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Chip Amps Amplifiers based on integrated circuits

Jolida JD 1301 with LM1875T
Jolida JD 1301 with LM1875T
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Old 8th October 2019, 07:30 PM   #1
jjasniew is offline jjasniew  United States
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Join Date: Feb 2002
Location: Roy, WA
Default Jolida JD 1301 with LM1875T

I went fishing inside my JD 1301 amp, originally in an attempt to locate the LM1875T feedback resistor. (I was hoping to make the feedback remote sensed)

While there, I caught a couple of 100 uF Aluminum-Electrolytics in the feedback network, to ground. I never liked the sound of this amp compared to my Jolida JD 102B, an all tube unit I have running JJ EL84s in triode mode. I figured out why.

I appreciate this forum where I read that it's basically OK to eliminate these caps. I did so and measure ~30mV on each channel's speaker outputs, which I also read wont be a problem.

Making this one change has rendered the little amplifier wonderful sounding, on par with the tube unit if not better. I assume the idea behind this amp is a tube stage providing some 2nd order harmonic distortion, followed by a power amplifier with zero distortion - at least at the levels I listen at.

This architecture works across a wide range of system implementations, from a little easy-peasy amp the JD 1301 is, to that which was reviewed in this Stereophile article Lamm Industries L2 Reference preamplifier | Stereophile.com

There, we have a $13,600 preamp with "an absolute dominance of the second-order harmonic", followed by a pair of Krell 350MC monoblocks. Not claiming the little Jolida has the same sound as $50K worth of hardware, only the same idea.

And once I got rid of those little 100uF Al-Els in-the-signal-chain buggers, it sounds a lot more in the right direction. Just in case anyone has this amp, or is using the "stock" LM1875T circuit, that cap makes a difference. It should be eliminated, or of the same quality as the best cap in the signal chain.

Yes, I should probably put fuses in the speaker wiring - and hope the coupling caps from the tube stages never leak. I realize the LM1875 part is coupled all the way to DC now, opening the door for such trouble.

If I knew that cap was there, I would have never bought the amp in the first place. They put polyprops coupling the tube outputs, then these little POS - with AC audio across them (worst case) no less, as it's a +/- power supply to the chip. Hope this helps someone. Thanks for the info being here and to the folks putting that here over the years.
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Last edited by jjasniew; 8th October 2019 at 07:38 PM.
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Old 14th October 2019, 07:25 AM   #2
jjasniew is offline jjasniew  United States
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Join Date: Feb 2002
Location: Roy, WA
Today I modified the LM1875 feedback section to be similar to this;

The amp's stock feedback component values were 22k, 1k to a 100 uf cap to ground.
I replaced the 100uf with a 1k resistor. I connected a second 22k to this junction, then to the speaker side of a 0.27 ohm resistor I put in series with the amp's output (+).

The feedback arrangement apparently combines voltage feedback with current feedback, which I assume also increases the amp's output impedance. Replacing the upper 1K resistor with a potentiometer, I assume one could then make the output impedance vary from a voltage amp towards a current amp.

Maybe not all the way to infinity, but enough to more closely emulate (and perhaps go well beyond) 4-8-16 Ohm amplifier output impedances associated with transformers and tubes.

For now, I think I was able to substantially improve the sound of this little amp by simply adding in a few resistors I happened to have lying around in the right places. I really like this kind of mod.

The music definitely takes on a bit more life and I easily notice that recording quality is revealed - both ways; lousy and great - listening to jazz streams of songs I've never heard before. Head over the Radio Paradise flac streams, where I might catch an old classic from my generation, it's fun to realize I dont think I've ever heard "Funk 49" that clearly - it's easy to be hit like that.

I'll for sure be experimenting with bigger / better current sense resistors. Perhaps adding a Linkwitz damped output inductor would help to shield that speaker side feedback input from noise on the speaker cables.
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File Type: jpg LM1875-DC-current-of-the-negative-feedback-circuit.jpg (69.2 KB, 51 views)
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