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DSE amp to power amp mod.

Discussion in 'Electronics & Electrics' started by TERRA Operative, Aug 25, 2006.

  1. TERRA Operative

    TERRA Operative Member

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    Well I decided to go get one of those Dick Smith amplifiers (A2760) and mod it into a power amp to go with my modified Chaintech AV-710 soundcard.
    Shouts go to the various members at the diyAudio Forums , for the basis of this mod.

    Oh, and remember, this WILL void any form of warranty you have for this product.


    Step 1:

    Open the amp (unplugged of course).
    You will see the main sections of the amplifier. To the right is the toroidal transformer that supplies the amp and pre-amp with power. To the bottom left is the actual amplifier circuitry itself, with the input/switching section at the bottom and the heatsinks in the centre.
    And at the top left is the pre-amp. This we will remove, along with all the pesky sound-degrading tone controls.

    [​IMG]
    Click to view full-sized image!

    See those four shielded leads that connect the pre-amp to the amp? (the grey, black and brown ones in my pic.) Pull them all out.


    Step 2:


    Now, remove the fascia and unscrew all the nuts holding the pots on. Also, you will see a C-clip on the rod that extends to the rear of the amp, remove this too. This allows you to pull the rod out of the way of the preamp circuit board when you take it out.
    Unplug the green and brown power wires from the fuse board, and unscrew the ground wire (I put the screw back in the hole so I could use it for a possible future pre-amp mod). The pre-amp should just be able to be gently lifted out.

    Step 3:

    Now it's time to decide if you want volume control on the amp or not. I did, but I'll show you both ways. No volume control is the easiest. (you can control the volume on the computer or external preamp or whatever).

    [​IMG]
    Click to view full-sized image!

    Take two of those shielded wires you pulled out earlier, the shortest one and another. Now, look at where you unplugged them from, they will be labelled with an L-out for left out, R-in for right in etc. What you have to do is connect L-out with L-in and R-out with R-in. Simple.
    Just re-align the selector knob and re-attach the fascia. You will have some empty holes, which you can use you're imagination to fill (frosted perspex, new CNC drilled faceplate, wood panelling etc.)
    Put the lid on and you're done!

    Step 4:

    For those that want volume control, there are a few more steps...

    [​IMG]
    Click to view full-sized image!
    [​IMG]
    Click to view full-sized image!

    Desolder the 100K pot used for the volume control. Take four 25K 1% metal film resistors (I used .25W ones from DS) and make the circuit shown below.
    This is known as a variable shunt series attenuator, and basically lowers the signal as you turn the volume down.

    [​IMG]
    Click to view full-sized image!

    Take two 25K resistors and twist one side of their legs together to make a V shape. Do this with the other two 25K resistors, and solder them to the left most legs of the pot, as looking at the shaft.
    Take the shortest and longest shielded wire and cut the plug off one end of both. Make sure to cut right near the plug, as you will need all the length you can get.
    Strip the wires a little (about 5mm max) and twist the shields together. Solder this to the middle leg on the pot closest to the shaft. Now solder the centre wires to the ends of the resistors on the corresponding 'level' of the pot.
    Repeat for the other wires.

    You should have something that looks like this:

    [​IMG]
    Click to view full-sized image!

    Step 5:

    Take your new attenuator and screw it back into the volume hole on the front plate. Do it loosely so you can position it easily, make sure the wires are to the left, as in the pic below. Now put the front fascia and volume knob back on and align the knob in the hole. Remove and tighten the nut when the pot is central. Turn the pot down and align the dot on the knob with the mark on the fascia.
    Plug the shorter lead into the top socket on the amp board, plug the other wire into the corresponding socket (R-in to R-out etc). Do the same with the other leads. It may be a strech, just try not to put any force on the leads or resistors.
    When you're done it should look like this:

    [​IMG]
    Click to view full-sized image!

    Stick the lid on and test!
    You will have holes in the fascia, which will need to be covered (unless you just don't care, but you can't leave it half finished! :p )

    [​IMG]
    Click to view full-sized image!

    I'm planning on a CNC drilled faceplate, once I get around to designing it. I'll update this post once I'm done.



    I hope this is useful to those who also have one of these amps. I unfortunately don't have any before-and-after comparison, as I leapt into this as soon as I got home. :D If anyone completes this mod and does a comparison, I'll be more than happy to post it here.
     
    Last edited: Sep 2, 2006
  2. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    NEW MOD!!


    I noticed some caps on the input stage of the amp, and decided to try bypassing them.
    They are C212 and C240, and are both 4.7uF 50V non-polarized electrolytics. They are located directly alongside the L-IN and R-IN sockets.

    [​IMG]
    Click to view full-sized image!

    I just soldered a bridge across their legs on the track side of the board to effectively remove them from the circuit.

    Results? Bass seems more 'there' and extends nice and low.
    I dunno what the cutoff frequency was for these caps, but I read in the above linked diyForums thread that the input impedance is 47kOhm. Anyone want to work it out?

