Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

Jolida JD 1301 with LM1875T
Jolida JD 1301 with LM1875T
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 8th October 2019, 07:30 PM   #1
jjasniew is offline jjasniew  United States
diyAudio Member
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.
Joe Jasniewski

Last edited by jjasniew; 8th October 2019 at 07:38 PM.
  Reply With Quote
Old 14th October 2019, 07:25 AM   #2
jjasniew is offline jjasniew  United States
diyAudio Member
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.
Attached Images
File Type: jpg LM1875-DC-current-of-the-negative-feedback-circuit.jpg (69.2 KB, 51 views)
Joe Jasniewski
  Reply With Quote


Jolida JD 1301 with LM1875THide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Fake LM1875T - have I been had? Pentode Chip Amps 13 26th September 2019 07:28 AM
Molex 71694-1301 MiniFit Jr. IDC IDT 4-Pole Connector Lot of 373 Pieces HonorPC Vendor's Bazaar 0 21st December 2018 12:24 AM
TDA2040 vs LM1875T for guitar amp djduck Instruments and Amps 6 1st November 2015 07:23 AM
Hum on LM1875T - I can not get rid of it mertol Chip Amps 15 10th December 2011 07:31 PM

New To Site? Need Help?

All times are GMT. The time now is 09:34 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio