New "Kit" on the Bolck - BrianGT /LM3886 Upgrade

[danielwritesbac]

Useful input impedance for chipamp.com LM3886 and the audiosector LM3875, is from 4.7k to 22k. The center point, about 10k, is usually desirable and at line level spec.

A "Protect the Potentiometer" scheme can provide a nice 11k input load:
Here's how
Put a 1m as a load (from + to -) upon and directly at the RCA jack.
Put a 100pF to 220pF as a load upon and directly at the RCA jack.
Put a quality 4.7uF cap onto center pin of the RCA jack and in series with the signal+ so as to block DC.
A 22k potentiometer is now protected from DC damage.
A 22k potentiometer is now directly parallel with the 22k input load resistor onboard the chipamp.com kit, resulting in 22k//22k = 11k input load.

This is documented in the audiosector support forum to set DC offset at zero.
Of course there are many many options--I just wished to convey an example of input load.

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On your schematic at Picasa, if you appreciated replacing R1 with an LDR, then you would like to replace R3 with TWO Series carbon resitors that add up to approximately 680R. The task is identical albeit less cost. Both techniques fix insufficient isolation.

As for Ci being too small on the schematic, the wrong size is optional, but safety is not. 47uF is a bass blocker there. The problem is kind of like tiny lawnmower wheels on a big pickup truck. Instead of that, I would use a suitable size(s). In fact, I would try 47uF//330uF//4.7nF, which is, at least, big enough.

RZ is too small for anything other than supporting a lossy polyester dip cap (blob). If you have the more decorative box cap, which is seriously more efficient, then change the resistor to about 5.6R or any value that represents a reasonable load. The problem is kind of like Gain detergent, which doesn't work in the tumble dryer.

I have a competitor to your LDR circuit, and while mine doesn't use an LDR, both do attempt to make "a loss that isn't" so as to get a less forwards sound that is nicer to hear. I'll try to find the sketch of it. Both can be used on one amp, if you like.

[danielwritesbac]

Here is my BlareBuster circuit Although my triple parallel amp never did get popular, the much easier 2-chip parallel version has been doing well. That thread does not specify small signal parts, since it is merely a build method that works with most designs.

Frequency response?
Noticeably, neither my amplifiers nor the chipamp.com kit is a gainclone. They are chip amp (same power circuit as discrete amp). If level frequency response is desired, these chip amplifiers are reliant on a CRC power supply so that some extra large high quality capacitance is located not more than a couple of inches away from the amplifier. You can replace the "R" in the CRC with ordinary cables, which is useful if you have separate enclosures for power and amp--in any case, big caps are still located within a few inches of the amplifiers. This helps level the frequency response. See also an authentic CarlosFM power supply since it has all of the parts, very much different than the power supply that comes with kits.

[woodturner-fran]

I received my resistor replacer kits from Uriah today - although I could have done them on some vero board, I went for the kits from Uriah because they are pretty neat and small. One of mine had to fit into a fairly tight space, so I thought it would be a good idea. Plus they cost buttons.

Anyway, I soldered them up, hooked them up to my bench supply and adjusted them to 680R. No problem with this, although the resistor/zener does get warm (@30V supply). I wasn't expecting much of a difference - I had kiwames in there from Peter Daniels premium 3875 kit. I'm surprised though, there is a positive difference. I think its down to slightly better resolution. Soundstage seems more defined and wider. I thought maybe the bass was a bit deeper too, but now Im not convinced on that.

Seems like a very well worth while experiment, I'm glad I did it - plus now I can vary that resistor over a wide enough range to change the gain, without having to think about buying fancy brands.


Fran

[bcmbob]

Hi Fran, So glad to hear you like the upgrades. My take on the bass was the removal of some "mud" that allows one to hear lower tones better. I haven't done any adjusting but will give it a shot soon.

Regards,

[syklab]

Hi Bob,
Is the RR resistor replace kit available for order? Can this be applicable for other amp instead of Chip amp?

Thanks

[bcmbob]

Email Uriah Dailey at diyldr@gmail.com. It is his product and he can give you all the information you need. He had started applying this concept in other places but I'm not sure of the progress.

Here is his web site.Build An Amp

[danielwritesbac]

Originally Posted by syklab

Can this be applicable for other amp instead of Chip amp?

Creative stopper resistors are very much applicable to other amplifiers, especially fet amplifiers.
The IN+ and IN- connections of transistor/chip amplifiers are a good spot for trying either two resistors in series (instead of just one) or an LDR.
CDS is also workable when combined with some clear glue, some black paint and a flat top LED, thus creating a DIY LDR.

[troystg]

Has Uriah tried using an LDR in the feedback look to control volume that way?

[udailey]

Troy
I havent measured the voltage across the feedback resistor yet. I worry that it would be to high to be useful. LDRs like 2.5V or less across the resistive side. I think it could work for the feedback resistor that goes to ground though.
Uriah

[udailey]

Skylab
This resistor replacer can be used anywhere that 2.5V or less is across the resistive side and where the resistance is 40R-10k. Probably should really only be 40R-5k but depending on the precision needed you could go up to 10k and wobble around by about 5% on average. You have to determine if thats okay for your application.
Uriah