One Channel Lower Bass/Mid Response

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System: All Chinese gear

Speakers Two-way bookshelf with Kasun outboard crossovers. Heavily modified from the ground up retaining only parts of the cabinets. $70 for all parts for the pair.

Solid State Amplification Two 110v constant v PA amps 50w that run in mono that have a 4-16ohm post pair for each. $26 each. $52 for the monoblock set up. Cheap Chinese mosfet output transistors provide the power.

Tube preamp stage One Mercator Little Dot 6J1 clone as preamp. $27. When used alone or as headfi amp, all channels good, balanced and FR SQ is even.

Digital Source DMP $15 with 3.5mm output split to RCA to the preamp. FLAC, MP3 on USB. 3.5mm output from the Mercator tube preamp, L to one amp, R to the other.

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Tweeter high end is unaffected through all.

When I play a USB source through the DMP only and run that output to the two amps, mid and bass response is equal and ok. Audibly and by visual observation of the 3" midrange. However, when I insert the Mercator clone preamp, one amp always has reduced bass response.

I have swapped out all cables, swapped speakers, reversed channel on all connections, reversed polarity, changed sources - it only occurs when one amp is used with the tube preamp. The other amp is fine, both speakers are ok, and when using a DMP as the preamp source the issue does not arise. I have checked the wires with a DMM, the xo, the output and input of the xo - it's all fine when swapping out the bad amp. All amps are running cool to the touch, no blown fuses anywhere.

The problem goes away when the tube preamp is removed from the chain.

Reading up this could be out of spec capacitors or transformers that are responding oddly to the tube preamp load. Considering the cost of everything involved, I will simply buy a stereo amp.

I was hoping that this would proof of concept a cheap monoblock setup that I could scale up to a multi-amp active crossover setup. It appears that one of the links in the chain is too weak for this.

It is a shame because the sound is **very good** - the tube preamp adds a lot of bass response, oompf and detail in the upper bass ranges on electric bass. Those two small speakers sound very good indeed for $175 all in.

Maybe if I get a larger amp, 80 or 100 or 200 w per side, this will not occur. Or it could just be this one amp. There are many many issues with gear in China. One of the dozens is that normal receivers and stereo amplifiers simply do not exist. There are KTV amps, 5.1 and 7.1 theatre amps, and 110v amps for small businesses to use, and then the high end, much of that big names you know that are double or treble the price. A stereo SS 2.0 amp simply isn't to be found. Tubes? Many.

Much as I would like to multi-amp an active crossover setup using only 6 or 8 Krell Monoblocks, that is not a possibility so a more cost effective solution is my goal. I don't want to have 4 or 8 tube amps running mains as I don't want to keep up with tube pair matching and replacement. However, a very nice tube preamp to soften the sound? Yes.

Any insights would be welcome, but the gear involved is so low price that it will be faster and cheaper to just buy a replacement rather than trouble shoot.
 
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