How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

[tyger23]

All,

A few weeks back, at the Lone Star Audio Fest, a forum member (TC) gave me his broken down and shorted AudioSource AMP-100. His only request for me was to fix it and "pay it forward". Well, fixing it was easy, but I wasn't satisfied with the performance.

There was a significant volume difference between the left and right channels and the audio sounded a bit muddied and vague. So, to pay it forward to the community, I set forth on an "optimization" project. This has taken me several weeks of off-and-on testing and listening, but I think I've nailed down the changes necessary to turn this sleeper into a killer amp-on-a-budget. I tried to stick with changes that were BOM options only. I didn't tackle any modifications to the layout. I was NOT impressed with the layout of this board, as several optimizations could be had. However, I wanted to keep this fairly easy to accomplish for someone with a good soldering iron, a soldapullt, and some time on their hands.

My initial tests on the AudioPrecision unit basically confirmed my listening concerns - a 0.6dB difference between the left and right channels, a relatively poor frequency response (0.1dB drop at 20Hz and 1dB drop at 20KHz and falling off rapidly past that), and a THD+N graph that was all over the place. After spending quite a bit of time reverse engineering the circuit, the main issues became immediately evident. There's quite a bit of RF suppression with high-value capacitors, which leads to high frequency droop and phase issues. Also, some of the cap values used in the circuit were acting as a high-pass filter, causing the droop evident in the bass frequencies.

Anyways, enough chatting. I'll break these changes up into a few posts. Here's the changes that I propose for the rear panel. Note that there are two options - one if you want to keep the "auto switching circuit" and one if you don't. The only real advantage to not keeping the auto-switching circuit is that it costs slightly less and it removes the turn-on thump you hear when the amp first gets power.

Rear Panel Rework (Keeping the auto-switching circuit):

1. Change the pull-down resistors from 100K to 10K to improve the thermal noise. R426, R427, R428, R429 change to 10K, 1%. [Mouser PN 271-10K-RC]
2. Because pull-down resistors changed, also need to change value/quality of series caps. Change C411, C412, C413, C414 to 100uF, 16V Nichicon KA. Use KA because of temperature. [Mouser PN UKA1C101MDD1TD]
3. C408, C409 are the supply caps for the buffer op-amps. Change to audio quality caps, use Nichicon KA for temperature. Ordered 470uF, 16V. [Mouser PN UKA1C471MPD1TD]
4. IC4, IC5 are the buffering op-amps for the left and right signals. Change these to LM4562 for higher quality and higher GBW (helps avoid phasing issues). [Mouser PN LM4562NA/NOPB] Could also possibly use OPA2134, but bandwidth is not as high.
5. Relay DY1 needs to change to higher quality relay with lower contact resistance. Use Panasonic DS series. [Mouser PN DS2Y-S-DC12V]

Rear Panel Rework (Deleting the auto-switching circuit):

1. This rework will delete the auto-switching circuit. It costs less, but you lose the functionality of the LINE IN 1 jacks.
2. Remove the excess circuitry associated with the LINE IN 1 switching. This involves removing the DY1 relay, C411, C412, R426, R427, and IC5.
3. Change the pull-down resistors from 100K to 10K to improve the thermal noise. R428 and R429 change to 10K, 1%. [Mouser PN 271-10K-RC]
4. Because pull-down resistors changed, also need to change value/quality of series caps. Change C413 and C414 to 100uF, 16V Nichicon KA. Use KA because of temperature. [Mouser PN UKA1C101MDD1TD]
5. C408, C409 are the supply caps for the buffer op-amps. Change to audio quality caps, use Nichicon KA for temperature. Ordered 470uF, 16V. [Mouser PN UKA1C471MPD1TD]
6. Change IC4 to LM4562 for higher quality and higher GBW (helps avoid phasing issues). [Mouser PN LM4562NA/NOPB] Could also possibly use OPA2134, but bandwidth is not as high.
7. Because IC4 handles the “left circuit” and IC5 handles the “right channel”, we have to do some modifications after removing IC5 and the relay:
   • Using the attached relay pinout diagram at the bottom:
   • Run a wire from pin 4 to pin 6 of the removed relay
   • Run a wire from pin 13 to pin 8 of the removed relay
   • Connect a wire from R429 (the side closest to C413) to the location where R426 was (on the side closest to C411). Remember, R426 should be removed.

Relay Pinout:


Final Rear Panel showing all components while keeping the autoswitch:


Final Rear Panel with autoswitch deleted:

[tyger23]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

EDIT 5/1/15 - I added the resistor "RL1" to the reworks in step C of optimizing the amps. This is required for bridging mode. RL1 is located next to the grey bridging wire in the top right corner of the main board.

