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Thread: Advice on reducing the volume on my bass drivers

  1. #1
    Join Date: Apr 2016

    Location: Bishops Stortford

    Posts: 1,119
    I'm Chris.

    Default Advice on reducing the volume on my bass drivers

    I have recently replaced my mid drivers in my open baffles with something of better SQ but of lower output - perhaps about 3db down.

    I am now finding the 15" bass driver to a be a bit too loud and want to turn it down about 3 db. I think an Lpad seems to be the best choice, but need some help in selecting values

    Alternatively mono volume pots looks like they would give more scope while I optimise the balance, but they seem even more difficult to select for the novice with things like resistance and log or linear to consider - any recommendations?

    Thanks Chris
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

  2. #2
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 5,641
    I'm Lawrence.

    Default

    Weren't you actively amplifying each driver in your speakers, you bought my passive pre to attenuate the bass amp, you could use that?

    Sent from my BKL-L09 using Tapatalk

  3. #3
    Join Date: Apr 2016

    Location: Bishops Stortford

    Posts: 1,119
    I'm Chris.

    Default

    Quote Originally Posted by Lawrence001 View Post
    Weren't you actively amplifying each driver in your speakers, you bought my passive pre to attenuate the bass amp, you could use that?

    Sent from my BKL-L09 using Tapatalk
    That's true Lawrence but things move on.
    For me its all about minimising the component count to reduce anything that compromises SQ no matter how small. For a while I was running the bass driver from its own amp, passive pre and MiniDSP to set the low pass. With cables this amounted for each channel to
    1. Three interconnects
    2. One speaker cable
    3. One attenuator
    4. One power amp
    5. A component containing digital processing flanked by an A/D and D/A converter.

    Just too much

    So out it all came and I am once again running the bass from the same amp as the mid/tweeter giving a total of just one quality inductor and capacitor for low pass.

    So you can see how I get to the current situation. Unless I'm guided otherwise I can't see how the (phono) attenuator can be used in the speaker cable.
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

  4. #4
    Join Date: Dec 2017

    Location: limerick

    Posts: 177
    I'm charles.

    Default

    If you intend inserting an L-Pad into the 416 circuit, use an 8 ohm unit. I'm assuming you have an 8 ohm driver. If you are happy with the result a suggestion is to then measure the series and parallel resistance of the setting you choose and to then replace the L-Pad with those two resistors. 2 quality fixed resistors are superior to an adjustable pot.

    As the L-Pad is rotated it selects different values for the series and parallel elements in conjunction with the 8 ohm driver such that the combination with an 8 ohm driver is maintained at 8 ohms. If the driver is 4 ohm, get a 4 ohm L-Pad. The values are chosen to provide different amounts of attenuation whilst maintaining 8 ohms, or as close as possible with standard values.

    For simplicity lets attenuate an 8 ohm driver by placing an 8 ohm resistor in series with it, resulting in 16 ohms. If this is in the crossover it would change the XO frequency! Ok, so lets put a 16 ohm resistor in parallel with the (driver + 8 ohm resistor) bringing you back to 8 ohm.

    Bear in mind that this will decrease the amp's damping factor which probably is not such a big deal with your valve amps but you will be losing some useful power. L-Pads are mostly found in mid and tweeter circuits.

    As Lawrance mentioned, attenuate before the amp. This is a far more elegant approach.

    I don't know how much headroom you have but it may be possible to increase the output to the other drivers by about 3 dB. 1.8W does not leave you with much to play with though.

  5. #5
    Join Date: Apr 2016

    Location: Bishops Stortford

    Posts: 1,119
    I'm Chris.

    Default

    Quote Originally Posted by sailor View Post
    If you intend inserting an L-Pad into the 416 circuit, use an 8 ohm unit. I'm assuming you have an 8 ohm driver. If you are happy with the result a suggestion is to then measure the series and parallel resistance of the setting you choose and to then replace the L-Pad with those two resistors. 2 quality fixed resistors are superior to an adjustable pot.

    As the L-Pad is rotated it selects different values for the series and parallel elements in conjunction with the 8 ohm driver such that the combination with an 8 ohm driver is maintained at 8 ohms. If the driver is 4 ohm, get a 4 ohm L-Pad. The values are chosen to provide different amounts of attenuation whilst maintaining 8 ohms, or as close as possible with standard values.

    For simplicity lets attenuate an 8 ohm driver by placing an 8 ohm resistor in series with it, resulting in 16 ohms. If this is in the crossover it would change the XO frequency! Ok, so lets put a 16 ohm resistor in parallel with the (driver + 8 ohm resistor) bringing you back to 8 ohm.

