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  1. Old Comment

    Wharfedale Dovedale cone rubber repair

    Thought of a more thorough way to recon the outer surround so I removed it completely. That was a bit of a task as the glue was rather hard. Achieved removal with minor damage which will not matter after reinstalling.
    Used
    1. 2 large ziplock bags. Big enough to allow the surround to sit flat and stay that way during the heating phase. New bags are best as there will be few pin holes or compromised welds
    2. ~20mL Oil of Wintergreen (OoW)
    3. ~250mL water
    4. Paper towels
    5. Rubber gloves
    6. Large microwave oven

    Placed surround, concave up, into first bag.
    Taped the bag's corners inwards to prevent fluid from residing there.
    Add the OoW and water and seal.
    Placed that into second bag as first was leaking a little. Seal it.
    Covered the glass turntable with paper towels.
    Placed into large microwave oven but on top of a small plastic lid to raise the centre of the bags and force the fluid to the perimeter.
    Set 1min High, 3 min Low and 20 min V.Low and Start.
    Checked fluid temp a few times by hand. Always hot but did nor appear to boil.
    Removed and allowed to cool.
    Removed paper towels containing leaked fluid and thoroughly cleaned oven. A little pong from there.

    Preliminary result was that the surround is very compliant but a little expanded such that the flats at inner and outer are not flat. The roll was a little distorted.
    Cleaned off the softened glue from both flats. Behaved like it was contact adhesive which, if so, might be ok in a factory with jigs but which I will not repeat given the forgiving nature of the adhesives currently available.
    Laid out as flat as possible on clean paper.

    Now for a long wait to see if enough shrinkage occurs and the flats resume being flat and the hard won compliance remains.

    Results with some pics then.

    Roj
    permalink
    Posted 18th June 2017 at 01:34 AM by Roj Roj is offline
  2. Old Comment

    Wharfedale Dovedale cone rubber repair

    Thought of a more thorough way to recon the outer surround so I removed it completely. That was a bit of a task as the glue was rather hard. Achieved removal with minor damage which will not matter after reinstalling.
    Used
    1. 2 large ziplock bags. Big enough to allow the surround to sit flat and stay that way during the heating phase. New bags are best as there will be few pin holes or compromised welds
    2. ~20mL Oil of Wintergreen (OoW)
    3. ~250mL water
    4. Paper towels
    5. Rubber gloves
    6. Large microwave oven

    Placed surround, concave up, into first bag.
    Taped the bag's corners inwards to prevent fluid from residing there.
    Add the OoW and water and seal.
    Placed that into second bag as first was leaking a little. Seal it.
    Covered the glass turntable with paper towels.
    Placed into large microwave oven but on top of a small plastic lid to raise the centre of the bags and force the fluid to the perimeter.
    Set 1min High, 3 min Low and 20 min V.Low and Start.
    Checked fluid temp a few times by hand. Always hot but did nor appear to boil.
    Removed and allowed to cool.
    Removed paper towels containing leaked fluid and thoroughly cleaned oven. A little pong from there.

    Preliminary result was that the surround is very compliant but a little expanded such that the flats at inner and outer are not flat. The roll was a little distorted.
    Cleaned off the softened glue from both flats. Behaved like it was contact adhesive which, if so, might be ok in a factory with jigs but which I will not repeat given the forgiving nature of the adhesives currently available.

    Now for a long wait to see if enough shrinkage occurs and the flats resume being flat and the hard won compliance remains.

    Results with some pics then.

    Roj
    permalink
    Posted 18th June 2017 at 01:30 AM by Roj Roj is offline
  3. Old Comment

    Discrete Op Amp measurements

    Well done indeed !
    permalink
    Posted 13th June 2017 at 02:22 PM by kasey197 kasey197 is offline
  4. Old Comment

    Superlux HD668B SE classA amp

    Richard, today I have tested a few LPF options on a balanced transistor I/V for tda1543, single supply. With no LPF the sound is quite enjoyable off axis: I might prefer no filter if I am away from the speakers (cooking for example).

