audiosource amp100 (tda7294) mods

I just cracked open an audiosouce amp100. Some interesting things that would be easy to improve include:

1. power supply caps - existing caps are only 6800uF per rail. With both channels driven, I measured 52 watts into 8 ohm at 1 kHz, but only 42 watts at 20 Hz.

2. op amps - I found 5 - 3 "5532" and 2 "4580". All but one (a 5532) appear to be easily removed from the signal path by bypassing the circuit board that does the input selection and auto-on functionality. The audio inputs an be wired directly to a main board connector that would otherwise be connected to the board being bypassed. I'm not sure, but the remaining 5532 might be used to invert the input audio signal when the amp is configured in bridged mode (I haven't yet traced the circuit).

The transformer is a 53.6V CT 3.2 amp toroid.

Has anyone else modified and audiosource amp100? What changes would you suggest? Are the later amps chips like TDA7293 and TDA7294S sonically superior? Maybe modifying the circuit (hopefully just tieing off some pins) to use the newer parts would make sense.

I've seen factory refurbished units selling for$75-$85

Based on my twiddling with my 7294 amp, there are options that can bring it to sing quite sweetly:

- usual power supply optmisation, including soft recovery or schottky rectifiers, bigger / better caps, snubbers
- generous bypass of power supply pins; RC snubbers on power pins
- use of quality (film or BG non-polar) cap for dc block
- lower rail voltage -- brought mine down from 40V to +/- 29V, and it seems happier driving 4R speakers

One of the major upgrades was to *add* a buffer after the volume pot. With just a volume pot w/o buffer, bass and midbass was wooly. With a good buffer, the sound improved a lot. You might think of optimising rather than removing the linestage.
I'm interested in grabbing one of these amps as a backup for any downtime when I've got my tube monoblocks on the bench. Does anyone happen to have a schematic of this thing or shots of the inside? Maybe I could squeeze in a tube front-end using a low-profile toroid... :scratch2:
Has there been any updates on modding this amp?

I have one stashed away in my closet that I'm going to pull out tomorrow. I'll open it up and take some detailed pics of the innards and post them here.

I'm thinking of "hot-rodding" it and using it as my main amp in my system. Since I already have a sweet Accuphase C-200 preamp, I can bypass not only the volume control, but also the balance and auto input selector features as well as the auto-on feature. Of course, I'll be looking to upgrade any caps in the unit that I can, both in the PS section and in the signal path. After I do all of that, I'll swap it into a new chassis with more room to breath. Besides, I'll need the room for all those new larger caps! :D

If there's any good info on modding this amp, please let me know!
post #2 sounds good. Reducing the supply voltage might only insure that the chip doesn't go into protection mode with 4 ohms which possibly sounds raspy(?). However it will of course still clip if over driven.

Might be better to stay with the higher voltage and not turn up the volume till clipping or get into over current mode.
Wonder if the protection circuitry will let 'some' clipped signals through for a few miliseconds before it kicks in. The dc blocking cap at the input can be quite critical !

About supply caps. For any significant change maybe you will need to double it's value but that also depends on the capability of the supply transformer. With small transformers increasing the supply caps might not help at all. There is always an optimum value that you might need to test and find out for your particular amp.
I brought an AudioSource Amp100 for use with a pair of ROX Studio 9 monitor speakers and a Flat Panel TV. Out of the box I was not real impressed. I decided to modify the unit to my liking. The mod was fairly easy and rendered worthwhile results. I am going to list the changes that I made to my unit for anyone who wants to paint by the numbers and upgrade their unit. I am assuming that you are experienced with equipment repair, good soldering practices and you have the right set of tools. If you are a novice don’t even try this mod you will end up with an expensive non working amplifier. Remover the Input PC board from the back of the metal case. Remove the Main PC Board from the bottom of the case, pay attention to where the screws and other parts came from. On the Main PC board Change C104 and C207 to 10uf at 16volts non-polar. Remove IC#1 (5532) and install an 8 pin dip socket (don’t be cheep here) Replace IC#1 with a Burr Brown OPA 2134. Replace C101 and C201 with 4.7uf 16 volts non-polar cap.
On the Input board Change C411, C414, C413 and C412 to a 100uf 16volt non-polar cap. Remove IC #4 and #5 (5523) and install an 8 pin dip socket Replace IC#4 and #5 with a Burr Brown OPA 2134. Change R426, R428, R429 and R427 to a 10K 1% resister. I think if you want to further improve the amp after you have made modifacations that are outlined here, A complete new PC board would be nessisary. It may be worth doing because you can't buy a case, power transformer and all of the pysical stuff (conectors and so) on for the 100 or so dollars that the amp 100 cost. Remember, this is what I did to my unit and your outcome may be different so I can not take any responcibilitys for your results.
You can get all these parts at DigiKey or Mouser. I hope this helps.
David Eltzroth
Rox Electronics, Inc.
Repairing blown Amp100

David (or anyone else with this amp), can you tell me what the original values of C401 and C402 (on the amp board) are? I have one of these amps that's had a meltdown and I'm trying to piece it back together.

On a related note, does anyone have a schematic for this amp or know where I can get one? If I can't get the amp working, I'd love to use the input board for another project since it has the built in bridging capability.
Yet another zombie thread, this one was replied to with a 'for sale' advertisement ;) Good luck getting much for a half-functioning cast-off.

For anyone who is interested in buying or modding the Amp100, fully working examples can be had for <$40 secondhand. There is a guy on the Parts Express tech talk who has modified this amp, and come up with a how-to-improve list...
How to turn your AudioSource AMP 100 into a real amp. (Paying it forward))
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