... I didn't consider it........
    I'll make other test to see if i hear a difference between the Markbass and the Demeter .......

    You're right. I always turn on full when i record.

    I forgot to write, before, that i have the rolloff problem only when playing live. And only in some situations (not all).

    Very very interesting ..... I've never investigated upon this......
    This was useful .....

    Which could be some standard values for these components to solder? ......

    Heh.... it's a push-pull .......

    Thanks for your reply. It was useful.
    I'll try to search for info regarding this "treble bleed", and if it has downsides.....

    Greetings !
  14. SGD Lutherie

    SGD Lutherie Inactive Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products

    You can change the cap value to tun the treble frequency. Smaller values equals higher frequency. If you make them too low you down get a good volume taper. You really want to bring through the high end that's missing.

    The resistor allows some lows back in. You can also that value too. I've seen a version of this sold with trim pots to adjust the amount of bleed.

    The values listed are probably good.
  15. A big thank you !

    Surely i'll try to solder these..... and i'll listen (and obviously report) if i'll hear the same treble loss or not.......

    TNX, greetings ... :)

Share This Page