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Old 10th April 2011, 12:57 PM   #1
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Join Date: Sep 2008
Default Gainclone with tube preamp - too bright sound - need HELP!!!

Hi, everyone !
I recently built (with the help of the professional) a gainclone by purchasing Chinese kits on ebay.There's the 2 pcs of PCBs, ea. with a single LM3886 + the psu PCB (4x10000 mF Rubycons) + the tube preamp referenced to Marantz 7C (3x12AX7B tubes). The reason I entered this project was to build a spare amplifier which I'd be using after getting "tired" with too boomy/dirty bass produced by my push-pull 150 W RMS all-tube mono blocks. I have THIEL CS 1.6 speakers and basically I couldn't say that they have the "bright" character, but with this new gainclone the entire sound can be described as such. This especially counts when the volume control goes to a "10 o'clock" or higher position. The bass is definitely cleaner comparing to what it was with tube amps, it is quite deep - yet, all at the cost of mids and highs being too emphasized.
I must say that I expected "a good mixture of an old/tube technology with the
modern one like the LM3886s", but the result is far behind from my expectations. I am most kindly asking for the advice of what to do, now !?

Should I focus on the preamp, only or both (preamp and power amp section) ?
Should I replace all metal-film resistors to graphite, and which ones (manufacturer). ?
Should I replace the caps, which ones to which ?
Should I replace the Chinese tubes with other ones, and which ?

Please, help with your advice - in order to bring the sound "darker" and more pleasant to the ears.

Thank you all ahead of time.
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Old 10th April 2011, 01:23 PM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
I suspect your Chinese amplifier is on the point of oscillating. Changing a few components is NOT what you should be doing.
Investigating the Stability Margins is where you should be going.

If you can't do that yourself, then post lots of detail of the amplifier and we may be able to help.

I just don't understand why any beginner would ever trust a Chinese rip off to be designed properly and go ahead and buy knowing there will be no support from the retailer.
regards Andrew T.
Sent from my desktop computer using a keyboard

Last edited by AndrewT; 10th April 2011 at 01:38 PM.
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Old 10th April 2011, 01:32 PM   #3
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Join Date: Sep 2008
Thank you Andrew - I shall contact that guy who handles all the work and measuring, for me. I will also wait for a few more replies in order to collect all valuable information from you guys and talk to my friend. Then I shall come back, with all details - and photos from his instruments so that we can start from there. Thanks a lot !!!

Links for the photos:
Picture 055_m.jpg
Picture 056_m.jpg
Picture 057_m.jpg
Picture 058_m.jpg
Picture 059_m.jpg

Last edited by dadoarica1; 10th April 2011 at 01:45 PM. Reason: adding photos
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Old 10th April 2011, 02:50 PM   #4
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Join Date: Oct 2006
Do you have the links to the kits or the circuit diagrams that came with them? Whilst seeing the photos of your nice looking amp is very pleasant, we won't be able to assist you without the details of the build.

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Old 10th April 2011, 03:37 PM   #5
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Join Date: Sep 2008
Hi - thanks to both;
here's the link to the particular LM3886 kit with "connection schematics", but no real
el.schematics provided. (High-Performance 68Wx2 LM3886 Audio Amplifier Board REG | eBay)

The links to preamp's documentation:
M7C Tube Preamplifier User Manual.pdf

Last edited by dadoarica1; 10th April 2011 at 03:39 PM. Reason: wrong link posting
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Old 14th April 2011, 02:33 PM   #6
AUDIODH is offline AUDIODH  United States
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Location: Mayberry RFD, Alabama
I would suspect the tube pre-amp design. As AndrewT suggested, a real schematic would serve all well in determining the problem. I suspect the cathode has a parallel circuit to ground that set's the hi-pass frequency and the inter-stage coupling caps are likely suspects.
Oftentimes in many of these circuits you can use a simple equation to calculate the -3dB point: 1/2PI x (RxC) or roughly 6.283 time the sum of the R/C combination. It's a simple and easy place to start. If no schematic can be obtained.....reverse engineer it and draw one. Then we can help out.
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Old 14th April 2011, 06:59 PM   #7
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Join Date: Jun 2010
Location: St. Petersburg
Film decoupling capacitors could sound bright. Foil-paper-oil is better imho or you can try foil teflon instead.

However I would check your audio source first. In case very revealing premap-amp is in use it could let all source imperfections be audible.
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Old 14th April 2011, 07:31 PM   #8
AUDIODH is offline AUDIODH  United States
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Default hybrid amplifier

I don't think it is important to chase styles of capacitors until the design issue is resolved. I assume the amp section is flat, and also assume the tube circuit is contributing to the sound. I still contend a schematic is of best help, so the frequency response throughout the signal chain can be calculated and verified to be a good design, or perhaps, a wrong part value installed.
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