    Using the formula:

    C = 1/(2*pi*f*R)

    I get around 7Hz.


    Now, back to looking for more things to mod on this amp! :D
     
  3. Dr feelgood

    Dr feelgood Member

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    aren't those input caps there to block dc from your source, Without them present it may push your outputs bias high. Of course this depends on the rest of the setup and amps topology.

    good work so far. :thumbup:
     
  4. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Yea, I'm running the amp from the Chaintech card modified in the other post (link at top). So I'm not too worried about DC offset at the moment.

    I'll be sticking in some polypropylene caps before I connect anything else. (Just that I couldn't get them today... :p )
     
  5. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    WOOT! or whatever.
    I found a schematic!! eeeeeeeeeeee! :D

    And some other info. It's actually a rebranded KODA Electronics KD-260.

    As the diagram is actually for the KD-261 (the DSE amp is a KD-260) It's got a few small differences (things like component values etc.) but it's a workable diagram.
    I'll probably make corrections as I find them, so I'll update once it's done if people are interested.

    [​IMG]
    Click to view full-sized image!

    This *should* print out at A4 landscape at 150dpi. It worked for me at least...
     
  6. mAJORD

    mAJORD Member

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    great guide Terra :p

    Ditto the comment by DR on input caps, only for the benefit of others who may try it - only recomended if you know what you're source is.

    nice find with the schematic. Soft start/prot circ down the bottom?
     
    Last edited: Aug 26, 2006
  7. Goth

    Goth Grumpy Member

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    It's basically an anti-thump circuit, the relays patch in the speakers after the amp has settled down into a steady state.
     
  8. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    That's right, the front panel LED even flashes to let you know it's 'timing out'. Once it's all ready, the LED stays on.

    Got any idea what R238 and C214, and R286 and C239 on the input stages on the amp section do? It looks like some sort of bypass RC network.
    The caps are ceramic (along with caps C213 and C238), a type of cap that is not too well renound for its accoustic properties. I was toying with the idea of trying a polystyrene or polypropylene cap there. They don't seem to be in the direct signal path, so I'm not sure how much change there will be.
     
  9. Dr feelgood

    Dr feelgood Member

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    I believe r238 and c214 are there to filter out frequencies above 20Khz, that sort if thing is mainly to block interferance/oscilations from surrounding gear. A similar filter for below 20hz is not needed due to the 4.7uF dc block cap.
     
  10. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Ah yes, I see. I guess then I'll leave those caps alone as they don't directly effect the signal entering the amp.

    [EDIT]

    I have found another NP electro. There is one per channel, on the schematics they are almost smack in the middle of the amp section. Labelled as C205 and C233, they are marked on the schematic as polarised (incorrect).
    They seem to be bypass caps of some sort, maybe to prevent DC from getting to the next stage of the circuit? Would it be worth putting poly caps here?
     
    Last edited: Aug 27, 2006
  11. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    MORE MODS!!!!! eeeeeeeeeeeee!

    Ok, here is a list of further mods I have done...

    - Replaced all electrolytics in the amp stages with 100V Nichicon FG series (Fine Gold) caps.
    - Replaced C213 and C238 with silver mica caps of same value.
    - Replaced C214 and C239 with polystyrene caps of same value.
    - Replaced all other ceramic caps to class1 NP0 (or C0G) rated ceramic capacitors.
    - Replaced all polyester capacitors to same of better quality.
    - Replaced R209 and R257 with 1K trimpots and adjusted to read 55mV across R213 and R210, and R260 and R258. This is to set Vbias, and help eliminate crossover distortion. (The point at which the transistors turn on and off as the waveform crosses 0V, I think...)
    - Replaced R230 and R253 with 1.8K resistors. This lowers the gain of the amp from 48 to 27.1, enabling more 'turn' on the volume control.


    Mods still to do:

    -Build a stepped attenuator, probably a 23 step ladder type.
    -Build a new faceplate (CNC/laser milled and etched)
    -Possible modification to the SOAR circuitry.
    -Replace non-polarised electrolytics (C205, C212, C233, C240) with polypropylene caps of same value.


    Damn, It's gonna be a totally new amp once I'm finished. :)
    I'll have to get another so I can do a side by side comparison (my brother wants one for his computer). I'm not much of a critic, but it is sounding better, a cleaner sound with more prominent bass and smooth highs.
    I'll have to work out a decent way of measuring the performance of this amp. I might just have to invest in a CRO. :leet:

    Oh, and thanks to Greg Erskine, for his help on a number of these mods (resistor mods mainly). :thumbup:
     
  12. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Well, I went to Dick Smith's today and purchased another amp (damn they're getting hard to find!) as my brother is looking at modding it for his system, and I took the chance to do a side by side comparison.

