I bought what I thought was a decent used amplifier, a Niles Audio SI-275. However, the damn thing is noisy. When I connect my speakers to it, I hear a distinct 60hz hum, as well as HF noise.
I know it's not a diy amp, but I want to fix/improve this amp on my own, which I hope is diy'ish enough to post here ;-)
So where do I start?
SPECIFICATIONS
Design Principle: Linear voltage/current amplification.
Continuous Average Power Output
* 75 watts per channel RMS at 8 ohms.
* 100 watts per channel RMS at 4 ohms.
Input Impedance: Approximately 15K ohms
Input Sensitivity: .985V for 8 ohm rated output
Overall Voltage Gain: 30 dB
Frequency Response: Bandwidth Limited from 10 Hz to 100 kHz
Total Harmonic Distortion:
8 Ohms - .05% THD from 20 Hz to 20 kHz @ full rated power
4 Ohms - .06% THD from 20 Hz to 20 kHz @ full rated power
Here are some pictures of it's guts. I can post much higher res pictures upon request.
This is what the noise looks like when recorded in Arta.
And finally, a link to the manual
Any advice is appreciated.
Dan
I know it's not a diy amp, but I want to fix/improve this amp on my own, which I hope is diy'ish enough to post here ;-)
So where do I start?
SPECIFICATIONS
Design Principle: Linear voltage/current amplification.
Continuous Average Power Output
* 75 watts per channel RMS at 8 ohms.
* 100 watts per channel RMS at 4 ohms.
Input Impedance: Approximately 15K ohms
Input Sensitivity: .985V for 8 ohm rated output
Overall Voltage Gain: 30 dB
Frequency Response: Bandwidth Limited from 10 Hz to 100 kHz
Total Harmonic Distortion:
8 Ohms - .05% THD from 20 Hz to 20 kHz @ full rated power
4 Ohms - .06% THD from 20 Hz to 20 kHz @ full rated power
Here are some pictures of it's guts. I can post much higher res pictures upon request.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
This is what the noise looks like when recorded in Arta.
An externally hosted image should be here but it was not working when we last tested it.
And finally, a link to the manual
Any advice is appreciated.
Dan
Hi,
welcome.
your query is certainly DIYaudio.
Nice output stage.
Shame about the power supply.
The smoothing caps could usefully be doubled, or tripled and my brute force design method says +-40mF to +-50mF is needed for each channel.
That three pair is capable of a lot of current even with a hot heatsink. It is capable of at least 140W/4ohms with a highly reactive load (probably nearer 180W if it was not strangled by lack of current supply).
But that has nothing to do with your hum and/or noise problem.
Can you check resistance from the input RCA ground to audio ground and to safety earth?
Can you post a pic or two of how the grounding has been arranged?
welcome.
your query is certainly DIYaudio.
Nice output stage.
Shame about the power supply.
The smoothing caps could usefully be doubled, or tripled and my brute force design method says +-40mF to +-50mF is needed for each channel.
That three pair is capable of a lot of current even with a hot heatsink. It is capable of at least 140W/4ohms with a highly reactive load (probably nearer 180W if it was not strangled by lack of current supply).
But that has nothing to do with your hum and/or noise problem.
Can you check resistance from the input RCA ground to audio ground and to safety earth?
Can you post a pic or two of how the grounding has been arranged?
Using my DMM I measured no resistance between input RCA ground to audio ground. The power connector for this amp only has two prongs, so I guess there is no safety earth. Could that be part of the problem?
Are you referring to the three chips labeled A1386? There are a total of 12 of these (four groups of 3). The power supply has 4 x 10,000 uF 63v capacitors, are those the caps you recommend upgrading?
I'll take more detailed pics later today, as soon as the wife returns with our digital camera. For now you can see a high res version of the first pic I posted.
2560 x 1920 pic of internals - 2.5 MB
Dan
Are you referring to the three chips labeled A1386? There are a total of 12 of these (four groups of 3). The power supply has 4 x 10,000 uF 63v capacitors, are those the caps you recommend upgrading?
I'll take more detailed pics later today, as soon as the wife returns with our digital camera. For now you can see a high res version of the first pic I posted.
2560 x 1920 pic of internals - 2.5 MB
Dan
Here is a link to a page with 6 high resolution pictures of the guts of this amp. It's about 13mb of images, so 56k beware.
Link to pictures of internals. 13mb of images
In case it matters, my source is my computer. The soundcard, an M-Audio Audiophile 2496, shows no signs of noise when doing loopback tests.