    At the listening position some filtering is needed to make the sound more natural (my speakers are quite forward). Right now I have two Vishay KP 2.2nF 250V connected in series (acts like a 1.1nF therefore). Connected across the phases (between I/V resistors) and the midpoint I can connect to gnd, vcc or leave disconnected. Once again I find the sound more enjoyable and easier to follow with the midpoint disconnected. Itīs that old story: donīt filter "to ground" with these chips!

    I will place a jumper between the two caps and keep casually comparing filtered vs non filtered.

    Another idea is a common mode choke before the filter cap, then take the output from the cap. I have a ferrite toroid that might be suitable (1 inch diameter). No idea about the parameters of it though... What do you think Richard?

    BTW my amp is completely reworked, its a single supply bridged THS4131 plus two LM3875, all inverting, all chips receive the same 32V regulated supply (zener -> 2K2 -> 1200uF -> triple EF), and the raw supply is 40V 20mF capacitance. Sounds very good.
    permalink
    Posted 11th June 2017 at 12:55 AM by Alexandre Alexandre is offline
    Updated 11th June 2017 at 01:01 AM by Alexandre
  5. Old Comment

    Discrete Op Amp PCB Build

    you're running this in inverting mode i assume ?

    non-inverting introduces unnecessary common mode distortion imo..
    permalink
    Posted 9th June 2017 at 08:17 AM by kasey197 kasey197 is offline
  6. Old Comment

    Discrete Op Amp PCB Build

    nice going and its looking good
    permalink
    Posted 9th June 2017 at 08:14 AM by kasey197 kasey197 is offline
  7. Old Comment

    Discrete Op Amp PCB Build

    Only G=2 at present - I have been too busy with work. I hope to get to this.

    At G=2:
    - I tweaked the diff pair current to 1mA
    - I set the slew rate to 8v/us, or miller C = 180pF
    - I set the VAS current to 2-3mA (there are a couple of versions)
    - I set the output current to 4mA per device or 8mA total.

    Most of the above were to manage thermals - with the higher bias currents everywhere on such a tiny board, things got toasty warm. I was not really happy with this given this device is likely to live its life in a confined unventillated space... The curse of CLASS A!


    With this:
    - the distortion is above the self noise of my distortion test set on loopback.
    - The test set, at the output level it is running at, is around -115dB across 20Hz to 20kHz
    - The op amp adds a couple of dB to this, or 0.0005%
    - this was measured at about +/-5Vpk into 1K Ohms

    The high level of NFB makes this measurement hard. I think G=10 will make things a lot clearer.
    permalink
    Posted 8th June 2017 at 01:27 PM by googlyone googlyone is offline
    Updated 8th June 2017 at 01:30 PM by googlyone
  8. Old Comment

    Discrete Op Amp PCB Build

    this is pretty interesting - have you had any time to do the measurements yet ? Say at G=10 and G=2?
    permalink
    Posted 7th June 2017 at 12:12 AM by kasey197 kasey197 is offline
  9. Old Comment
    rjm's Avatar

    Gustard H10 Headphone Amplifier

    I don't remember it being difficult to remove to top lid. It is held in place by two screws on the rear plate, and one screw on each side just behind the front panel.
    permalink
    Posted 3rd June 2017 at 10:46 PM by rjm rjm is offline
  10. Old Comment
    abraxalito's Avatar

    Superlux HD668B SE classA amp

    Having looked at the BSP92 DS I can see why you'd be worried about not having enough rail voltage - because of its higher breakdown voltage the on-resistance is considerably higher. Although the gm plot looks good my guess is it was made at a fairly high Vds (probably 10V at least) and when operating with only a couple of volts across the FET the gm is going to be considerably lower.
    permalink
    Posted 29th May 2017 at 01:21 AM by abraxalito abraxalito is online now
  11. Old Comment

    Superlux HD668B SE classA amp

    Ok thanks ! that's sounds perfect.... don't keep the suspense going for too much longer tho ;0 let me know and I'll put this into my next parts order
    My fave p-ch has been the bsp92 for some time now - time to give it some competition (pdiss notwithstanding) !!
    permalink
    Posted 28th May 2017 at 03:10 PM by kasey197 kasey197 is offline
    Updated 28th May 2017 at 03:13 PM by kasey197
  12. Old Comment
    abraxalito's Avatar

