Recently I purchased a Dick Smith 2 channel integrated amplifier via the StereoNET classifieds. For those outside Australia, Dick Smith is an electronics retailer. I bought this amp for use in my next speaker project - active speakers. It's a discontinued amp, but they still go on ebay from time to time for around $80.
How good can a cheap amp be?
General consensus seems to be that it's good for the price. It's hard when listening to such an amp to not have a tarnished impression simply by a little expectation bias. In my first listening session with it, I found the sound wasn't appealing. I believe this is due to removing EQ settings I normally use, resulting in sound that isn't quite right. Since then I've had a chance to spend some time with it and I haven't found anything to complain about. It seems to be quite transparent and has potential to work in a high end system. Of course, that comes with a word of warning - nothing suits every system.
In a blind shootout, this amp came second among a handful of more expensive amps.
What makes this amp worth using?
This amp is ideally suited to a simple light duty mod project. Apparently the preamp is the weakest link. A few minutes under the hood and you can bypass the preamp and have a dedicated power amp. All you have to do is switch the jumpers internally. They are labelled on the PCB.
The idea is to modify the amp for improved sonic performance and give it a facelift.
- new binding posts
- new RCA inputs - input selection removed
- re-painted enclosure
- minimalist face plate - just one knob
- remove preamp
- a minor tweak to lower crossover distortion
- reducing input gain
1. Power amp conversion
This is easy to do by simply re-routing the jumpers - no soldering necessary. I went to the next step and hard wired the connections with light duty hookup wire in twisted pairs.
Preamp board removed - it gets in the way and I may find another use for it. DC input to the preamp disconnected. This is also simple as the jumpers can be removed, again without any soldering.
2. Binding posts upgrade
Existing damaged binding posts were replaced with new binding posts.
The damaged posts are shown above. I had to add a sheet of black acrylic to mount the new binding posts.
3. RCA inputs
I didn't like the look of the cheap RCA inputs and there is no need for multiple inputs. The existing rotary switch can create a nasty sound when switching, but I would never use it for a power amp. So this was a good chance to dress it up with some better quality RCA gold plated inputs. I had to attack the chasis to allow the new inputs to be spaced differently, then I placed some acrylic sheet over the previous inputs. I was lucky to have some on hand, and after cutting with a hacksaw, I filed then sanded it smooth, followed by cut and polish to get a nice finish.
4. Volume pot
A volume control isn't necessary in a power amp, but I prefer to have a passive attenuator for this application. As a power amp driving a tweeter directly in an active setup, it's useful to be able to turn down the input. This helps with determining if there is a problem with turn on/off thumps without full volume. It also enables level matching.
Inputs now are fed directly to the pot, then outputs from the pot feed the power amp. Elliot Sound Products have some good information on pots. If you are trying to get your head around how to wire it up, look no further. You'll also find information on how to improve a standard pot. If you find a better diy electronics resource online, please tell me about it. I haven't found anything that comes close.
5. Reducing gain
The gain of this amp is insane. I've never experienced an amp that tries to murder your speakers in the 9 o'clock position.
This is easily fixed by replacing R230 & R253 - the 1k is replaced with 1k8. Yes, that means 1.8k ohms.
The gain of this amplifier is determined by resistors:
Gain = 1 + R231/R230
Gain = 1 + 47k/1k
Gain = 48
Gain = 1 + R231/R230
Gain = 1 + 47k/1k8
Gain = 276. Reducing crossover distortion
Crossover distortion is one of the amplifier specs that actually matter. It is a form of distortion that occurs in conventional solid state class AB amplifiers when switching between class A and B operation. The amp starts in class A at low power then switches to class B as the power increases. We can reduce crossover distortion at the cost of running the amp hotter. For my application the extra heat is a non issue - an active speaker driving a compression driver with 108 db sensitivity doesn't need much power. The full power output of this amp here is enough for a commercial cinema!
There are two options for reducing crossover distortion. One involves replacing R209 and R257 with 160 ohms. This is the simple solution. I chose the better and slightly more difficult option - using 1k trim pots. The value is set by adjusting the trim pot until the voltage is 55mV while the amp is on and at normal operating temperature.
Please note: working on an amp while it is powered up is dangerous. There is a risk of electric shock. If you choose to do it, then it is at your own risk. Please read the disclaimer at the bottom of this post.
7. Modified face plate
I wanted a more high end feel for the amp as well as a customised version. I decided to strip the face plate with the belt sander, then with some sanding by hand. It had some damage, so I filed and sanded the edges to smooth this out. All text was stripped back and give the amp a more minimalist look. I placed black acrylic behind the holes that are no longer used, attached with contact adhesive.
Above: Face plate stripped and holes filled in with acrylic sheet
8. Modified case
Due to scratches, I decided to sand the case back and repaint, as seen in the first photo. I prefer matt black to the stock silver case.
Do you have one of these amps?
I'd like to hear from anyone who has a DSE A2760 - feel free to leave comments below on your experiences with this amp. You might also like to submit a link to your modified version so that this can be a good resource for modders.
There are all kinds of dangers associated with taking on a project like this. If you aren't sure of your capabilities, then don't try. Even when disconnected from the mains, power supply caps can retain enough voltage to give you a shock if you touch certain parts inside. Nothing written in this post should be considered as advice that anyone should undertake a project like this. It is at your own risk. There may be other risks but don't ask me, I'm no expert!
The details of this mod are based on instructions here:
Overclockers Amp mod wiki
Greg Erskine >
Before asking techy questions, keep in mind I'm not an electronics geek. If you want to discuss the circuit design of this amp then I'm happy for you! You should make new friends at diyaudio.com >