If you made a little mistake then just fix it or acknowledge Andrew's help. It was not a negative comment. This is a forum that should be based on fact and when a statement is wrong but not corrected most will view it as fact.
The DC offset on this amp is abysmal. I think you will be very unhappy with it as long as you have DC offset levels like those quoted. I enjoy my LM3886 amp with 2 and 3mV offset. Offset pushes out the drivers for 100% of the time the amp is powered. It lowers your dynamic range and you will actually miss much of the music.
I did a little research and found, guess who... AndrewT explaining the problem for us. He is followed by another explanation from another knowledgeable member so read on from the link.
http://www.diyaudio.com/forums/chip-amps/205087-dc-offset-gain-gc-lm3886-2.html#post2877824
The offset must be fixed. You could place a huge cap at the output or vary some of the resistors values to see if you can reduce it.
Uriah
The DC offset on this amp is abysmal. I think you will be very unhappy with it as long as you have DC offset levels like those quoted. I enjoy my LM3886 amp with 2 and 3mV offset. Offset pushes out the drivers for 100% of the time the amp is powered. It lowers your dynamic range and you will actually miss much of the music.
I did a little research and found, guess who... AndrewT explaining the problem for us. He is followed by another explanation from another knowledgeable member so read on from the link.
http://www.diyaudio.com/forums/chip-amps/205087-dc-offset-gain-gc-lm3886-2.html#post2877824
The offset must be fixed. You could place a huge cap at the output or vary some of the resistors values to see if you can reduce it.
Uriah
i'm telling you the truth🙂 this thing is heavier to drive than the hifiman he-6
this is a link to the specs of my cans: Wharfedale ID1 Isodynamic - Wikiphonia
peter daniel does not use capacitor at all in the signal path. i followed his recommendation
as peter daniel suggests, i only soldered R1(BG NX 22uF), R2, R3, & Rf in the amp board. what do you think may cause this problem?
DC coupling, or mixed AC & DC coupling, is likely to result in highish output offsets.
These offsets are very much more likely to vary depending on volume controls and whether the sources are connected or not connected.
These offsets are very much more likely to vary depending on volume controls and whether the sources are connected or not connected.
On page 5 of this thread I'm showing that offset (in this particular circuit configuration) depends greatly on a chip itself, as well on input impedance (R2): http://www.diyaudio.com/forums/audi...-kit-building-instructions-5.html#post1524877
To get lower offsets you need to ask me for sorted chips.
The 6moons review of my first amp shows that GC is actually quite a good match for AKG K1000 (which I own myself as well): 6moons audio reviews: Audio Zone AMP-1 by Audio Oasis This amp was using only BG STD 1000/50 in PS filtering
You can still get better results in balanced configuration and one particular amp I've built recently was specifically dedicated to heaphone use: http://www.diyaudio.com/forums/audi...kit-building-instructions-69.html#post2432391
BTW, you will find a lot of additional info browsing links thread: http://www.diyaudio.com/forums/audio-sector/149343-useful-links-audiosector-not-only-projects.html
To get lower offsets you need to ask me for sorted chips.
The 6moons review of my first amp shows that GC is actually quite a good match for AKG K1000 (which I own myself as well): 6moons audio reviews: Audio Zone AMP-1 by Audio Oasis This amp was using only BG STD 1000/50 in PS filtering
You can still get better results in balanced configuration and one particular amp I've built recently was specifically dedicated to heaphone use: http://www.diyaudio.com/forums/audi...kit-building-instructions-69.html#post2432391
BTW, you will find a lot of additional info browsing links thread: http://www.diyaudio.com/forums/audio-sector/149343-useful-links-audiosector-not-only-projects.html
Beats me. In PD's manual he states in the last sentence on pg 45 that he doesnt know how to fix offset. He also says dont go to low with Rin which you dont have.
In the datasheet for LM3875 on page 16 toward the bottom Nat Semi suggests high values of feedback resistors to beat DC offset. For instance they mention 100k and 5k1 for Rf and R3.
I have done well over an hour or so of research and have not found ANY results that specifically address removing offset on a non-inverting lm3875 that already has a DC blocking cap in the signal.
Wish I had more for you.
One thing to consider before you go ahead..
Your headphones need 30mW to go to 95dB. They are 120 Ohms. You need about 2V into your cans to get more than 30mW. 2V/120R=.01667 amps multiply that by your voltage and you have .032W or 32mW. You are connecting it to an amp capable of driving more than 10W into your cans and completely rendering you deaf at the same time destroying your cans. Its possible to alter the input section of this amp to keep it in the range of normal and safe listening but I dont think you should be attempting this at your current level of DIY expertise. At least not with these expensive phones and never actually placing them on your ears until you do get it working. Also the gain is much to high right now to be using this as a headamp.
Uriah
Edit : From Peter's last post I see he is using similar cans. My math seems right to me but I could be wrong. It doesnt make sense to me to use a full blown chip amp for this job as its to easy to overdrive them. Maybe I can be set straight about this.
In the datasheet for LM3875 on page 16 toward the bottom Nat Semi suggests high values of feedback resistors to beat DC offset. For instance they mention 100k and 5k1 for Rf and R3.
I have done well over an hour or so of research and have not found ANY results that specifically address removing offset on a non-inverting lm3875 that already has a DC blocking cap in the signal.
Wish I had more for you.
One thing to consider before you go ahead..
