DIGI-125 Kit Amplifier Module

[mrfeedback]

Hello All, Found this whilst out trawling.

Australian, Graham Dicker designed, Aust $39.00, DIGI-125 amp module kit, ( http://www.bettanet.net.au/gtd/kits.html ).
7 transistors, 2 diodes, 8 resistors, 3 capacitors, no adjustments, +\- 38 Vdc supplies, 125w/4ohm, (200w/2ohm modification).
0.06% at rated power, 0.01% 10Wrms, 100Khz power bandwidth, DampingFactor - 90 wrt 8 ohms.
Looks to me like an easy, cheap, quick way to build acceptable surround/triamp setups etc, economically.
I am keen to hear experiences, and sonic critiques, and mods -
27,000 sold can't be all wrong maybe.?

Regards, Eric.

[tvi]

<a href="http://www.alphalink.com.au/~cambie/">www.alphalink.com.au/~cambie/</a> has the full article for the ETI-480 an earlier design by the same designer as the Digi 125.

Regards
James

[djk]

"Digi 125 power amp module small in size easy to build with no alignment Since first publication in 1987 over 27,000 have been built to the authors knowledge. The amplifier features low cost with an output power of 200 watts rms into 2 ohms 125 watts rms into 4 or 70 watts into 8 ohms from a +/- 33 volt power supply. The shortform kit has the PCB. and all components less heatsink and power supply." Let's see 70W X 8R = 560 and the square root is 23.664V RMS times 1.414 = 33.461V peak.From a +/- 33V supply? Tell me another one.Oops, I forgot to allow for the volage drop in the output transistors and the emitter resistors.I'm sure this thing will work and sound OK for what it is.Just don't expect the world from it.With a typical un-regulated supply like you see in commercial audio gear it will deliver about half the power it claims.I'm sure that this is the basic design: http://sound.westhost.com/project03.htm

[mrfeedback]

Thanks tvi and djk, I have alredy been to the sites that you have suggested.
I am more fishing for actual experiences of these amps as opposed to theoretical.
I built a couple of ETI-480 modules 20 years ago, modified, and they sounded pretty good at the time.
This system did a million parties till real late, people dancing on the tables etc, but never any noise complaints from neighbours!
I am interested in the digi-125, partly for simplicity sake, partly for economic considerations.
I am planning to build a set of seriously good active DJ/party hire cabinets, also tri-amp modules for my existing home cabinets.
I envisage idiot proofing in form of limiting etc, but below overload level, I would like big clean nice sound, unlike most DJ systems.
I figure MJL 1302/1381 output pair is probably useful mod ( 2n3055/2955 in audio amp ?), and tweaking of PSU voltages to ensure decent power output.
I consider that amplifier THD performance is not the most serious factor, and IMD and load depedance to be far more damaging to the end system resultant sound.
By virtue of circuit simplicity, maybe these amps are suitable.
Still seeking constructors comments.

Regards, Eric.

[junglejuice]

i am interseted in this amplifier and was wondering if anybody has the schematic for it that they are willing to share with me

many thanx in advance

[palesha]

junglejuice,
Refer to the message from tvi. Go to www.alphalink.com.au/~cambie for more information.

Secondly nobody has suggested any changes or modifications or additions. There are so many experienced people at DIY. We need their help & suggestions to go ahead with the digi - 125.
Mahendra Palesha

[syl]

Here are a few improvements for you to explore.

- local degeneration of the input differential. 220 ohms on each emitter. That pulls down the open loop gain quite a bit but will provide a healthy amount of overload margin for the input stage. If needed the collector resistor can be increased for higher gain in the input stage. But my advice is to stay below a gain of 6 (15 dB) for the input stage. The calibration criteria for the emitter resistors is based on dynamics. As the emitter resistors gets too high the sound will no longer improve and the subjective dynamics will be reduced, so use that as the stop criteria. I think 220 ohms is about right based on own experience.

- local degeneration of the VAS. 47 - 68 ohms on the emitter is a good start. Same thing here, the stage is much linearized.

- insert a low pass RC filter for the supply voltage for the input stage and the VAS, this is VERY audible. Use 12 ohm/1W & 4700 uF on both the positive and the negative supply. This will lower the ripple voltage 30 dB at 100 Hz. This mod is probably the most important one.

- If the amp will drive complex loads DO NOT omitt the output inductor with the resistor in parallel. It is there for stability.

- Place 1N4001 in parallel with each output device to protect them from reverse current spikes from inductive loads.

Happy tweaking.

[paulb]

Are we talking about the ETI-480 here, or the Digi-125? I couldn't find a schematic for the latter.

[palesha]

I am sorry if anybody thinks that i am violating some copyright. It is just to help someone on the list. But i think i am not doing illegal or harm to anyone. Hope everyone understands,
MP

[mrfeedback]

Thankyou Palesha for posting the schematic, and thankyou SYL for your modification comments.
Syl, I agree that your mods ought to make a wortwhile improvement.
Question is - Has anybody amongst us built one, and what sort of sonic results did you get ?

Regards, Eric.