Dan
Link to pictures of internals. 13mb of images
In case it matters, my source is my computer. The soundcard, an M-Audio Audiophile 2496, shows no signs of noise when doing loopback tests.
Dan
Maybe an oversight but its complimentary pairs 2SA1386 and 2SC3519 (3X 1386 & 3X 3519 per channel)owdi said:
I am not recommending that you change the capacitors. The other changes to allow that to work may be extensive. I just wonder why they have specified a super output stage that cost signifcant money and PCB space and heatsink area and yet skimmped on the transformer/rectifiers/smoothing caps. It all comes down to cost. The way they have done it will minimise warranty returns. That may be very important to them.owdi said:
The output stage is as Bigred identified.
Download the datasheet from Sanken, these are nice 130W 15A transistors. Can you identify the gain selection letters on them?
The power supply is fine, a pair of 10,000µF per 75W channel.
He has a ground loop with his computer.
Everyone needs one of these in his toolbox:
http://www.edcorusa.com/products/matchers/s2s10k-10k.html
He has a ground loop with his computer.
Everyone needs one of these in his toolbox:
http://www.edcorusa.com/products/matchers/s2s10k-10k.html
It might just be me, but I cannot see anywhere where the mains safety ground connects to the amp chassis, even though a 3-pin IEC socket is used.
The manual also suggests a 2-prong cable, so the chassis is not earthed. The amplifier might well be picking up hum in this case.
The manual also suggests a 2-prong cable, so the chassis is not earthed. The amplifier might well be picking up hum in this case.
AndrewT said:
Sanken 2SA1386 datasheet. Pardon my noobness, but how do I find the gain selection letters?
djk said:
I have a similar device, designed to isolate components. I opened it up and it's nothing more than a pair of transformers, one for each channel. I tested it using RMAA, and SNR and stereo separation became about 20db worse. I'd rather not use it, if possible.
jaycee said:
The socket only has 2 pins.
Dan
"I opened it up and it's nothing more than a pair of transformers, one for each channel. "
How else could it work?
Since the hum is 60hz, it cannot be the supply caps (that would be 120hz).
How expensive are your transformers?
The ones in the units sold for computer and car stereo use are the size of a pencil eraser and have limited bass. The one I recommended is much better. You want the best, go Jensen.
http://www.jensen-transformers.com/iso_aud.html
A good read (all the white papers and app notes are too):
http://www.jensen-transformers.com/an/ts_guide.pdf
"and SNR and stereo separation became about 20db worse. I'd rather not use it, if possible."
The SNR sounds like a measurement error. You care about separation? That's a red herring. A good phono sounds much better than most digital, and separation on a good cartridge seldom measures much past 20dB.
Do you have any cable lines hooked up?
Try the cheap transformer you have. It may not sound the best, but unless you don't also have a cable line in it should fix the problem. If it fixes the problem then consider a better transformer.
The Edcor units have enough core size for good low end, but have M6 steel cores for low cost. The most expensive units have high nickel cores to reduce H2 distortion. The small amount of H2 added by M6 steel cores is actually part of the 'sound' of some expensive pro equipment. Replacing M6 with high nickel in these products may result in loss of the 'warm, lush' sound that people buy them for.
Excerpts from a john curl thread:
"You people worry too much about superficial things."
"Listening is more important than measuring."
"Absolute THD is not very important to me, (because) it is usually below what most references would consider audible."
"I would prefer a transformer to an IC chip. I once designed out (removed from the design) the best IC chip that I could find.... when it failed critical listening tests."
How else could it work?
Since the hum is 60hz, it cannot be the supply caps (that would be 120hz).
How expensive are your transformers?
The ones in the units sold for computer and car stereo use are the size of a pencil eraser and have limited bass. The one I recommended is much better. You want the best, go Jensen.
http://www.jensen-transformers.com/iso_aud.html
A good read (all the white papers and app notes are too):
http://www.jensen-transformers.com/an/ts_guide.pdf
"and SNR and stereo separation became about 20db worse. I'd rather not use it, if possible."
The SNR sounds like a measurement error. You care about separation? That's a red herring. A good phono sounds much better than most digital, and separation on a good cartridge seldom measures much past 20dB.
Do you have any cable lines hooked up?
Try the cheap transformer you have. It may not sound the best, but unless you don't also have a cable line in it should fix the problem. If it fixes the problem then consider a better transformer.