Main Board Reworks:

1. Replace POTs with better quality. The stock pots on my unit had horrible channel matching. These may not be significantly better, but they make a big difference in volume levels.
   a.) Volume pot is replaced with Bourns PDB182-K430K-104A. Note that this shaft is too long and you must cut down the shaft. Had to order longer length because that’s all Mouser carried.
   b.) Balance pot is replaced with Bourns PDB182-K220K-104B
   
2. Replace series capacitors with audio quality or remove them all-together.
   a.) Remove C101, C102, C201, C202. Short across on bottom of board where C101 and C201 were removed. Series caps are not needed in this location because there are already blocking caps on the rear panel.
   b.) C104, C204 = Replace with 100uF, 16V Nichicon KA. KA used for temperature (close to heat sink). [Mouser PN UKA1C101MDD1TD]
   
3. Change out the op-amp with better quality
   a.) Use an LM4562 for audio path (IC1) [Mouser PN LM4562NA/NOPB]. You could possibly use OPA2134 if preferred, but bandwidth is not as high.
4. Remove the RF immunity trap to help with high-frequency phase by removing C100 and C200. Note that this will make the amp more susceptible to RF noise. If RF noise experienced, replace caps with 220pF ceramic multi-layer caps. [Mouser PN FK28C0G1H221J]
5. Increase the gain level and fix the frequency response though the filter.
   a.) Change negative input impedance to 3.9K, 1% (R104, R204). Lowering impedance helps with thermal noise. [Mouser PN 271-3.9K-RC]
   b.) Change feedback resistors to 34K, 1% (R103, R203) [Mouser PN 271-34K-RC]
   c.) Remove the feedback capacitors (C103, C203)
6. Optimize the Amps:
   a.) Change Negative FB Capacitors (C107 and C207) from 100uF to 470uF, 16V Nichicon KA. KA used for temperature (close to heat sink). [Mouser PN UKA1C471MPD1TD]
   b.) Change Bootstrap capacitors C106 and C206 to Nichicon 100uF, 50V. I recommend using KA due to proximity to heat sink. [Mouser PN UKA1H101MPD1TD]
   c.) Change the FB resistors and input shunts to 27K (R106, R206, R107, R207, RL1). This will bump up the gain a little bit and put the amp in a “happier place”. [Mouser PN 271-27K-RC]
   d.) Remove the RF filter trap capacitors (C105, C205).
   e.) Change the FB shunt resistors to 732 ohms (R108, R208). [Mouser PN 271-732-RC]
   NOTE: Removing and replacing R107, R108, and R208 can be a bear due to the heat sink. I recommend NOT disconnecting the heat sink to avoid the hassle of cleaning of and replacing the factory thermal grease. You CAN remove these resistors without disconnecting the heat sink. You MAY be able to replace them. I was able to replace all but R207 on the top side. I replaced R207 by placing the resistor in the exact spot, but on the bottom side of the board. You can use a similar trick on any of these resistors for ease of installation. If you disconnect the heat sink, you should wipe off and replace the factory thermal grease with high-quality grease like Arctic Silver 5. Also, be sure to note the mica insulator between the amp and the metal. It must isolate the amp from the heat sink when you re-install it.
7. Optimize power supply
   a.) Change the Bulk Caps to Nichicon KW 6800uF, 50v. [Mouser PN UKW1H682MRD] (C901, C902). Note that these caps are too tall for the amp and have to sit at an angle tilted towards the heat sink. DO NOT cut the leads too short!!!
   b.) Op-amp supply caps to Nichicon KW, 470uF, 25V. [Mouser PN UKW1E471MPD] (C905, C906)
   c.) Diode bypass caps to X7R, 0.1uF, 50V. [Mouser PN FK28X7R1H104K] (C907, C908)

Here's the bottom of the main board after the reworks:


Here's the top of the main board, near the pots, after the reworks:


And here's the main board near the amp section:

[johnnyrichards]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

I now wish I had not sold mine. This looks like a fun project, and a great intro to modifying.

[tyger23]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

Edit 5/1/2015 - Cart updated to reflect the new requirement for 5x 27K resistors (change RL1 as mentioned in Post #2).

All told, these reworks come in at $35.27 in parts with today's pricing. That's a small price to pay for the large increase in performance I see.

With these changes, I now have much better matching between channels <0.3dB for most of the volume control, much better gain through the system (which helps when using low-level audio output devices like an iPhone or iPod), significantly better behaved harmonic distortion, and a much better frequency response.