Your headphones need 30mW to go to 95dB. They are 120 Ohms. You need about 2V into your cans to get more than 30mW. 2V/120R=.01667 amps multiply that by your voltage and you have .032W or 32mW. You are connecting it to an amp capable of driving more than 10W into your cans and completely rendering you deaf at the same time destroying your cans. Its possible to alter the input section of this amp to keep it in the range of normal and safe listening but I dont think you should be attempting this at your current level of DIY expertise. At least not with these expensive phones and never actually placing them on your ears until you do get it working. Also the gain is much to high right now to be using this as a headamp.
Uriah
Edit : From Peter's last post I see he is using similar cans. My math seems right to me but I could be wrong. It doesnt make sense to me to use a full blown chip amp for this job as its to easy to overdrive them. Maybe I can be set straight about this.
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Peter I read your headphone amp post. So, we can drive HD800 at safe levels with a normal input voltage like 1-2V by only changing R3 to 1k2? Is this the only change and this headphone amp does not damage the phones at max volume?
Uriah
Uriah
You can drive any headphones to a safe level as long as you are using volume control. More info: http://www.diyaudio.com/forums/headphone-systems/62828-lm3875-headphone-amp-another-try.html
To be accurate, I present few ways to decrease the offset by means of selecting parts or adjusting components values. In a minimized GC circuit, there's no other ways to control the offset, which will mainly depend on chip itself:
In real life, the offset is quite low, especially with amp connected directly to preamp stage with low impedance buffered output. I often measure chips for offset and it's quite possible to select them for less than 5mV offset in a given configuration. Ci cap is in a signal path, and it will influence the sound basically same way as input coupling cap. That's why I prefer not to use it. In a minimized GC circuit, I don't see a way to adjust offset with a trimpot. Lowering the input resistance will decrease the offset, but you might not want go too low with it.
OTOH, I had a Gaincard amp for testing and that amp, although using datasheet schematic and input coupling caps, was still showing the offset in a range of 80mv.
If the DC coupled chipamp input sees a near constant source resistance, then it is possible to adjust the offset downwards.
The NFB upper and lower leg resistances load the -IN PIN.
Match the -IN PIN resistance to the +IN PIN source resistance.
If the +IN PIN sees a variable source resistance (eg volume control or disconnected source, or relay muting), then that matching method does not work well.
The NFB upper and lower leg resistances load the -IN PIN.
Match the -IN PIN resistance to the +IN PIN source resistance.
If the +IN PIN sees a variable source resistance (eg volume control or disconnected source, or relay muting), then that matching method does not work well.
@udailey: thanks for your reply. it doesn't make sense to me as well, but i have a pretty sensitive hearing, yet i have to crank up the volume to 1-2 o'clock
good news, i cleaned the amp and rectifier board with acetone earlier this evening, and it yields quite significant result: the dc offset reduced to 17mV and 35mV respectively 🙂
funny thing happened this evening, while cleaning the pcb with acetone, i removed the BG NX as well, replacing it with a piece of wire (i use potentiometer, so it counts as R1 as well)
the sound of this amp now sounds a little more harsh than with the BG NX. it's supposed to be the other way around, isn't it? lol
but without the input caps, the treble sounds a little more crispier and airier..
good news, i cleaned the amp and rectifier board with acetone earlier this evening, and it yields quite significant result: the dc offset reduced to 17mV and 35mV respectively 🙂
funny thing happened this evening, while cleaning the pcb with acetone, i removed the BG NX as well, replacing it with a piece of wire (i use potentiometer, so it counts as R1 as well)
the sound of this amp now sounds a little more harsh than with the BG NX. it's supposed to be the other way around, isn't it? lol
but without the input caps, the treble sounds a little more crispier and airier..
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Wow, thats great! I have to doubt for a moment that cleaning made the difference. I will bet that you have a cold solder joint somewhere and that you pushed the solder connection around a little with cleaning. I suggest to go back and warm each of the solder joints with your iron til they flow and then remove the iron. If I am right about this you will get your DC offset problem again if you dont reflow the joints.
Great that you have found or are close to the solution.
Uriah
Great that you have found or are close to the solution.
Uriah
well, i did reflow the solder joint, but the leftover flux is pretty messy as well. so i guess both actions contributed to the lower dc offset? 😉
as i previously mentioned, without the caps, the treble sounds crispier and airier, but produce a little sibilance in the upper midrange. i hope it will be gone within a few days of burn in
That is quite normal, as BG NX tend to warm and smooth the sound by itself, it's what they usually do.
What value linear pot and law faking resistor do I need to make a 20k audio pot?I was listening to some vinyl this evening using my version[copy] of Peters Gainclone amp and during quiet passages there was the occasional breakthrough of a German radio station;never experienced that before. Any pointers as how to eliminate this would be much appreciated. tia.
so i have listened again to the gainclone (or patek clone?) after i swap the capacitor on the rectifier board with BG STD 1000uF
now my configuration is like this:
trafo: talema 2x25V 4.5A (2A slow blow fuse)
amp board caps: BG F 100uF/50V
rect board caps: BG STD 1000uF/50V
R1: shorted
R2, Rf: caddock mk132
R3: audionote tantalum
V+ & V- wire from rect to amp board: mogami 2534 (2 wires/channel)
now i find the sound very pleasing.. treble is smooth and very extended. airy and crispy. mid is very open and detailed, not forward nor laid-back. just perfect. bass note is very detail and nice
but there's one problem. i still find the low bass is lacking the punch i want. the low bass detail is there, in fact, every little detail is there.. but there's almost no impact/punch in it..
is there any way i can fix this small (yet noticeable) flaw?
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