    Superlux HD668B SE classA amp

    My first four prototypes I built with the FET in a UFM (2mm long) package which is rated at 500mW with about 600mm^2 of copper on the pads. Subsequently I've found the same die can be gotten in a true SOT-23 where the dissipation rating goes up to 1W with the same amount of copper. So if you need 2W of dissipation (which would correspond to 1W of classA output roughly) then three transistors paralleled should be able to manage comfortably. The 1W rating is only valid at 25oC ambient which would be very hard to achieve inside a case.
    The opamp-based current source is using the same P-channel MOSFET so you'd need a total of 12 transistors for a stereo amp. Fortunately they are extremely cheap
    permalink
    Posted 28th May 2017 at 01:50 PM by abraxalito abraxalito is online now
    Updated 28th May 2017 at 01:56 PM by abraxalito
  13. Old Comment

    Superlux HD668B SE classA amp

    Thanks for looking for that fet in sot-223 ;0 let me know if you think I can get away paralleling maybe three of the smaller ones?
    permalink
    Posted 28th May 2017 at 11:23 AM by kasey197 kasey197 is offline
  14. Old Comment

    3 way speaker

    How do I know if my 20000uf 100v capacitor ir ok.
    I put a tester on it it was reading 1.64
    permalink
    Posted 27th May 2017 at 11:16 PM by clemmtec clemmtec is offline
  15. Old Comment
    abraxalito's Avatar

    Superlux HD668B SE classA amp

    I used a charge pump converter on a classAB headphone buffer (elsewhere on my blog) to get 7.2V - that one had much lower swing due to the use of cascoded output devices.
    I searched for some higher dissipation FETs which might be suitable for your application, drew a blank as none in SOT-223 came close for gm to this one I'm using. I figure to go up in output power its probably possible to parallel these babies. Since the threshold voltages are bound to vary rather a lot, paralleling them by AC coupling is probably going to work best. If I had some headphones that needed more grunt I'd even build a prototype, but right now its only my 600ohm DT880s that can't be driven by this. Those Beyers need much higher swings.
    permalink
    Posted 27th May 2017 at 03:36 AM by abraxalito abraxalito is online now
  16. Old Comment

    Superlux HD668B SE classA amp

    That's understood and agreed on the device dissipation. Looking at the chart you posted 1amp at 3vds I thought this was a little bigger :00

    I would approach this design only very slightly differently - with higher rail voltages from a dc-dc convertor after the 18650.
    You know your stuff though so good luck with this I'll be watching with interest
    permalink
    Posted 27th May 2017 at 02:56 AM by kasey197 kasey197 is offline
  17. Old Comment
    abraxalito's Avatar

    Superlux HD668B SE classA amp

    A watt or two is going to be too much for this little chappie, he's only a slightly under-sized SOT-23. Sounds like you'd need SOT-223 as a minimum.
    I haven't done any measurements yet but what I've done in sim is subtract output from input and looked at the residual. I'm in general not confident that sims return accurate enough THD and anyway consider that number mostly irrelevant to gauging audio quality. Its a good check if there are any design mistakes though.

    Low Vds is inevitable with such low voltage rails as from a single cell.
    permalink
    Posted 27th May 2017 at 01:26 AM by abraxalito abraxalito is online now
  18. Old Comment

    Superlux HD668B SE classA amp

    Around a watt or two is about what I need ...

    sperately don't you think running yours at such low Vds is inviting distortion ? Or does it measure ok ?
    permalink
    Posted 26th May 2017 at 02:51 PM by kasey197 kasey197 is offline
  19. Old Comment

    MJL21194 Transisors: Real, Second Source and Fake

    I inherited an amp with a bunch of MJ21194/94 that previous owner used in his unsuccessful attempts to fix the amp.

    The reason of the failure is on the pic below.

    Left - fakes.
    Righ - original.

    Click the image to open in full size.
    permalink
    Posted 26th May 2017 at 10:39 AM by eponomarenko eponomarenko is offline
    Updated 26th May 2017 at 10:48 AM by eponomarenko
  20. Old Comment
    abraxalito's Avatar

    Superlux HD668B SE classA amp

    I'd want to be sure that the part I'm using is suitable for your application. What power dissipation do you need?
    permalink
    Posted 26th May 2017 at 12:43 AM by abraxalito abraxalito is online now
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