Joe Rasmussen said:
Thanks Joe, I was afraid the answer was already posted somewhere in the thread and I simply had missed it. I knew about the 18 K value being the value of 220 K in parallell with 22 K so my comment should rather have read:
Does the 18 K resistor also degrade the sound quality and if so how do I remedy that?
Joe, actually I think KYW states that increasing the feedback resistor remedies the sound degradation of the 22 K from the negative input. I would also like to know why exactly that is but also as in my previous reply to you the question is how, and if, the 18 K from the POSITIVE input degrades the sound and how to remedy that.
I did some listening comparisons, and the sound is different with 250K and 300K (what I used). You might try it yourself and decide which you prefer. I'm also using log pot and the control range is very good, both with average efficiency speakers and horns (100db efficient). So you might also consider using that type of pot instead of linear.
Peter Daniel said:
How about shorting it?
Simple answer, but with the manufacturing drift these chips have you may have from as little as 7~8mv DC Offset to around 50mv.
And I love my speakers.
Kuei Yang Wang said:
I did a board with LM1875s and 330k as feedback resistors.
I used the 22k resistor as always, but for initial tests I ommited the 18k (direct wire).
I had almost 40mv DC.
So, I removed the direct wire and put 50k multi-turn pots.
Final resistance of the pots at 0.0mv: 18.9k.
But the 22k resistor remained in place, it's not as you're saying, Kuei.
Maby removing the 22k resistor...
Ahhh... nevermind, the board is finished and looks loovely with the 50k multi-turn pots.
Kuei Yang Wang said:
OK, let's get a handle on this, we seem to be discussing moving goalposts.
On the basis of above re 22K & 18K, it should look like this now: (?)
An externally hosted image should be here but it was not working when we last tested it.
Won't this now give us 300mV DC on the output? The circuit (my apologies for amending it) seems to be missing something?
IF we left the 22K from (-) to ground, then we'd get 22mV DC Off-set, that is... not great, but not that bad.
Peter Daniel said:
Sure does change the sound... seems maybe there is a consensus that +30dB is good? If so it's come to the same one I made about 10 months ago.
IF I had 100dB sensitivity speakers, I'd choose a Log pot too, but with normal sensitivity - circa 90dB or a little less - I'd stick with Lin.
Joe R.
one thing I had already wondered about.
On the schematic Joe had posted. The wiper is essentially grounded, for A/C signal, by a combo of the input resister (10K) and feedback resistor (220K/300K) in parallel (the small speaker impedance not withstanding). so the wiper has a A/C load of less than 10K.
so even if the pot is log, is the output really log given that the wiper is loaded by a <10K resistor network?
On the schematic Joe had posted. The wiper is essentially grounded, for A/C signal, by a combo of the input resister (10K) and feedback resistor (220K/300K) in parallel (the small speaker impedance not withstanding). so the wiper has a A/C load of less than 10K.
so even if the pot is log, is the output really log given that the wiper is loaded by a <10K resistor network?
Konnichiwa,
Well, enough folks build the same circuit with a 220K feedback resistor and have low offset. Going up from 220k to 330k (NOT 300k - I'm not sure why keep insisting on that value) will increase the offset at the worst by 1/3rd.
No, it is "hyper log", if you so want. Also, the fairly low AC load will cause all of sorts of interesting effects if conductive plastic (YUCK) pot's are used, as the wiper impedance on these is dependent on the current through the wiper....
Sayonara
Joe Rasmussen said:
Well, enough folks build the same circuit with a 220K feedback resistor and have low offset. Going up from 220k to 330k (NOT 300k - I'm not sure why keep insisting on that value) will increase the offset at the worst by 1/3rd.
millwood said:
No, it is "hyper log", if you so want. Also, the fairly low AC load will cause all of sorts of interesting effects if conductive plastic (YUCK) pot's are used, as the wiper impedance on these is dependent on the current through the wiper....