The changes enable the amp to have 0.01dB droop at 20Hz and 0.025dB droop at 20KHz. Even at 100Khz, the amp is only dropping 0.45dB now, where as before it was almost completely gone by 100Khz.

Listening to the amp, it's much crisper now with significantly better highs and mids, less muddy bass, and better rapid transitions. I haven't compared this to my NAD just yet in a head-to-head battle, but I'll get to it shortly and post back.

Overall, this is $35 well worth spending on this amp. Given that you can buy the AMP 100 for about $80 in many places around the web, this means that you can have a true hi-fi quality amp for only about $120. I hope you all enjoy this, and let me know if you have any questions.

Thanks!
Ty

(oh, and assuming nobody makes any changes to it, here's the pre-built cart for the Mouser shopping basket: https://www.mouser.com/ProjectManage...sID=9566F60591)

[MSaturn]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

What kind of FETs are they using? Why not upgrade the power supply to 10,000uF a channel?

Also, I wish you could +1 users around here. This is a fantastic write-up.

[tyger23]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

What kind of FETs are they using?

No fets - just chip amps (TDA7294).

Why not upgrade the power supply to 10,000uF a channel?

A few reasons:
1. The 10,000uF would have been even harder to fit in physically.
2. The 10,000uF would have been more expensive.
3. I don't personally believe that 10,000uF would have bought you anything in this design. I actually considered dropping back to 4,700uF instead of the stock value of 6,800uF. Ended up sticking with the stock value, though.
4. The larger caps put additional charge and discharge stress on the rectifier circuit that I didn't want to account for.

You'll note that I didn't make any changes to the rectifier circuit, mostly because I don't think it would net any real performance increase. The current design uses 6A4 rectifiers. Changing to FREDs or something similar just isn't necessary on a linear power supply.

Also, I wish you could +1 users around here. This is a fantastic write-up.

Thanks!

[marscoast]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

What a cool project - and a great write-up! Kinda makes me want to send you my broken AMP-200! One channel died a few years back and it's been sitting on the shelf waiting for me to learn a thing or two (or fifty) about circuit troubleshooting.

[johnastockman]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

Nice work and great write-up! Even with my limited skills, it looks like I could even do that. I see these show up occasionally in thrift stores and pawn shops...might be a fun project to tackle and improve/test my electronic repair skills. Thanks!


John A.

[DanP]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

Nice job Tyger.

Dan

[Steve Henry]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

Nice job Tyger.

Dan

+2

[PWR RYD]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

+3

[kevinr]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

+4
Thanks for sharing and identifying these issues . Lots of these amps around ..... like one at my house .
Maybe this explains why it sounds underwhelming .
I was just going to replace mine with a Topping but maybe not now .

[bkeane1259]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

+5

Many thanks for the well thought out posts. Perfect!! This is a wonderful contribution.

[JRT]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

... Replace POTs with better quality. The stock pots on my unit had horrible channel matching. These may not be significantly better, but they make a big difference in volume levels.

a.) Volume pot is replaced with Bourns PDB182-K430K-104A. Note that this shaft is too long and you must cut down the shaft. Had to order longer length because that’s all Mouser carried.
b.) Balance pot is replaced with Bourns PDB182-K220K-104B

.

Looks like a good project. I was thinking about putting together a system for one of my nieces. She seems to be gaining an interest in better sounding playback.

I saw an earlier comment of yours bringing up use of LDRs in another thread. Would it be worth the added complication to remove the pots from the signal path and instead use the pots to control diode protected NPNs to control cap bypassed LEDs optically close coupled to LDRs to attenuate input? Bad idea?



.

[tyger23]

Re: How to turn your AudioSource AMP 100 into a real amp. (Paying it forward)

OK - so I've done some listening comparisons to my NAD amp (C 315BEE) on my Continuums, and it's not too shabby. The NAD still wins on clarity. It seems to have just a touch more treble and just a tad more punch in the low end. However, this AudioSource costs less than half (brand new + modifications).

The Audiosource comes across as "warmer". Almost like it's a tube amp or a class-A in comparison to the NAD. This likely has to do with the overall distortion. While the AudioSource is not bad ~0.04% THD+N across the board, it's not up to the distortion levels of the NAD (less than 0.01%).

I don't want to really say one is a winner over the other, as some people like warmth and some like clarity. Where the NAD is punchy and crystal clear, the AudioSource is mellow with an ever-so-slightly more pronounced mid-range. The NAD can actually come across as harsh with poorly recorded music, whereas the AudioSource doesn't.

Sadly, I don't have a factory AudioSource to compare it to, but I can definitely say that I'm more than pleased with these changes and highly recommend them! Enjoy!