DSE A2760 Amplifier Modification
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The DSE amplifier (cat. no. A2760) is actually a rebranded Koda KD-261. Despite first looks it is actually a rather good quality amplifier for the price, incorporating such things as a toroidal transformer, 40,000uF total capacitance for power supply filtering, genuine Toshiba output transistors etc etc. However, there are things that can be improved, that's what you are about to learn.
First of all, the usual disclaimer applies. CONTINUE AT YOUR OWN RISK!! Any damages to your own person, equipment etc etc. is your own responsibility. If you are unsure of anything, ask someone who knows a bit about electronics and/or ask in this thread.
Oh, and always unplug the amplifier BEFORE you work on it....
The first mod that should be undertaken is removal of the preamp stage and conversion to a power amp. The stock preamp is the main let down of this amp. It uses the NEC NE5534 Op-Amp, one not well renowned for audiophile status. Swapping of preamps has been tried, with little success. It seems the preamp circuitry has been optimised for the NE5534 and anything else just gives lacklustre results. By completing this mod, you will lose all tone, balance and associated controls, however, volume control will remain, if you wish to incorporate it. This shouldn't pose too much of a problem for most users, as pretty much all current media players offer advanced eq controls that more than make up for the loss on the amp. Those who use a CD player etc. can be happy knowing they are getting a more pure, undistorted sound...
Mod 01: Conversion to poweramp.
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 heat sinks in the centre.
Remove the pre-amp, along with all the pesky sound-degrading tone controls, and the four shielded leads that connect the pre-amp to the amp? (the grey, black and brown ones in my pic).
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.
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).
Basically all that needs to be done is to hook up L-IN and L-OUT together and R-IN and R-OUT together as per the photo. Just use the cables that were used to connect the amp stage to the preamp. Easy.
For those that want volume control, there are a few more steps...
Desolder the 100K pot used for the volume control, a desoldering tool helps make short work of this. Take four 25K 1% metal film resistors (I used .25W ones from Dick Smith) and make the circuit shown below. If 25K resistors cannot be found, the closest equivalent will do, as long as they are all the same value. This is known as a variable shunt series attenuator, and basically lowers the input signal as you turn the volume knob down.
Take two of the 25K resistors and twist one side of their legs together to make a V shape. Do the same with the other two 25K resistors also 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.
Take your new attenuator and screw it back into the volume hole on the front plate. Only tighten it loosely so it can be positioned easily and make sure the wires are to the left, as in the picture 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 stretch, just try not to put any force on the leads or resistors. When you're done it should look like this:
Stick the lid on and test!
Alternate Shunt Mod.
This version will allow for a better quality pot to be used and better tracking of the volume control. It is based on the 'better volume control' at the ESP website. http://sound.westhost.com/project01.htm
A suitable linear 2-gang pot can be purchased from http://www.jaycar.com.au Part no. RP-8712.
I have yet to test this shunt with the DSE amp. Feel free to add to this section if you do so. :)
Mod 02: Removal of DC blocking capacitors
WARNING. Only complete this mod if you are SURE your source unit/s do not produce a DC offset. Failure to do so could result in the death of your amp!
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.
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. The input impedance of this amp is 47kOhm. Using the formula:
C = 1/(2*pi*f*R)
The input cut-off frequency is around 7Hz, so it's not a significant mod in regards to bass extension. what it does achieve however is the removal of an electrolytic from the signal path, which is a good thing. They tend to introduce distortion when used.
Mod 03: Direct replacement of capacitors.
- Replaced all electrolytic caps 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 C205 and C233 with polypropylene of same value from speakerbits. (tight fit!)
The only capacitors I didn't swap were those in the SOAR circuitry (centre of the PCB) as these have no bearing on sound reproduction at all. Size comparison between polypropylene and electrolytic capacitors
Mod 04: Adjustment of Vbias.
This modification will allow the adjustment of Vbias, which will help eliminate crossover distortion as one output transistor turns off and the other turns on. This mod will increase the heat output of the amplifier as the transistors will be turned on for a longer period of time. The aim is to adjust the voltage across the 2 emitter resistors from 47mV to 55mV.
There are two methods to this modification. One is a direct resistor replacement, recommended for most modders. The other is a trim pot mod, allowing full adjustment but also allowing the deep frying of the output transistors....
Vbias is temperature dependent so it must be adjusted when the amp is at normal operating temperature. A good starting point is to replace resistors R209 and R257 with resistors with a value of 160 ohms.
1K trim pots can be installed to allow precise control of the voltage, measuring across resistors R208, R209 and R256, R257.
-pic to go here-
Mod 05: Adjustment of Input Gain.
The gain of this amplifier is determined by Resistors R230, R231 and R253, R281. Gain can be calculated with the following formula:
Gain = 1 + R231/R230
Gain = 1 + 47k/1k
Gain = 48
By replacing the resistors, we can change the gain to suit our needs.
Gain = 1 + R231/R230
Gain = 1 + 47k/1k8
Gain = 27
Once again, trim pots can be installed to allow for variable gain control if desired.
To be continued....