    The setup was run to my computer with one amp on the left channel and the other on the right. This allowed me to pan left and right to compare the two amps.

    All I can say is wow. I didn't think that the modifications would make such a difference. The bass is more powerful and defined, the highs are clear but not overpowering and the mids are smooth but not muddy. The original amp just sounds cheap by comparison. The bass is weak, highs indistinct and overall the music sounds 'compressed', like a badly recorded mp3.

    All I can say is, if you have the technical competence, do these mods! It makes a good cheap amp sound like something multiple times it's price.
     
  13. zeeps

    zeeps Member

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    Nice work :thumbup:

    How much did you spend on parts and time? And what speakers are you using with the amps?

    I'll have to try these mods out soon.
     
  14. mAJORD

    mAJORD Member

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    wow, you really are going all out. great stuff.

    unfortunatly a cro won't help you for anything but power measurments.

    a distortion anaylizer is what you need, but thankfully, if you have a soundcard with decent in/outs you can use a simple voltage divider to bring an amp's output back down to life and use your soundcard + software of your choice instead. (RMMA is a good basic choice for comparisons)

    I think I briefily explained it in another thread, but it has some risk involved, and unless your soundcard has differential inputs or outputs (at least one), then it can give poor readings if you create a ground loop, or if your amp is not single ended in/out. (meaning both input and output are referenced to ground) .

    If you're keen to try it I can help you there. I've read many other's have used buffers to prevent any risk to there soundcard, but I prefer to be extra carefull instead.. saves buffers adding extra distortion+N into the readings
     
  15. PooJou

    PooJou Member

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    You going to do the dual lineout mod as well? That is, I have the i think its record-out or something running as a 2nd line out to my subwoofer.

    Great going so far! If only I had the same soldering skills lol...
     
  16. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    I'm not sure how much I spent, Some of the stuff I had to buy in multiples, even though I only needed 2 etc. I'll have to work it out.

    The speakers are just some Panasonic (Technics drivers) 3-ways from my brothers stereo. Nothing spectacular, but I've modded them too. :)

    I think that the inputs and outputs are referenced to ground. If you could supply some more info, that would be great. I currently have an unmodified amp here, so I can run comparison tests.

    PooJou, can you post details? I am interested in the line out mod. Is it simply a pass through connection? How did you combine the left and right channels but maintain channel seperation? (If you did).
    The soldering is pretty basic, the layout of the PCB is wide open, so there's no fiddly stuff to do.
    You may have noticed, but there is a removeable panel allowing acces to the underside of the main PCB. That makes it heaps easier to work on. :)
     
  17. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Just a bit of an update.

    I purchased a heap of 4.7uF polypropylene capacitors from Speakerbits.com.au as they were on sale and I needed some for a crossover etc.

    So I decided to stick a few in the amp. :D

    [​IMG]
    Click to view full-sized image!
    Here is a size comparison. Original electro on the bottom, new poly in the centre. The AA battery at the top gives you an idea of just how big these things are...

    [​IMG]
    Click to view full-sized image!
    Here they are in circuit. Damn they're big! They dwarf everything around them! I put them in place of the 2 NP electros C205 and C233. I decided not to put them in place of the DC blocking caps C212 and C240 as my soundcard already has a set of Nichicon Fine Gold series caps which I installed for DC blocking, and I reckon no cap is better for signal quality preservation than even the best cap avaliable.
    I'll be squirting a bit of hotmelt glue around to support these things too.
     
  18. alfalfa

    alfalfa Member

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    Hi Terra. Great thread. Have you been enjoying the sound of your modded amp?

    I just bought a dse amp and would like to mod the volume control to decrease the gain like you have done.

    - Replaced R230 and R253 with 1.8K resistors. This lowers the gain of the amp from 48 to 27.1, enabling more 'turn' on the volume control.

    How do you find the volume knob now? Is 1.8k the best size resistor to use?

    I've never done much soldering before. Is this a pretty simple mod?

    Cheers
    Alf
     
  19. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Wow. Blast from the past!

    Yea. the amp sounds awesome. The gain mod has been changed, I found the values to be too low, so I have soldered in some trimpots instead. That way I can adjust the gain as I want. I wouldn't go too much lower than what is already in there, otherwise you will loose too much 'turn' on the dial.

    It's easy to work on though. Just take the cover off, and the bottom access panel too, so you can access the underside of the PCB. There's no need to remove the PCB from the chassis.

    The volume control as I have modded it is starting to get a little dirty. There is a bit of crackle when the knob is turned, but a bit of back and forth fixes it. I still have to get that stepped attenuator....

    I also have plans to get a new custom front face laser cut with provision for VU meters, just for a bit of bling. :leet:
     
    Last edited: Nov 21, 2006
  20. OP
    OP
    TERRA Operative

    TERRA Operative Member

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    Started a WIKI entry here
    Feel free to edit or add info as you see fit. :)
     
    Last edited: Nov 24, 2006

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