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Old 12th April 2008, 08:31 AM   #41
djk is offline djk
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Let's see, 200W/2R is 14.14A peak collector current. The DC current gain is rated at 15, AC gain will be lower.

How much current is the driver transistor rated at? The gain?
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Old 12th April 2008, 11:06 PM   #42
sandyK is offline sandyK  Australia
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Default Digi 125

Secondly nobody has suggested any changes or modifications or additions

If Q1 and Q2 are matched for both HFE and VBE, temporarily fit a 10K trimpot in series with the collector of Q2. Adjust so that the difference between the collector voltages of Q1 and Q2 is as close as possible to 0mV. Then replace the trimpot with 1 or 2 MF resistors (series or parallel connected) , chosen to be close to the trimpot resistance used to obtain balance.
Yes, I built several of these many years ago from PCBs purchased from Graham Dicker, and they were just mediocre in their original form.

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Old 18th April 2008, 04:26 PM   #43
gain is offline gain  United States
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I was thinking about building this (Digi-125) for my first amplifier project, as it has the advantage of being very simple and (hopefully) easy to build, while still producing what many have claimed as decent sound. I plan to fab the PC board myself, so as to beef up the inadaquate solder traces as noted by G.Kleinshmidt. I was considering the following mods, some taken from previous posters on this thread, and wanted to get you all's oppinion on them.

- 2SC5200/2SA1943 for output devices.

- BC556/BC546 for the predrivers. BF459 for the VA transistor.

- 2N5401 for the input stage differential transistors.

- Replace the two 1N914's in the collector of the VA stage with an amplified diode, for more accurate control of the bias. Also mount the amplified diode transistor to the heatsink for better thermal compensation.

- Lower the value of the pole cap in the VA stage to 68pF (possibly replace with a trimmer cap for experamentation)

- Add an output inductor and a zobel network to improve stability.

- Add capacitive coupling at the input.

- Add decoupling caps and/or a low pass filter on both rails to improve power supply rejection.

- Metal film 1% resistors throughout.

Once again i am sort of a noob at this so please be gentle. Thank you to all who have contributed to this tread ... it has been a very intresting read.
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Old 18th April 2008, 06:26 PM   #44
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Sry but I'd rather suggest some more sophisticated design, especially if it's your first project.
Digi-125 likes oscillating (one reason for that is the OP/driver stage has gain), especially with the fast 2SA/2SC OP devices you mentioned.

Try Rod Elliott's P3A or something similar. Choose a straightforward, stable, and TESTED design. Digi-125 with the modifications you mentioned isn't tested yet. You would be the first prototyper! As first project it's a bad idea.
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Old 18th April 2008, 08:50 PM   #45
sandyK is offline sandyK  Australia
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Default DIGI-125 Kit Amplifier Module

Gain

Sorry, but I agree with Andy. I think you will run into trouble.
If you really want to try it, don't deviate much from the original design and components.
Also,what we thought sounded decent back then, was very different to what many of would regard as decent these days.

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Old 18th April 2008, 09:31 PM   #46
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hi gain,

If you look at posts #25 and #26 you will see I did similar mods to what you are suggesting. I found the project very satisfying, starting with the basic circuit (with 2SC5200/2SA1943) and changing or adding extra features as suggested by members on the forum.

I found that increasing the bias and adding a bootstrap to the VAS made the most difference, the other changes only made minor improvements.

I changed the VAS trnasistor from a TO92 because it got hot.

So, if you are more interested in learning about amp design and building then do what you suggested, if you want a guaranteed result look for other options.

Soundwise, the final result was surprisingly good, but not in the same league as SandyK amps.

WARNING: I only ever tested with 25 volt rails.

regards
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Old 18th April 2008, 09:39 PM   #47
gain is offline gain  United States
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thank you Andy and SnadyK for your input. yes this is most likely taking on too much for a first project. you guys are absolutely right i should not be trying to prototype something new before i at least get a few successful amplifier builds under my belt ... thanks for setting me straight on that. i suppose i was drawn to this circuit because it was the first amp schematic that i could look at and understand the purpose of every single component (im a CS major and not EE but i will be taking some EE courses next year so hopefully will learn much more).

thanks Andy for the suggestion to try the PA3
http://sound.westhost.com/project3a.htm

i will look more into that amp over the weekend or if i do decide to go with the digi125 i will implement the original circuit with no mods.

You mention the digi125 had a tendency to oscillate ... was there ever a solution developed for this? would reducing the gain in the OPS or mabye increasing negative FB help?
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Old 18th April 2008, 09:41 PM   #48
sandyK is offline sandyK  Australia
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Default Digi-125

So, if you are more interested in learning about amp design and building then do what you suggested, if you want a guaranteed result look for other options.

Good advice Greg !
But try it in the original form first, so you can gauge the extent of the improvements. Also it is best to do one change at a time, then listen. Some changes don't always give the expected results, and this way you will know which change to reverse.

SandyK
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Old 18th April 2008, 09:54 PM   #49
gain is offline gain  United States
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hey Greg,

yeah your work on modifying this was one of my primary inspirations for wanting to try the digi125. my goal is to learn, but if i can happen to create a decent sound system in the process then all the better!

i will most likely build the circuit with the original design, get that working (i hope), then try swapping the output devices for the 2SC5200/2SA1943 ... both you and Tomcat said that made a big improvement.

at any rate i will get the original circuit working, then do one mod and get that working, then do another single mod and get that working ... etc.

regards
-John
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Old 18th April 2008, 09:57 PM   #50
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Quote:
Originally posted by gain

- 2SC5200/2SA1943 for output devices.

- BC556/BC546 for the predrivers. BF459 for the VA transistor.

- 2N5401 for the input stage differential transistors.

- Replace the two 1N914's in the collector of the VA stage with an amplified diode, for more accurate control of the bias. Also mount the amplified diode transistor to the heatsink for better thermal compensation.

- Lower the value of the pole cap in the VA stage to 68pF (possibly replace with a trimmer cap for experamentation)

- Add an output inductor and a zobel network to improve stability.

- Add capacitive coupling at the input.

- Add decoupling caps and/or a low pass filter on both rails to improve power supply rejection.

- Metal film 1% resistors throughout.
The output transistor choice is good.

I don't know how he got away with those little BC transistors for drivers, you could try ZTX653/753 as beefier but still small package. You could also try ZTX653 as the VAS transistor.

Keep the BC transistors for the LTP as they have better noise performance and gain than 2N5401.

Using an amplified diode is a good idea.

Using a trimmer capacitor for stability is probably a bad idea, I dislike trimmers be them capacitors or resistors, unless absolutely necessary as they can go noisy.

By the time you add the other modifications you have changed the amp quite a lot and you might as well throw the Digi-125 name to the distant memory.

The output stage does not have voltage gain, only current gain. The oscillation problems may have been due to lack of Zobel and poor PCB design.

Feel free to add your mods, but as has been pointed out by experienced tweakers already, make sure you do things incrementally so you can observe the differences.
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