The Edcor units have enough core size for good low end, but have M6 steel cores for low cost. The most expensive units have high nickel cores to reduce H2 distortion. The small amount of H2 added by M6 steel cores is actually part of the 'sound' of some expensive pro equipment. Replacing M6 with high nickel in these products may result in loss of the 'warm, lush' sound that people buy them for.
Excerpts from a john curl thread:
"You people worry too much about superficial things."
"Listening is more important than measuring."
"Absolute THD is not very important to me, (because) it is usually below what most references would consider audible."
"I would prefer a transformer to an IC chip. I once designed out (removed from the design) the best IC chip that I could find.... when it failed critical listening tests."
Hi,owdi said:
look for the hFE rank O, P, Y in the box labelled a or b shown in the package diagram.
djk - I'll try the isolation transformers and listen to how it sounds. I paid $40 for the unit I had, but it was from a car stereo shop, so I doubt I got my dollars worth.
AndrewT - I think the hFE rating is O, which according to the spec sheet is 50-100. You can see it in this picture.
Dan
AndrewT - I think the hFE rating is O, which according to the spec sheet is 50-100. You can see it in this picture.
Dan
If theres no chassis earth, how can it be a ground loop ?
owdi, try it with some portable device such as a CD walkman, MP3 player etc.. something battery operated. If you still get hum, then the amp is picking it up through the case, in which case a cure is to ground the case - perhaps swap out the IEC connector for a 3-pin type.
owdi, try it with some portable device such as a CD walkman, MP3 player etc.. something battery operated. If you still get hum, then the amp is picking it up through the case, in which case a cure is to ground the case - perhaps swap out the IEC connector for a 3-pin type.
I spent a few hours today taking some more measurements, in order to get a good handle on what problem I am trying to fix. It turns out the problem is me, and the stupid way I took my earlier measurements.
I like a quiet listening environment, so I have my computer in another room from my speakers. I just use a long RCA cable. To perform today's tests, I moved the amplifier into the same room as the computer, and plugged it into the same power strip. Surprise surprise... I couldn't measure any noise. Looks like the quality of the power wiring in my apartment is not very good.
I did get some interesting results when measuring my isolation transformers, so this has not been a total waste. Those will be posted shortly.
Dan
I like a quiet listening environment, so I have my computer in another room from my speakers. I just use a long RCA cable. To perform today's tests, I moved the amplifier into the same room as the computer, and plugged it into the same power strip. Surprise surprise... I couldn't measure any noise. Looks like the quality of the power wiring in my apartment is not very good.
I did get some interesting results when measuring my isolation transformers, so this has not been a total waste. Those will be posted shortly.
Dan
All tests were performed with an M-Audio Audiophile 2496 @ 16bit/96khz
Loopback frequency response
Frequency response with isolation transformer
Frequency response with Niles SI-275 amplifier, no isolation transformer
Loopback harmonic distortion
Harmonic distortion with isolation transformer
Harmonic distortion with Niles SI-275 amplifier, no isolation transformer
I also took measurements with the amplifier and isolation transformer, but it looked just like when the isolation transformer was plugged in by itself. I was pretty shocked just how much odd order harmonic distortion the isolation transfomer caused.
Dan
Loopback frequency response
An externally hosted image should be here but it was not working when we last tested it.
Frequency response with isolation transformer
An externally hosted image should be here but it was not working when we last tested it.
Frequency response with Niles SI-275 amplifier, no isolation transformer
An externally hosted image should be here but it was not working when we last tested it.
Loopback harmonic distortion
An externally hosted image should be here but it was not working when we last tested it.
Harmonic distortion with isolation transformer
An externally hosted image should be here but it was not working when we last tested it.
Harmonic distortion with Niles SI-275 amplifier, no isolation transformer
An externally hosted image should be here but it was not working when we last tested it.
I also took measurements with the amplifier and isolation transformer, but it looked just like when the isolation transformer was plugged in by itself. I was pretty shocked just how much odd order harmonic distortion the isolation transfomer caused.
Dan
"If theres no chassis earth, how can it be a ground loop ?"
Capacitance between the power supply components and the chassis allows current to flow down the shield to the computer which has a grounded plug.
Capacitance between the power supply components and the chassis allows current to flow down the shield to the computer which has a grounded plug.
"If there's no chassis earth, how can it be a ground loop ?"
or two or more routes inside the amp that are connected, eg, two channel inputs both directly connected to power ground/audio ground.
or two or more routes inside the amp that are connected, eg, two channel inputs both directly connected to power ground/audio ground.
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