    Bear in mind that this will decrease the amp's damping factor which probably is not such a big deal with your valve amps but you will be losing some useful power. L-Pads are mostly found in mid and tweeter circuits.

    As Lawrance mentioned, attenuate before the amp. This is a far more elegant approach.

    I don't know how much headroom you have but it may be possible to increase the output to the other drivers by about 3 dB. 1.8W does not leave you with much to play with though.
    Thanks Charles

    I have settled on the 300B Elekit (about 8 watts) as my amp of choice so I have a bit of headroom to play with over the 45 triodes. Thanks for explaining about the L Pad. It makes more sense now. My drivers are indeed 8 ohms

    A calculator I found on line suggests that to attenuate by just 3db (half the volume) requires:
    1. A 3.3 ohm ohm resistor in line - making the overall impedance about 11 ohms which I guess is within the tolerance of the amp.
    2. An L-Pad of 2.3 ohms in series with 19.4 ohms in parallel - which returns it to 8ohms.

    I am nervous of most electronics, but option 1. seems OK and the simplest, so I will do my evaluation there. If satisfactory I will institutionalise it by moving to the L Pad with high quality resistors.

    Cheers Chris
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

  6. #6
    Join Date: Dec 2017

    Location: limerick

    Posts: 177
    I'm charles.

    Default

    Reading again what I posted I feel I may not have explained very well the operation of a variable L-Pad which is essentially a voltage divider.



    So as the shaft is rotated different values for the 2 legs are varied offering less or more attenuation while maintaining 8 ohms. Once you have played with it for a while remove the L-Pad without disturbing the shaft position. Measure between terminals 2 & 3 for the value of series resistance and between 2 & 1 for the parallel resistance. These then are the values of the fixed resistors to permanently install.

    Choose standard values closest to the measured values. To get closer to required value, resistors can be paralleled which is what I do because it reduces inductance which is not an issue in the woofer circuit and it raises the power handling. Although the 15" will be driven by a low power amp (8W?) the higher power rating will limit thermal compression.

    I have some L-Pads knocking around somewhere. There are a pair of Technics and pair of Altecs and ?
    I can send you a pair if you like and you can return them when done.

  7. #7
    Join Date: Apr 2016

    Location: Bishops Stortford

    Posts: 1,119
    I'm Chris.

    Default

    Quote Originally Posted by sailor View Post
    Reading again what I posted I feel I may not have explained very well the operation of a variable L-Pad which is essentially a voltage divider.



    So as the shaft is rotated different values for the 2 legs are varied offering less or more attenuation while maintaining 8 ohms. Once you have played with it for a while remove the L-Pad without disturbing the shaft position. Measure between terminals 2 & 3 for the value of series resistance and between 2 & 1 for the parallel resistance. These then are the values of the fixed resistors to permanently install.

    Choose standard values closest to the measured values. To get closer to required value, resistors can be paralleled which is what I do because it reduces inductance which is not an issue in the woofer circuit and it raises the power handling. Although the 15" will be driven by a low power amp (8W?) the higher power rating will limit thermal compression.

    I have some L-Pads knocking around somewhere. There are a pair of Technics and pair of Altecs and ?
    I can send you a pair if you like and you can return them when done.
    Thanks for the offer Charles, but I have finally found a Monacor 8 ohm L Pad that I bought a fair time ago and put away in a safe place which I then forgot! I will modify one speaker and play mono music whilst alternating between this and the unmodified speaker. This will give me a fine handle on what is happening. When I've finished I will take measurements as you suggest and buy some quality resistors.
    Source
    SW1X Universal Music Server UMS I Signature with Power Supply Unit PSU I Signature
    SW1X USB II
    SW1X DAC III Special
    Amps
    Pre amp -- Hi fi Collective twin mono ladder stepped attenuator, with Charcroft Z-foil and silver wired.
    Power amps -- Welborne 45 SET monoblocks 1.8W / Decware Taboo 6W / Elekit 300B TU-8600SVK plus further improved components 9W / ICE Power 1000W
    Speakers
    Highly modified Endorphin P17 open baffle speakers containing both vintage and modern alnico drivers and paper cones. All silver wired - 8" Cube Audio FC8 full range drivers and vintage 15" Altec VOTT 416 bass drivers. All sat on Townsend Audio Podium seismic isolation platforms.
    Cabling
    Silver mains cables, interconnects and speaker cables by SW1X
    Headphones
    HRT HeadStreamer and SennHeiser HD650 headphones

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