Sayonara
Maybe more yawn than yuck
Walt Jung reported on the wiper contact nonlinearity for carbon pots over 25 years ago in The Audio Amateur PAT-5/WJ-lA preamp mod article as I remember, and recommended several Meg as a minimum load. I changed my 100K Alps Black Beauty pot for law faking cermets and will never go back, The Alps is very colored in tonal balance but does complement slightly hard or bright electronics. the loss in resolution is what really made me give up on them. There are some cheaper better sound conductive plastic pots than the Alps pot, but I take cermet pots over them as well.
Walt Jung reported on the wiper contact nonlinearity for carbon pots over 25 years ago in The Audio Amateur PAT-5/WJ-lA preamp mod article as I remember, and recommended several Meg as a minimum load. I changed my 100K Alps Black Beauty pot for law faking cermets and will never go back, The Alps is very colored in tonal balance but does complement slightly hard or bright electronics. the loss in resolution is what really made me give up on them. There are some cheaper better sound conductive plastic pots than the Alps pot, but I take cermet pots over them as well.
I thought it wouldn’t be bad to tell this before someone tries that. 
Joe Rasmussen said:
in the particular schematic, the inverting input is essentially DC grounded via the feedback resistor (the speaker DC resistance is too little vis-a-vis the feedback resistor). National says that the typical input bias current for LM3875 is 0.2ua. and since the non-inverting end is grounded, so the output offset is 0.2ua*300K=60mv.
it would then reason that for lower DC offset, one shouldn't be using the coupling cap on the input so that the input resistor would help lower the resistance from the input to ground, thus lowering DC offset.
DC gain gain basics
Eliminate the coupling cap on the input and you then have a DC gain equal to the AC gain instead of unity gain as exist in the present topology. There is also a contribution from offset voltage between the inverting and the non-inverting inputs which is not a function of input bias current through the feedback resistors. Remove the cap and you also have the contribution of the preamp offset voltage times the closed loop gain. This will vary with pot setting. Offset due to bias current through the pot will change for different volume settings as well. This is all very basic op amp theory which is in most op amp tutorials. You will very likely be better off with an input capacitor for low and consistent offset voltage, which also can be nulled further by a DC voltage on the non-inverting input. This is very basic stuff that one should really make an effort to understand if your going to build even very simple Chip amplifiers.
Eliminate the coupling cap on the input and you then have a DC gain equal to the AC gain instead of unity gain as exist in the present topology. There is also a contribution from offset voltage between the inverting and the non-inverting inputs which is not a function of input bias current through the feedback resistors. Remove the cap and you also have the contribution of the preamp offset voltage times the closed loop gain. This will vary with pot setting. Offset due to bias current through the pot will change for different volume settings as well. This is all very basic op amp theory which is in most op amp tutorials. You will very likely be better off with an input capacitor for low and consistent offset voltage, which also can be nulled further by a DC voltage on the non-inverting input. This is very basic stuff that one should really make an effort to understand if your going to build even very simple Chip amplifiers.
Peter Daniel said:
As they said at Pearl Harbour... too many Zeros.
I posted that 300mV figure just as I was about to hop into the car and ten minutes later...
. My JLTi & The DIY Hybrid IGC uses 1M feedback resistors and gives 100mV DC Off-set if left uncorrected - so naturally 300K should give 30mV and not 300mV!In fact the input currrent is 0.1uA, so:
0.0000001A x 300000R = 0.03V or 30mV, so this tallies nicely with your 28mV (250K, right).
While I don't think that is great, it is not a major downer. In fact, even if we had 4 Ohm speakers with 3 Ohm DCR, it is only 0.3mW dissipation in the voice coil (and a very slight pull of centre which will reduce effective Xmax), and it won't affect weeney tweeter voice coils due to high-pass C/O.
One thing I've noticed, the DC parameters of the 3875 are rather good, in fact especially that this isn't just an opamp but a power opamp!
Joe R.
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